We have an old house in the Philadelphia area which we bought last year. This has a slate roof and we were told by our home inspector that we'd need to consider replacing it in 10 years. It is the original roof and has been on the house ~70 years so I think the estimate of 10 years is conservative. I plan to continue having the roof checked out and slates replaced as necessary.

Spray foam insulation is attractive because we are installing an AC system and it'll bring this within the thermal envelope of the house, albeit at a premium cost. Assuming the lifetime is so short (and all the insulation will be out when a new roof is installed) I'd expect that I could have roof leaks or other issues before the roof is replaced.

Does this type of insulation add major overhead to what would be simpler tasks? How do leaks manifest with spray foam insulation? Do they hide themselves for a longer period?

2 Answers 2


Spray foam is fine here. Hire it out, don't try yourself. You won't save much if any money and this stuff is better installed by pros. Make sure the roof decking is bone-dry on the day they install it. Determine what kind of R-value you want, too. The stuff is so expensive that installers will often try to convince you that 2 to 4 inches is sufficient for a variety of phony-baloney reasons; ignore that. If you want an R-50 roof (which is a good idea, and only slightly higher than code-minimum for your area in the latest version of the energy code), that'll require 7" of foam, which can very expensive. Typical price is $2+ per inch thick per square foot. for an 800 square foot attic, 7" of foam would cost more than $11,000!

Spray foam may be unnecessary. If you don't install the AC unit and its ductwork in the attic, then you can skip the spray foam and pile cellulose or fiberglass on the attic floor to get better performance for 1/3 to 1/10 the price. And it's easily DIY-able.


It really sounds like spray foam insulation is a great option if you're looking for a little less mess in your attic. There are so many different options though that it really just comes down to whether or not you're looking for someone to do it for you, or whether you would like to do it yourself. I personally like the idea of spray foam since it can be a one day and done kind of deal.

  • This really doesn't address the questions in the question (for reasons obvious looking at the edit history.)
    – Ecnerwal
    May 28, 2015 at 14:25

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