It has been a week since the spray foam insulation was installed for my upstairs bedroom renovation project, so it’s time for me to make good on my pledge to provide a follow-up post on the issues of smell and satisfaction.
That New Insulation Smell
Venting the newly-foamed areas with fans was essential for the first 24-48 hours after the insulation was installed. At the end of last week, the weather here turned cold and I could no longer leave the windows open but by then the smell had dissipated. An odor is still slightly noticable when you are in the project space, but that is all– and the smell was never anything that made me or my family complain. I would not be surprised if people with greater sensitivities had a different experience, however.
When Is A Hot Roof Cold?
As I have mentioned in previous posts, my foam insulation was installed directly to the underside of the roof deck as a “hot roof.” A hot roof is so named because it has no ventilation on the underside of the roof deck to keep the shingles cool on hot days– though, as my buddy who is a roofer in Charlotte, NC can tell you, any attic on a sunny summer day gets plenty hot.
But with winters as long as they are here in Minneapolis, it is more important how insulation performs on the coldest days than on the hottest ones. Warm air escaping through attic insulation can melt snow on the shingles leading to ice dams and possibly indoor water damage. Moisture in that escaping air can also contribute to rot in rafters and roof decking. Because spray foam insulation seals the warm, moist air inside in winter, it should keep the roof deck cool in winter, even when applied as a “hot roof.”
As luck would have it, we got a bit of snow here in the last few days which allowed me to compare the difference in cold-weather performance between the new foam-insulated area of the roof and the part that is ventilated and insulated with cellulose and fiberglass.
In this photo, taken the day after a light snowfall, the area on the left side of the roof labeled “A” is the foam insulated hot roof. There appears to be some slight melting above each of the rafters probably due to some retained heat in the mass of the wood. By contrast, the section of roof labeled “B” has melted through much of the snow cover, and the rafter lines appear to be the places where the snow is thickest– the inverse of section A. Clearly in this case the spray foam hot roof is doing a better job of keeping the roofing cool.
I can’t wait to see exactly what this will mean for my heating bill this winter.