One tide mark, one beeping instrument, one four-figure quote
The wall is the party wall of a 1900s terrace, the stain runs about 900 mm up from the skirting with a ragged upper edge, and the plaster behind the sofa has blown in two patches the size of dinner plates. Somebody has been round with a pin meter, pressed it against the wall at half a dozen heights, watched it read high near the floor and low near the picture rail, and written rising damp on the form. The quote that followed is for a drilled chemical damp-proof course the full length of the wall, plaster hacked off to a metre, and a waterproofed sand-cement render to replace it, guarantee attached.
That treatment addresses exactly one of the four things that put water into a wall, and it is the least common of the four in a house of that age with a functioning original slate course. If one of the other three is what is actually happening, the injection will go in, the render will hide the symptom for two or three years, and the fault will still be running underneath it.
So diagnosis comes first, and diagnosis means separating four mechanisms that all end in a damp wall and a mould smell: water rising out of the ground, water crossing the wall from outside, water condensing on the room face, and water condensing inside the build-up where nobody can see it. Each has a condition it cannot do without. Find the missing condition and the mechanism is eliminated, without having to prove anything about the others.
What each mechanism has to have before it can happen
Rising damp is capillary transport. It needs an unbroken path of fine pores from ground moisture into the masonry, uninterrupted by a working damp-proof course — or by one that has been bridged by raised external ground, a rendered plinth carried down past it, or a screed poured over it. It also has a ceiling. Water rises until evaporation from the face balances the supply through the pores, and BRE Digest 245, Rising damp in walls: diagnosis and treatment, describes that limit as rarely more than about a metre in ordinary brickwork. A band that starts at skirting level and stops with a defined upper edge is consistent with it. A band starting at first-floor sill height is not, whatever a meter says.
Penetrating damp is bulk water crossing the wall laterally through a defect, and its distinguishing feature is that it keeps time with the weather. Cracked render, open perpends, a cavity bridged by mortar snots or by insulation that got wet on installation, a blocked tray over a window head, a downpipe joint spraying brickwork for the length of a storm — all wet a wall during driving rain and dry back between events. BS 8104, Code of practice for assessing exposure of walls to wind-driven rain, is what puts a number on how hard a given elevation gets hit, because the same detail behaves very differently on a sheltered inner-city terrace and on a west-facing gable in Cornwall.
Condensation needs no external water source at all. It needs indoor air carrying a dew point above the temperature of the surface it touches, which makes it a problem of two numbers that have nothing to do with the ground and everything to do with heating, ventilation and how cold the coldest bit of the room gets. Interstitial condensation is the same physics moved inside the wall: vapour arriving at a cold layer within the build-up faster than the layers outboard of it can pass it on. Both are covered by BS 5250, Management of moisture in buildings, which is the code of practice worth reading before any of this gets argued about with a contractor.
| Mechanism | Cannot happen without | How it behaves across a year | What rules it out |
|---|---|---|---|
| Rising damp | A continuous capillary path from ground moisture, with no working damp-proof course or a route around one | Present in every season, most visible in the heating months when the face evaporates fastest; a stable upper edge rather than a creeping one | A wall with no ground contact, anything above the ground floor, or a damp band whose lower edge starts above skirting level |
| Penetrating damp | A defect that lets bulk water cross the wall — failed pointing, cracked render, a bridged cavity, a discharging gutter | Follows rainfall and wind direction; darkens within hours of a storm on the exposed elevation and dries back between events | A wall that wets in a dry cold spell, or damp on a sheltered elevation while the driven-rain elevation stays clean |
| Surface condensation | Room air whose dew point is above the temperature of the surface it is touching | Arrives with the heating season and clears in summer; worst in unheated bedrooms, behind furniture and in external corners | Damp on a surface logged as staying above the room's dew point through a winter week |
| Interstitial condensation | Vapour reaching a cold plane inside the build-up faster than the layers beyond it can release it | Slow and cumulative, with no seasonal pattern at the surface until the finish, the fixings or embedded timber give it away | A build-up whose layer temperatures have been checked against the room's dew point at design conditions |
What the meter in the surveyor's hand is actually reading
A pin-type moisture meter measures electrical conductance between two probes, and it is calibrated for timber. On plaster and masonry it is not reading moisture content at all; it is reading whatever conducts. Hygroscopic salts — chlorides and nitrates carried out of the ground and deposited at the evaporation front — conduct enthusiastically, and they hold water out of the room air at ordinary humidities with no liquid supply behind them. A wall that was wet thirty years ago, dried out, and kept its salt band beeps exactly like a wall that is wet today. Foil-backed plasterboard and metal lath do the same for entirely different reasons.
