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Does an ICF Wall Need Extra Insulation? No, and Here Is Why: R-22 and Up With No Bridging, No Poly on the Wall, the Ceiling Poly That Remains, and Where People Waste Money Adding Foam to ICF
Insulation & Air Sealing
ICF Is Full Insulation: Nothing Goes Inside It, Nothing Goes Outside It, and the Poly Goes on the Ceiling
I get this question from every third client and most of their drywallers. An ICF wall is 2.5 in. of EPS foam, 6 in. of reinforced concrete, and 2.5 in. of EPS foam again, from the footing to the roof. It starts at about R-22 effective for a standard 6 in. core, R-24 is the common manufacturer figure, and 8 in. cores or thicker-foam forms run R-30 and up. It has no studs to bridge it, no cavity for air to move in, and a vapour retarder built into both faces. Adding a batt wall inside it, a layer of foam outside it, or a sheet of poly over it does not make it a better wall. It makes it a more expensive one. Here is where that money goes instead.
What the wall already is
Take the effective R-value table and look at the ICF row. The nominal R and the effective R are the same number, because the foam is continuous and there is nothing in the wall that conducts better than it. A 2×6 batt wall loses five points to its studs and comes out at R-17; a 6 in. ICF wall at R-22 to R-24 loses nothing. Above the basement slab, below grade, behind the siding, behind the drywall: it is the same wall all the way up, which is why I say ICF is full insulation. The ICF R-value page has the manufacturer figures by core size and foam thickness.
The concrete does something the R number does not show. Six inches of concrete at the middle of the wall is thermal mass; it takes most of a day to change temperature, so the wall rides through a cold night on the heat it stored in the afternoon. An ICF house in January holds its temperature for hours after the furnace stops. A batt wall does not. The ICF energy savings calculator puts a number on it for your house; the mechanism is on the ICF basics page.
The five ways people add insulation to ICF, and what each one buys
| What gets added | What it costs | What it does |
|---|---|---|
| 2×4 stud wall and R-14 batts inside the ICF basement | $4.50 + $1.10 = $5.60 per sq ft | Adds about R-11 to an R-22 wall and takes 4 in. off every room. The wall was already above the OBC R-20 for a full-height basement. Drywall strapped to the webs does the same job for the drywall. |
| Closed-cell spray foam over the inside foam | $2.50 per sq ft at 2 in. | Foam on foam. Adds R-12 and a thermal barrier problem, since the new foam is exposed and the drywall still has to cover it. |
| 1.5 in. of rigid foam outside the ICF at siding | $4.50 per sq ft plus $150 per opening | Adds about R-6 on an R-24 wall and moves every window and the soffit out. On a batt wall this is the best upgrade there is; on ICF it is the worst. |
| 6 mil poly over the inside foam | $0.50 per sq ft | Nothing useful. The EPS and the concrete are already the vapour retarder. Poly over it traps whatever gets between them and gives the drywaller a slippery wall. |
| Batts in a strapping cavity on the inside face | $1.10 to $1.50 per sq ft | The strapping is 3/4 in.; the batt is compressed to nothing. Paid for and gone. |
On a 36 x 28 ft house with 984 sq ft of basement wall, the stud-wall-and-batt line alone is $5,510 before HST, for a basement that already met code with nothing in it. The basement section runs the poured-wall assemblies it is being compared to.
Why there is no poly on an ICF wall
The OBC asks for a vapour barrier on the warm side of insulation in a frame wall because batts are vapour-open and the sheathing behind them is cold. Neither is true of ICF. The inside foam is 2.5 in. of EPS, which is already a vapour retarder; behind it is concrete, which is warm because it is inside the outer foam; behind that is more foam. There is no cold surface for vapour to find and no open cavity for it to travel through. The code’s own clause allows the vapour barrier to be a material other than poly if it meets the permeance limit, and the foam does.
