Ontario's Plain-English Guide to Custom Home Buildingπ 705-533-1633 | βοΈ 1-866-868-6606 | βοΈ info@icfhome.ca
Effective R-Value and Thermal Bridging: Why a 2×6 Wall With R-22 Batts Is Really R-17, the Table for Eight Ontario Walls, and How Continuous Insulation and ICF Fix It
Insulation & Air Sealing
The Wall Is Not the Batt: A 2×6 R-22 Wall Is an R-17 Wall, and Here Is the Arithmetic
Pick any 2×6 exterior wall in Ontario and about 23% of it, by area, is wood: studs at 16 in., top and bottom plates, headers, corners, the studs around every window. Wood is about R-1.2 an inch, so a 5.5 in. stud is R-6.6 sitting in a cavity that is R-22. The heat goes around the batt through the stud the way water goes around a rock. Run the arithmetic the way the code does and the wall comes out at about R-17 effective. That is the number the SB-12 form uses, the number the heat-loss calculation uses, and the number nobody prints on the bag.
The table
| Wall | Nominal R | Effective R | Why |
|---|---|---|---|
| 2×4, R-14 batt, 16 in. o.c. | R-14 | about R-11 | The 1970s wall. A quarter of it is R-4.2 wood. |
| 2×6, R-22 batt, 16 in. o.c., 23% framing | R-22 | about R-17 | The standard Ontario wall. SB-12 accepts it in most packages as “R-22 nominal”; the form still carries the effective value. |
| 2×6, R-22 batt, plus 1 in. R-5 continuous exterior foam | R-27 | about R-21.5 | The foam covers the studs too. R-5 outside adds R-4.5 effective, nearly all of it. |
| 2×4, R-14 batt, plus 1.5 in. R-7.5 exterior foam | R-21.5 | about R-18 | An old 2×4 house at re-siding. Beats a new 2×6 batt wall. |
| 2×6, full-depth closed-cell spray foam, 5.5 in. | R-31 | about R-24 | The foam is R-6 an inch; the studs are still R-1.2 an inch and still 23% of the wall. $6.90 a square foot for R-7 effective over the batt. |
| 2×6 flash and batt, 2 in. closed cell plus R-14 batt | R-26 | about R-19 | Two points over the batt wall, with the air seal and the dry sheathing. The flash-and-batt page. |
| Double-stud wall, 12 in. dense-pack cellulose | R-44 | about R-40 | Two 2×4 walls with a gap; the only wood that bridges is the plates. Expensive in framing and floor area. |
| ICF, 6 in. core, 2.5 in. EPS each face | R-22 to R-24 | R-22 to R-24 | Continuous foam both sides, no studs, no bridging; nominal and effective are the same wall. 8 in. cores or thicker foam run R-30 and up. |
These are the eight rows the wall assembly comparison calculator prices against each other for your wall area. The basics of what R means are on the R-value page; the ICF figure and where it comes from are on the ICF R-value page.
The arithmetic, once, so you can check anyone
Heat has two paths through a framed wall: through the batt, and through the wood beside it. The code method adds them in parallel, which means you add the conductances, not the resistances. For a 2×6 wall at 23% framing: the batt path is 77% of the area at R-22, which is 0.77 divided by 22, or 0.035. The wood path is 23% at R-6.6, which is 0.23 divided by 6.6, or 0.035. Notice the two are the same size; a quarter of the wall is moving as much heat as the other three quarters. Add them: 0.070. One divided by 0.070 is R-14.3 for the insulated layer.
Then add the layers that are the same across the whole wall, in series: drywall, sheathing, siding and the air films on both faces, about R-2.5 together. R-14.3 plus R-2.5 is about R-17. That is the whole method. A 2×4 wall with R-14 and 3.5 in. studs at R-4.2 comes out at about R-11 the same way. The double-stud wall at 12 in. only bridges at the plates, so it loses almost nothing. Continuous insulation outside the studs is added in series after the parallel step, which is why nearly every point of it counts.
Rule of thumb: framing takes about a fifth off any batt wall. Foam in the cavity does not change that, because the studs are still there. Only insulation that covers the studs, outside, inside or instead of them, gets you the number on the bag.
Why it shows up on the drywall and the heating bill
The studs are the cold lines in the wall. On a January night at minus 20 C, the drywall over a stud in a 2×6 batt wall runs 2 to 3 C colder than the drywall over the batt. That is where dust sticks, where the paint ghosts in stripes behind the dresser on the north wall, and where a thermal camera shows the framing like an X-ray. It is also where condensation starts in a humid house. An infrared image of an ICF wall is a blank; there is nothing in it to show.
