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ICF vs Conventional Basement Insulation in Ontario: An ICF Basement Needs Nothing Added Against a Poured Wall With Foam, Studs and Batts, on Cost, R-Value, Moisture and Time
Insulation & Air Sealing
ICF Basement vs Poured Wall With Insulation: One of Them Is Finished the Day the Concrete Cures
A conventional basement wall is poured, stripped, waterproofed, backfilled, and then it waits for four more trades: the foam, the framing, the batts and the drywall, and the question about the poly. An ICF basement wall is poured inside 2.5 in. of foam on each face, and when the forms come off, which they do not because they stay, the wall is at R-22 and up effective, continuous, full height, with nothing to add. I have built them both ways since 1979 and only the second way since 1995. This page puts the two beside each other on the same 36×28 house: what the insulation costs on each, the R you actually get, what moisture does, and how long the basement takes to be a room. The ICF wall costs more to pour; that number lives on the ICF pages, and I will tell you exactly where to look for it.
The two walls, layer by layer
| Poured wall, Assembly A | Poured wall, Assembly B | ICF wall, Assembly D | |
|---|---|---|---|
| The wall | 8 in. poured concrete, damp-proofed outside | 8 in. poured concrete, damp-proofed outside | 6 in. concrete core inside 2.5 in. EPS each face, with plastic webs; membrane outside |
| Added inside | 2 in. XPS taped, 2×4 wall at 24 in., R-14 batt, 6 mil poly | 3 in. closed cell, 2×4 wall at 24 in., nothing else | Nothing. Drywall screws to the webs. |
| Insulation cost per sq ft | $9.60 | $8.75 | $0 |
| On 984 sq ft with the rim | $10,150 | $9,314 | $704, the wood rim only |
| Effective R | about R-21 | R-19 to R-21 | R-22 to R-24, R-30 and up on thicker forms |
| Thermal bridging | At every stud, mostly behind the foam | At every stud, behind the foam | None. The foam is continuous through the wall and the slab edge. |
| Air barrier | Taped foam, if the tape holds; the poly, if sealed | The foam, complete | The concrete and the foam, complete |
| Vapour barrier | The foam; the poly over it is the argument | The foam | The foam on both faces; no poly |
| Floor space lost | 5.5 in. around the room | 6.5 in. | Nothing inside the form |
| Weeks from pour to drywall-ready | Foam crew, framer, insulator, three scheduling gaps | Foam crew, framer, two gaps | Zero. Strip the bracing and hang drywall. |
The basement cost page has the per-line arithmetic for A and B; the ICF R-value page has the effective R by form thickness.
Cost: the $10,150 that disappears, and the wall that costs more
On the 36×28 house, the conventional basement spends $10,150 before HST to get a poured wall and its rim to R-20 effective full height in Assembly A, or $9,314 in Assembly B. The ICF basement spends $704 on the wood rim joist and nothing on the walls, because the walls arrived insulated. That is the honest insulation comparison and it is the whole of what this page is allowed to price.
The ICF wall itself costs more to build than a bare poured wall: the forms are the insulation, so you are buying the foam, the webs and the labour to stack them on the day the foundation is built rather than four months later. How much more depends on the form, the height and the crew, and the ICF vs block basement page is where that number lives. What I will say here is that the premium on the wall and the $10,150 of finishing it saves are the same order of money, and that the ICF basement also skips the framer, the second foam visit and the drywall furring that a poured wall needs before the drywaller can start. When people price an ICF basement against a poured one and forget the $10,150, they are comparing a finished wall to a bare one.
R-value: why nothing added is already more
The standard 6 in. core ICF form has 2.5 in. of Type II EPS on each face, 5 in. of foam, and it starts at about R-22 effective; R-24 is the common manufacturer figure for the 6 in. form and the 8 in. core forms with thicker foam go R-30 and up. The word that matters is effective. There are no studs, so there is nothing bridging the foam; the concrete between the two layers is thermal mass that evens out the day, and the foam runs continuous past the slab edge to the footing. Assembly A gets to about R-21 effective, which is a good number, by stacking three products and hoping the tape holds; the ICF wall gets to R-22 and up with one. The what are ICFs page explains the form, and the wall assembly comparison calculator runs the same house four ways with the effective R beside the installed cost.