Radio-frequency capacitance meters avoid the pin holes and read a shallow depth rather than a surface, which is useful for mapping the shape of a wet area quickly. They are still comparative instruments. Neither type produces a number that can be put in a report as a moisture content, and BS 6576, Code of practice for diagnosis of rising damp in walls of buildings and installation of chemical damp-proof courses, is explicit that installation follows a diagnosis rather than substituting for one. A quote that arrived after a ten-minute visit with one instrument has skipped the part of the standard that the standard is mostly about.
The measurement that settles the argument is destructive and cheap. Drill samples at two depths — one just inside the plaster, one well into the masonry — catch the dust in sealed tubes, and have them tested for total moisture content by drying and separately for hygroscopic moisture content by re-equilibrating at a known humidity. Free water high in the deep sample points at a real supply. Moisture that is almost all hygroscopic points at a salt band with nothing feeding it, which is a plastering problem rather than a damp-proofing one. That pair of numbers, from three or four positions including a control on a wall nobody is worried about, costs less than the site visit.
The other half of the evidence is time. Condensation is seasonal, penetrating damp is event-driven, and rising damp is neither, so a single afternoon's readings cannot distinguish them by construction. Two logging hygrometers and a surface-temperature probe left in the room through a fortnight of the heating season separate them more reliably than any instrument used once.
- Photograph and dimension the damp band on every affected elevation before anything is touched, with the date and the previous week's weather on the same sheet.
- Go outside to the same position: ground level against the damp-proof course line, render or paint carried past it, pointing, and the gutter and downpipe above.
- Use the conductance meter as a screening pass, recording readings at stated heights rather than a verdict.
- Drill samples at two depths through the affected band, plus a control set from a wall with no history, into sealed containers.
- Have each tested for total and hygroscopic moisture content, so free water is separated from salt-held water.
- Leave logging hygrometers and a surface probe in the worst room for two weeks of the heating season before anybody prices a remedy.
You cannot argue about a cold surface without a number for the wall
The condensation case stands or falls on how cold the room face of that wall gets, and that follows from the resistance of everything between the room air and the outside. Solid brickwork has very little: a 225 mm wall plastered on the hard is doing almost nothing thermally, which is why its inner face in January sits far closer to the outdoor temperature than most people assume. An unfilled cavity adds a modest amount, a filled one adds a great deal, and an internal lining adds it in the place that matters most for surface temperature and least for the health of the masonry behind.
Resistances in series add, which is the one piece of arithmetic that has to be right before anything else is argued. BS EN ISO 6946, Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods, is the governing document, and it counts the surface films on both faces because on a poorly insulated wall they are a non-trivial part of the total. Build it from what is actually there — plaster, masonry, cavity, lining, insulation at its real installed thickness — rather than from the specification of a wall the same age somewhere else, and remember that the total is a property of the plane away from junctions. The corner, the reveal and the joist end all behave worse, and they are where the mould turns up.
Put in the resistances that are genuinely in this wall — it adds the surface films itself, so do not enter them twice — and the surface-temperature argument later on rests on a total rather than on an assumption about what a house of that era should have.
The R-value of your main insulation layer, printed on the product.
The R-value of a second layer, like exterior sheathing.
The R-value of interior finish material, like drywall (typically about R-0.45 for 1/2 in).
Total assembly R-value
15.8 R-value
- Sum of material layers
- 14.95 R
- Air film allowance
- 0.85 R
They open the calculator with your figures already in it
R-Value Calculator: 15.8 R-value — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 15.8 R-value — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Layers are summed straight through the insulated cavity. Studs, joists, plates, headers and rim areas conduct several times faster than the insulation between them and are not deducted here, so the real whole-wall or whole-ceiling figure is lower than this total. Run the Thermal Bridging Effective R-Value Calculator on the same wall to see by how much.
- The figures are imperial R-values in hr·ft²·°F/BTU, and so is the fixed R-0.85 air film allowance. Metric RSI values in m²K/W are about 5.68 times smaller, so entering those makes the film allowance alone roughly six times too generous. Convert before you type.
- The air film allowance is one fixed number written for a wall: still indoor air with sideways heat flow, and an outside face exposed to wind. It does not change for heat flowing up or down, for a surface facing a vented attic or an enclosed crawl space, or for a reflective low-emissivity face, all of which shift the film values.
- Product R-values are lab ratings for material at full thickness with no gaps. Batts compressed under wiring, voids at plates and corners, and loose fill that has settled all deliver less than the printed number, and nothing here downgrades the total for installation quality.
