After seeing problems in a few straw bale buildings, I’ve been thinking about this lately: is it a truly durable building system? By which I mean, will a straw bale house measure its lifespan in centuries rather than decades? I’ve concluded that most will, some won’t. The ones that won’t are predictable, however, and for the most part they break the rules.
Architects occasionally design straw bale homes with no roof overhang, for instance. I’ve seen this twice, and in both cases an overhang was added before construction was completed. In one of them there were already some moisture issues a year or so after the wall was closed in. Water was sheeting down the wall in spring rain storms and working in through cracks. These were a few horizontal cracks which had reopened after crack filling. Straw at the base of the wall was saturated and had to be replaced – which was not as hard as I thought, and in a weird way I found that encouraging for the question of longevity. With the overhang in place I think this will be one that does last.
Other houses that I worry about don’t break the rules so blatantly, rather they push them a little, but they are on exposed sites. Driving rain is the enemy of straw bale houses, and gable ends are particularly susceptible. If you’re thinking of building a straw bale house on an exposed site – a hill or a lakeshore, or any site where you might consider using a wind turbine – your design must be impeccable. You might want to consider a bungalow with good overhangs all the way around, you should certainly avoid a large gable end on a windward side of the house. Gable ends in general should have some kind of skirt roof, and you may want to consider siding the upper part if it’s large or particularly exposed.
Cement-lime plaster tends to make things worse. There’s an unfortunate tendency to gravitate towards cement-lime on very exposed sites because it is the most durable plaster. Cement-lime won’t erode away under driving rain, but it will trap in moisture more effectively than any other plaster. High lime content helps a lot, but pure lime is better, or an earth-lime hybrid system; in rare cases exterior earth plasters may even work on their own (note that the right paint is important for earth and lime plasters). In any case, if you’re very worried about your plaster eroding under driving rain, you probably have a design problem and cement-lime plaster is likely to make it worse. You need to redesign, or possibly you just shouldn’t be building a straw bale house there. An oft-overlooked alternative that can eliminate most external moisture issues, even on exposed sites, is to use siding or rainscreen over bale walls. And keep in mind that whatever you build on an exposed site, bale or otherwise, you’ll need good design and attention to detail.
Cracks must be filled. I’ve seen a house that went maybe 8 years without crack filling and painting, and it was fine! But I’ve also seen disastrous results from unfilled cracks. Again, the site seems to make all the difference, but there’s no sense pushing your luck. Fill your cracks within a few months, or if you plaster in the fall, wait until the following spring or early summer – but not years.
This sounds like a whole lot of bad news, so why build straw bale at all? Is it worth the hassle, and is it really a sustainable wall system? To put this in perspective, when a 100-year-old hay-bale house was dismantled in Nebraska the hay was in such good shape that cows ate it. Or consider that straw bale building is not alone in having had its share of mistakes – modern building practices have created a “perfect storm” of stucco failures on conventionally built homes. In some ways, bale walls are better, they can be more resilient than some conventional wall systems. As soon as you add insulation to a wall you’re inviting moisture problems – the more insulation you use, the harder it is for the wall to dry out if any moisture gets in, because the middle of the wall tends to stay cool. Superinsulated homes are built to have very low air leakage for energy efficiency, but also because air leakage can cause moisture problems if water condenses in the wall.
Straw bale walls can likely handle small to moderate moisture loads better than conventional wall systems because of the vapour permeable plaster skins on either side, and because the straw itself can act as a large reservoir for moisture without ill effects, so long as it does not exceed an upper limit, and the conditions occur for drying. It’s still very important to air seal a straw bale home properly, and many natural builders have been slow to realize how important air sealing is. In my experience those days are over and air sealing is a priority for most natural builders, which means some kind of air fin behind all plaster joints, and of course good detailing around electrical boxes etc.. This is not just a question of energy efficiency, but also is likely to extend the life of the home.
There are other benefits to straw bale, of course, that I should mention briefly: A relatively high R value (at least double that of a 2×6 stud wall with batt insulation, but still less than most superinsulated homes); low embodied energy and local sourcing of the building materials; and aesthetics. Straw bale is not for everyone, and is certainly not the only ecological way to build, but it has a role to play when done correctly.
A literature is beginning to develop around moisture control in straw bale walls. Here’s a short list of important resources
Many of the best practices of design, air detailing, flashing, and other details of conventional homes also apply to straw bale homes, and for this one of the best resources is the Builder’s Guide to Cold Climates.