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Insulating a House in Ontario: What Goes Where, What the Code Requires, and the Mistakes That Waste It
Heating and efficiency Insulation From a builder
Insulating a House in Ontario: What Goes Where, What the Code Requires, and the Mistakes That Waste It
A house dressed for an Ontario winter wears three layers: a shell to stop the wind, a sweater to hold the heat, and a liner to keep the sweat off the sweater. Most insulation problems I see are not too little sweater. They are a missing shell, or a sweater with holes, or the liner on the wrong side.
Short version: Insulation only works as part of a system: a continuous air barrier (the shell), the insulation itself (the sweater) with no gaps, voids or compression, a vapour retarder on the warm side (the liner), minimal thermal bridging, and a way for the assembly to dry. In Ontario the Building Code’s SB-12 sets minimums by package, roughly R-60 in a flat attic, an effective R-22 or better in walls (which usually means 2×6 batts plus continuous exterior insulation, or ICF), R-20 full-height in the basement, and under-slab insulation in most packages. The attic is the cheapest place to add more; the basement is where most existing Ontario houses are worst; the walls are where thermal bridging through the studs quietly takes a third of the number on the bag.
The system, not the product
| Layer | Job | Common failure |
|---|---|---|
| Air barrier | Stops wind and stack effect moving air through the assembly | Not continuous at top plates, rim joists, pot lights, hatches, windows |
| Insulation | Slows conduction | Compressed, gapped, wet, or bridged by framing |
| Vapour retarder | Keeps warm moist indoor air out of the cold side of the wall | On the wrong side, or doubled up so the wall cannot dry |
| Thermal breaks | Keep framing from short-circuiting the insulation | Studs at 16 inches with nothing outside them |
| Drying path | Lets any moisture that gets in get out | Two vapour barriers, or impermeable layers both sides |
The R-value on the bag assumes a perfect installation in a lab. Insulation R-value explained covers nominal versus effective; the short version is that a 2×6 wall of R-22 batts tests around R-17 effective once the studs, plates and headers are counted, and worse if the batts are stuffed or gapped.
What the Code wants, by assembly
Ontario’s energy requirements for houses live in Supplementary Standard SB-12. You pick a prescriptive package (a table of values that go together) or prove the design by the performance path. Typical package values for a gas- or heat-pump-heated house in most of southern Ontario:
- Flat attic: around R-60 (loose-fill cellulose or fibreglass, or batts in layers); cathedral ceilings less, with a vented or unvented detail that has to be done right.
- Above-grade walls: an effective value in the low R-20s, which in wood frame means 2×6 batts plus continuous exterior insulation (R-5 or more), or 2×6 with R-24 batts and a package that trades elsewhere; an ICF wall meets it by construction.
- Basement walls: R-20 effective, full height, inside, outside or both; ICF basements comply by construction.
- Under the slab: R-10 or so in many packages where there is a heated slab or a walkout; always under a radiant floor.
- Windows: a maximum U-value, which is why the window spec and the wall spec travel together.
- Air tightness: a blower door test earns credit on the performance path and is required in some packages. SB-12 explained has the mechanics; attic R-value in 2026 goes deep on the attic.
Attic
The cheapest big win in any house. Air seal the ceiling plane first (every penetration, the top plates, the hatch), then blow it full. Baffles keep the soffit vents open; raised-heel trusses in new construction let the insulation stay full depth right out over the wall, which is also the cure for ice dams. Do not bury recessed lights that are not rated for it, and do not store the Christmas decorations on top of the insulation.
Walls
In new wood-frame construction: fill the cavity properly (batts cut to fit, split around wires, not stuffed behind boxes), or dense-pack cellulose, or spray foam; then continuous insulation outside the sheathing, taped as the air barrier or with a separate wrap detailed as one. Studs at 24 inches where the engineering allows cut the bridging. In an existing wall, dense-pack through holes is the usual retrofit; a solid-brick house needs a new insulated wall inside or outside. Spray foam and closed-cell vs open-cell cover the foam options. An ICF wall skips the whole conversation: two continuous layers of foam, a concrete core as the air barrier, no studs to bridge. ICF R-value explains why it performs above its label.
Basement and slab
Outside is best from a building-science view: rigid or mineral wool board against the foundation keeps the concrete warm and drains water, and it goes on with the waterproofing. Inside is cheaper and more common: rigid foam against the wall (not batts against bare concrete), then a stud wall, then more insulation if you want it, with no polyethylene trapped against the foam. Fix water first; insulating a wet basement makes a mouldy one. Attic insulation cost and why new basements leak are the two pages to read with this one. Under the slab: rigid foam over a drainage layer and vapour barrier, especially under radiant tubing, where the slab is the heater and the ground is the enemy.
The mistakes that waste it
- Insulating before air sealing; wind through a batt is a batt doing nothing.
- Compressed batts, batts cut short, batts behind wiring instead of split around it.
- Polyethylene on both sides of a wall, or poly inside and foil-faced foam outside with no drying path.
- Fibreglass against bare concrete in a basement, with poly over it; it is a mould farm.
- Blocked soffits, buried hatches, no baffles.
- Counting the number on the bag and ignoring the studs, headers, rim joists and corners that carry a third of the heat out.
- Thin insulation at the eaves because the truss is low there.
Related guides on this site
NEW: The whole Insulation and Air Sealing section
Attic, spray foam, basements, walls, air sealing, rebates and contractors: fifty-three pages with October 2026 Ontario prices, three calculators and a $99 quote review.
Insulation and air sealing hub · Insulation cost in Ontario · Basement insulation cost
Sources and further reading
Questions people ask
How much insulation does an Ontario house need?
Under SB-12 packages, roughly R-60 in a flat attic, an effective value in the low R-20s for above-grade walls (2×6 batts plus continuous exterior insulation, or ICF), R-20 full height in the basement, and under-slab insulation in many packages, with window U-values and air tightness tied in. The performance path can trade one against another.
What is the best way to insulate a basement in Ontario?
Rigid foam against the foundation wall (outside is better, inside is common), then a stud wall if you are finishing, with no polyethylene trapped against the foam, and under-slab foam in new construction. Never fibreglass batts against bare concrete. Fix any water first.
Does insulation work without an air barrier?
Poorly. Air moving through fibrous insulation carries the heat with it, and warm air leaking into a cold attic or wall condenses there. Air seal first, then insulate.
Is spray foam better than batts?
Closed-cell foam gives the most R per inch and an air seal in one step, at the highest cost; open-cell and dense-pack cellulose are cheaper and need a separate air barrier. Properly installed, all of them work; the installation and the air barrier decide more than the material.
Building in Simcoe County or on Georgian Bay?
We are ICFhome.ca, a family builder in Tiny Township with 45 years of custom homes behind us. Pick the path that matches where you are.
Reviewed and updated October 2026. Costs are Ontario ranges, not quotes; code references are to the Ontario Building Code in force at that date.
I appreciate that you explained how the real R-value of the insulation is lowered by compressing it, leaving air holes surrounding it, and allowing air to travel inside it. This advice will be very useful because my sister and I intend to construct a home for our parents. I’m hoping we can eventually locate a residential insulation business to perform this for the house.