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Water, Water Everywhere

“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.

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.

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.


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.

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

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.

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

Insulation and your attic hatch

Since Autumn is fully upon us and it’s starting to get quite chilly out there, let’s talk about energy efficiency.  Over the years I’ve told thousands of clients that they should insulate their attic hatch.  I hope they’ve all taken my advice, but for anybody who still needs convincing let’s put some numbers to the problem.

First a quick insulation lesson.  Insulation is rated by its R-value, which is a measure of the insulation’s resistance to heat flow.  A higher R-value means better insulation because it provides more resistance to heat flow.  So we want a high R-value insulation at our exterior walls and ceilings.

In contrast, U-value is a measure of how well a material allows the flow of heat.  Windows and doors are rated by their U-value (sometimes called U-factor), and of course you want a low U-value which would mean very little heat flow through the door or window.

R-value and U-value are reciprocals: R=1/U  and U=1/R

The International Energy Conservation Code currently requires R-49 insulation in the attic in the Chicagoland area.  This is a fairly new requirement, however.  For many years only R-38 was required in the attic, so if that’s all you’ve got it’s still pretty good.

When you add layers of insulation you just add the R-values to determine what the new total insulating value is.  So if you have only R-20 insulation on your attic floor, and you blow in another R-29 of insulation, you now have R-49 insulation in your attic.

But when you have different areas of the attic with different levels of insulation, finding the total average R-value over the entire area is more complicated.  The formula for finding the total average R-value when different areas have different insulation levels is:

UtAt = U1A1 + U2A2 + U3A3 + . . . . You find Ut and then convert that to Rt with the formula above, R = 1/U.

Let’s do an example and see how important it is to insulate your attic hatch.

Let’s say your attic floor covers an area of 800 square feet, and you have R-38 insulation over that area.  But your attic hatch, which is 6 sqaure feet, is only covered with a piece of plywood.  That plywood only has an R-value of about R-1.  But it’s only a samll area, less than 1 percent of the total floor area of the attic, so it can’t make much of a difference, right?  Let’s see.

Here’s the formula again:

UtAt = U1A1 + U2A2 + U3A3 + . . .

In our example:

Total —  At = 800 square feet, and we’re trying to find Ut

Insulated area — A1 = 794 square feet,  R1 = 38, U1 = 1/38

Uninsulated area — A2 = 6 square feet, R2 =1, U2 =1

Ut*(800) = (1/38)*(794) + (1)*(6) = 26.8947

Ut = 26.8947/800 = 0.0336    and Rt=1/Ut, so

Rt = 29.7

Wow, a small little uninsulated area reduced the total average R-value of your attic insulation from R-38 to R-29.7, a reduction of 22%.  That’s a terrible waste.

If you put just a single layer of 1-inch rigid insulation on the hatch, with a value of R-5, the new number for total attic R-value would be R-36.5.  That’s a huge improvement.  Just two inches of rigid insulation and you’re at R-37.3.

So please insulate your attic hatch.  Save some energy, and save yourself some money.