Straw Bale Houses
STRAWBALE BUILDING IN A WET CLIMATE –
Warm and cosy, but is it durable in this climate?
We certainly think so. 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 were never evangelists of this as a construction method, but we believe the way in which we have used the straw makes it a sustainable and suitable cladding system. 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.
Since we built our house, many new materials have come on the market for low energy building.
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, preferring not to rely on the straw structurally. 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.
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.
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.
We since applied a siloxane based sealant. We used Aquaseal ™ which is available in local hardware stores. 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.
There are areas at the bottom outside of the wall which still show high moisture, but there is no sign of deterioration and the walls look and feel as good as new. We believe that this is because we put the bales on edge. The straw lies vertically, so there is no wicking effect to encourage the moisture to travel into the wall, so there is a small area close to the outside of the wall that may be damp, but the moisture goes no further and doesn’t seem to do any damage to other straw or to the lime render.
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).
Straw bales provide a low cost insulation material for external walls. When plastered they have tremendous aesthetic appeal. We would recommend the use of a frame and putting the straw in vertically. It may also be cheaper to clad the wall externally, rather than use lime render.
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.
Interesting test on strawbale houses in earthquakes. Click here..