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Wood Frame vs ICF Cladding Installation: Fastening to the Webs, Strapping, Stucco Straight on the Foam, Brick Ledges, Windows, and Everything That Changes When the Wall Is Concrete and Foam
Exterior Finishes
Cladding an ICF Wall: No Studs, No Sheathing, No Wrap, No Poly, and a Siding Crew That Has to Be Told All Four
An ICF wall is 6 or 8 in. of concrete with 2.5 in. of foam on each face and plastic webs running through it every 6 or 8 in. There is nothing in it to nail to and nothing in it to rot, which changes the siding job in both directions. The fasteners are screws into the webs, not nails into studs. The foam is the drainage plane, the air barrier and the vapour retarder, so the wrap and the poly stay on the truck. Stucco goes straight onto the foam. Brick sits on a ledge poured into the wall. I have been putting every cladding on this wall since 1995, and the mistakes come from crews that treat it like a wood wall, not from the wall.
What changes, line by line
| Wood-frame wall | ICF wall | |
|---|---|---|
| What you fasten to | Studs at 16 or 24 in., through sheathing | Plastic webs at 6 or 8 in. on centre depending on the block, marked on the foam face |
| Fastener | Nails, siding nails, roofing nails | Screws, coarse thread, into the web flange; nails do not hold in plastic |
| Second plane of protection | Sheathing membrane over the sheathing, lapped and taped | The foam face itself, joints taped, openings flashed to it; your block maker’s evaluation report and the building department settle whether a membrane is also wanted |
| Air barrier | Poly inside, or taped wrap outside, if anyone finishes it | The concrete and foam, continuous; the openings are the only place it can be broken |
| Vapour barrier | 6-mil poly on the warm side | None added. The foam is the retarder and the wall is not a diffusion problem. |
| Rainscreen cavity | Strapping where the cladding wants it, $2.50 | Same, screwed to the webs; there is no sheathing behind it to save, so it is for the cladding’s sake |
| Exterior insulation | Added at re-siding, $4.50 to $7.00 | Already there: 2.5 in. each side, about R22 for the wall as built |
| Stucco | Needs an insulation board or a drained EIFS build-up, $13 to $18 a sq ft | Base coat and mesh on the foam, $11 to $16, about $2 a sq ft less |
| Brick | Ledge on the foundation wall or a shelf angle | Brick ledge block poured into the wall, at the height you want it |
| Stone (adhered) | Wrap, lath, scratch coat | Mesh and base coat on the foam, or lath screwed to the webs, then stone |
| Window openings | Flange on the sheathing, pan and head flashing | Buck in the opening, flange on the buck or the foam face, same pan and head flashing |
Siding: find the webs, use screws, do not over-tighten vinyl
The webs are the whole fastening system. Every block maker moulds marks on the foam face where they are, and a chalk line down each one before the first panel saves the crew from fishing. Screws go into the web flange, which is about 1.5 in. wide, with enough thread to bite; 1 5/8 in. is the usual length for siding direct on the foam. A screw that misses the web is holding foam, which is holding nothing, and it is why I walk the wall and pull on the strapping before the siding goes up.
Vinyl on ICF is nailed loose on a wood wall and screwed loose on an ICF wall: the head 1/32 in. off the panel, centred in the slot, every 16 in., which on an 8 in. web spacing means every second web. A crew used to nailing cannot help themselves with a screw gun and runs it tight, and a tight vinyl panel on a south wall ripples the first hot afternoon. The vinyl problems page explains why. Fibre cement and engineered wood can be screwed direct to the webs if the siding maker’s sheet accepts the spacing and the screw; most do with conditions, and strapping over the webs at $2.50 is how I take the conditions out of it and get the rainscreen gap at the same time.
Stucco: the easy one
Acrylic stucco on a wood wall needs an insulation board or a drained EIFS assembly between the sheathing and the base coat, because stucco on house wrap on OSB is how the 1990s lawsuits happened. On ICF the foam is already there and it is already the right substrate: rasp the face flat, embed the mesh in the base coat, finish coat. No board, no extra membrane, no drainage mat. That is the $2 a square foot the price schedule takes off: $11, $13 and $16 by grade instead of $13, $15 and $18. The details, bands, frieze, quoins and colour changes, cost the same on either wall and they are where two stucco quotes for the same house land $8,000 apart. Stucco on ICF goes through it with the house priced.
The one thing to watch is water at the bottom. The base coat stops at a weep screed or a starter track above grade, with the foam below it protected by a parged or coated band, because foam at grade is foam the mice and the weed trimmer get at.