- R-value covers conduction only. It says nothing about air leakage, wind washing through the insulation, or moisture in the assembly, and this total is not a code compliance check: the required figure depends on climate zone and on which element you are building, and codes are frequently verified against a whole-assembly U-factor that includes the framing this sum leaves out.
The criterion is a temperature factor, and it is written in U
Mould does not wait for liquid water. BS EN ISO 13788, Hygrothermal performance of building components and building elements — Internal surface temperature to avoid critical surface humidity and interstitial condensation — Calculation methods, sets the surface criterion at 80% relative humidity at the surface rather than at visible condensation, because that is where germination becomes possible. The metric that carries it into practice is the temperature factor at the internal surface, fRsi: surface temperature minus outdoor temperature, divided by indoor minus outdoor. It is dimensionless, it is a property of the detail rather than of the weather, and BRE Information Paper IP 1/06, Assessing the effects of thermal bridging at junctions and around openings, is where the value commonly required for dwellings comes from.
That side of the argument is written in transmittance, not resistance, because heat loss multiplies cleanly through U times area times temperature difference and because regulations and product specifications are denominated that way. The conversion is a pure reciprocal, and the error that wrecks it is doing it too early — resistances add across layers, transmittances do not, so total the layers first and invert once at the end. Check which U you have been handed, too: a metric figure in W/m²K and an imperial one in BTU/hr·ft²·°F differ by a factor of about 5.68, enough to make a wall look five times better than it is.
Total the assembly first, then invert once — this is the step that puts the wall into the same units as the specification, the regulation and the heat-loss calculation somebody will run against it.
The assembly's total thermal resistance.
U-value
0.05263 BTU/hr·ft²·°F
U = 1 ÷ R. The two are exact reciprocals of one another. R-values add across layers; U-values do not. Sum R first, then take the reciprocal once.
They open the calculator with your figures already in it
R-Value to U-Value Calculator: 0.0526 BTU/hr·ft²·°F — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 0.05263 BTU/hr·ft²·°F — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The conversion trusts the number you type and cannot tell a whole-assembly R from a product label. Insulation, sheathing, cladding, linings, cavities and the interior and exterior surface films all have to be totalled before the reciprocal is taken; a batt's rated R entered on its own returns the U-value of a wall that does not exist.
- Thermal bridging is not in it. Studs, plates, headers, rim joists, cladding fasteners and metal furring carry heat around the insulation, so a framed assembly performs worse than the reciprocal of its clear-field R. That figure comes from a parallel-path or isothermal-planes calculation, not from this one.
- A U-value describes steady-state, one-dimensional conduction and nothing else. Air leakage, wind-washing through loose-fill, convection in unsealed cavities, moisture in the material, thermal mass and solar gain all sit outside it, so a heat loss worked out as U x A x deltaT is the conduction share of the load rather than the whole load.
- R-value is quoted at a rating temperature and does not hold across all service conditions. Foil-faced polyiso in particular delivers less R per inch as the assembly gets cold, and foam boards drift as their blowing agent ages, so the R that applies on a design winter night may not be the one printed on the board.
- This is arithmetic, not a compliance check. Energy codes set maximum U-factors against specifically defined assemblies, and windows and doors are rated as whole units including frame and edge-of-glass, so inverting a glazing R will not answer either question. Everything on this page is imperial: a metric U in W/m2K is roughly 5.68 times the imperial figure shown here.
R and U describe the same physical property from opposite ends: resistance to heat flow, and the rate of heat flow itself. Insulation is marketed in R because bigger sounds better, while heat-loss calculations run on U because it multiplies straight through the Q equals U-A-delta-T relationship. The conversion is a pure reciprocal, and the mistake that ruins calculations is doing it too early. Resistances add in series — batt plus sheathing plus cladding plus air films — so the correct order is to total every layer's R and invert once at the end. Summing the individual U-values of each layer produces a number that is not merely imprecise but wrong, typically by a large margin.
Why the mould draws lines and fills corners
Look at where the growth actually is. Vertical stripes at a regular spacing on a battened or timber-framed wall, a grid on the room face of a wall with dense mortar joints, blooms in the top corners of an external angle, a band along the ceiling perimeter under a cold roof, a solid patch behind a wardrobe pushed tight to the plaster. None of those patterns is produced by ground water. They are a map of where the surface runs colder than the plane around it.
Repeating bridges — studs, battens, ties, mortar joints — are what a parallel-path calculation can screen. It splits the wall into the fraction taken by the conductive element and the fraction taken by insulation, and returns an effective figure often much worse than the number on the packaging, particularly on timber framing with the corners, headers and plates counted honestly. It is a screening tool and says so: BS EN ISO 10211, Thermal bridges in building construction — Heat flows and surface temperatures — Detailed calculations, is what a real junction gets modelled to, and a geometric bridge like an external corner is outside what a framing fraction can represent at all.
