Minus Twenty-Five Is an Instruction to the Ground, and It Never Gets Cancelled
The bay is 30 by 24 metres in the corner of an existing distribution shed, and the client wants it at minus twenty-five for frozen pallets. Panel supplier appointed, refrigeration contractor appointed, racking layout drawn, a go-live date in the diary. The floor is a line on a drawing that says 150 mm power-floated slab on insulation, and the two subcontractors who care most about what is under it — the refrigeration engineer and the ground worker — have never spoken to each other.
Everything else in that room can be taken back out. A panel can be unbolted, an evaporator swapped, a rack bay relocated, a door leaf replaced on a Sunday. The floor cannot. Once the wearing slab is down, every layer beneath it — the boards, the membrane, the heating grid, the sand it sits in — is sealed under 720 square metres of concrete with racking bolted through it, and the only way back to any of it is a demolition contract and a shut store.
That asymmetry is the whole argument for spending a week on the build-up before anybody prices it. A freezer floor is not an insulated warehouse floor with a colder room on top. It is a permanent, one-directional heat sink installed over ground that has spent its entire existence above freezing, and the ground's response to that is slow, cumulative and structurally destructive.
Ice Lenses Need Very Little Water, and They Do Not Stop
Heave is not the nine per cent expansion of the water already in the soil. If that were all it was, the movement would be trivial and self-limiting. What actually happens is that a freezing front sitting in a soil with fine enough pores generates suction, draws liquid water toward itself from the unfrozen ground below, and freezes it out as a discrete lens. The lens grows by import, the front creeps deeper, another lens forms, and the ground swells by far more than the water it started with. ASTM D5918, the standard test methods for frost heave and thaw weakening susceptibility of soils, exists because this behaviour cannot be predicted from a grading curve alone.
Three things have to be present together, and all three are ordinary. A frost-susceptible soil — silts, silty sands and lean clays, classified to ASTM D2487 or the BS 1377 equivalent, are the classic offenders because they are fine enough to hold capillary water and open enough to move it. Water within reach of the front, which in most of the UK and northern Europe means anything within a few metres of a normal water table. And a sustained sub-zero temperature at the freezing front.
Under a road or a footing, that third condition is seasonal. It arrives in December, reaches whatever depth the winter allows, and reverses in spring, and the design response is to get the founding level below the frost depth and stop worrying. Under a freezer it is permanent. The front advances every month of every year, the bulb grows downward and sideways past the wall line, and there is no thaw to relieve it. Cold stores have lifted floors by tens of millimetres, pulled racking out of plumb and jammed doors — and the guidance on this is not new, it is in the refrigerated-facility design chapter of the ASHRAE Handbook—Refrigeration and it has been for decades.
| Condition | What it means under a freezer | Can you remove it? |
|---|---|---|
| A frost-susceptible soil | Silt, silty sand or lean clay anywhere within the depth the front will eventually reach | Only by excavating and replacing it to a depth nobody has costed, and the front keeps going deeper |
| Water within reach of the front | A water table, perched water, or capillary rise through the natural ground below the sub-base | Rarely, and never permanently — under-floor drainage lowers the risk without ending it |
| A sustained freezing temperature at the front | The room itself, held below zero for the life of the building with no seasonal relief | Yes. Put heat under the insulation and hold the sub-grade above zero. This is the one the industry actually uses |
| Time | Not a fourth condition, but the reason the problem is invisible at handover | No. The bulb takes seasons to develop, so the floor that heaves is signed off as perfect |
Insulation Sets the Timetable, Not the Outcome
The first instinct is to insulate the problem away, and it is worth being precise about why that does not work, because the reasoning is what justifies the cost of everything in the next four sections. Insulation is a resistance, not a source. It reduces the rate at which the room extracts heat from the ground; it cannot supply heat to replace what is extracted. In the middle of a 720 square metre floor, far from any edge, the only heat arriving from below is the geothermal flux, which is a fraction of a watt per square metre — one to two orders of magnitude below the loss through even a heavily insulated freezer floor. The sub-grade under that point has one long-run destination and it is below zero. Board thickness decides whether it takes three years or fifteen.