Adding insulation to an ICF wall is where people waste money. A finisher used to Assembly A will quote a 2×4 wall and batts inside an ICF basement because that is what he does, and a foam salesman will offer to spray the inside face. Neither is needed: the wall is past R-20, there is no cold concrete to isolate, and a stud wall inside it only costs floor space and $4.50 a square foot. The only reason to frame inside an ICF wall is to run plumbing and wiring where the webs will not do, and a 2×2 strapping or a chase does that for less.
Moisture: the whole poly argument goes away
Every mould problem on the vapour barrier problems page comes from organic material touching cold concrete with a sheet of plastic over it. On an ICF wall the concrete is between two layers of foam. The inside face of the inside foam is at room temperature all year; summer humidity finds no cold surface, winter vapour cannot reach the concrete, and there is no batt to hold water in the first place. The foam on both faces is the vapour retarder, so there is no poly on the wall, and the inspector who asks for it gets shown 9.25.4 and the manufacturer’s CCMC evaluation.
The concrete still has to be kept dry from outside, the same as any foundation: a membrane over the outside foam, weeping tile, clear stone, grading. ICF does not make a wet site dry. The difference is what happens inside when the outside is right: on the ICF wall, nothing, for the life of the house.
Time, trades and the day the basement becomes a room
- Poured wall: pour, strip, damp-proof, backfill. Then wait for the house to be closed in. Then the foam crew for the rim and the walls, or the carpenter for the foam board. Then the framer for the stud wall. Then the insulator for the batts. Then the electrician, because the wiring goes in the stud wall. Then the drywaller. Three to five scheduling gaps, each a week or two in a busy season.
- ICF wall: stack, brace, pour, strip the bracing, membrane and backfill. When the house is closed in, the electrician cuts chases in the foam with a hot knife, and the drywaller screws to the webs. One trade fewer, two fewer visits, no foam truck in the basement at all except for the rim.
- The rim is the same on both. A wood floor on an ICF wall still has a wood rim, and it still gets 2 in. of closed cell or cut foam, $5.50 a linear foot. The rim joist page. The ICF houses I build get the rim foamed when the attic crew is there, so there is no separate minimum.
- The slab is the same on both. 2 in. of Type II EPS and poly under it, R-10 if heated, and on the ICF wall the slab edge is already isolated by the form.
- The drywall is thinner on ICF. No furring; 1/2 in. drywall straight to the webs at 6 or 8 in. on centre. It is also the thermal barrier over the foam, which the code wants on both walls.
The houses this describes are on ICFhome.ca, and the walls and foundations on their own are on ICFPro.ca, which is where the wall price belongs rather than here.
Questions people ask
Does an ICF basement need extra insulation?
No. The form is 2.5 in. of EPS on each face, R-22 and up effective, continuous and full height, with no studs to bridge it. No stud wall, no batts, no poly, no spray foam on the inside face. Drywall goes straight to the webs.
Is an ICF basement cheaper than a poured wall with insulation?
The ICF wall costs more to pour; the poured wall costs $10,150 to insulate afterward on a 36×28 house. The two numbers are the same order of money and the honest comparison includes both. The wall premium is on the ICF basement pages; the $10,150 is on this one.
What is the effective R-value of an ICF basement wall?
About R-22 to R-24 for the standard 6 in. core form with 2.5 in. of foam each side, R-30 and up for 8 in. cores or thicker foam panels. Effective, because there is nothing bridging it; a 2×4 wall with foam behind it lands about R-21 effective.
Do you put a vapour barrier on an ICF basement wall?
No. The foam on both faces is the vapour retarder and meets 9.25.4 without poly. In the ICF houses I build the only poly is on the ceiling under the roof.
Can an ICF basement get mould?
Not from the wall assembly. There is no batt and no cold concrete face inside the room. It can get wet from outside like any foundation if the membrane, weeping tile or grading is wrong, so the waterproofing is not optional on either wall.
Run the same house both ways
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 side by side with the effective R beside each, so the ICF column shows what the other three are paying for. If you have a builder quote for a poured basement, the $99 review will show whether the $10,150 of finishing is in it or waiting for you.
Sources and further reading
- ICFhome.ca: where I build the ICF houses these insulation numbers come from.
- ICFPro.ca: ICF walls and foundations: the effective R-values, SB-12 compliance and the details at the rim and the slab.
- Canadian Construction Materials Centre: National Research Council: the product evaluations behind spray foam and other insulation products, and what an evaluation number means.
