We are building a strawbale timber frame Passivhaus in Norfolk, and had the chance to test it for airtightness before the plaster goes on. We knew it would not give a good airtightness result – this requires the door and window installation to be complete – but we wanted to know how the straw on its own was performing, and how our standard design details work without using tapes.
What we found out was that the vast majority of our compressed straw walls were airtight without any render, and that our standard details are good.
Watch the test results here:
In a few places there was air passing through the bales themselves – this was in the gable end, where it is not possible to reach the same level of compression as in the rest of the walls. There was also air passing through a few joints between bales in the body of the wall – this was due partly to some stuffing having been pulled out when trimming the face of the wall ready for plastering, and partly where dressing of the bale had not been done very well, or stuffed enough.
There was air passing through some parts of the junction between the timber base plate and the foundation – in our standard detail here we cover this with a render stop, and skirting on the inside. For the Norfolk house this will be taped over. There was air passing around the fixing post at one bale for a door and a window – where the notch in the straw had not been done well enough – but all others were good. Our normal practice is to deal with this at the preparation for plastering stage, with a long straw/lime mix.
What this test tells us is that our standard details and above all the quality of our straw installation is really good. The straw walls are mostly airtight even without render. This confirms what we thought, that it is important that the straw is compressed, even when used for infill, otherwise it is only the render/plaster that prevents air from entering the building, with associated problems should there be cracking.
It also confirms that we are correct in the way we apply the key coat of render, by using it to reveal any imperfections in the straw wall rather than to cover them up, and then packing any revealed holes with dry straw before the body coat is applied, ensuring that we find all areas that may not be airtight and deal with them as part of the plastering/rendering process. It also confirms that joints between timber and straw work well if the straw is under compression, but joints between timber and other materials need to be designed to be airtight.
However, a word of caution: the standard of our designs and installation is extremely high. Straw Works is very particular about teaching best practice, to dress bales properly, to stuff walls adequately, and to pay attention to details. Our high quality is the result of many years of experience, our trainers and builders have all been apprenticed for some time before we allow them to work on our behalf, and they all have a very good understanding of the material and of how to use it. Not everyone has this expertise; it cannot be acquired by doing a few courses or reading books.