That is not an argument for thin insulation. The board is still cutting a refrigeration load that runs continuously for the life of the building, and its thickness comes out of the engineer's optimisation of floor gain against capital cost rather than out of a code table — landing, for a low-temperature room, a long way above what a frost-protected shallow foundation ever asks for.
There is a materials trap hiding in the R-value figure itself. Rated thermal resistance is measured at a stated mean temperature — ASTM C518 by heat flow meter, ASTM C177 by guarded hot plate, BS EN 12667 for the European equivalents — and the headline number on a data sheet is normally quoted around room temperature. Extruded and expanded polystyrene get slightly better as they get colder. Polyisocyanurate does the opposite, because its blowing agent begins to condense, and its performance at freezer temperatures is materially worse than its catalogue figure. That is the main reason cold store floors are built from XPS or high-density EPS and not from the polyiso that would be the obvious choice on a roof.
Ask the supplier for the conductivity at the mean temperature this floor will actually run at, and work the board's resistance back from that rather than taking the headline one. Then size a single layer at a time. Watch the scale while you do it: the box below is written in imperial R and R per inch, not in the square-metre-kelvins per watt this article works in, and one metric unit is 5.68 of them — an RSI-6 build-up is R-34. It stops at R-25, and a freezer floor is normally specified above that, which is not a problem because it is laid in two or three layers with staggered joints anyway. Divide the design total by the number of layers and size the layer.
Enter one layer's share of the design R-value, not the whole stack, in imperial R rather than the square-metre-kelvins per watt used above — multiply an RSI figure by 5.68 first — and take the board's R per inch at freezer mean temperature rather than the data sheet headline. The frost-protection method this box was written for keeps heat in the ground under a building, which is the opposite problem to the one you have.
The total R-value required by your frost-protected shallow foundation design.
The rated thermal resistance per inch of thickness for the specific rigid insulation board.
Required insulation thickness
2 in
Target R-value for frost-protected shallow foundations (FPSF) depends on your local frost depth/climate zone per IRC Appendix AF or local code — confirm the required R-value for your specific location before sizing insulation thickness.
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Foundation Frost-Protection Insulation Depth Calculator: 2 in — 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 — 2 in — 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 rated R per inch is a dry laboratory figure at a mean temperature around 75°F, and buried foam is neither dry nor at 75°F. Boards in wet ground take up water and lose resistance as they do, extruded polystyrene sheds blowing agent over decades toward a lower aged value, and polyiso — which is what the top of this input's range describes — absorbs water badly and is generally not accepted below grade at all. Choose a product the FPSF detail permits underground first, then read its R per inch off that product's sheet.
- The answer is a raw division, not a stock thickness. Foam comes in fixed thicknesses, so 3.33 in (85 mm) means two layers, and two layers only deliver the calculated figure if their joints are offset — aligned seams leave a straight thermal path through the whole assembly at every one. The portion above grade also needs a protective covering against sunlight and impact, which adds thickness the trench width and the finish detail have to accommodate.
Three Ways to Keep the Sub-Grade Above Zero
Having established that something has to put heat under the insulation, there are three families of answer in common use and a fourth that avoids the question entirely. All four are recognised practice; the choice is made on room temperature, plan area, climate, running cost and how much the client's operation can tolerate a failure that cannot be reached.
