Winter Winds and Withering Wallets

As a little kid, I was pretty sure that Winchester got its name from the windiness of our winters. When your wet hair freezes at the bus stop and you go to sleep listening to the whistling around the windows, it makes sense. It turns out that my childhood etymology was a bit off; nevertheless, the windchill factor around here undeniably turns frigid into freezing.[1]

For the budget conscious, blustery breezes have a direct effect on how well we can keep our homes warm in the winter – and how much we have to pay to achieve this.

A few weeks ago, we took the whole-house fan out of the big grey house. To be blunt, whole-house fans are obsolete and let all the heat out. While we’re working on other projects, we insulated from the top and put plastic sheeting over the hole to act as an air barrier until we can come back and put in new drywall. The plastic sheeting is, of course, much more flexible than drywall, and when the wind whistles through the attic you can actually see the negative pressure being exerted. In the video below, notice how the plastic gets sucked up when the wind blows.

Besides the rustle of the plastic, you can actually hear the suction of the air. (No. That’s not me mouth breathing)

Now, we all know how insulation works, right? It keeps cold air from getting through. It’s like an itchy blanket for your house you never want to touch. Well, yes and no. In most homes, the insulation in the attic and elsewhere is usually fiberglass, mineral wool, cellulose, or sometimes a spray-in foam. Regardless of type, what these materials do is create small pockets of air that resist the transfer of temperature. Air itself is an effective insulator. The trick is that you have to keep it from moving. Unfortunately, the air barrier that makes this possible is the part of the insulation system that most frequently gets overlooked.

Most of us have seen house-wrap on new homes. Some wraps are moisture resistant, but their primary job is to stop air flow, like a windbreaker jacket. This allows the insulation to hold stable pockets of air that resist temperature transfer. While wrapping the vertical walls of the house is pretty straight forward, the situation isn’t quite the same in the attic. The roof and walls redirect outside winds, and if the windows are shut, wind should not be blowing against the ceiling below the attic. Still and air barrier is needed.

The biggest variable when it comes to air movement in the attic is its venting. Attic’s have vents, either windows, gable vents, soffit vents, ridge vents, or some combination of these, to allow hot air to escape in the summer. In the winter these same vents allow glacial gusts to enter into the attic. It might seem weird, but this is not a bad thing. If a house has enough insulation, the rooms below will stay toasty. Still, for the insulation to do its job, an air barrier must be present to stop the movement of air from the insulated part of the house to the attic.

In most houses this air barrier is the drywall or plaster ceiling. Unlike house-wrap on the exterior walls, ceilings don’t have to have an impermeable membrane because they don’t get direct exposure to wind. However, if the ceiling air barrier is compromised, the warm air in the rooms below the attic can get sucked out of the house. When wind does whip through an attic it creates negative pressure that, along with heat’s natural inclination to rise, will draw air – the warm air for which you pay so dearly – into the attic.

The big grey house has gable vents so there is little to impede the wind in the attic, but the point of this post is to help folks understand how heat can be lost though attic accesses, poorly insulated HVAC ducts, cracks in older homes, or really any opening in the ceiling. It is also important to understand that for insulation to be effective there must be something to stop the flow of air through it. Over the hole where the whole-house fan sat are batts of R-30, the recommended insulation rating for this area. The batts, though, can only do so much to stop the flow of air. It takes a certain depth of insulating material and an air barrier to maximize the effectiveness of the insulation system.

I have a feeling I’ll write this again, but remember that, in the long run, insulation is more cost effective than turning up the thermostat.

 

[1] Current thinking claims that the name comes from either Brittonic word ouenta (favored place) or venta (town, meeting place) being paired with the Latin castrum, meaning camp. Old English: In Old English this became Uintancæstir.