Furniture and curtains matter more than they sound. A wardrobe against an external wall does not insulate it, it isolates it: the room's warm moving air stops washing the plaster, the effective surface resistance rises well above the value the ISO 6946 total assumed, and the plaster behind the wardrobe therefore runs several degrees colder than the wall either side of it while the air trapped against it stops carrying moisture away. That combination is why the first mould in a house very often appears on a surface nobody was looking at. It is also why a diagnosis that ends at the wall build-up misses half the cases.
The practical consequence for a homeowner holding a quote is that a cold-bridge pattern is a strong argument against a chemical DPC, because the two are answers to different questions. A wall can have a perfectly sound original slate course, no ground contact issue whatever, and grow mould in eight places, and injecting it will change none of them.
Screen the repeating bridge first: the effective figure with the framing fraction counted is usually the one that explains a stripe pattern, and it is worth having before anyone is asked to model a junction properly.
The R-value of the insulation filling the stud cavity.
The R-value of solid wood at the stud's thickness.
The percentage of the wall's area taken up by studs, headers, plates, and other framing rather than insulated cavity.
Effective assembly R-value
8.73 R (effective)
This parallel-path method is a widely used simplification — it doesn't account for more complex heat flow effects like point thermal bridges at intersections, which a full 2D/3D heat transfer model would capture more precisely.
- Nominal cavity-only R-value
- 13 R
They open the calculator with your figures already in it
Thermal Bridging Effective R-Value Calculator: 8.73 R (effective) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 8.73 R (effective) — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- This is the framed insulation layer only. Drywall, sheathing, cladding, air gaps, interior and exterior air films, and any exterior continuous insulation are not in the arithmetic, so the figure is not the wall's total R-value and should not be read against a target stated for a whole assembly.
- The parallel-path split describes timber. Steel studs, Z-furring, spacer bars and brackets, and masonry ties spread heat sideways into the surrounding material, so the bridged area behaves larger than its geometric fraction and this method will not give you the right penalty. For metal, use a steel-specific method or the system supplier's tested U-value.
- One framing fraction is applied evenly over the whole wall. Corners, headers over openings, rim joists, the wall-to-floor and wall-to-roof junctions, and anything penetrating the envelope lose more heat than a plain stud bay, and none of them are resolved here.
- The cavity is assumed filled to its rated R-value everywhere. Gaps at the edges, batts compressed behind wiring and pipework, settled blown insulation, and air moving through the framed layer all put real performance below this number, and none of them are inputs.
- Not a compliance calculation. An energy code submission or a declared U-value needs a whole-assembly figure produced by the method your code names, with junction losses handled separately. Use this to see how much of the batt's label the framing takes back, not as the document you submit.
The condensation that leaves no mark until the wall is opened
Interstitial condensation is the mechanism that gets missed, because for years it produces no stain. Vapour moves from the warm side toward the cold side through whatever the build-up allows, and it condenses wherever it reaches a plane below its dew point. If the vapour control layer is on the warm side of the insulation, the cold plane sits beyond the barrier and the assembly is fine. If the layer sits partway out — behind the insulation, or on both faces, or missing and replaced by a low-permeability paint film — then a cold, impermeable surface is receiving vapour and there is nowhere for it to go.
This is what internal wall insulation causes when it goes in without a hygrothermal check, and it is common enough that PAS 2035, Retrofitting dwellings for improved energy efficiency, puts the assessment ahead of the product specification. The tell is a damp complaint that began within a year or two of an upgrade, or growth on the lining rather than behind furniture, or a lifted board finding wet insulation and dark timber against masonry that looks dry from outside. ASHRAE Standard 160, Criteria for Moisture-Control Design Analysis in Buildings, is the equivalent document on the North American side.
The steady-state check is crude and still worth running before anything is cut. Put in the total assembly resistance and how much of it sits between the room and the plane you are worried about, then read that plane's temperature against the dew point of the air the room actually holds — the logged figure, not the design assumption. If it sits below dew point, moving the barrier toward the warm side or adding resistance on the room side of it are the two levers, and both are far cheaper to pull on a drawing than on a finished wall.
It is the barrier's position in the build-up, not the total insulation thickness, that decides whether it sits above or below the dew point — this puts a temperature on that plane so the discussion stops being about opinions.
The assumed indoor air temperature.
The outdoor winter design temperature for the site's climate zone.
The full wall assembly's total thermal resistance, interior surface to exterior surface.
