Sealing and Conditioning a Crawlspace

What this calculation does not cover

• Piers, columns and interior footings carry no term in the arithmetic: the total is the floor footprint plus perimeter times wall height, so liner wrapped around a masonry pier or carried up an interior support is extra material this figure does not contain.
• A single wall height is applied around the entire perimeter, so a stepped footing, a sloping site, or a run that drops into a full-height basement wall has to be split and estimated in sections — and the height field caps at 2.5 m (about 8 ft), short of a true basement lift.
• Floor area and perimeter are read as two independent numbers and never reconciled, so nothing objects when the perimeter you enter could not physically enclose the floor area you enter, and a transposed digit in either still returns a plausible-looking total.
• The waste allowance is one multiplier on the combined floor-and-wall total rather than a separate overlap on each surface, it is already folded into the headline number with only the wall area broken out beneath it, and it stops at 20% — below what a crawlspace crowded with ducts, plumbing drops and pier blocks can actually consume.
• The perimeter you enter serves only to generate wall area and is never returned as a linear run, so the seam tape, adhesive and top-of-wall termination that make an encapsulation airtight — quantities that follow seam length rather than area — sit outside this answer.

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.

What this calculation does not cover

• Board thickness is not an input, so the count is the same for a 25 mm board as for a 100 mm one, and it covers a single layer. Nothing here tests the resulting R-value against what the rim joist is expected to achieve, and building up two layers to reach a target doubles the sheet count.
• Thickness is also what keeps the rim board dry, and that is not checked either. Foam too thin for the climate leaves the inner face of the rim board cold enough for indoor air to condense against it, so the penalty for getting thickness wrong here is damp timber rather than a slightly higher heating bill, and the thickness needed to hold that face above the dew point depends on the climate zone.
• The 10% allowance covers ordinary offcuts, not the yield lost to cutting a whole sheet down into bay-sized pieces. A 4x8 ft sheet cut into 9.25 in by 14.5 in rectangles — 2x10 joists, the default here, at 16 in centres — gives 30 pieces, about 87% of the sheet, with the rip and end strips discarded, so a long band worked out from area alone can come up a sheet short.
• Only the boards are counted. The canned foam that seals the perimeter of every cut piece, the sealant at the sill plate, and any covering the foam needs where it is left exposed are all outside the number — foam plastic inside a dwelling normally has to sit behind a thermal or ignition barrier, and whether the rim joist band is granted an exception is jurisdictional.
• Perimeter times joist height treats the band as an unbroken rectangle. Nothing is deducted for basement windows, walkout doors or beam pockets, and nothing is added where a cantilever, bay window or bump-out carries the band past the foundation line that the help text asks you to measure.
• The band is assumed to be exactly as tall as the joist. Many rim joist details are insulated from the underside of the subfloor down over the sill plate to the top of the foundation wall, which is taller than the joist depth entered here.

What this calculation does not cover

• Building volume here is nothing more than floor area multiplied by average ceiling height. That ignores the thickness of intermediate floor assemblies, flattens stairwells and double-height spaces into an average, and omits any conditioned basement or attic whose area you did not enter. The target moves in direct proportion to the volume, so a volume measured ten per cent differently from the tester's gives a CFM50 limit ten per cent different — agree the volume basis with whoever will run the test before you build to this number.
• Nothing here defines the tested envelope: whether an attached garage, an unheated basement, a conditioned crawl space or a plant room sits inside or outside it. That single decision changes both the volume in the denominator and the leakage the fan has to move. Two people can test the same building to two different ACH50 figures purely by drawing the boundary in two different places.
• ACH50 normalises leakage against volume. UK and much European testing, and some North American programmes, instead specify air permeability or leakage per unit of envelope area. This page never asks for envelope area, so a target handed to you in those terms cannot be met by hitting this CFM50 figure.
• The number assumes a properly conducted test at 50 Pa. It carries no correction for wind, stack effect or baseline pressure, no averaging of a depressurisation result against a pressurisation one, and no allowance for how the building is prepared — which trickle vents, flues, extract fans and drain traps get sealed and which are left as found. Those are the usual reasons two tests of the same building disagree.
• This is a target airflow, not a ventilation or combustion-air check. A building tightened towards a low ACH50 needs mechanical ventilation sized and commissioned for it, and any open-flued or naturally aspirated appliance inside the envelope needs its own combustion air and spillage assessment. Hitting this figure says nothing about either, and those are the parts that carry a safety consequence.

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.

What this calculation does not cover

• Free area is not the size of the hole you cut. A grille or louver passes only its open fraction — roughly three-quarters for metal, as little as a quarter for wood — and an insect screen behind it takes more again, so a 100 in² (645 cm²) free-area requirement can need an opening of 130 to 400 in² (840 to 2,580 cm²) depending on what covers it. Size from the louver's published free area, or the appliance is starved through an opening that measures correctly.
• The openings only work if the space on the other side is large enough. This method assumes the adjoining room, plus everything freely communicating with it, holds at least 50 ft³ (1.4 m³) per 1,000 BTU/hr (0.3 kW) of combined appliance input; cutting two grilles into the partition of a small closet does not create combustion air, it shares one shortage between two rooms.
• Nothing here accounts for air being pulled out of the space. A kitchen hood, a clothes dryer or a bath fan can drop the room below atmospheric pressure and reverse an atmospheric flue no matter how generous the openings are, which is how a correctly sized opening still ends with a CO alarm — a house with substantial mechanical exhaust needs the appliance's air supply looked at as a pressure balance, not as an area.