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Moisture Behind Vapor Barrier
Hi! Not sure if this is a problem or not.
I'm here in Minnesota and I have 2 year-old house. Currently, my walkout
basement is unfinished and is partially insulated. Insulation is only at
where wood frames are and not where concrete blocks are.
Last summer, I noticed that the area between the insulation and the vapor
barrier is wet. Is this a problem? If it is, what do I need to do?
Dennis
Lately
I have read considerable debate about vapor barriers. When I started
college in the early 1980's it was gospel that you needed a vapor
barrier to prevent not only loss of indoor humidity, but also the heat
that such vapor carries along with it as it wafts out of your walls.
And the rule is:
- put the vapor barrier on the warm side of the structure, that is:
- on the inside for houses in the north, which need heating more than
cooling,
- on the outside of houses in the south where the predominant need is
for air conditioning.
But... what happens when the conditions are reversed? Like Minnesota, we
get some pretty hot and humid days here in Northern Michigan. Anybody up
north with an air conditioned house with a sheet plastic vapor barrier
is likely to have condensation just behind the vapor barrier.
And whether or not your house has air conditioning, your basement has a "de
facto air conditioner" because it is kept cool by the all the
massive amount of masonry and cool earth nearby.
So you will probably see moisture behind that vapor barrier whenever the
weather has been warm and humid for a few days. Is this a problem?
That's a tough question to answer because it depends on many variables.
Since the moisture probably is not going to occur very often, I'm
tempted to say it's not a problem. But then, consider this: if
the moisture content of an untreated piece of wood gets above 29
percent, fungal growth WILL begin. It's not a matter of
cleanliness... the fungus spores are everywhere. If the wood dries out
quickly then the fungal growth stops soon and rot never has a chance to
occur. But in a tightly sealed wall, the condensation may not dry out
very fast, giving the fungi a chance to grow and maybe cause decay. It
could also invite termites. My experience is it takes several weeks,
possibly months, of persistent wetness to make wood decay.
One reason many old houses have lasted so long is that with no
insulation the inevitable rain intrusions get dried out by all the
draftiness. Fill the wall cavities with insulation and add a vapor
barrier and now those little drops of water can't evaporate into the
house, so they've lost one of two escape routes. Not the end of the
world, just a reason for caution when insulating.
And it's possible that the moisture you've seen is caused by rain
intrusion that evaporated and condensed on the vapor barrier. If this is
true you should see the condition more often that just the dog days of
summer, however.
So... what should you do? Perhaps nothing is the best choice. Monitor
the situation, check the walls once in a while to see if the moisture
only occurs after a few days of hot and humid weather (I bet that's the
case). When you see water droplets, check the walls every day or two to
see how long they linger.
If water stays there longer than a couple of weeks, I would consider
some action. One thing you could do is violate the vapor barrier in a
controlled manner. What I'm thinking is: take a dinner fork and poke
some holes near the bottom of the insulated wall section, and some more
at the top. You'll need to poke a couple of hundred tiny holes to be
effective.
Another option is to change the vapor barrier. Polyethylene vapor
barriers (such as Visqueen) are virtually impervious to water vapor. But
there are materials that are vapor retarders. The paper facing on
kraft-faced insulation is one such example. This paper has a thin layer
of asphalt on the back side, which deters but does not stop vapor
transmission. With such a material there should be less condensation, if
any. I think you can buy rolls of this kraft paper vapor retarder, but I
can't say I've seen it. One other product that is a vapor retarder is
roofing felt (a.k.a. tar paper). But this gives off an odor so you might
not like it.
There may be other products sold (look beyond Home Depot and Lowes, look
at the contractor-oriented lumber yards) that allow some vapor to pass
through. I'm just not aware of all the products out there. I see things
advertised in trade magazines and I know that those lumberyards carry a
lot more than Home Depot would ever stock.
Maybe you won't see any more water until next summer. This could be a
trivial issue or something bigger. I'd hate to take chances though, so
keep an eye on the situation.
Bruce W. Maki, Editor.
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