The design duty is smaller than people expect, and the arithmetic is worth doing early because it changes the conversation about running cost. Thirty kelvin of difference across six square-metre-kelvins per watt of board is about five watts per square metre in steady state — for a 720 square metre bay, under four kilowatts, and only when the controller calls for it. The installed grid is larger than that, because it also has to bring a sub-grade up during initial pull-down, cover the perimeter where losses are higher, and carry on working with one circuit dead.
| System | Where it fits | What it costs you later |
|---|---|---|
| Electric heating cable grid in a sand bed or lean-mix sub-slab | Most freezer floors: cheap to install, simple to control, easy to zone | Running cost is continuous, and a cable not laid in a withdrawable conduit is unrepairable |
| Warm glycol or warm-fluid loop under the insulation | Larger stores, and any plant with usable heat reclaim from compressor discharge | Pumps, a heat exchanger, a plant room and a leak risk under a slab — but near-zero energy cost if the heat is reclaimed |
| Ventilated ducts through the sub-base, passive or fan-assisted | Milder climates and stores where ambient air stays reliably above freezing | Ducts silt up, get blocked or ice over, and in a hard winter the incoming air itself needs heating |
| A structurally suspended floor over an open ventilated void | Very cold climates, deep freezers, sites where the ground cannot be trusted at all | The highest capital cost by a wide margin, and the only one with nothing to fail |
A Grid You Will Never Lift Again
Take the electric grid, because it is what most of these bays get. The spacing follows from an output, not the other way round: watts per square metre equals the cable's rated watts per metre divided by the spacing in metres. A cable rated six watts per metre laid at 300 mm centres puts twenty watts per square metre into the sub-grade, and the length of cable that buys is simply the area divided by the spacing — 2,400 metres across a 30 by 24 bay. Every one of those metres is a metre you will never see again.
So lay it in conduit. Pulling constant-wattage cable through PVC or HDPE ducts cast into the sub-slab, with both ends brought out to accessible pits beyond the freezer envelope, converts an unrepairable buried component into a serviceable one: a failed circuit is withdrawn and a new one pulled through the same route on a Sunday morning. Every joint, termination and gland lives outside the footprint. Run spare empty ducts on the same grid while you are at it — they cost almost nothing at this stage and they are the only spare capacity this floor will ever have.
Sensors deserve the same treatment and rarely get it. The sub-grade temperature has to be measured where it is coldest, which is the middle of the floor, and again near the perimeter where the bulb spreads sideways; both sensors go in their own sleeves so they can be replaced. The control target is a sub-grade held above freezing with a working margin, and the alarm that matters is not the heating failing — it is the sub-grade temperature drifting down over weeks, which is the only symptom you will get before the concrete starts moving. The heated area also runs past the insulated wall line rather than stopping at it, because the frost bulb does not respect the panel.
Electrically it is fixed heating equipment cast into a floor. In the UK that is BS 7671 Section 753, heating cables and embedded heating systems, which requires 30 mA residual current protection and a record of the installed cable; the cable itself is normally made to IEC 60800. Under NFPA 70 the relevant article is a question worth settling with the inspector before ordering rather than at first fix, because Article 424 for fixed electric space heating and Article 426 for embedded de-icing installations are both argued for on freezer sub-floor heaters and they are not identical.
- Insulation-resistance test every circuit on delivery, before a metre of it is unrolled, and record the reading against the drum number.
- Test again once the cable is laid and fixed, before anything covers it.
- Test again after the bedding layer or sub-slab has gone over it, and stop the job if a reading has moved.
- Test again after the wearing slab is poured and before energising — four readings, all on the same certificate.
- Mark every cable and duct run on an as-laid drawing dimensioned off two permanent gridlines, not off a wall that a panel will later cover.
Enter the zone and the centres as designed — 300 mm and wider is normal for slab heating laid to stop heave, and both boxes now take it. Run one CIRCUIT'S zone at a time rather than the whole bay: cable comes in fixed lengths with a rated maximum per loop, so a freezer floor is laid as several zones and each is ordered on its own figure. The sum itself is area divided by spacing, so it scales without caring how big the slab is.
The total floor area to be heated.
The centre-to-centre spacing between adjacent runs of the cable loop.
