Straw Bale Houses

STRAWBALE BUILDING IN A WET CLIMATE –

Warm and cosy, but is it durable in this climate?

Straw bales are an attractive building material for a number of reasons. Straw has fantastic insulating properties, and with the increasing use of slatted sheds, it has almost become an agricultural waste product.

However, in Irish climates using straw for walls is relatively unproven, and when choosing it as a construction material for our new house, we had some reservations – three years on, we are still not evangelists of this as a construction method. While putting the bales into the wall, we incorporated moisture sensors so we could assess the suitability of straw in West Cork where we live.

The conventional wisdom among strawbale builders in Ireland and the UK seems to be that if the house has a generous roof overhang, the lime render will protect the straw, allowing some moisture in during high rainfall, but evaporating it back out during the dry spells. This may well be true in some areas, but West Cork rain comes almost horizontally and protection from a roof overhang would be limited to say the least.

Timber Frame or Load Bearing?

There are two options – using straw as a load-bearing wall to hold up the wall, or building a timber frame and using the straw as “infill�? to clad the outside of the house – we opted for the latter. This meant that should the straw fail for whatever reason, external walls could be re-clad where necessary with timber and insulation. This would be quite a job, but the house wouldn’t be a write-off. Also, using straw as a load-bearing structure would require a long dry spell during which the walls could be built, roof timbers and roofing put in place, all without the straw getting a good soaking. In recent years, such weather windows have not been a very safe bet.

We opted for a post-and-beam frame using local Douglas Fir timber, which is structurally strong, but easy to work with, and aesthetically pleasing. This frame supports the roof, and additional timber framing supports windows and doors. Once the roof was on, we were able to use tarpaulins (used advertising hoardings) to keep things dry while we put in the straw and plastered it.

Moisture detection

We put moisture sensors into the walls at 8 different locations. At each location, three sensors were put on top of the first bale, one near the inside of the wall, one in the middle and one near the outside of the wall. We repeated this half way up the wall, and again under the top bale giving us a total of 9 sensors at each location (72 sensors in all).

We bought a £100 timber moisture meter which has two probes that are normally stuck into the timber a couple of centimetres apart. The moisture meter measures the electrical resistance between these two sensors to give a reasonably accurate reading of the moisture level in the timber (wetter timber conducts electricity more easily). To make moisture sensors we simply put screws in a piece of timber the same distance apart as the sensors would otherwise be, and attached wires to these screws. All the wires were brought back to one location and wired to switches from which the meter can take readings.

We had been warned that the moisture levels would be high initially – water in the lime render would take time to dry out, so the readings would be high and meaningless for the first year or so. And sure enough they were high, but gradually falling. But after a couple of years, the levels stopped falling at a level way above the 20% moisture considered to be totally safe for straw. There are different opinions on what is a safe level – some saying that within a wall, a level of up to 25% or even 30% would not cause the bales to rot, but we had some readings that failed to go below 30% and even 35% on occasions.

With the configuration of the 9 moisture sensors at each location we could make an educated guess as to the cause – moisture could be caused by vapour coming out, rising damp, or even leaky roofing – a look at the profile of the readings (see chart below) showed the most likely cause to be rainfall penetrating the lime render system. Moisture was higher as you moved down the wall, and the inside was drier than the outside. North walls where there was little of no rainfall were lower, so the problem wasn’t vapour. Clearly the culprit was Mother Nature’s teardrops.

There is a need for some caution here – moisture meters provide reasonably reliable readings at levels between 10% and 18%, but above that level, their accuracy falls off, and it is easy to get erroneous readings as high as 60%. In the figures we show here, we disallowed readings above 38% – at this level the readings are very unreliable. Regardless, we had moisture levels in some areas that were likely to be too high, and boring holes in the walls at these locations revealed that the straw in these areas was indeed decomposing. Something would have to be done.

Moisture Protection Options

We consulted with Ritchie Murphy who had worked as a strawbale builder in the USA, and who had built a post-and-beam straw clad house in Waterford with his wife Linda. He in turn spoke to Dan Smith, an architect working on straw houses in California. This whole area has been a talking point from California to Canada, but oddly enough nobody around here seems to be quite as concerned. Dan believed that at a moisture level of 30%, the temperature would need to be high for an extended period for rot to really take off. But we were above that in places.

In California, there are two suggestions. The more common seems to be to render using cement with waterproofing additive. While this might be anathema to the purists, the reality is that if houses were built with 1�? of cement render either side of a well-insulated straw wall, it would be infinitely better than 4�? of concrete block either side of a draughty 2�? piece of aeroboard, which is how most houses are currently built. If cement render is used, your walls would lose their vapour permeability, and it would be important that cement was used both inside and outside to prevent internal vapours from condensing into water where the straw joins the outside cement render.

The second option is to use a siloxane based sealant to protect the lime render. This is a synthetic sealant that is painted on, lasting for up to 10 years. It allows vapours to pass through, but prevents water droplets from getting through to the lime.

A third suggestion would be to clad the house externally using timber cladding, leaving a vented space between the straw and the cladding. This would work, but care would have to be taken to ensure that rodents or bees could not get under the cladding, and strong wire mesh would be needed at the bottom and top of the vented area. While the timber cladding would be expensive, there would be substantial savings on the cost of the lime render system outside the house.

At this stage we opted for the siloxane based sealant. We used Aquaseal ™ which is available in local hardware stores and has an odour somewhat pleasing to anybody with a glue-sniffing habit. It may not be ideal to have to use a synthetic paint every ten years, but cladding around the walls, and particularly windows and doors at this stage is a job we are unwilling to take on.

So far, the indications are that this has worked. Moisture sensors are very slow to respond to changes and it isn’t worth looking at them every month.

A low cost option?

A last word about the cost… There are numerous claims that straw bale building is a cheap construction method, and claims that a three bedroom house could be built for £30,000 were common when we started out. This was patent nonsense – in a typical house-build, the walls account for between 10% and 15% of the finished cost – even if they were for free, this couldn’t have a huge effect on the finished house price. Straw can usefully be part of a low-cost build with voluntary labour, but only if all the other materials are cheap or salvaged. Either way, but lime render is expensive to buy (we spent €2,000 on lime alone). You’re not likely to get truck-loads of volunteers for a cement rendering weekend, but building a structural strawbale house that relies on lime render in a wet climate might prove to be a false economy in the long term.

We hope we’re wrong on this, but our moisture levels suggest that anyone who has done this already should bore a few holes in the outside of their walls and see what is happening under their local weather conditions.

Conclusion

Straw bales provide a low cost insulation material for external walls. When plastered they have tremendous aesthetic appeal. We would recommend external cladding with a ventilated space between the cladding and the straw. This would save the cost of the external render, and would be quick and easy to incorporate into the design, guaranteeing a waterproof, warm and dry house for generations to come.

Footnote:

August 2007

Since writing the above, I have been told of another straw bale house nearby, built just before ours. They took out some straw to build an extension, and were pleased to find that the straw was in excellent condition.

A second house which had similar construction to ours also did a renovation, and they found that although the outside few inches of the bales were damp (as ours are) and decomposing, the rest of the bale was in good condition.

It seems that the problem is one that would not cause a long-term problem for houses that have a frame construction, but I would not be too happy in this climate building without a frame.

If I was building again, I would still use straw – it is the cheapest well insulated construction material I know of, but I would use a frame to support the roof for a number of reasons, and I would probably clad externally rather than use lime render.

Earthquakes

Interesting test on strawbale houses in earthquakes. Click here..