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?

Detailing the Top of a Straw Bale Wall Where it Meets the Roof

We stressed the importance of considering this straw bale wall to roof connection during our workshop last week. For better or worse, our roof framing presented a big detailing challenge. (Good for a demonstration, bad for the hours we spent dealing with the detailing!) Because the house we’re building has exposed rafters inside that continue through the plane of the wall, we have a lot of thermal bridging at the top of the wall and many linear feet where the plaster can pull away from the roof rafters and deck. That’s bad news for thermal efficiency.

The tar paper is stapled to the roof deck and around each rafter

However, we took a good cue from The Natural Building Companion and designed air fins to provide a secondary barrier against air penetration at this point in the wall. The material we used was tar paper, cut to fit around each individual rafter and along the deck between pairs of rafters, providing a continuous barrier behind the plaster and against the straw bale wall, where air could be stopped in the event where the plaster pulls away from the wood. It’s like a secondary defense system — of course, plaster is the first defense against air infiltration. Yet it’s impossible to stop plaster from shrinking as it cures. Shrinkage is not a problem along a clear run, but wherever plaster meets a window frame, the roof, or anything similar, you have a point where air infiltration is likely to occur.

Finished air fin with straw in place — tar paper runs continuously under roof framing, and expanded lathe provides key for plaster where loose straw was stuffed between rafters

As you can imagine, the difficulty of installing this air fin was exacerbated by the fact that our rafters were exposed. If we had a soffit, it would be a different story. We wouldn’t have to wrap each rafter individually. And so thousands of hours later (well, not really thousands, but that was the quote we kept repeating as we cut piece after piece of tar paper, and many staples later), we finally wrapped the entire bottom of the roof / top of the straw bale wall. A soffit would simplify this greatly, because each individual rafter would not have to be flashed in tar paper. Did I mention we’re using rough sawn wood, which further complicates the air fin detail? (That’s because no two pieces of rough sawn wood are the same exact dimension, unlike the milled lumber you would buy at the big box home store.) Anywayyy…

In summary, it’s good to think as far ahead as possible when you’re doing your house design work, always remembering that the junction between materials is where you’ll spend most of the construction time, and that these are also the places where maintaining good performance is key. Straw bales are highly insulative, but if you have a ton of area where cold air can blow inside the house because of spotty junctions, you’re kinda missing some of the benefit, right? An air fin is a great solution to include as a secondary barrier, and the easier it is to install, the better!

Food for thought.