Brick and stone: the ledge is poured, not bolted
On a wood-frame house the brick stands on the foundation wall or on a steel shelf angle bolted to it; adding brick to an existing house means a ledge or an angle at $45 a linear foot before the first brick. On ICF the ledge is a block: a brick ledge form that steps the concrete out 4 in. at whatever height the brick starts, poured with the wall. The ledge is concrete, it cannot rot, and it is exactly where the drawing says. Through-wall flashing goes over it to the outside with weep holes every 800 mm, and the 1 in. air space behind the brick is the same as on any wall. The brick support page covers ledges and angles on both kinds of wall; the ledge block itself and the wall under it are the business of ICFPro.ca.
Brick ties on ICF are screwed to the webs, not nailed to studs, and they are the one place I see masons improvise. A tie into foam is not a tie. The mason wants the web lines chalked the same as the siding crew does, and he wants to know the spacing before he prices the job, because ties at 8 in. horizontally and 16 in. vertically on an 8 in. web wall land on every web, which is easy, and on a 6 in. web wall they do not.
Adhered stone goes on the foam the way stucco does, with a base coat and mesh as the scratch coat, or on lath screwed to the webs. Either way the top of the stone gets its metal cap or Z-flashing before the siding above it, same as a wood wall.
Windows: the only place an ICF wall can be got wrong
The wall itself does not leak. The openings can, and they are built differently: a buck, wood or vinyl, lines the opening and the window fastens to it. The sill pan, the jamb tapes and the head flashing are the same as on a wood wall, lapped to the foam face, with the head flashing tucked under the taped foam joint above or under a strip of membrane that does the same job. The ICF window page is the full detail. On a wood wall the flashing protects the sheathing; on an ICF wall it protects the buck and keeps water out of the inside finishes, and that is still worth $85 an opening.
- Tell the siding crew it is ICF before they quote. Screws, webs, no wrap. A crew that finds out on the first morning charges you for the morning.
- Chalk the webs. Every one, floor to eave, before any strapping or siding.
- Pull-test the first ten screws. If they turn in the hand, they missed.
- No house wrap unless the building department asks, and then it is a question for the block maker’s evaluation, not the siding crew.
- Protect the foam at grade with parging, a cementitious coating or a metal termination; bare foam below the siding is the one ICF detail that looks bad in five years.
The houses this comes from are on ICFhome.ca, most of them finished in acrylic stucco, stone and siding over exactly this wall.
Questions people ask
How do you attach siding to an ICF wall?
With screws into the plastic webs moulded into the foam, at 6 or 8 in. on centre depending on the block, either direct or through strapping screwed to the webs. Nails do not hold in the webs and nothing holds in the foam.
Does an ICF house need house wrap under the siding?
Usually not. The foam face with taped joints and flashed openings is the second plane of protection, and there is no sheathing behind it to protect. Some building departments ask for a membrane anyway; the block maker’s evaluation report is the document that answers it.
Is stucco cheaper on ICF?
About $2 a square foot cheaper than acrylic stucco on a wood-frame wall, $11 to $16 by grade against $13 to $18, because the foam is already the base and no insulation board or EIFS build-up is needed. The bands, frieze and trim details cost the same on either wall.
Can you put brick on an ICF house?
Yes, on a brick ledge block poured into the wall at the height the brick starts, with through-wall flashing and weep holes over it and ties screwed to the webs. It is easier than adding a ledge to an existing wood-frame house, which is $45 a linear foot of angle before the brick.
Does an ICF wall need a vapour barrier?
No. The foam on each face is the vapour retarder and the wall is not a diffusion problem the way a batt-filled stud wall is. No poly inside, no wrap outside. The windows and the top of the wall are where the air barrier can be broken, and they are the details to watch.
Price the cladding on your wall
The exterior cladding cost calculator has an ICF switch that takes the insulation board out of the stucco and the wrap out of the siding, so the number is for the wall you have. If you have stucco or siding quotes for an ICF house and one of them has house wrap and foam board in it, the $99 review will say so.
Sources and further reading
- Ontario’s Building Code, Section 9.27 Cladding: the first and second planes of protection, sheathing membrane, flashing at openings and transitions, and the clearances from grade and roofs that every siding and stucco job has to meet.
- ICFPro.ca: the ICF walls, brick ledges and attachment details that decide how cladding goes on an ICF house.
- James Hardie installation instructions: the clearances, fastening, flashing and primed-cut rules a fibre-cement warranty depends on; the installer is held to this sheet.