The portion of the total R-value between the interior face and where the vapor barrier sits.
The dew point temperature of the interior air, based on its temperature and relative humidity.
Temperature at vapor barrier location
55.9 °F
The temperature at this location stays above the interior dew point shown below, so condensation is not predicted under the design conditions entered. No risk predicted under these conditions is not the same as none. The conditions are the ones you entered, and one surface is not the assembly.
- Interior air dew point
- 50 °F
They open the calculator with your figures already in it
Vapor Barrier Dew-Point Condensation Risk Calculator: 55.85 °F — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- Air leakage, not diffusion, is what usually wets a wall, and there is nothing about it here. The gradient gives the temperature at the plane; it cannot say how much moisture arrives there. A wall that passes this check and leaks warm interior air through a top plate, a service penetration or an unsealed electrical box deposits far more water at that plane than vapor diffusion through an intact assembly ever could.
- The R-values entered describe the clear field of the wall, between the framing. The path through every stud, plate and header is colder than this straight line says, and at a steel stud or an uninsulated slab edge much colder — so an assembly that passes in the middle of a bay can be sitting below dew point on the back of the sheathing at every framing member, which is exactly where mold turns up.
- It is one snapshot at one pair of temperatures, and it totals nothing. What damages an assembly is how many hours a year it spends below dew point and whether it dries out in between. A wall that dips below on a few cold nights and recovers is not the same wall as one that stays below for a month, and this returns the identical verdict for both.
- The obvious fix for a failing result can produce a wall that cannot dry. Moving the barrier inboard is right only if the outboard side is open to vapor. Where a low-perm layer already sits outside — exterior foam, a self-adhered membrane, an impermeable sheathing — a second one inside traps whatever gets past either of them, and the assembly then passes this temperature check with no drying path in either direction.
Where the water in the air comes from, and it is not the wall
A household generates a surprising quantity of water vapour every day, and almost none of it comes through the masonry. People breathing and perspiring, cooking, washing, showering, and above all drying laundry indoors or venting a tumble dryer into the room are the sources. Flueless bottled-gas and paraffin heaters are a category of their own: burning a hydrocarbon produces water as a combustion product, and a flueless appliance puts every gram of it into the room it is heating, which is why the damp complaint so often traces to the one room with a portable heater in it.
Extract at the source beats dilution everywhere else, because moving humid air out while it is still humid removes far more water per unit of airflow than replacing already-mixed room air. That is why the regulations treat wet rooms separately: Approved Document F, Ventilation, tabulates minimum extract rates by room type for dwellings in England, and ASHRAE Standard 62.2, Ventilation and Acceptable Indoor Air Quality in Residential Buildings, does the equivalent job in North America. Whichever applies, the figure is a minimum for a working installation, not a target for a fan painted over, ducted through six metres of flexible hose with four bends, or fitted with a grille that has rusted shut — so check the installed airflow with an anemometer or flow hood before specifying anything. A fan measuring a third of its plate rating is not a ventilation strategy; it is a noise.
Size the extract against the room's volume and its use before comparing it with the code minimum — a bathroom that produces the moisture and never removes it will keep every other surface in the house damper than it should be.
Different rooms have different recommended ventilation rates.
The length of the room.
The width of the room.
Floor-to-ceiling height.
Recommended airflow (CFM)
85.3 CFM
Based on general air-changes-per-hour targets, not a code-mandated minimum for your specific jurisdiction — check local building code for required bathroom/kitchen exhaust minimums.
- Room volume
- 640 cu ft
- Target air changes/hour
- 8 ACH
They open the calculator with your figures already in it
CFM Ventilation Calculator: 85.33 CFM — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 85.3 CFM — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- This sizes the airflow the room needs, not the fan you buy. Manufacturer CFM ratings are measured against a static pressure far lower than a real installation imposes, and duct length, elbows, flexible duct, the interior grille and the exterior cap all cut delivered flow. None of that is modelled here.
- This is not a code compliance check. Codes commonly set a flat minimum exhaust rate for bathrooms and kitchens, plus separate whole-dwelling ventilation rates driven by floor area and occupancy, and a small bathroom or WC will compute below those minimums. Size on whichever figure is larger and confirm it against the code your work is inspected under.
- Makeup air is not accounted for. Air pulled out has to be replaced, so a fan sized from this figure with no door undercut, transfer grille or trickle vent will move less than the number says, and in a sealed room it can depressurise the space enough to backdraft an open-flued gas or solid-fuel appliance. Combustion safety is a separate check, not something this result covers.
