Archive for Straw Bale Techniques

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.

Window shaping in straw bale homes: a how-to and slideshow

Window curves are one of the most distinctive features in straw bale homes, and are often a big consideration in the choice to go with straw bale over other forms of construction. But information on how to shape curves is sparse, so I thought I’d share some of what we’ve learned over years of doing bale work.

The first thing you really need to think about is radius of curve. To visualize this take a string nine inches in length, pin it at one end and attach a pencil to the far end. Now draw a quarter of a circle – this is roughly what a nine inch window curve looks like. Try the same thing with a 16″ length of string, and you’ll see how dramatically different this window curve would look. Generally we find large radius curves that open out quickly from the window are more popular because they let in more light and make the window seem larger. However many people really like windows that come out straight and then have a smaller curve at the end. The sides of windows usually have curves anywhere from 9″ to 16″ radius.

Window tops and ledges can be straight or curved. Flat tops generally need some extra wood framing.

There are three steps to shaping a window – carving, stuffing, and attaching mesh.

First you need to carve the bales to match the shape of the curve. We try to do the bale work fairly tight to the window, but typically the bales are set back 1.5″  because we push them up against the framing without notching. In preparation for shaping we cut one string, and carve the bale to the shape of the curve using a grinder with a lancelot blade. Cut a plywood template that you can hold up against all your curves as you grind them, to help keep them uniform over the curve and consistent from one to the next. Stand back and look at it from a few different angles. Try to get it perfect, but when in doubt err to the side of too low, you can always fill with plaster.

Next stuff loose straw in the voids. If the carving was done well, you hopefully won’t need too much stuffing. To finish the stuffing, you’ll need to attach the mesh to hold the loose straw in place.

We do a lot of our window shaping with tenax mesh instead of metal lath. If a curve is done well it is easily shaped with plastic mesh and a little stuffing, and is actually easier to plaster than curves shaped with metal. Curves that need a lot of stuffing are formed using metal lath, also called diamond lath (and sometimes ‘blood lath’ because it tends to draw blood from those working with it).  Keep a consistent staple line with your mesh or lath, make sure it is on straight and even, and you should end up with an good curve.


Straw Bale Building checklist

Recently my friends asked me for advice on building a small straw bale building (a garage with a living space above). They asked me how it will compare, timewise, to conventional framing. The answer: it will definitely take longer. They will save money on materials (some) but it will only be worth it if they really want the extra insulation value. They do, and they want to try building with bales as a test run for a future house. Also they want to use post and beam anyway, which means they need some kind of secondary wall system – for this reason, and aesthetic reasons, bales and timber frame go well together. I wanted to give them a good idea of what they were getting into, so I compiled a step-by-step checklist of exactly what you need to do to build an infill straw bale wall.

Here it is:

Bale building checklist

Before bales (after foundation and basic structure)

  •  Roof must be finished. Really finished!
  •  Bale curbs installed
  •  Top plate (never OSB – plywood OK)
  •  Framing for doors and windows
  •  Pre-hang mesh if / where needed
  • plan air sealing details which may need to go in before bales
  •  Electrical?
  •  Bracing for windows or guides for bales if needed
  • Scaffold? Set up before baling if possible. Plan tarping accordingly
  •  Tarping (tarping that rolls up can be nice). Attach to soffit area, not facia
  •  Get bales

Bale install

  • Check bales for moisture as you go
  • Pre-fill voids where corner of bales meet
  • Notch / install bales. Check plumb as you go. Tie into posts as needed (frequent)
  • Cut curves / angles for windows – can be done before or after bale placement
  • Dipping bales? Need to pre-mix clay slip and mask finished wood
  • Straighten walls / weed whack if not dipped
  • Stuff all voids / check stuffing everywhere

Plaster Prep

  • Cover all wood that will be plastered with typar / tyvek,
  • Lath over wood & typar, hanging over bale transition by 1.5 to 2″
  • Lath over air sealing materials
  • Tenax mesh / stitching (recommended if not dipping bales)
  • Flashing completed everywhere plaster will meet
  • Plaster stops?

Plastering

  • Process site clay?
  • All materials and mixer on hand
  • chopped straw (save from windows)
  • Make wheat paste where needed
  • Mix
  • Apply scratch coat inside and out
  • wait for drying / curing (1-3 weeks earth / 3 weeks lime / 1 day cement-lime)
  • Finish coat in and out
  • De-masking / clean up / detailing
  • Caulk joints
  • Crack filling (minimal with earth / more with other plasters)
  • Paint