[Greenbuilding] Subject: mounting rigid insulation above sloped framed un-vented roofs.

[RT]

On Thu, 05 Jul 2012 07:09:00 -0400, Alan Abrams  
<alan at abramsdesignbuild.com> wrote:

> a PH consultant and owner of 4.75 High Performance Building
> Products, recommends (and sells) vapor barriers with variable  
> permeability

> Permeability drops in winter, helping to keep
> moisture out of the cavity (which could otherwise condense on colder,
> exterior surfaces).  Permeability increases in summer, allowing the  
> cavity  to dry to the interior.

Okay. I have to confess that I harbour a prejudice against  
PassiveHouse/Haus because I think that it's very confused and when I saw  
the above claim the needle on my built-in BS meter made a few twitches to  
the right.

I had to wonder if that "variable (vapour) permeability" was more a  
function of temperature rather than some magical property of the material.

ie In when cycling strenuously in winter I often find that there will be  
accumulation of condensation on the inside of supposedly vapour-permeable   
Gore-tex outerwear whereas in warmer weather, there will be none of that  
condensation.

I just assumed that the condensation in winter was due to my body not  
providing enough heat loss to keep perspired moisture in vapour form for  
long enough to drive it out through the Gore-tex.


-- 
=== * ===
Rob Tom					AOD257
Kanata, Ontario, Canada

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