Total cable length needed
330 ft
Confirm your specific cable product's minimum bend radius and total length/wattage rating against your circuit before finalizing spacing — spacing that is too tight risks exceeding the cable's rated length for a single loop.
They open the calculator with your figures already in it
Radiant Floor Heating Cable Spacing Calculator: 330 ft — 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
- Spacing is a heat output decision and this sum treats it as geometry. The cable has a fixed rating per unit length, so the spacing chosen sets the floor's output per unit of area - halve the spacing and you double it - and the figure that should drive it is the room's heat loss, or the holding load of the slab in a cold store. Pick a spacing to make a cable length come out neatly and you have quietly changed the floor's output, which shows up later as a room that never reaches temperature or a slab costing far more to run than it needed to.
- Nothing here caps the floor surface temperature. Finishes set their own limits - engineered wood and vinyl are commonly held to around 27 degrees C at the surface, and comfort guidance for occupied floors sits close to that, with a higher figure allowed in bathrooms and at perimeter strips - and that limit binds whatever spacing the arithmetic will accept. A tight spacing under a timber floor cooks the adhesive and opens the boards long before it heats the room properly, so the output is pulled back to suit the finish, not the cable.
The Vapour Drive Runs Inward, Which Turns the Detail Upside Down
In a heated building the vapour pressure is higher inside than out, and every retarder detail you have ever drawn assumes that. A freezer reverses it completely. The air in a room at minus twenty-five holds almost no water; the ground and the ambient warehouse outside hold a great deal. The drive is inward and it is relentless, and it is the single most damaging thing that happens to a cold store envelope over its life.
Which puts the vapour barrier under the insulation, on the warm side, and makes that position non-negotiable rather than a preference. Put it above the boards and vapour migrates up into the insulation from the ground, reaches the freezing plane inside the board thickness, and condenses as ice. Ice cannot dry out — there is no season in which the room gets warm enough to reverse the flow — so the moisture accumulates. Wet insulation conducts better than dry, so the loss rises, so more vapour is drawn in, and the assembly degrades along a curve that only ever goes one way. Insulation lifted out of failed freezer floors comes out as blocks of ice, and the standard test method for characterising exactly this kind of moisture-driven thermal degradation is ASTM C1512.
Specify the membrane by permeance rather than by gauge. ASTM E1745 sets a maximum water vapour permeance for under-slab vapour retarders, measured to ASTM E96, and grades them into classes by puncture and tensile resistance; ASTM E1643 covers how they are selected, laid and inspected. For a low-temperature room the sheet is normally a good deal heavier and tighter than the six-mil polythene a builders' merchant reaches for, and it is often a reinforced or foil-laminated product. There is also a second sheet in this floor that is not a vapour barrier at all — a slip membrane over the boards, which stops wet concrete running into the board joints and lets the slab move independently of them. Count it separately, because it is the same area again.
Run the floor area twice — once for the vapour barrier under the boards and once for the slip membrane over them — then re-divide both by the roll size your specified product is actually sold in, because the coverage assumed here is a standard poly roll and a cold store vapour barrier is neither that material nor that width.
The total crawlspace floor or basement wall area to cover.
Sheet spent where seams overlap before they are taped.
Vapor barrier rolls needed
2 rolls
- Area to cover (with overlap allowance)
- 1,188 sq ft
They open the calculator with your figures already in it
Vapor Barrier Calculator: 2 rolls — 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
- The roll is fixed at a 10 ft x 100 ft (3 m x 30 m), 1,000 sq ft (93 m²) sheet. The calculator never asks what you are buying, so a 12, 16 or 20 ft wide roll, a 50 ft roll, or a reinforced 10-20 mil barrier will not divide into this count. Take the area figure from the breakdown and divide it by your own product's stated coverage.
- The overlap allowance is for flat-plane seams only. Nothing is added for turning the sheet up the foundation wall, wrapping piers and columns, sealing around penetrations, or the off-cuts an irregular crawlspace footprint produces. Measure and add those separately.
