House Wrap vs Air Barrier vs Vapour Barrier: Three Different Jobs, Which Layer Does Which, and Where the Poly Goes in an Ontario Wall

The three jobs side by side

Rain: the sheathing membrane

House wrap is a rain coat for the sheathing, and like a rain coat it only works if it is on the right way round. Every sheet laps over the one below it, so water running down the wall runs onto the next sheet and never behind it. The window sill pan goes under the wrap below the window; the head flashing goes under the wrap above it; the tape goes on the top edge of the flashing and never on the bottom. Get the order wrong at one window and you have built a funnel into the wall, which is most of what the flashing page is about.

Wrap passes vapour on purpose. A wall with wet sheathing has to dry somewhere, and in Ontario it dries outward in the heating season because the poly inside will not let it dry inward. Put a non-permeable layer outside, which some peel-and-stick membranes are and which XPS foam nearly is, and you have changed the rules; the ratio rule on the insulation page is how the code manages that. Asphalt building paper, the black stuff on a 1970s house, is a perfectly good sheathing membrane as long as it is lapped and intact, and on a re-side it often is not.

The wrap is also not a cavity. Siding nailed tight to it bridges water across by capillary action, which is the argument for a rainscreen gap in front of it.

Air: the barrier most houses do not actually have

Air leakage moves far more water into a wall than diffusion does. A square inch of hole at a top plate, with warm house air pushing through it all winter, can deposit litres of water in the insulation above it; the same wall with no hole and no vapour barrier would pass a cupful by diffusion. Which is why the OBC in 9.25.3 requires a continuous air barrier and is less fussy about what it is made of than about whether it is continuous.

In the Ontario houses of the last forty years, the air barrier was supposed to be the poly, sealed with acoustical sealant at every lap, plate, box and penetration. Go into any attic and look at the top of the wall and you will see how that went. On a re-side you can move the air barrier outside: tape every seam of the wrap, seal it to the window flanges and the foundation, foam the rim joist, and you have an air barrier you can see and reach. The attic ceiling is still the attic ceiling, and ice dams come from the holes up there, not from the walls; attic ventilation covers that end of the house.

Vapour: where the poly goes, and where it does not

Vapour moves from warm and humid toward cold and dry, which in an Ontario January is from the living room to the outdoors, so the vapour barrier goes on the living room side of the insulation. That is OBC 9.25.4 and it is why 6-mil poly is stapled behind every drywall sheet in a wood-frame house. The rule of thumb older than the code: one vapour barrier per wall, on the warm side, and everything outside it more permeable than it is, so the wall can dry outward.

Where poly does not go: on the outside of a wall; behind an interior basement wall where the warm side in summer is the concrete, which is why basement poly grows the mould it was meant to prevent; doubled, as in poly inside and a foil-faced foam or a non-permeable peel-and-stick outside, which is a wall with no exit. Smart membranes, which tighten in winter and open in summer, are the answer where the wall has to dry inward some of the year, and they cost more than poly for good reason.

On an ICF wall there is no poly and no wrap. The foam is the vapour retarder, the concrete and foam together are the air barrier, and the outer foam face with taped joints and flashed openings is the second plane. The ICF cladding page covers what that means for the siding, and the ICF window page covers the openings, which is where even an ICF wall can be got wrong.

What to ask for on a re-side

1. New wrap, as its own line. $0.85 a square foot, 2,400 sq ft is $2,040. A quote that re-sides over torn 1978 building paper without a line for wrap is re-siding over torn 1978 building paper.
2. Lapped shingle-fashion and taped. Ask which tape. The wrap makers’ own tape or an acrylic flashing tape; duct tape has been found on more walls than you would think.
3. Flashed to every opening before the siding. Sill pan, jamb tapes, head flashing with end dams, in that order, $85 per opening on a re-side. Window leaks are a wrap problem more often than a window problem.
4. Sealed to the foundation at the bottom and the soffit at the top if the wrap is the air barrier. Otherwise it is a rain coat with the zipper open.
5. Leave the poly alone from outside. Nobody fixes a vapour barrier by removing siding. If the wall has no poly at all, a 1950s house for example, the answer is usually to keep the inside finishes vapour-tight with paint and go on with life; adding poly to a wall that has dried inward for seventy years can do more harm than good.

Questions people ask

Sources and further reading

• Ontario’s Building Code: the official page with the current regulation; cladding is Part 9 Section 9.27, masonry veneer 9.20, attic ventilation 9.19.
• Keeping the Heat In: Natural Resources Canada: walls, exterior insulation at re-siding, air and vapour barriers, and what changes at the windows and soffits when the wall gets thicker.
• Canada Mortgage and Housing Corporation: the Canadian wall-assembly and moisture research behind rainscreens and the second plane of protection.