[Greenbuilding] PU foam

[Rob Tom]

On Thu, 06 Sep 2007 18:01:45 -0400, Keith Winston  
<keith at earthsunenergy.com> wrote:

>  for roofs, when doing a non-vented roof assembly, you don't want high  
> vapor permeability: you
> want a vapor barrier

>
> 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.


Moisture transport into the roof assembly via vapour diffusion is seldom  
an issue with low-rise residential construction so the vapour permeance of  
the pee-you foam should be neither here nor there from the building  
inspectors' point of view.

OTOH, bulk moisture transport via air leakage can be an issue and  
sprayed-in-place pee-you foam is very good at minimising the potential for  
air leakage to occur.

Bottom line: Pee-you foam is effective both as a "vapour barrier" (in  
OldSpeak, or "VDR" in technically-correctSpeak)) and as a component of the  
air barrier strategy.

-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at   C h a f f Y a h o o  dot  C a  >
(winnow the chaff from my edress in your reply)