Just kidding about the sexy part. Here’s the question of the day: Should you open your basement windows to get out the musty smell in the cellar when it is hot and humid outside? Everybody does that, right? That’s why we have operable basement windows. Isn’t it?

Nope. Let’s assume that it’s a lovely summer day, 80° and perhaps a bit humid, but still a golden opportunity to open the basement to the outside and dry things out before you go outside to work on your tan. Deciding to air out the basement is a bad choice masquerading as a good one. We can show that it’s a bad idea by using a psychrometric chart (also known in the trade as a psychiatric chart because it looks like it could drive you crazy).

This chart tells you, among other things, when moisture in the air changes from a vapor to a liquid. That’s called the “dew point.” 

But here’s the good news: You don’t need to use this bewildering chart. You’ll do just fine with a couple rules of thumb. As Bones McCoy would have said to Captain Kirk on Star Trek: “Damn it, Jim, I’m a remodeler, not an engineer!”

Remember this essential fact: If it is 80° outside and 80% humidity, the dew point occurs at 73°. When you go into an unfinished basement with no insulation at the foundation, look around the bottom of the basement wall and especially in the corners, and you are likely to see staining. This is almost always the result of condensation.

Keep in mind that warm air rises so cold air settles, and the bottom of the basement is the lowest, coolest place in the house.

Once the moisture turns from vapor to liquid on a surface that is cooler than the dew point, mold will grow. (It feeds on organic material that exists on most surfaces in the house. Such material includes sloughed off skin cells and a lot of other stuff we prefer not to think about.)

Therefore, as the mathematicians like to say, “airing out the basement,” actually makes the condition you are trying to make better get worse. This is a pretty good definition of a bad idea that is right up there with the more famous “Stupidity is doing the same thing over and over and still expecting a different outcome.”

Consider instead having one operable cellar window so that long stuff can be gotten in and out. The window should be one that seals tightly because we definitely don’t want air leakage into the basement. Replace the rest with glass blocks. They don’t leak air (although I have detected minor air leakage through mortar, so I like the ones that are assembled with caulk/adhesive/sticky goop). Don’t be talked into using vents in those windows. They take us back to the most basic concept: keep the freaking outdoor air outdoors and the freaking indoor air indoors.

There will be moisture in the basement anyway. It can travel through concrete pretty easily. Ever store a cardboard box on the concrete floor? Don’t even think about using carpet on one.

The solution is one I hate because it uses energy, but it is what it is: a cost benefit analysis. Use an Energy Star dehumidifier (include one in your work scope) that can be set to go on at 50% relative humidity. Do not expect people to empty the “bucket” that collects the water. That task rarely scores in their top three most important chores.  Deliver the water to a genuine drain in the floor. This could be a literal floor drain or a sump.

If you would like to be able to calculate the dew point without looking at what might easily be a plate of spaghetti (the psychrometric chart), I am told by the good folks at Therma-Stor that they have put up an app for the iPhone and that they were working on one for Droid. My inner geek thinks that’s really cool.

And please remember that every detail counts when you are a person who cares.