Is spray foam the right insulation for your house?

What is spray foam insulation?

Spray foam is made on site by mixing two components that react and expand to fill gaps in a wall, roof, rim joist or floor. Unlike batts, it bonds to the surface and fills awkward shapes, so it also works as an air barrier. That dual role is its main selling point. In older houses where leaks are the bigger loss, a foam layer in the right place can change comfort quickly. See our guide to air sealing a house for why leaks matter so much.

What is the difference between open-cell and closed-cell foam?

Open-cell foam has tiny cells that are not fully sealed. It is lighter and softer, expands a lot and has a lower R-value per inch. It lets water vapour pass through and is mostly used inside walls and in unvented roof assemblies where the design allows. Closed-cell foam has sealed cells, is denser and stiffer, and has a higher R-value per inch. It also acts as a vapour retarder and adds strength to the surface it covers, which is why it is used in basements, crawl spaces and rim joists.

Closed-cell foam costs more per square foot, and blowing agents vary between products. Ask the installer which product is used and request the technical data sheet.

Where does spray foam make sense?

• Rim joists and crawl space walls. Closed-cell foam handles damp and seals the awkward gap where the floor meets the foundation, covered in crawl space insulation.
• Cathedral ceilings and tight roof spaces. Where there is no room for thick batts, foam gives good performance in a thin layer.
• Irregular framing in older houses. It fills gaps that batts miss.

In an open, easy-to-reach attic, blown-in insulation often does the job for much less money, a point made in attic insulation in Canada. Comparing it with fibre products is the topic of cellulose versus fibreglass.

What are the drawbacks and safety points?

Cost is the obvious one, and prices vary by region, product and thickness, so get written quotes. Quality depends heavily on the installer: wrong mixing ratios, cold or wet surfaces and thick single passes can cause poor adhesion, shrinkage or strong odours. Occupants and pets normally have to stay out during the work and for a period afterwards, following the manufacturer's re-entry instructions. Foam on the room side usually needs a code-approved thermal or ignition barrier.

It can also hide problems. Foam on a roof deck covers leaks, and removing it later is slow and messy. Cutting into foam for wiring or plumbing repairs is harder than lifting a batt. Fix moisture issues before foaming, and ask your insurer and your roofing warranty provider whether foam under the deck affects terms.

What should you ask an installer?

Ask about training, insurance and conformity with the Canadian standards for spray polyurethane foam, which cover products and installer quality programs. Ask for the product name, the thickness and R-value to be installed, how many passes are planned, the re-entry period and the warranty. Request photos at each stage. Then compare the proposal with recommended levels for your region in R-values by region. Rules on permits and fire barriers vary, so also ask your municipal building department.

Quick answers