On the bill, the difference between R-17 and R-22 effective is about 23% less heat through the walls, and the walls are roughly a third of an older house’s heat loss, so call it 7 or 8% of the heating cost. That is not nothing, but the attic and the air leaks are bigger and cheaper, which is why the air sealing page comes first in this section. Where effective R really matters is sizing: the heat-loss calculation that picks the furnace or heat pump works from the effective wall, and a wall entered as R-22 that is really R-17 under-sizes the equipment by a step. OntarioHeatLoss.ca does that design from the real assemblies.
How the bridging gets fixed
- Continuous insulation outside the sheathing. The code allows R-19 plus R-5 continuous in place of R-22 in the SB-12 packages for exactly this reason. On an existing house the only cheap day to do it is re-siding day; the exterior insulation page has the $4.50 and $7.00 rates and the vapour ratio rule.
- Cover the studs inside. A layer of rigid foam under the drywall, or strapping the wall out with foam between, works and eats floor space and every electrical box depth. Rare in Ontario for good reason.
- Fewer studs. 24 in. on centre with single top plates and two-stud corners, the advanced framing package, brings framing from 23% to about 16% and a 2×6 batt wall to about R-18.5. Free if the framer will do it; most will not.
- Two walls. The double-stud wall at R-40 effective. The thermal answer with a framing bill and a lost 6 in. around the whole house.
- No studs. ICF: foam both sides of concrete, continuous, R-22 to R-24 effective for a 6 in. core, more with thicker forms. Nothing to add inside or out, and the ICF extra-insulation page explains why people who add it are wasting money.
Reading a quote or a plan
A quote that says “R-22 walls” is quoting the batt. A plan that says “R-22 eff.” is quoting the wall, and the two are different things by five points. The SB-12 compliance form on a new house lists every assembly by its effective value and names the package it meets; ask the builder for that sheet, because it is the only place the real numbers are written down. The SB-12 page explains how the packages trade one element against another, so an R-17 wall with triple-glazed windows and an R-60 attic can pass where an R-17 wall with R-50 and double glazing does not.
Where homeowners go wrong is paying full-depth spray foam money, about $6.90 a square foot, for a wall that comes out at R-24 instead of R-17, when R-5 of exterior foam at $4.50 gets R-21.5 and a flash-and-batt at $3.60 gets R-19 with the same air seal. Foam in the cavity is the right answer for the rim joist and the basement; in an above-grade stud wall it is paying to insulate the batt and leaving the studs alone.
Questions people ask
What is the effective R-value of a 2×6 wall with R-22 insulation?
About R-17, with studs at 16 in. on centre and 23% framing. The batt is R-22; the wood beside it is R-6.6; added in parallel with the sheathing, drywall and siding, the wall comes out at R-17. That is the number the SB-12 form carries.
What is thermal bridging?
Heat taking the easier path through a material that conducts better than the insulation around it: a wood stud through a batt wall, a steel stud through anything, a concrete slab edge through a foundation. The fix is a continuous layer of insulation that the bridge cannot get around.
Does spray foam fix thermal bridging?
Not in a stud wall. Full-depth closed cell in a 2×6 wall is R-31 nominal and about R-24 effective, because the studs are still 23% of the wall. It does seal the air and dry the sheathing, which batts do not. Continuous foam over the studs fixes bridging; foam between them does not.
What is the effective R-value of an ICF wall?
R-22 to R-24 for a 6 in. core with 2.5 in. of EPS each side, the same effective as nominal because there is no framing to bridge it; 8 in. cores and thicker-foam forms run R-30 and up. The concrete mass adds performance the R number does not show.
Does the Ontario Building Code use nominal or effective R?
Effective. SB-12 lists its packages in nominal terms for convenience, such as R-22 walls or R-19 plus R-5, but the compliance form and any performance path work from the effective value of each assembly, and the heat-loss design does the same.
Price the eight walls on your house
The wall assembly comparison calculator takes your wall area and prices a 2×6 batt wall, batt plus exterior foam, flash and batt, and ICF, with the effective R beside each cost. That is the only fair way to compare a $2.00 wall with a $6.90 wall. If you already have quotes, the $99 review will say which R each one is actually selling.
Sources and further reading
- Ontario’s Building Code, Supplementary Standard SB-12: the energy-efficiency packages that set the R-values for attics, walls, basements and slabs in a new or renovated Ontario house.
- Keeping the Heat In: Natural Resources Canada: the plain-language Canadian guide to air sealing, insulation materials, basements, walls and ventilation.
- Building Science Corporation: the cold-climate basement, wall and roof assembly guidance that explains where the condensation goes and why foam goes against the concrete.
