Floor Insulation.
A floor over unheated space in Zone 7A has to reach effective RSI 5.02 (R-28.5), with or without a heat recovery ventilator — the HRV trade-off that relaxes ceilings and walls does not apply to floors.
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Cold floors are usually an air problem wearing a temperature costume. The joist bays over a garage, a cantilever or an unheated crawl space are open to outside air at their ends, and no amount of R-value in the middle of the bay fixes an open end.
Floors on the ground and heated slabs read from an entirely different table with lower numbers — the full Zone 7A set, above grade and below, is in the code guide with the placement rules that go with each one.
Which Floor, Which Number.
The word floor covers at least four different code requirements in Zone 7A, and they are not close to each other. Naming the right one is the first part of the job.
- Floors over unheated space: effective RSI 5.02 (R-28.5), Table 9.36.2.6.-A and -B. Unlike ceilings and walls, this figure does not change when an HRV is installed.
- Heated floors in contact with the ground: effective RSI 2.84 (R-16.1), Table 9.36.2.8.-A, and Sentences 9.36.2.8.(5) and (6) require insulation over the full bottom surface including the edges.
- Slabs-on-grade with an integral footing: effective RSI 3.72 (R-21.1), insulated under the entire slab and around all edges but not under the integral footing, plus skirt insulation of the same effective resistance under Sentence 9.36.2.8.(9).
- Unheated floors on ground above the frost line: effective RSI 1.96 (R-11.1). Below the frost line the table permits them uninsulated.
- Alberta relief worth knowing about on hydronic work: STANDATA 23-BCV-003 is a province-wide variance permitting less than RSI 2.84 under a heated floor in contact with the ground, on the basis of Alberta's own analysis that going beyond RSI 1.32 (R-7.5) has marginal effect on comfort and annual energy.
- Sealing the ends of the joist bays, the cantilever return, and the rim at the perimeter — the parts of a cold floor that actually move air.
- Foam kept 75 mm clear of any device with a surface temperature above 80 °C, per manufacturer instruction, which matters where hydronic manifolds or recessed fixtures sit in the same cavity.
- Spray foam applied in lifts of no more than 50 mm, the CAN/ULC-S705.2 baseline, with cooling time between passes — thick single lifts in a confined joist bay are the classic exotherm risk.
Working Out Why the Floor Is Cold.
- 01
Identify what is under it
Unheated garage, vented crawl space, cantilever over outside air, or ground. Each of those is a different table, a different number and in two cases a different placement rule.
- 02
Close the ends before filling the middle
A joist bay open at the rim is a duct. Sealing the perimeter and the cantilever return usually changes the feel of a floor more than the last RSI 1.0 in the middle of the bay does.
- 03
Respect what is in the cavity
Plumbing on the cold side of the insulation is a freezing risk in a Zone 7A winter, and heat-sensitive services need thin first passes and cooling time before full thickness goes on.
- 04
Cover the foam where the code says to
Foam in a concealed floor assembly is carved out of Article 9.10.17.10. Foam exposed on the ceiling of an occupied garage or basement room is not, and needs an interior finish or a listed thermal barrier.
Frequently Asked Questions About Floor Insulation
What insulation does a floor over an unheated garage need?
Effective RSI 5.02, about R-28.5, in Climate Zone 7A. That figure sits in Table 9.36.2.6.-A and it is repeated unchanged in Table 9.36.2.6.-B, so unlike ceilings and walls it does not relax when a heat recovery ventilator is installed. It is also the same number the code sets for cathedral ceilings and flat roofs, which surprises people.Why is my floor still cold after it was insulated?
Almost always because the ends of the joist bays are open to outside air, or because the insulation is not in contact with the underside of the subfloor. Batts that have slumped leave an air gap at the top of the bay and a convective loop inside it, and neither shows up on an invoice. Effective RSI assumes the insulation is where the drawing says it is; a cold floor is usually the assembly telling you it is not.Does a heated concrete floor need as much insulation as a floor over a garage?
No, and the difference is large. A heated floor in contact with the ground is effective RSI 2.84, about R-16.1, under Table 9.36.2.8.-A, against RSI 5.02 for a floor over unheated space. Alberta has gone further on hydronic work: STANDATA 23-BCV-003, issued in May 2024, is a province-wide variance permitting less than RSI 2.84 below a heated floor in contact with the ground, on the strength of an Alberta energy analysis finding that going past RSI 1.32 has only marginal effect on comfort and total annual energy use.Can spray foam be applied to the underside of a floor in winter?
Yes, subject to the substrate. Every closed-cell product publishes a substrate and ambient temperature range, and winter grades typically run from about -10 to 10 °C, with the coldest published Super Winter grade at -20 to 0 °C. The substrate also has to be at least 3 to 5 °C above the dew point, frost-free, at 19 percent moisture content or less, with relative humidity under 80 percent. In an unheated crawl space or an open garage in January those conditions are made, not found — which is why enclosure and temporary heat are engineering steps rather than an upsell.How thick can each pass be?
CAN/ULC-S705.2's baseline is a maximum of 50 mm per pass, with cooling time between passes, because the reaction is strongly exothermic. Published CCMC exotherm testing on a high-lift product recorded peak in-foam temperatures of 176.5 °C over wood and 178.5 °C over gypsum at 125 to 127 mm, and more than six hours for the foam to cool back to 21 °C. One product is evaluated for single passes up to 127 mm under its own CCMC technical guide, and that report states plainly that anything sprayed beyond 140 mm must be removed immediately because of the risk of spontaneous combustion. The exception exists precisely because the general rule is real.