Post-Insulation Follow-up

by Josh on October 14, 2009 · 5 comments

in Bedroom,Projects

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.

Insulation Comparison

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.

{ 5 comments… read them below or add one }

Holyoke Home October 15, 2009 at 8:07 am

Hi! Though I am certainly not an expert, my research shows that dense pack cellulose provides an R rating equal to foam and WAY better than fiberglas. With a recycled content of 83% and no petroleum, I think that’s the way we’re going to go for our attic.



reuben Collins October 15, 2009 at 10:00 am

NEAT! Hopefully you can save some $$. I’m really into this hot roof idea. It’s probably the best option for many of these 1.5 story homes where the 2nd story sloped ceilings are attached right to the underside of the roof structure – talk about hard to ventilate!

I really want to do something like this to my home, but it looks like I’d have to gut the whole upper story of my home to do it – and… well that’s not an option right now…


Shane October 15, 2009 at 3:23 pm

Holyoke Home: From what I have found, dense pack cellulose only provides an R-value of 3.5-4. That is half of what closed cell foam insulation provides.


Stacy October 19, 2009 at 9:22 pm

so what happens to the shingles on a hot roof during the hot summer??? It does get really hot up in my currently un insulated 1/2 story attic area during the summer, i can not imagine it getting hotter. That insulation looks like it would really work for my house.


Josh October 20, 2009 at 9:48 pm

@Holyoke Home- I hear you about the green value of cellulose’s high proportion of recycled content, but I think Shane is right that it doesn’t quite match the R-value per inch of foam. It’s still an excellent insulation option, just not my top choice for this project.

@Reuben- Good luck with your project! The closed-cell foam hot roof is about the best way I found to insulate a 1/2 story with 2×4 rafters.

@Stacy- Actually, the inside of the attic should be cooler during the summer because the hot roof insulation cuts down the interior heat gain and makes the entire attic “conditioned space.” Because the hot roof has no air circulation under the roof deck to cool the shingles, the shingle manufacturers void their warranties on homes with hot roofs. However, as you’ve experienced, many ventilated attics still get very hot–often much hotter than the outdoor air. In those cases, the real difference in shingle temperature compared to a hot roof is probably minimal.


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