- Only the sheet is counted. Seam tape, mastic, mechanical fasteners and termination bar are not in this estimate.
- This is a quantity take-off, not a vapour-control design. It says nothing about the permeance, thickness or puncture class the barrier has to meet, or which face of the insulation it belongs on. In a cold store or an unvented crawlspace the wrong side traps moisture inside the assembly no matter how many rolls you order.
- It is not a radon or ground-gas membrane specification. Those are designed, jointed and verified systems with their own material, welding and testing requirements, and a 6-mil poly roll count does not substitute for one.
Laps, Collars, and the Junction Nobody Owns
A cold store envelope is a closed bag or it is nothing, and the seam between the floor membrane and the wall panels' own vapour seal is where the bag is usually left open. It falls between two subcontractors: the ground worker lays the floor sheet weeks before the panel erector arrives, the erector seals panel to panel and panel to slab, and the tie between the two belongs to neither of them unless somebody drew it and named an owner. Settle that on a drawing before the membrane is ordered, not on site with a roll of tape and two foremen.
The run-to-run seams are the easy part and the calculator handles them: sheet width against slab width gives the number of parallel seams, each one the full length of the bay. Add the perimeter turn-up separately. Penetrations should be a much shorter list here than on a normal floor, because a freezer floor is designed with as close to none as can be managed — a drain through a freezer slab is a cold bridge, a frost path and an ice plug all at once, and where the operation genuinely needs one it gets its own heated and separately detailed assembly rather than a taped collar. If your penetration schedule for this floor is long, that is a design review rather than a tape order.
Slab width against your sheet width gives the parallel seams and runs each the length of the bay; add the perimeter turn-up and the tie into the wall panel line on top, and treat any long list of penetrations as a design question rather than a quantity.
SettingsSettings for this calculation
Waste is set to 5% by hand. Pick a tier above to replace it, or keep your own figure.
The overall width of the slab area being covered by the vapor barrier.
The width of a single vapor barrier roll or sheet.
The overall length of the slab area, running the direction of each seam.
Extra tape to allow for overlaps, patches, and cut waste.
Seam tape needed
154.3 ft
- Number of seams
- 3
They open the calculator with your figures already in it
Under-Slab Vapor Barrier Seam Tape Calculator: 154 ft — 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
- The sheet count divides the slab width by the full roll width, so nothing is deducted for the side lap where adjacent sheets overlap each other. Laps are typically specified somewhere between 150 mm (6 in) and 300 mm (12 in) depending on the barrier manufacturer and the project specification, and once that is taken off the effective coverage of each sheet a wide slab can need one more sheet, and one more seam, than this calculation reports. Enter the lapped coverage width rather than the printed roll width if you want the lap counted.
- Only the longitudinal seams between sheets laid side by side are counted, and each is taken as running the full slab length in one unbroken run. If the roll is shorter than the slab, every run also carries an end lap partway along it, and there is no roll-length input here, so that transverse tape is absent from the total.
- The figure is seam tape between sheets and nothing else. Sealing the barrier to the footing, the foundation wall or the slab edge, and taping around pipes, conduit, column bases and other penetrations are all excluded and have to be estimated separately. On the default 20 m by 15 m slab the perimeter alone is 70 m against the 47.25 m of seam tape returned, so the excluded work can be the larger quantity.
- Which dimension you enter as the width decides the answer, and the calculator does not compare the two lay directions for you. Running 6.1 m sheets across the 20 m side of a 20 m by 15 m slab gives three seams and 47.25 m of tape, while running them across the 15 m side gives two seams and 42 m. The geometry also assumes one plain rectangle, so an L-shaped or stepped slab has to be broken into rectangles and the results added.
