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Insulation R-Value Explained: Nominal vs Effective, What the Ontario Code Requires, and Why Air Sealing Comes First
Heating and efficiency R-value From a builder
Insulation R-Value Explained: Nominal vs Effective, What the Ontario Code Requires, and Why Air Sealing Comes First
R-value is the number everyone asks about and the number that tells you the least on its own. It measures one way heat leaves a house, in a lab, through a perfect sample. Your house is not a perfect sample, and heat has two other ways out. Here is what the number means and how to use it.
Short version: R-value is resistance to heat flow by conduction through a material (RSI is the metric version; R = RSI x 5.68). The number on the bag is nominal; the number the wall delivers is effective, after the studs, plates, headers, gaps and compression are counted, and a 2×6 wall with R-22 batts delivers about R-17. Ontario’s SB-12 now specifies effective values: roughly R-60 attics, walls in the effective low R-20s (2×6 batts plus continuous exterior foam, or ICF), R-20 full-height basements, insulated slabs in many packages. Insulation only slows conduction; air leakage through the assembly can carry away as much heat as the walls lose, so air seal first, then insulate, then ventilate on purpose. More R pays back fastest in the attic and under the slab, and slowest past about R-30 in a wall that already has no thermal bridges.
What R-value is, and is not
R-value is the resistance of a material to heat flowing through it by conduction, measured on a sample in a hot-box at steady temperatures. Higher is slower heat loss, like a better thermos. Insulation sold in Canada is labelled with R and RSI. It is a fair way to compare materials and a poor way to describe a wall, because a wall is not a sample: it is studs, plates, headers, boxes, wires, gaps, and whatever the installer did on a Friday afternoon. R-value also says nothing about the other two ways heat leaves: air moving through the assembly (convection and leakage) and radiation through glass. A house with R-40 walls and a leaky ceiling loses more heat than a house with R-20 walls and a tight one.
Nominal versus effective
| Assembly | Nominal (label) | Effective (whole wall) |
|---|---|---|
| 2×4 wall, R-12 batts, studs at 16 in | R-12 | About R-10 |
| 2×6 wall, R-22 batts, studs at 16 in | R-22 | About R-17 |
| 2×6 wall, R-22 batts plus R-5 continuous exterior foam | R-27 | About R-22 |
| 2×6 wall, R-24 batts, studs at 24 in, R-7.5 exterior | R-31 | About R-26 |
| ICF, 2.5 in EPS each side | R-22 to R-24 | R-22 to R-24 |
| Attic, R-60 loose fill, raised-heel truss | R-60 | About R-55 to R-60, less at the eaves on a standard truss |
| Steel-stud wall, R-22 batts, no exterior foam | R-22 | About R-10 to R-14 |
Wood is about R-1.25 per inch and makes up 20 to 25 percent of a framed wall; every stud, plate and header is a bridge for heat, and the batts between them only count where they are full, uncompressed and unbroken. Continuous insulation outside the framing (or a wall with no framing through the insulation, which is what ICF is) is how the effective number catches up to the nominal one. ICF R-value is the case where the two numbers are the same; steel frame homes is the case where they are furthest apart.
What the Ontario Code requires
Supplementary Standard SB-12 of the Ontario Building Code sets the energy requirements for houses and, since the 2017 and 2024 updates, writes them as effective values. You comply by choosing a prescriptive package (a table where the attic, wall, basement, slab, window and mechanical values all go together) or by the performance path (an energy model showing the design uses no more energy than the reference house). Typical package values in most of southern Ontario:
- Attic (flat ceiling): about R-60.
- Above-grade walls: effective low R-20s, which in wood frame means batts plus continuous exterior insulation; ICF complies by construction.
- Basement walls: R-20 effective, full height.
- Under the slab: R-10 or so in many packages, and under any heated slab.
- Windows: a maximum U-value (lower is better), typically around 1.4 W/m2K or better.
- Air tightness: credited on the performance path and required in some packages, with a blower door test. SB-12 explained covers both routes; attic R-value in 2026 goes deep on the attic line.
Air sealing: the half of the story R-value leaves out
Every cubic metre of warm air that leaks out through the top plates, the pot lights, the rim joist and the window frames is replaced by a cubic metre of outdoor air that the furnace has to heat from minus 20. In an ordinary new frame house that leakage is worth as much as the conduction through the walls; in an older house it is worth more. Insulation that air can blow through (batts, loose fill) does almost nothing where air is moving. So the order is: a continuous air barrier (taped sheathing, sealed poly or airtight drywall, every penetration foamed or caulked), tested with a blower door; then insulation; then mechanical ventilation so the tight house breathes on purpose. Air sealing an Ontario home is the full argument and HRV vs ERV is the ventilation half; in an ICF house the concrete core is the air barrier and most of this work disappears.
Where more R-value pays back
- The attic, to R-60: cheap per R, big area, biggest temperature difference at the top of the house.
- Under the slab and at the basement walls: most older Ontario houses have nothing there, and a new house with radiant tubing needs it.
- The eaves and the rim joist: the thin spots on every frame house.
- The walls, to the effective low R-20s with no bridges: after that each additional R-5 buys less, and the money does more in windows.
- Windows: an R-3 window in an R-22 wall is where the heat goes; triple-pane glass is the upgrade that shows up on the bill. How to buy windows.
- Past about R-30 effective in a wall with no bridges, the returns are small in our climate; spend it on the air barrier and the glass instead. Insulating a house in Ontario has the assembly-by-assembly detail.
Related guides on this site
NEW: The R-value with the studs counted
An R-22 batt in a 2×6 wall is about R-17 as built. The new section shows the arithmetic, compares seven wall assemblies by effective R and cost, and has a depth-to-R chart for the attic.
Effective R-value and thermal bridging · Wall assembly comparison calculator · Depth to R-value chart
Sources and further reading
Questions people ask
What R-value does the Ontario Building Code require?
Under SB-12 packages, roughly R-60 in a flat attic, an effective value in the low R-20s for above-grade walls (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 difference between nominal and effective R-value?
Nominal is the label on the insulation; effective is what the whole assembly delivers once studs, plates, headers, gaps and compression are counted. A 2×6 wall with R-22 batts is about R-17 effective; an ICF wall delivers close to its nominal value because nothing bridges the foam.
Does more R-value always pay back?
No. The attic to R-60, the slab and the basement pay back fastest; walls past the effective low R-20s with no thermal bridges pay back slowly in Ontario. Air sealing and better windows usually beat adding wall R-value.
Is R-value the most important number for a house?
It is one of three: conduction (R-value), air leakage (blower door result) and radiation through glass (window U-value and solar gain). A tight house with good windows and R-22 walls outperforms a leaky house with R-40 walls.
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Reviewed and updated October 2026. Costs are Ontario ranges, not quotes; code references are to the Ontario Building Code in force at that date.