- The air change rate is one fixed value per room type, and only four room types exist. It does not move for the number of occupants, shower versus bath use, gas versus electric cooking, cooktop or hood width, an openable window already in the room, local humidity, or whether the fan runs continuously or on a timer.
- Room volume is length x width x height and nothing else. A sloped or vaulted ceiling, an L-shaped room, a dropped bulkhead, or a space open to an adjoining one such as an ensuite off a bedroom or a kitchen open to the dining area has no honest set of three numbers to enter, and the volume has to be worked out separately.
The house was fine until somebody made it airtight
A recurring case history: new double glazing, draught strips on the doors, a filled cavity, a chimney capped, and mould in the bedrooms the following winter in a house that had none for eighty years. Nothing was done wrong in isolation. What changed is that the building's uncontrolled ventilation — the draughts that were quietly exporting the household's moisture all along — was removed and nothing replaced it, so the same daily litres of vapour now sit in the house instead of leaving it.
Airtightness is measurable and it is the one figure that puts this conversation on a footing. A blower door pressurises or depressurises the house to 50 pascals and reports the airflow needed to hold that difference; dividing by the internal volume gives air changes per hour at 50 Pa, which is the comparable number between buildings of different sizes. It is not the natural infiltration rate — the relationship between the two depends on climate, exposure, shielding and building height, which is what the Lawrence Berkeley infiltration models exist to handle — but it is repeatable, and a before-and-after pair around a retrofit says exactly what was done to the building.
Read the result against intent rather than against a league table. A house at eight air changes per hour at 50 Pa has plenty of accidental ventilation and its damp problem is probably not a ventilation problem. A house that has been driven down to three without a mechanical system installed at the same time has been sealed without being given lungs, and the fix is ventilation with heat recovery or a properly commissioned continuous extract, not a dehumidifier running in a corner forever.
Turn a blower-door reading and the building's volume into the ACH50 figure, which is the only airtightness number that compares one house with another or the same house before and after the work.
The total conditioned (heated/cooled) floor area of the building.
The average ceiling height across the conditioned space.
The airflow reading from a blower door test at 50 Pascals of pressure difference.
ACH50 (air changes per hour at 50 Pa)
10.42 ACH50
Fairly leaky — typical of an older, un-air-sealed home; many energy codes target under 3-5 ACH50 for new construction.
- Building volume
- 8,640 ft³
They open the calculator with your figures already in it
Air Changes Per Hour (ACH50) Calculator: 10.42 ACH50 — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 10.42 ACH50 — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- ACH50 is leakage measured while a fan holds the building at 50 Pa, not the rate at which air actually changes in service. Real infiltration is a fraction of this figure and varies with wind, stack effect, exposure, shielding and building height, so this result is not a ventilation rate and not a number to feed straight into a heat-loss or plant-sizing calculation.
- The test behind the reading is run with trickle vents, extract terminals and other intentional openings closed or sealed, so a low result is not evidence the building is ventilated. A dwelling driven down towards the tight end without mechanical ventilation added at the same time has been sealed, not improved.
- The calculation divides whatever CFM50 you type; it does not check the reading. Temperature and wind correction on the day, a single-point reading versus a multi-point fit, pressurisation versus depressurisation, and how thoroughly the crew sealed intentional openings all move the flow figure, and none of them are visible here.
- Volume is taken as floor area times average ceiling height, and the fields only reach dwelling scale — a floor area above 1,000 m² (about 10,760 sq ft) or a ceiling above 6 m (about 20 ft) is replaced with the limit. Whether a conditioned basement, a room in the roof or the space between storeys belongs inside the tested volume is settled by the test protocol and by the two numbers you enter, so the ACH50 on a certificate for the same house can differ from this one.
- This is not a compliance check. It applies no code target, no climate zone and no certification scheme's pass mark, and it does not produce air permeability in m3/(h.m2) of envelope area, which is the figure a UK or European air-leakage test is judged against. The flow field is CFM only, so a certificate quoting m3/h at 50 Pa has to be converted before it goes in the box.
Weighing the moisture load with a machine you already own
Here is the finding that reframes most condensation arguments. The air in a room holds far less water than people expect — a whole living room's worth of air carries barely two hundred millilitres between 70% and 55% relative humidity at ordinary room temperature, which a modest dehumidifier removes in under an hour. If the tank is still filling day after day at several litres, the air is not the reservoir. Something is supplying it continuously, and identifying that supply is the diagnosis.
So use the machine as an instrument rather than as a treatment. Run it in a closed room with the door shut and the extract off, empty and log the tank daily for a week, and note the outdoor weather alongside. A daily yield that falls away steeply after the first day and then sits low is an air-and-contents load that has been dried out. A yield that stays high and steady is a source: laundry, an unvented dryer, a leak, or masonry giving back water it took in. A yield that spikes after rain is penetrating damp announcing itself with no drilling required.