Board That Has to Hold a Rack Leg for Thirty Years
Under a freezer floor the insulation is not a lining, it is a structural bearing layer. The slab above it does not rest on the sub-base; it rests on the boards, and the boards rest on everything else. That changes what the slab designer is working with — the effective modulus of subgrade reaction under a slab on insulation is governed by the board, not by the compacted stone under it, and it is a much softer number. ACI 360R, the guide to design of slabs-on-ground, and Concrete Society Technical Report 34 for industrial ground floors are where that design is done; the point here is only that the board specification is an input to it.
Short-term compressive strength is the wrong figure to buy against. ASTM C578 classifies rigid cellular polystyrene into types by compressive resistance measured to ASTM D1621, and BS EN 13164 declares a CS(10\Y) level for XPS tested to BS EN 826 — all of those are the resistance at ten per cent deformation under a load applied over minutes. A rack leg applies its load for thirty years. What governs is compressive creep, measured to BS EN 1606, and manufacturers publish a long-term design stress that is a fraction of the short-term figure. That is the number the rack base plate has to be checked against, and it is the number a substituted board will quietly fail to match while showing an identical headline strength.
Water absorption matters for the same durability reason the vapour barrier does — BS EN 12087 for long-term immersion is the test to ask about — and it is one more argument for closed-cell XPS at the bottom of the stack. Lay the boards in two or three layers with joints staggered in both directions and between layers, tight-butted, on a surface flat enough that they do not rock. A continuous joint line through 200 mm of board is a continuous thermal path and a continuous plane of weakness under a wheel load, and it is created by one person laying a whole floor from a single corner.
The stack below is the whole floor as separate purchases. Read it as a bill of orders in seven different units rather than as a construction detail, because that is the way it gets mispriced: an estimate covers the concrete and the boards and treats the four things between them as sundries.
Seven layers, and the one that stops the ground freezing
- Wearing slab — carries the racking legs and the truck wheels, and shortens as it is pulled down to temperature rather than only as it dries Concrete Calculator
- Slip membrane — keeps wet concrete out of the board joints and lets the slab contract without dragging the insulation with it
- Rigid insulation, staggered layers — a structural bearing layer as much as a thermal one, bought against long-term compressive creep rather than headline strength Foam Board Insulation Calculator
- Vapour barrier, warm side — under the boards and turned up at the edge, because the drive runs inward and anything that gets into the insulation freezes there permanently Vapor Barrier Calculator
- Sub-floor heating grid — the only layer here that adds heat rather than resisting its loss, laid in withdrawable ducts and extended past the wall line Radiant Floor Heating Cable Spacing Calculator
- Compacted sub-base — placed and compacted in lifts a plate can reach through, and checked against a density reference before anything covers it Standard/Modified Proctor Compaction Percentage Calculator
- Natural formation — the frost-susceptible material the whole build-up exists to keep above zero for the life of the building
Run the bay area once per layer rather than entering the total board area — a 200 mm build-up in two staggered 100 mm layers is two separate deliveries of full-area boards, and the count for each has its own cutting waste.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
The total wall, foundation, or roof deck area to cover.
Offcuts from fitting boards between framing, around openings and at corners.
Foam board sheets needed
15 sheets (4x8 ft)
- Area to cover (with waste)
- 473 sq ft
They open the calculator with your figures already in it
Foam Board Insulation Calculator: 15 sheets (4x8 ft) — 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
- This counts sheets to cover an area. It does not choose a thickness or R-value, check your climate zone's requirement for continuous insulation, or confirm the board's compressive strength grade suits a load-bearing position under a slab or screed.
- The count is for a single layer. A staggered two- or three-layer build-up is a separate full-area order for each layer, so run the area through once per layer rather than entering the combined thickness.
- The waste allowance is a flat uplift on area, not a cutting layout. Rafter bays, hips, curved walls and dense penetrations throw off more offcuts than that, and offcuts from one bay are often unusable in the next. A wall whose window and door openings you did not deduct is over-ordered by roughly their area.
