Blowing insulation into closed walls: density, access holes and fixing moisture first

Older houses with empty or half-filled walls are tempting candidates for blown-in insulation: no demolition, a few holes, a day's work. NRCan's Keeping the Heat In explains what that can and cannot achieve, and the details that decide whether the walls end up filled or full of voids.

What to expect

NRCan says professional contractors can dense-pack cellulose, glass fibre or approved foam into partially insulated wood-frame cavities, compressing existing batts, filling voids and improving air sealing. Stud spaces are only about 89 to 102 mm (3.5 to 4 in.) deep, so gains may be modest, especially where batts are already present. Before deciding, turn off the power and look behind an electrical outlet cover to see what is already in the wall.

Fix moisture first

NRCan is direct: address any moisture or structural problems before insulating. Warning signs include staining, mould, rot, cracks, and doors or windows that stick because they are out of square. Filling a wall that already gets wet removes the air space that let it dry, so the leak, the missing flashing or the failed gutter has to be dealt with before the insulation goes in.

Holes and coverage

In most cases one to five holes are drilled per storey in each cavity, including above and below windows and doors, so the spaces under sills and over headers are not missed. Interior holes are 15 to 50 mm (5/8 to 2 in.) and must be fully plugged and sealed. In balloon-framed houses, where the cavity runs from basement to attic, filling from above or below is often easiest, but windows, doors, fire stops and cross braces have to be allowed for or the fill stops short.

Density goes in the contract

NRCan says to specify the manufacturer's recommended density in the contract. Too little leaves voids or settling gaps at the top of the wall; too much can bulge walls or push insulation where it is not wanted. Ask the contractor how they will check that each cavity is full and what they do if a cavity takes much less material than expected, which usually means a blockage.

Vapour control and other options

Old plaster with several coats of paint may need to be removed or perforated, and NRCan reminds readers that the vapour barrier belongs on the warm third of the finished wall. If walls are being opened anyway, NRCan describes rebuilding them with new cavity insulation, horizontal strapping to cut thermal bridging and a sealed vapour barrier, or building a new insulated wall inside the old one with electrical boxes moved into sealed poly pans. On re-siding, insulating from outside is often better; see ArtBuild's article on that.

Prevention Checklist

• Look behind an outlet, with the power off, to see what is in the wall.
• Fix leaks, rot and structural problems before insulating.
• Plan holes above and below windows and doors, and around fire stops.
• Write the manufacturer's density into the contract.
• Ask how full fill will be checked and how holes will be sealed.
• Keep vapour control on the warm third of the wall.

References

1. Natural Resources Canada. Keeping the Heat In - Section 7: Insulating walls, Web page, 7.1 blown-in insulation; 7.2 renovating the interior. Accessed 2026-11-04.

Next on ArtBuild