When it comes to insulating a roof, there aren’t many options… and in the case of a flat roof, the options are even fewer. A traditionally pitched roof has a large ventilated space that sits between the insulation (typically cellulose or fiberglass) and the top of the roof. With a flat roof, there obviously isn’t room for the same ventilated space and you’re forced to take a slightly different approach. While it is possible to install a ventilated option, we opted to make the entire roof cavity airtight with R40 worth of BASF Walltite.

According to their website, Walltite is “is a polyurethane insulation/air barrier system intended for institutional, commercial, industrial and residential use. It provides a gap-free, airtight, monolithic envelope of low permeability that adheres tenaciously to virtually all surfaces, smooth or irregular.”

There are actually 2 kinds of polyurethane spray foam; Open-cell (or 1/2 pound), expands to about 150 times its original volume, however it is not considered a vapor barrier and only R3.6 per inch. This means you need a lot of it to achieve the R-Value you desire, plus you need to then install and tape 6 mil plastic as the vapor barrier. Closed-cell (or 2 pound) expands to about 40 times its original volume, is considered a vapor barrier and is about R5.5 per inch.

On the day of installation (which has to be above -10°C), a crew of sprayers arrived at the house, covered everything that wasn’t being sprayed and promptly filled every void with the bright blue foam. Apparently this stuff doesn’t come off once it’s applied and the last thing we wanted was blue stains on your newly-installed windows.

The garage ceiling was sprayed to R30, including the main HVAC stack and ductwork that runs along the inside wall. The floors of the exterior overhangs were both sprayed to R40, while the joist ends were sprayed to R30. We also sprayed all of the steel posts to ensure a thermal break was achieved.

In an ideal world, we would have sprayed everything… however, our pocketbook disagreed ever so slightly. In the end, we settled on a compromise of sorts and went for a mix of Walltite, Roxul and 2” of rigid exterior foam.