Will straw bale buildings last?

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.

This wall is ready to be replastered after wet straw was removed. It had no overhang at all.

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

Design of Straw Bale Buildings

Moisture Movement and Mould Management in Straw Bale Walls for a Cold Climate

Moisture Properties of Plaster and Stucco for Straw bale Buildings

Building Science for Strawbale Buildings

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.

11 comments

  1. […] A track record in Ontario has shown that cement-lime plastered buildings can work in our climate, if designed properly. However I would agree with Racusin and McArleton that cement-lime plaster is not the best choice […]

  2. Eddy Wilbers says:

    Two years ago, I purchsed a straw bale home in Minnesota. When I closed the house was in poor shape and the exterior had to be rebuilt. Our first winter in the house was cool and drafty and we were spending a fair amount on heat. We did as much research as we could, but until I read this article I had never heard the term \\\”rainscreen\\\” mentioned.

    Fortunately when we removed the old rainscreen the bale was in good shape. It had not been plastered and the bale had settled in some places leaving air gaps. We repacked the gaps before residing the house with Hardiboard. There is a Tyvek moisture barrier between the bale and the siding, but the bale remains unplastered.

    I thought that this would make a big difference in our heating costs, but last winter we continued to spend much more than we anticpated to heat the home. I suspect that the bale in the roof has settled as well, but I am loath to remove the tin roof to fix the bale.

    I was wondering about blowing some kind insulation into the gaps. I also had an energy audit done on the house and they found that the house exchanges air 11.6 times per hour. The second floor ceiling is rough beam and there are gaps between the beams. On top of the beams there is a layer of felt beneath the bale. The inspector recommended caulking between the beams to help reduce the air exchange. I am worried that this could cause moisture to build up under the bales in the roof.

    I would really like to shore things up before the snow flies again in order to reduce our natural gas usage this winter. Any advice would be appreciated.

    Thanks,

    Eddy

  3. Mike Henry says:

    Hi Eddy, this might not be what you want to hear, but unplastered bales function very poorly as insulation, something that owner-builders discover whenever they try to overwinter in an unplastered bale building. No amount of housewrap will seal them as well as the plaster, so I would try to get a plaster coat on wherever and whenever possible, which sounds like it may be a big job, and possibly impractical. Other than that, caulking is probably going to be a good friend – I\’ve never used bales as roof insulation, but it seems to me that the caulking would cause less moisture problem than the warm moist air that can find its way through those gaps? 11.6 is pretty leaky, close anything you can! The Natural Building Companion will give some good insight on how things should have been done on your home, and then you just have to figure out where and how to work backwards.

  4. Karen says:

    We have recently considered buying a straw bale house and noticed there was no over hang on the roof. The house was also built on a hill, with a void beneath where the bales to the floor were exposed. Also the floor of the void contained huge puddles, some quite deep. Is the a property best avoided, it’s in the South of France.

  5. Mike Henry says:

    I would be quite concerned about the lack of overhang. I’m not sure I understand what’s going on with the floor. I can’t really give good advice on a house I haven’t seen, but if you can smell wet straw at all, that would be a sign of major issues. Even if you don’t, it could be worth using a moisture probe though the plaster in any areas of concern before buying a house without an overhang etc. – this may be impractical as the holes would need to be patched. There are some very good french straw bale builders you could potentially bring in to assess it.

  6. Carol says:

    I want to build an A-frame tiny house. I’m considering building the frame and stacking the bales on top of the frame for insulation. I will plaster the bales inside and out, then add a metal roof. I don’t plan to use bales on the gable sides. Will that be an efficient and long lasting way to do it?

  7. Dawn Diovera says:

    Hi:
    We are considering a straw house in Paso Robles. I read your article. After reviewing the photos of the house, I only see an extended roof on the lower level. Is this what you mean? Any ‘sure signs of issues” other than wet straw smell – to look for? Here is the website with the photos: http://www.realtyonegroup.com/homes-for-sale/CA/Paso_Robles/93451/6255_Buckhorn_Ridge_Place/243_PR1053247/

    • Mike Henry says:

      If it’s a very dry climate it may not be as essential to have the extended roof overhang. You could consult a local builder.

  8. Haley says:

    I want to build a straw bale house with large over hangs,and a living roof system. I really want it to look like a hobbit house, but I know the bales need to breathe, so a truly enclosed bale house (think earth sheltered walls), wouldn’t be as breathable as a strawbale wall needs. I’ve been thinking around this problem, and have possibly come up with a solution….extreme overhangs that extend to the ground in a seamless transition from the roof to the garden ground.
    This structure is tiny, less than about 200 sf, with timer beams supporting the roof. Will this idea work? It would trap a few feet worth of air between the living overhang and the walls, which would be plastered on both sides with earthen lime plaster.

Leave a Reply

Your email address will not be published. Required fields are marked *