Dew Point? Not at Me, You Don't

We promised to grapple this time with the issue that lives at the center of new, tighter construction standards: trapped moisture, which leads to rot.

In a previous post we touched on the subject of dew point, which is briefly defined as the temperature at which moisture begins to condense from the surrounding air. When air holds lots of moisture (we call it humidity) that moisture begins to condense at higher temperatures, sometimes as high as 75 degrees Fahrenheit. Air holding less moisture (lower humidity, think Arizona) condenses at a lower temperature, sometimes as low as 15 degrees F. Don't worry about the math, unless you spend your evenings that way, but just get it that air can be wet and ready to give up its moisture at a wide range of temperatures, and when it does, the quantity of water condensed can be enough to wet down any available surface.

When air from your house gets into your walls, and almost all houses exhale air through their walls, it cools or warms to imitate outside temperature as it travels outward. In winter, especially a cold winter like this last one, and in Connecticut, where I live, the outdoor temperature can be 15 degrees on most winter nights.

So here's the picture: fairly warm, moist air (you probably maintain a fairly low humidity, but there's still a lot of moisture in your air, or your lungs would complain) from your living space is traveling out into your walls, cooling as it goes, and it reaches its dew point and drops its load of moisture. Don't think thunderstorm here, think more like the bathroom after a teenager's shower. The moisture can be deposited on your actual inner walls (I've seen it, it's awful), in your insulation (if you have any), on your structural framing (2x4s, outer sheathing) or, in extremely embarrassing cases, on the outside of the house near the site of a terrible air leak. Your attic is the most likely source of terrible air leaks and condensation, since it's probably vented to the outside. Warm air escapes through lighting boxes and structural cracks, cools to dew point in the attic, and the moisture settles on rafters or forms icicles under your eaves. Note the difference: icicles that form at the gutter or drip edge are probably due to daily warming of roofs and refreezing of drippy drops. Icicles forming under eaves or around air vents are probably due to moist air escaping into the attic and outside. That leads to trouble.

In this photo, note that some of the icicles are on the eaves and others are attched to the attic vents under the eaves. Those inner icicles spell trouble due to condensation in the attic.

Why am i nagging you about icicles in this glorious weather? Does the name Eeyore ring a bell? It was supposed to be about dew point, remember, but I always get carried away. And when it's 90 degrees this summer, with 90% relative humidity, and you've got your head in the refrigerator for some relief, and your shirt is stuck to you but the shower is still steaming up the bathroom but it won't go away, you're still dealing with dew point. It's an everyday thing.

So Mr. Natural says, tighten up your house to prevent excessive air leakage to the outside. You don't have to do the math; just do the caulking, foaming and light carpentry that helps you save energy and keep your walls dry and safe from rot.