[Greenbuilding] Difference between an Air Barrier Strategy vs. AB materials (was Re: Water Barrier)

[RT]

On Tue, 05 Mar 2013 12:07:55 -0500, Eli Talking  
<elitalking at rockbridge.net> wrote:

> The air barriers I am discussing are the non
> vapor barriers such as osb, plywood, building wrap.

Actually, the vapour permeance of even thin 3/8"-thick OSB or plywood are  
low enough for them to be effectively considered as "vapour barrier"  
materials.

Further, if the nominal R-value of the thermal envelope is relative high  
(as I suspect most "Green" builders would aim for), you'll find that the  
dew point for condensation will almost certainly be somewhere within the  
thickness.

Going back to the Gore-tex analogy that was mentioned earlier ...

I often wear an a Gore-tex garment over top of numerous breathable  
underlayers when it's really cold outside and if I'm doing something that  
is strenuous, it's not unusual to find so much condensation on the inside  
of the Gore-tex garment that some of the underlayers next to the Goretex  
garment will be soggy and the Gore-tex garment needs to be turned inside  
out so that the condensation on the inside can dry off.

The same thing can happen with "breathable" air barrier materials used in  
building assemblies. If the temp is cold enough, condensation can occur on  
the "warm" side and the insulation layer behind it may not allow  
sufficient heat loss to turn it back into vapour and be driven out ... so  
one gets the situation of the multiple layered leaky bags filled with  
water. ie water gets trapped between the layers and it will have a very  
difficult time getting out.
-- 
=== * ===
Rob Tom					AOD257
Kanata, Ontario, Canada

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