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Roof ventilation

Posted by JT on March 3rd, 1998 09:05 PM
In reply to Attic ventilation (or lack thereof) by Karen L. on March 1st, 1998 03:29 PM [Go to top of thread]

Karen, I don't think that lack of a vapor barrier is
the problem. The reason is that a vapor barrier
(properly called a vapor diffusion retarder) stops
diffusion (diffusion is when water molecules pass
though "solid" material) an diffusion is not the
main mechancism for moisture transport into an
attic. The contribution of diffusion is dwarfed by
air exfiltration i.e. warm, moist air driven by
stack pressure (the normal tendancy for warm air
to rise), forced air heating system duct leaks,
and now immensely strengthened by your attic
fan, finds it's way from your living area up into
the attic, or in your case roof cavity, where the
moisture condenses on the bottom of the roof deck.
Typicly there is a largish moisture source such as
a wet crawlspace or basement or a furnace humidifier
that is supplying the moisture. Even good ventilation
where outside air enters at the eaves and exits at
the ridge, can only carry off small amounts of
moisture; cold air just can't hold much water. If your
roof was rotted after 20 yr., you're probably pumping
a good deal of moisture up there. Unfortunately,
your attic fan, as I mentioned, will only draw
warm air and moisture from the house at a faster
rate; not what you want. The best solution to this
condition is to seal the paths of air exfiltration.
With a roof such as your's, the best solution is
probably to dense pack the rafters with celulose.
This will not only insulate but form an air barrier
which will stop the moisture entering. You will
also need to deal with the source of the moisture,
since once it can't escape through the roof, it
will cause the interior humidity to rise.

This concept may sound odd at first, but it works
where venting fails miserably. You can (and should)
do some further reading on the subject. Start at and then read the Air-
Infiltration thread on TOH BB at

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