“Water, water, everywhere” goes the old saying.  And that might be a good thing for a sailor, but not so much for a homeowner.
I tell all my clients that water is their home’s number one enemy. If you keep your house dry, it’ll last for a long long time.  If it gets wet, it goes downhill fast.

Of course, I’ve never met anybody who’d be happy going outside to the well every time they wanted a drink of water or to take a shower.  So we bring water pipes into our houses.  And we don’t (at least none of my clients) live in the desert, so rain water (snow too!) will fall on and around our houses.  We can’t stop water, we just have to control it.

It’s important to keep in mind that water comes in different forms.  We have to control liquid water of course, but we also have to control water vapor.  And that’s usually the hard part.
When I see mold in an attic I know that there’s too much water in the attic.  The problem might be a roof leak – liquid water getting in.  But the problem might also be water vapor coming up from a wet basement or crawl space.  I saw quite a few cases of moldy attics last year and in all but one case the culprit was pretty clearly a wet crawlspace.

Too much water in the attic. Notice that only the roof sheathing is affected, and not the rafters. That’s because the sheathing is the coldest part of the attic, so that’s where the water vapor condenses.

When the basement or crawlspace is wet that moisture easily evaporates into the air.  And that moist air diffuses throughout the entire house.  Water vapor wants to move from areas of high concentration and warm temperature to areas of lower concentration and colder temperatures.  The coldest and driest part of most houses is the attic (at least in the winter) and so the water vapor is going to be driven into the attic by natural forces.

Many people seem surprised and even a little dubious when I tell them that a wet crawlspace is the source of their attic moisture and mold problems.  But consider that in a typical house in this area you can expect anywhere from about 0.3 up to 0.7 natural air changes per hour.  This means that the total of cold air leaking in and warm air leaking out of your house will equal somewhere between about 0.3 and 0.7 times the size of your house.  So in about 1.5 hours up to maybe 3 hours the air in your house completely changes over.  Air moves, pretty fast.  And the water vapor in the air diffuses even faster than that.

Water in the crawlspace eventually ends up in the attic. Count on it.

So in short order that water vapor finds its way up into the attic.  When it hits the cold surface of the roof sheathing the water vapor condenses, and you get just the right conditions for mold to grow.

There are three general ways to control water: source control, ventilation, and dehumidification.  Source control involves stopping water from getting into the attic in the first place, and so stopping it from getting into the crawlspace.  Or we can ventilate the attic to bring in dry outdoor air and remove the moist air.  Or we can install a dehumidifier in the attic to remove moisture and drain it away.

Ventilation is important in an attic, and most home inspectors make a deal out of a lack of attic ventilation.  But I’ve seen so many attics with bad or no ventilation that had no problems.  And really, why let water vapor get into the attic in the first place and then be forced to deal with it.  Why not keep it out in the first place?

By far the best option is source control.  It’s always best to avoid the problem completely if you can.  Source control is the easiest, most dependable, and most robust solution.  Keep water out of the attic – and that means keeping water out of the crawlspace.

The way to keep your crawlspace dry is for another post, but here are the highlights.  There should be a vapor barrier on the ground to keep water from coming up through the ground.  (The ground is very wet – that’s why trees have their roots there!)  The gutters and downspouts should be in good condition to move rain water away from the house.  And the ground around the house should be sloped away so that rain water flows away from the house.
Stay dry my friends.  Your house will appreciate it.

Water, water, every where,

And all the boards did shrink;

Water, water, every where,

Nor any drop to drink.

“The Rime of the Ancient Mariner”  by Samuel Taylor Coleridge

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