The psychrometrics are worth understanding once, because relative humidity alone says nothing about how much water is present: a cool basement at 70% holds less per cubic metre than a warm room at 60%. The humidity ratio, and therefore the litres, follows from temperature and relative humidity together — the relations are in the ASHRAE Handbook — Fundamentals — which is why two rooms with the same hygrometer reading can be in completely different trouble.
Put the room's volume, temperature and the two humidity readings in and see how few litres the air itself holds — the gap between that figure and what the tank actually collects each day is the size of the source you are looking for.
The floor area of the space being dried.
Average ceiling height — area × height is the air volume that holds the water.
Air temperature in the space. Warmer air holds more water at the same relative humidity.
What the hygrometer reads now, in %.
Where you want to hold the space — 50–55% is the usual comfort and mould-control band.
The litres-per-day figure on the unit's plate or box.
How much of the rated capacity your room's conditions actually deliver.
Water to remove from the air
0.0666 gal
This is the water in the AIR alone. Wet walls, wet contents and the continuous load from infiltration and occupants add far more over a day than the air itself holds — the pull-down being quick does not mean the job is small.
- Room air volume
- 127.41 yd³
- Water in the air now
- 0.31 gal
- Water in the air at the target
- 0.24 gal
- Pull-down time at derated capacity
- 0.43 hours
- Effective extraction rate
- 3.7 gal/day
They open the calculator with your figures already in it
Dehumidifier Water Removal Calculator: 0.0666 gal — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Only the water suspended in the air is counted. Moisture soaked into plaster, timber, screed, carpet and stored contents is excluded, and so is the continuous load — air leaking in from outside, ground moisture through a slab or crawlspace, showers, cooking and occupants. That continuous load, not the air's own water content, is what fills the tank day after day and what sizes the machine.
- This is not a drying-out calculation. Reaching the target room humidity does not mean a wet subfloor, wall cavity or plaster is dry; only a moisture meter in the material tells you that, and water still held in the fabric will push the room back up as soon as the unit stops.
- The pull-down time assumes a closed space with nothing adding moisture while the unit runs and a single extraction rate held constant throughout. It does not model infiltration during the run, doors opening, extraction falling as the room dries and cools, coil defrost cycles, or the unit's own humidistat cutting out short of the target.
- One temperature and one humidity are applied to the whole volume. Real spaces stratify — cold corners, outside walls, the space behind furniture and under a floor sit colder and wetter than the meter in the middle of the room, and reach the target long after it does.
- Not a condensation or mould-risk assessment. Mould grows at the surface humidity against a cold wall, window reveal or uninsulated slab, which can stay high while the room air sits comfortably at 55%. Judging that needs the surface temperature and a dew-point check, not a room average.
When a machine is the remedy, and when it is an alibi
A dehumidifier is the right answer in a small number of honest cases: drying a building out after a flood or a plaster job, holding a cellar or store below the humidity where mould germinates when there is no economic way to insulate or ventilate it, and buying time while a genuine repair is scheduled. Sizing for those is a matter of the floor area and how wet the space actually is, and the answer should be checked against the space's temperature, because a refrigerant unit in a cold cellar delivers a fraction of its plate rating and a desiccant machine is usually the better choice below about 15 °C. Note which test standard the rating was measured under as well — the same hardware prints very different numbers under the older and newer test conditions.
It is the wrong answer, permanently, wherever there is a supply that can be cut off instead. Running a machine against a discharging downpipe, an unvented dryer, a bridged cavity or a failed cavity tray is paying an electricity bill to stand in for a repair, and the fabric keeps getting wet the whole time. The test is simple: if switching the machine off returns the problem within days, the source is still running, and the source is the job.
Capacity comes off the floor area and the dampness grade you logged on the first visit — worth settling before anyone buys a unit, and worth revisiting once the actual source has been shut off.
The floor area of the basement or room.
How consistently damp the space feels, especially in humid weather.
Recommended dehumidifier capacity
13 pints/day
This sizing chart assumes typical conditions — extra factors like an attached laundry area, frequent door traffic to outdoors, or a large uninsulated space can push actual needs higher.
- Room area
- 540 sq ft
They open the calculator with your figures already in it
Dehumidifier Sizing Calculator: 12.96 pints/day — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 13 pints/day — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The calculation takes floor area and nothing else. Moisture sits in the air volume, not on the floor plate, so a basement with a 3.5 m (11.5 ft) ceiling and a 1 m (3.3 ft) crawl space of the same footprint return the same answer, and neither the extra air nor the extra wall and slab surface behind it is in the number.
