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Drywall on ICF Walls: Screwing to the Webs, Whether to Fur, the Vapour Barrier That Is Already There, Bucks, Electrical and the Basement
Drywall, Taping & PaintingICF
Drywall on ICF Walls: It Screws to the Plastic, There Is No Poly, and Three Things a Wood-Frame Boarder Gets Wrong the First Time
I have built with insulated concrete forms since 1995, and every few years a new boarding crew walks into one of my houses, looks at a foam wall with no studs, and asks where the strapping is. There is none. ICF blocks have plastic webs every 6 or 8 in. running top to bottom, flush with the foam face, and the drywall screws into them exactly as it would into a stud. No furring, no poly, no shimming to flatten the wall, because a poured concrete wall in good forms is flatter than any framed wall I have seen. Here is how it goes, what is different, and what the code wants over the foam.
What is different, and what is not
| Wood-frame wall | ICF wall | |
|---|---|---|
| What the screws go into | Studs at 16 or 24 in. | Plastic webs at 6 or 8 in. (depends on the block brand), marked on the foam face. Coarse drywall screws, 1-1/4 or 1-5/8 in. |
| Vapour barrier | 6-mil poly over the insulation, under the board | None. The foam and the concrete are the vapour retarder; adding poly traps moisture against the foam. The code accepts this. |
| Flatness | Studs crown, bow and twist; the taper floats it | Flat. A straightedge across 8 ft of ICF touches everywhere. Joints finish faster. |
| Thermal barrier over foam | Not applicable | Required. The Building Code wants foam plastic insulation covered by a thermal barrier inside, and 1/2 in. drywall is the normal one. Every bit of foam gets board, including the basement. |
| Electrical | Boxes nailed to studs, wire through holes | Chases cut into the foam with a hot knife or a router, box set in the foam and screwed to a web or foam-glued, wire pushed into the chase. Done before board. |
| Windows | Framed opening, jamb extensions | A buck (wood or vinyl) cast into the opening; the board returns into the opening or the jamb extensions cover it. The wall is 11 to 13 in. thick, so the returns are deep. |
| Hanging pictures and cabinets | Find a stud | Find a web, or use foam-rated anchors anywhere. Kitchen uppers screw to the webs or to blocking set in the foam at framing. |
How it goes on
- Find the webs. Every brand marks them on the foam face. Snap a line down each one before the first sheet or you will be hunting with a screw.
- Hang it like a wall. 12-ft sheets horizontal, screws into webs at 12 in. vertically, 8 in. at the sheet edges. Coarse-thread screws bite the plastic. Do not overdrive; the web is tougher than a stud but the paper is the same paper.
- No adhesive is needed, and no poly. Some crews foam-glue the board to the ICF as well; it does no harm. Poly does. If an inspector or a boarder insists on poly over ICF, show them the block manufacturer’s installation manual and the code clause on foam-and-concrete assemblies.
- Return the board into the window openings or let the extension jambs do it, decided with the window installer. On my houses the deep returns are drywalled and corner-beaded, because a 12-in. drywall return with a wood sill looks like the wall was built that thick on purpose, which it was. Windows in ICF.
- Basement too. The foam in an ICF basement is exposed foam until it is boarded, and the code wants the thermal barrier whether or not the basement is finished. Board it, tape it to Level 2 at least, and it is a finished-looking basement for the price of the board.
Three mistakes a first-time ICF boarder makes
He furs the wall. Strapping over ICF adds a day, costs $0.50 a square foot, gives you a 3/4-in. gap that bugs and sound love, and solves a problem the wall does not have. The only reason to fur is to run services on the surface, and the chase does that better.
He screws between the webs. A screw into foam holds nothing. The sheet looks fine until it drums, then until the joint cracks. Snap the lines.
He runs poly. Covered above. Moisture that gets into the wall from the house side can no longer dry inward and sits on the foam. Leave the poly in the truck.
What it costs
The same as wood frame, or a little less once the crew has done one: the walls are flat, there is no strapping, and the taper has no crowned studs to float. The board is 1/2 in. regular; 5/8 is pointless on a wall that is already 11 in. of concrete and foam. The thing people notice is the quiet: an ICF exterior wall is around STC 50 before the drywall, which is why the inside of my houses sounds like the inside of a library, and why the interior partitions are the only walls that need soundproofing attention.
Questions people ask
How do you attach drywall to ICF?
Screw it to the plastic webs built into the block, which are every 6 or 8 in. and marked on the foam. Coarse drywall screws, standard spacing. No strapping and no adhesive are required.
Do you need a vapour barrier on ICF walls?
No. The foam and the concrete are the vapour retarder and the air barrier. Poly over ICF is wrong and the block manufacturers say so in the installation manuals.
Does ICF need drywall?
Yes, inside. The Building Code requires a thermal barrier over foam plastic insulation, and 1/2 in. drywall is the standard way to provide it, including in the basement.
How do you hang cabinets on ICF walls?
Screw to the webs where they line up, and set wood blocking into the foam at the cabinet rails during the build for the rest. Foam-rated toggle anchors carry smaller loads anywhere on the wall.
Building with ICF?
The ICF vs wood framing page covers the whole wall, ICFhome.ca is where I build them, and ICFPro.ca is the ICF foundation and wall contracting side for builders who want the shell done and the drywall left to them. The complete interior finishing calculator prices the inside of the house, ICF or frame, the same way.
Sources and further reading
- ICFhome.ca: where I build the ICF houses these details come from.
- Ontario’s Building Code: the official page with the current regulation. Fire separations and garage walls are in Part 9; interior finish flame spread is 9.10.17.
- Keeping the Heat In: Natural Resources Canada. The air barrier at the drywall, poly, and why the vapour barrier matters more than the paint.
