[Greenbuilding] PU foam

[Keith Winston]

Actually, Rob, its the other way around: for roofs, when doing a 
non-vented roof assembly, you don't want high vapor permeability: you 
want a vapor barrier that keeps any moisture from getting to the "dew 
point plane" located somewhere between the inside surface of the ceiling 
and the oustide surface of the roof. As long as no moisture/humidity 
reaches that plane, then there will be no condensation, and you don't 
need ventilation. That's the principle of a non-vented roof assembly.

So for that use, PU foam (or Spray Foam Polyurethane, SFP) functions as 
a vapor barrier. I also don't remember the numbers, but I would guess 
that beyond an inch or two of closed cell SFP, and maybe less, would 
have permeability ratings in the "vapor barrier" range. Of course, we 
haven't distinguished between open cell and closed cell foam, which 
behave quite differently, in terms of permeability and moisture 
absorption. Although the Icynene (open cell) folks will disagree, I 
believe closed cell is the preferred foam for non-vented roof 
assemblies, as per above.

Warmly, Keith


Vadurro, Rob, EMNRD wrote:
> Regarding vapor permeability and PU foam, I mentioned that PU foam was
> vapor permeable because our local inspectors allow PU roofs to be
> installed over un-vented roof joist space because they consider the PU
> vapor permeable enough to not require the additional joist venting. I
> don't know how this squares with the recent discussion here- perhaps
> we're talking about two different properties that have similar names?
>
> -Rob Vadurro
>
>
>
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