[GBlist] Roof vapor barriers.

["King & Liang" <lenking@blueridge.net>]

the following was written:

>I need a vapor barrier above a t&g decking ceiling under a
> thick layer of EPS roof insulation and metal roof.  I would like to use
> taped 30# felt rather than taped 6 mil poly because it will be much safer
> to walk on while the insulation and hold downs are being attached.  Will
> this provide an effective vapor barrier?

<<<It sounds like you will have a waterproof and vapor proof barrier above
the
insulation (e.g. the roofing underpayment as well as the roofing) and now
want to also install one underneath the roof insulation; sort of like
sandwiching the EPS insulation between a top and bottom layer of vapor
barrier> If so, I don't know if that is such a good idea.>>>

Absolutely. The roof must 'Breath'. Nothing should stop moisture vapor from
passing thru the ceiling and roof to the outdoors. A non permeable barrier
on the roof surface side will allow moisture to collect in the roof material
and insulation (if a catherdral) rendering it inefficient and eventually
causing damage.  A barrier on the ceiling side will stop the moisture in the
finish material of the interior ceiling and likewise cause eventual damage.
All of this is due to the fact that the interior warmer air rises to the
ceiling /roof carrying the moisture with it. Stopping it, although good for
thermal loss, is bad for the moisture problem. It must pass through. Attic
spaces have some other aspects but the principles are generally the same. So
to the original question- building paper is the way to go.


Len King, NCARB
Lake Lure, NC.


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