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Flooring Over a Heated ICF Slab: How We Build It, From the Foam Under the Concrete to the Last Plank, the Drying Time, the Moisture Test and the Order the Floors Go In
Flooring & TileICF
Flooring Over a Heated ICF Slab: What Is Under the Concrete Decides What Can Go On Top of It
Every flooring problem I have ever been called to look at on a heated slab started below the flooring. Not enough foam under the concrete, so the slab heats the gravel and the floor runs hot trying to keep up. No poly, so the slab never dries and the glue lets go. Heat turned on at day seven, so the slab cracked, or never turned on before the floor went in, so the moisture came up the first November. I have built ICF houses on heated slabs since 1995, and the flooring sequence starts the day the footings are poured. Here is the whole stack, bottom to top, the way we do it.
The stack, bottom to top
| Layer | What we do | Why it matters to the floor on top |
|---|---|---|
| Compacted granular | 6 to 8 in. of 3/4 in. clear stone or Granular A, compacted in lifts, laser-levelled | A flat slab starts with a flat base. Settling under the slab is a cracked slab is a cracked tile floor. |
| Under-slab insulation | 3 to 4 in. of Type II EPS, R-12 to R-16, R-20 on walkouts; the code minimum under a heated slab is R-10 | The heat goes up or it goes down. Every dollar of foam here comes back in a slab that runs cooler and a floor that runs where it was designed to. |
| Edge insulation | 2 in. at the slab edge and the footing, continuous with the ICF wall foam | The slab edge is where heat leaks to the footing and where the floor by the wall runs cold. |
| Vapour retarder | 10-mil poly, seams lapped and taped, sealed to the wall; code asks for 6-mil | The slab can only dry upward. Without poly it pulls ground moisture forever and no adhesive survives it. |
| Tubing | 1/2 in. oxygen-barrier PEX at 12 in. on centre, 9 in. near exterior walls and under big windows, clipped to the foam or tied to mesh, loops under 300 ft, pressure-tested to 100 psi and left under pressure through the pour | Even spacing is an even floor temperature, which is what lets wood and vinyl live under it. |
| Concrete | 4 in., 25 MPa, fibre-reinforced, 6×6 mesh where the structural engineer wants it, control joints cut or formed at 12 to 15 ft | The joints are where it cracks on purpose; tile gets an uncoupling membrane over them, wood and vinyl span them. |
| Floor covering | Tile, glued engineered, glued LVP, in that order of preference | Covered below. |
The heat loss that sets the tubing spacing and the water temperature is done room by room before the foam goes down. The slab and foundation side of this, for other builders, is ICFPro.ca.
The numbers the whole stack serves
The water in our slabs goes in at about 40 C and the floor surface runs 30 to 37 C, and that is what has been under the tile, vinyl and engineered hardwood in our houses for thirty years. The flooring sheets say 27 to 29 C under wood or vinyl, and the warranty hangs on that number, so a homeowner running warmer is trusting the builder and the product rather than the paper; say so out loud at design, and put a floor sensor or mixing valve on the zone so the number is known. The reason an ICF house keeps the floor from ever running away is that the walls and the slab are so well insulated the house needs very little heat, so the slab never has to run hot to supply it. A leaky house on a thin slab over bare gravel needs 50 C water to stay warm, and no wood floor survives that for long. The radiant compatibility article is the rule by floor type; this page is how the building makes the rule easy to keep.
Drying, heating and testing before the first tile
- No heat for 28 days. The concrete cures with water in it, and heating it early pulls the water out before the cement has used it. We pour, cover for a week, and leave the system cold for four weeks.
- Then heat on, low, and up slowly. 25 C water for a few days, then up 2 to 3 C a day to the design temperature. A heated slab is the best concrete dryer there is once the concrete is strong enough to take it.
- Two weeks of heat minimum before flooring. Three on our jobs. With the house closed in and the heat running, a 4 in. slab on poly reaches flooring moisture in 60 to 90 days from the pour. Without heat, double it.
- Test it, do not guess. In-situ probes to ASTM F2170, under 75% RH, or calcium chloride under 3 lb MVER; three tests for the first 1,000 sq ft and one per 1,000 after. Write the results on the slab in marker and photograph them. The adhesive maker’s number wins if it is lower.