So in the ICF houses I build there is exactly one sheet of poly, and it is on the ceiling, under the roof trusses, over the top of the walls, sealed to the top of the ICF with acoustical sealant. That is the lid of the house. The attic above it is blown to R-60 like any other attic, the hatch is gasketed, the pot lights are IC-rated and covered, and the ceiling poly with the sealing is about $1,200 on a 1,100 sq ft ceiling. Then the blower door runs, and the house tests well under 2 ACH50 because concrete does not leak and the only places that can are the lid and the openings. The ICF air sealing article walks through that day.
What an ICF house does still need
The walls are done the day the concrete is poured, above and below grade, so the insulation budget on an ICF house is short. On a 2,000 sq ft two-storey: attic, 1,100 sq ft to R-60 at $3.75, $4,125. Under-slab Type II EPS, 1,000 sq ft at $2.00, $2,000, and R-10 rather than R-8 if there is radiant in the slab, because SB-12 wants it under a heated slab. Ceiling poly, acoustical sealant, hatch and pot light covers, $1,200. Total $7,325 before HST. The same house in conventional frame carries $23,475 of insulation work, because the walls, the basement and the rim joists are all still to do; the new-home insulation cost page has both lists line by line.
The one place I will put foam on an ICF house is the rim joist where a wood floor system sits on a ledger, and the band where a garage or an addition in wood frame meets the ICF. Those are frame details and get frame treatment, 2 in. of closed cell. The ICF wall itself gets strapping for the drywall, screwed to the webs, and nothing in the strapping.
Where the money goes instead
- The attic to R-60, with the air sealing done first. The lid is the only insulation job on the house that is still yours to get wrong. The attic R-value rule is the target.
- Windows one grade better. With the walls at R-24 and tight, the windows are the weakest thing in the wall by a factor of ten. A U-1.4 window over a U-1.6 one is money that shows up on the bill.
- Under-slab foam before the pour, R-10 with radiant. The one day it can be done.
- The HRV, sized from the heat-loss design. A house under 2 ACH50 breathes through its HRV or it does not breathe. OntarioHeatLoss.ca does the stamped design and the ventilation sizing from the ICF assemblies.
- A smaller furnace or heat pump. The heat loss of an ICF house is a step or two below the same house in frame, and an installer who sizes from a rule of thumb will oversize it. Make him size from the calculation.
The houses themselves are on ICFhome.ca, and the forms, cores and foam thicknesses are on the building with ICF page in the ICF section of this site.
Questions people ask
Does an ICF basement need insulation?
No. The ICF wall is R-22 and up, continuous from footing to top of wall, which is above the OBC R-20 minimum for a full-height finished basement wall. Nothing is added; the drywall goes on strapping screwed to the webs. A poured or block basement needs a stud wall and batts or foam to get there.
Do you put a vapour barrier on ICF walls?
No. The EPS foam on each face and the concrete between them are the vapour retarder and the air barrier. The only poly in an ICF house is on the ceiling under the roof, sealed to the top of the walls.
Can you add spray foam to ICF?
You can, and you are paying about $2.50 a square foot to put foam on foam, then paying for a thermal barrier over it. The only foam on an ICF house belongs at a wood rim joist or where a frame addition meets the concrete.
What R-value is an ICF wall?
About R-22 effective for a standard 6 in. core with 2.5 in. of EPS each side, R-24 by most manufacturers, R-30 and up for 8 in. cores or thicker-foam forms. Nominal and effective are the same because nothing bridges the foam.
What insulation does an ICF house still need?
The attic to R-60, under-slab foam before the pour (R-10 with radiant), and the ceiling poly with air sealing, about $7,325 on a 2,000 sq ft house before HST, against about $23,475 for the same house in wood frame.
See the ICF wall beside the other three
The wall assembly comparison calculator puts an ICF wall next to a 2×6 batt wall, a batt-plus-foam wall and a flash-and-batt wall for your square footage, with the effective R and the installed cost of each. If a quote on an ICF house has an insulation line for the walls, the $99 review will find out what it is for.
Sources and further reading
- ICFhome.ca: where I build the ICF houses these insulation numbers come from.
- 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.
- ICFPro.ca: ICF walls and foundations: the effective R-values, SB-12 compliance and the details at the rim and the slab.