- It assumes every board is the market's standard full sheet. It does not cover the 2 ft wide XPS or 1200 x 600 mm half boards some ranges are sold in, and it does not deduct the overlap on tongue-and-groove or shiplap edges, which cover less than the board's nominal face area.
- Nothing beyond the boards is counted: seam tape, adhesive or foam, fixings and washers sized to the board thickness, furring, and any separate vapour or air control layer. Rigid foam is also combustible, and building codes generally require a thermal or ignition barrier between it and an occupied space. This calculator neither sizes nor includes that.
The Cold Edge, and Twelve Millimetres of Contraction
A warm warehouse floor shrinks as it dries and then stops. A freezer floor does that too, and is then taken from pouring temperature down to operating temperature, which is a change of around forty kelvin. Concrete moves roughly ten microstrain per kelvin depending on its aggregate, so that is 0.4 mm per metre — about twelve millimetres across a 30 metre bay, arriving over the days of pull-down and staying for the life of the store. Joint layout, joint width and the sealant specification all have to accommodate movement that a normal industrial floor never sees, and the sealant has to stay elastic at the room's temperature rather than at the day's.
At the perimeter that movement meets a wall panel that is not going anywhere, which is why the compressible edge detail is a linear item bought by the metre and one that an area-based takeoff drops every time. Count the full internal perimeter including every reveal, every column encasement and every return into a door pocket — 108 metres on a 30 by 24 bay before any of those returns are added. The strip runs from the top of the insulation to finished floor level so the slab surface is isolated too, and it is trimmed after the pour rather than cut down during it.
The bigger perimeter question is thermal rather than dimensional. The floor insulation has to be continuous with the wall panel's insulation, which in practice means it runs under and past the panel line rather than stopping at it, and the sub-floor heating runs past it as well. Where that continuity is broken you get a cold line at the base of the panel on the ambient side, which in a humid warehouse means condensation, then ice, then a wet floor at the one place forklifts turn. It is the most commonly reported defect on a converted cold store and it is decided entirely by a detail drawn before the boards are ordered.
Take the full internal perimeter including reveals, column encasements and door pockets — this is the one linear item on a floor whose every other line is an area or a volume, which is exactly why it is the one that goes missing from the order.
The total length of the foundation perimeter to be insulated.
The length of a single rigid foam board as sold.
Foam boards needed
17 boards
They open the calculator with your figures already in it
Under-Slab Perimeter Rigid Foam Board Calculator: 17 boards — 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
- The count divides perimeter by board length and nothing else. There is no upstand depth and no board width input, so it assumes one board covers one board length of run at the full depth required: a 2400 x 1200 mm (47 in) sheet ripped into four 300 mm (12 in) slab-edge strips actually covers 9.6 m (31 ft) of run, while a skirt deeper than the board width needs a second course and doubles the figure.
- There is no waste allowance and no corner allowance in this number. It assumes every offcut is carried onto the next run, which a plan with re-entrant corners, steps and door thresholds does not allow; the related area-based foam board calculator adds 10 per cent for exactly this reason.
- Where boards sit under a thickened slab edge they are a load-bearing layer, and this is a piece count that says nothing about compressive strength grade or long-term creep under sustained load. A board specified for a vertical face is not necessarily graded to bear beneath an edge beam.
- Nothing here covers protecting the foam once it is in. Exterior perimeter foam is a concealed route for termites, and jurisdictions differ on whether an inspection gap or termite shield is required and whether exterior below-grade foam is permitted at all; the above-grade portion also needs render or a protection board against UV and impact, which is a separate material line.
One Pour, Four Trades, and Nothing Reopened
Everything in the sequence below is unremarkable on its own. What makes it difficult is that four trades hand over to each other inside a few days, in an existing shed, with a concrete delivery booked at the end of it, and any one of them can bury a defect nobody detects for three winters. Written holds are the mechanism: a named person signs each stage before the next covers it, and there is no verbal handover.
