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,
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.
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.
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.
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
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