Two questions about closed cell foam inuslation

I've heard about all the advantages, but:
1. Could it be so heavy that it jeopardizes the stability of the roof in a roofline application? 2. The gas that it traps inside - I'm sure it slowly leaks over time - what type of gas is it and is it dangerous?
Thanks in advance,
Aaron
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Could it? If by that you mean "is it possible to add such a huge weight of foam that it would jeopardize the structure." Yes. But that is an extremely remote possibility. The stuff ain't free.

Are liquids poisonous? There are a number of different foams. DAGS: closed cell foam off gassing
R
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This is for a heated room underneath, right. Its very light it might equal 1/2" of snow, I am guessing, my roof might take 2ft+ load no problem. Some foams off gas little, some more, but the roof will breath it out. In sun in a hot area the roof temp will go up, roof life depending on type, may go down.
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The heaviest foams are about 2 lb per sq ft while roofs are supposed to be designed for loads from 25 to over 50 lbs per sq ft. So the foam won't overload your roof.

There are many different chemicals used for the foams but a good guess would be that the foams could off-gas. These foams are used in many houses so if you use one of the better known brands you would be safe. If this is intended to be a living space you could get additional gas proofing by using a vapor barrier over the foam. If it ia an open attis area then the normal ventilation should prevent gas build up.

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A square foot is an area, weights are measured by volume. You are also making assumptions about the theoretical roof, such as that it was properly designed to begin with, that there have been no modifications (reroofing, truss chords cut), etc.

Polyethylene is the standard vapor barrier used in residential construction. It is intended to drastically reduce water vapor transmission, not gas transmission. I do not know the gas permeability performance of polyethylene off hand, but it appears that you are making another assumption.
R
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You are aware that water vapor is a gas I assume? :)
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Typical closed cell polyurethane foam insulation has a density around 2 lbs per cubic foot. [For exposed roof applications, where the foam is the finished roof, the density can be higher.] So if you were to spray it 6 inches deep, which is a lot of foam, that would be 1 lb per square foot. By contrast, a typical design dead load would be 10 lbs per square foot, and a design roof live load would be 20 lbs per square foot, higher in areas with heavy snow.

The gas is called a blowing agent, you could try searching on that to see what information you get. It does slowly leak out over time, which is part of the reason there is some small degradation in R value over time.
Cheers, Wayne
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