Two practical points about the pour. Reinforcement chairs bearing on a slip membrane over rigid boards need plate feet or they punch into the insulation, taking the membrane with them where the two are adjacent; and a pump, conveyor or chute is nearly always needed, because a truck cannot drive over a floor made of foam board. The pour, the finish and the curing are the same job they are on any industrial slab, and that ground is covered elsewhere on this site.
- Formation stripped and proof-rolled, with soft spots dug out rather than blinded over — signed off before sub-base arrives.
- Sub-base placed in reachable lifts and compacted to a stated density, levels recorded on a grid rather than at the corners.
- Ducts, heating cable and sensor sleeves laid to the as-built grid, all terminations brought outside the footprint, insulation resistance recorded.
- Bedding layer or lean-mix sub-slab over the grid, cables re-tested, surface left flat enough that boards will not rock.
- Vapour barrier laid, lapped, taped and turned up, the tie into the panel line built rather than promised, photographed in full before anything covers it.
- Insulation laid in staggered layers, slip membrane over, reinforcement on plate-footed chairs, cables tested a third time.
- Pour, finish and cure, then a fourth cable test before the circuits are energised and the room is handed to the refrigeration contractor.
Pull-Down Is Measured in Days, and the Record Outlives Everyone
A new slab is holding a great deal of free water and it is going to a place where water turns to ice. Taking the room down too quickly freezes that water inside a slab that has not finished gaining strength, and it also imposes the whole forty-kelvin contraction on a floor whose joints have not yet opened. The refrigeration contractor's commissioning procedure sets the rate and it is properly measured in days rather than hours, usually with a hold at just above freezing to let the slab give up moisture before the room goes below zero. Get that procedure in writing at the same time as the concrete is ordered, because it is a programme item and it is invariably discovered late.
Start the sub-grade temperature record on day one of pull-down and never stop it. It is the only instrument this floor has. Nothing else will tell you that the heating grid has quietly lost a circuit, or that the frost bulb is advancing at the north end where the panel detail was compromised, and by the time a laser survey across the floor shows movement the argument has moved from maintenance to litigation. A trend that drifts down over a season is a repair; the same trend ignored is a demolition.
The handover file is short and it is worth more than the rest of the documentation put together: the as-laid heating grid dimensioned off two permanent gridlines, the four insulation-resistance certificates, the duct route plan with the spare ducts marked, the sensor positions, the board type and its long-term design stress, the membrane product and its permeance class, and photographs of the vapour barrier and the wall junction before they were covered. Everything in that list answers a question that can otherwise only be answered by breaking the floor.
The six orders a freezer floor is actually bought as
Priced as slab plus insulation, this floor comes in wrong. Quantify it as six separate purchases in five different units, and note that two of them — the heating grid and the vapour barrier — are the ones with no second chance in them.
- Insulation, per layer rather than per stack — Bay area run once for each staggered layer, with the board sized against the layer's share of the design R-value and specified on its long-term compressive creep, not its ten per cent strength.
- Heating cable and its ducts — Area divided by design spacing gives the cable; the withdrawable ducts, the sensor sleeves and the spare empty runs are counted alongside it and terminate outside the footprint.
- Vapour barrier, and the slip membrane above it — The same floor area twice in two different products, re-divided by the roll size the specified material is actually sold in rather than by a standard poly roll.
- Seam tape, plus the turn-up and the panel junction — Run-to-run seams from sheet width against bay width, with the perimeter turn-up and the tie into the wall panel's vapour seal added and given an owner on the drawing.
- Perimeter strip, by the metre — Full internal perimeter including reveals, column encasements and door pockets, at a height that reaches finished floor level so the slab surface is isolated as well as its edge.
- The commissioning programme, as a duration — Pull-down rate from the refrigeration contractor in writing, priced as days rather than assumed as an afternoon, with the sub-grade temperature record starting on day one.
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.