- Flatness 3/16 in. in 10 ft, 1/8 in. for large-format tile. A good finisher on a laser-levelled base gets there; the rest is $2.00 a square foot of grinding or $3.00 of self-leveller. Check with a straightedge before the flooring is ordered, not after.
- Mark the tubing. Photograph the loops with a tape in the frame before the pour, and again mark the manifold runs on the slab. Anyone drilling for a door stop or a toilet flange thanks you.
The order the floors go in
Tile first, because it is the dirtiest and the hardest to damage. Bathrooms, mudroom, laundry, and the kitchen if it is tile: an uncoupling membrane over the slab across the control joints, then porcelain. We use a membrane on every heated slab we tile because the slab will move at the joints and the membrane lets it. Then the drywall gets its final coat and prime, because tapers drip.
Glued engineered hardwood next, after prime and before final paint, with the heat turned down to warm for the install and ramped back after. Then glue-down vinyl plank in the basement rooms that get it, same conditions. Carpet, if any, last of all, after the painters leave, because carpet installers and painters do not mix. Baseboard goes on after the hard floors and before the carpet, and the flooring order page has the whole house sequence with cabinets and stairs in it.
What the floors cost over our slab, mid grade: 300 sq ft of porcelain with membrane at $19.25 is $5,775; 900 sq ft of glued engineered at $12.75 is $11,475; 500 sq ft of glue-down LVP at $7.00 is $3,500. Total $20,750 before HST and waste, for 1,700 sq ft of heated floor with nothing floating on it. The flooring cost calculator does your own rooms.
Where it goes wrong on other people’s slabs
R-5 under the slab, or none. The code says R-10 under a heated slab and plenty of basements got 1 in. of foam or a sheet of bubble wrap. The ground under the house becomes the heated object, the water temperature creeps up every year, and the wood floor on top gets a sheet of its own: a claim denial. Basement radiant covers the retrofit case.
Poly left out or shredded. Tubing crews walk on it, mesh tears it, and nobody tapes it back. The slab tests at 85% RH two years later and the glued floor bubbles. On a new build, walk the poly before the pour. On an old slab with no poly, a $4.00 a square foot epoxy moisture mitigation coating is the fix, and it is cheaper than the second floor.
Flooring in before the heat is commissioned. The schedule says floors in October, the boiler is not hooked up until December, the slab is at 90% RH and the floor is installed over it anyway. The first winter of heat drives the moisture into the adhesive. It is the commonest heated-slab flooring failure I see and it is purely a scheduling decision. If the heat is not running two weeks before the floor, move the floor.
Questions people ask
How much insulation goes under a heated slab in Ontario?
The Building Code minimum is R-10 under the whole slab when it is heated. We put R-12 to R-16 under our slabs and R-20 on walkouts, with 2 in. at the edges. The extra foam pays for itself in lower water temperature and a floor that stays in the comfortable range.
How long before you can put flooring on a heated concrete slab?
No heat for the first 28 days, then two to three weeks with the heat running, then a moisture test under 75% RH or 3 lb MVER. In practice 60 to 90 days from the pour with heat, longer without. The test decides, not the calendar.
Do you need poly under a heated slab?
Yes. Code asks for 6-mil; we use 10-mil, taped at the laps and sealed to the wall. The poly is what lets the slab dry and stay dry, and no glue-down floor lasts on a slab that pulls moisture from the ground.
Can you tile directly on a heated slab?
You can, and it will crack at the control joints and anywhere the slab does. We use an uncoupling membrane on every heated slab we tile, about $3.50 a square foot, and treat it as part of the tile price.
What flooring do you put in your own ICF houses?
Porcelain in the wet rooms over a membrane, glued engineered hardwood in the living areas, glued LVP in basement bedrooms and rental suites, and no carpet on a heated slab if we can talk the client out of it. The houses are on ICFhome.ca.
Building on a heated slab?
The radiant flooring compatibility checker tells you what can go on it and under what conditions. The slab system itself is priced on the hydronic radiant cost page.
Sources and further reading
- Ontario’s Building Code: the official page with the current regulation; stair dimensions, bathroom finishes and floor framing are in Part 9.
- Uponor Canada: hydronic radiant tubing and controls; their design manual is where the surface temperature and floor R-value tables come from.
- ICFPro.ca: the ICF foundations and slabs side of the business: what is under the floor.
