SB: Re: roof insulation and vapour

["Robert W. Tom" <archilogic@yahoo.ca>]

3/5/02 12:25:43 PM, Lakatos Géza <deru@nic.holop.hu> wrote:

>   Could be a solution for these problems to have 15 inch rockwool under the
>   membrane and 2 inch foam above the membrane? Would condensation occur at
>   the membrane if the foam wouldn,t let it be cold surface? (let,s assume
>   the vapour controll is not perfect).


Hello  Géza;

I don't know  how cold the winters in Hungary get so I really don't know
what I'm talking about WRT your specific scenario.

However, I can tell you that here, in this part of Canada, one tries to place
at least two thirds of the total R-value of an assembly on the cold side of
anything that might act as a vapour diffusion retarding  (VDR) plane.

The purpose of this placement is to minimise the amount of water vapour
that will travel past the dew point where it can cause trouble.

In the situation which you describe, the waterproofing membrane combined
with the relatively low-vapour permeance foam on top would constitute  a 
virtually  vapour-impermeable plane, and the dew point will be somewhere
within the rockwool with no means of escape. The effective insulation value 
of the rockwool will of course diminish with its moisture content.

If your interior air barrier is reasonably tight, the amount of water 
vapour getting into the rockwool can be reduced to minimal amounts
and in rockwool (unlike fibreglass) it will eventually drain and collect at 
some point determined by gravity.

If this condensation is anticipated in the design of the ceiling finish and 
perhaps if some means were incorporated to allow that collected moisture
to be disposed of,  the system could be made to work.





--- * ---
Rob Tom
Kanata, Ontario, Canada
<ArchiLogic@yahoochaff.ca>
(winnow the "chaff" spamguard from my edress in your reply)

Please visit http://www.theHungerSite.com daily



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