Note the tar paper “air fin”, a secondary barrier to air infiltration at the top of the bale wall

When we look at a building, we tend to focus on the finish details: the curve of a wall, the color and texture of the plaster, the tidy trim around a window or door, decorative mosaics, the feel of the floor underneath our feet. However, finish work is just one aspect of a good building. Often, little construction details go entirely unnoticed and unappreciated, yet it’s the details that can really enhance the actual performance of a good building. For example, unless windows and doors are sealed well, they may be an entry point for cold air or driving rain. And that’s a “feature” that no one really wants. Good window detailing is more or less invisible in the finished product, however, and it takes a lot of time to do well.

The top of a straw bale wall is another one of these areas that poses a great design challenge and a good opportunity to improve the overall performance of the house. This is true wherever two different materials meet. You need to consider the fact that the plaster will likely pull away from where it meets the roof surface as it dries and inevitably shrinks. This is a problem, because air will have a convenient point to infiltrate the wall, and that means decreased thermal efficiency. So what do you do? Ignore the problem, or address this issue from early on?

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