- The dampness level is a description of how the space feels, not a measured moisture load. Nothing here counts the actual sources - an unvented dryer, a bare-earth crawl space floor, an open sump, an adjoining laundry or shower, occupants, or humid outside air leaking in at the rim joist and the stair door. Two rooms of the same area carrying the same rating can differ several times over in the water they give up in a day.
- The per-500-sq-ft rate is applied strictly linearly, so 2,000 sq ft (186 m²) returns exactly four times the 500 sq ft (46 m²) figure. Published sizing tables do not scale that way - pints needed per unit of floor area falls as the space grows - so the further past 500 sq ft (46 m²) you go, the more this overstates the machine. At the largest area it accepts, on the wettest setting, it returns over 200 pints a day, far beyond any single residential unit.
- Air temperature is not an input. A refrigerant dehumidifier's plate rating is measured in warm air and its real extraction falls off sharply as the space gets colder, to the point where the coil frosts and the machine spends part of its cycle defrosting rather than pulling water; a basement, crawl space or garage that stays cold usually wants a low-temperature or desiccant unit, not a bigger conventional one. The pint figures here are also not tied to any stated test standard, and the standard changed - older ratings were taken in warmer air than current ones - so a 12-pint recommendation and a machine plated at 12 pints are not necessarily the same amount of water.
- This is a capacity figure only. It does not set a humidity target, estimate run hours or condensate volume, choose a drain route, or check the circuit will carry the machine, and it does not size drainage, a sump, grading or a vapour barrier. Where the 'extremely wet' level applies - standing water or active seepage - water is arriving faster than a dehumidifier is built to remove it, and the water path is the job before any capacity number means anything.
Reading the quote back against what you now know
Take the evidence to the quote in order. Bulk water first: ground levels against the damp-proof course line, which the guidance in Approved Document C, Site preparation and resistance to contaminants and moisture, puts at 150 mm above finished ground level for good reason, plus render bridging, pointing, gutters and downpipes. Then the fabric: surface temperatures, cold bridges, the state of any insulation. Then ventilation and the moisture load. Then, and only then, whether anything in the ground-water case survives. Most quotes for injection are written in the reverse order, starting at the remedy.
Understand what the two halves of an injection job do. The injected cream or fluid forms a water-repellent band in the masonry and can only address capillary rise; it does nothing about a bridged cavity or a cold corner. The hacking-off and salt-resistant replastering are what stop the salt band showing through the decoration, and that will make almost any damp wall look better for a while regardless of cause — which is the mechanism by which a wrong diagnosis passes its own guarantee period. Read the guarantee too: it usually covers reinstalling the injection, not the wall being dry, and it is worth nothing if the contractor stops trading.
None of which makes a chemical damp-proof course wrong. Where drilled samples show free water at depth, low down, on a wall in ground contact with no working course and no plausible alternative supply, it is the correct treatment and BS 6576 is the document it should be installed to. The point is that the evidence should have existed before the quote did — and that where the readings show a cold, under-ventilated room with a heavy indoor moisture load, spending the injection money on ventilation, a cold-bridge fix and a heating pattern that keeps surfaces warm will stop the mould, which the injection would not have.
The evidence to gather before a single hole is drilled
In the order it should be collected, cheapest and least destructive first. The workspace opens on the interstitial check, seeded for an internally lined solid wall on a January night with the vapour control layer sitting too far toward the cold side — a build-up that fails, so the effect of moving it is visible immediately.
- External walk-round, photographed — Ground level against the damp-proof course, render carried past it, pointing, and every gutter and downpipe above the stain. Half of penetrating damp is found here in ten minutes.
- Drilled samples, two depths plus a control — Total and hygroscopic moisture content from a laboratory, not a beeping surface reading. This is the only line on the list that produces evidence rather than an impression.
- A fortnight of logged temperature and humidity — In the worst room, through the heating season, with the outdoor weather logged alongside. Condensation, penetrating damp and rising damp keep different time, and one afternoon cannot tell them apart.
- Surface temperatures at the cold spots — External corners, window reveals, the ceiling perimeter and behind whatever furniture is against an external wall — measured, not inferred from the wall build-up.
- Measured extract airflow, not plate ratings — A reading at the bathroom and kitchen grilles. A fan delivering a third of its rating is the commonest single cause of a whole-house humidity problem.
- A week of dehumidifier tank yield — Litres per day in a closed room, against the weather. A steady high yield is a source still running; a spike after rain is penetrating damp identifying itself.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.