Frost in the attic? Time to insulate and ventilate

Posted Feb. 13, 2009, at 9:01 p.m.
Last modified Feb. 13, 2011, at 10:45 a.m.

Water is bad when it gets inside your house. If it is anywhere other than the sink, toilet, tub or washer, it is going to cause problems.

I get endless questions about frost in the attic. Frost in the attic comes from the house. The warm air you are paying for to heat your home is leaking into the attic because your house is not sealed from the attic. Warm air holds a lot of moisture. When it arrives at a cold surface — your attic roof this time of year, for example — the moisture condenses and freezes.

The frost accumulates and looks really scary to most folks. This is a common occurrence that you might miss if you don’t frequent the attic during the winter.

Many times it just builds up and then melts away and evaporates as it gets warmer outside. Sometimes there is so much frost that when it melts, it drips downstairs and alerts us to the problem.

If you are lucky, it never gets that bad.

The simple trick to avoid this problem is to seal the house from the attic. It also is necessary to ventilate the attic to remove the moisture buildup. The important thing,

though, is to seal the house from the attic. The access to the attic must be insulated and weatherstripped. Anytime air leaks up there, water gets into the attic. It is that simple.

Whenever I see an attic that is so bad that water is leaking down into the house, I usually wind up in the basement to see what is happening down there. A wet basement usually shows up in the attic with a lot of frost buildup. It is good to remember that hot air rises, and if that hot air is moist from a wet basement, it goes upstairs.

This whole thing is a system.

I took advantage of last week’s warm weather and took several loads of “dry” firewood down to my basement. It looks good and seems nice and dry and burns really well. It feels dry and it looks dry. A couple of hours after I brought it downstairs, I noticed that the humidity in the basement rose 10 percent. There was no snow on anything and it was all dry. Sort of.

The good news is that I have kept the house rather dry all winter, so the humidity is not a problem. The other good news is that there is spray foam and sheet foam between the house and my attic. If any moisture is landing up there, it has to pass through about 9 inches of solid foam insulation.

All that foam makes the humidity indicator go apoplectic whenever there is an insult like a big load of firewood landing in the basement.

Old drafty houses do not respond like this because all that dry winter air just washes through the house and the house stays dry. All the house humidity, wherever it comes from, goes out through the windows and walls and up into the attic.

Insulating the attic is the low-hanging fruit when it comes to attacking a house’s energy situation for the first time.

A lot of people who are building new homes that do not have drywall up yet call this time of year. That is because they always seem to peek in behind the fiberglass insulation.

And they freak out over the frost they see on the inside of the wall sheathing. Why is it there?

The fiberglass wall insulation does what it does quite well — it lets air pass right through it. New houses have tons of water in them. The concrete in the basement has hundreds of gallons of water in it. It only slowly releases all that water — into the house.

The wood that has been rained on for the couple months while the house is being built dries into the house. It might seem dry, but it is not. All that water has to go somewhere. It heads out to every cold surface it can find and turns to frost.

The good news is that like winter, the frost dissipates since the house exterior warms up and dries out and springtime turns to summer. And we forget about it till next winter.

The fun part is that a new house usually takes two years to dry out, so if you miss getting the drywall up the first year, you can still check it out during the second year.

Questions for Tom Gocze should be mailed to The Home Page, Bangor Daily News, P.O. Box 1329, Bangor 04402-1329. A library of reference material and a home-project blog are at bangordailynews.com/thehomepage.

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