Light Straw-clay
ASK THE EXPERT ADVICE
Links to the Ask the Experts page

Q: My question is about Light Straw-Clay. I am building a timber structure and I am going to do Light Straw-Clay infill. I attended one of 2 of your workshops and have a question as to how long I should let the clay slip straw sit around before tamping into forms. And should I use green or dried branch supports inside the walls?

A: The slip-coated straw can either be packed immediately into the wall cavity, or else left to set up for up to 24 hours. This supposedly helps the clay soak into the straw so the wall will hold together better, but I have not noticed that it makes a huge difference. In any case you want to keep the coated straw well covered so that it doesn't dry out, or you will need to moisten it again before packing it into the wall.

As far as the horizontal pins go, I have used both green and dry, both with good results. Probably dry is better so you introduce less moisture and movement into the wall as it dries. On the other hand, you want the sticks to be quite flexible, not brittle, or they may break when you tamp straw-clay over them.

Q: We are thinking about building a post and beam straw bale house and we are looking at options for the interior walls. Light straw-clay sounds like it could be a good option. Could it be built on a traditionally framed floor?

A: Yes, certainly. Light straw-clay can be built as a conventionally-framed stud wall with straw-clay infill. It is a bit heavier than a standard wall with fiberglass insulation and sheetrock, but not terribly so.

Drywall is such a pain to work with and we want something that will give a similar look to the exterior walls. Would it work OK for bathroom walls? I am assuming you couldn't have plumbing or electric inside the wall.

Sure you can put plumbing and electricity through a light straw-clay wall. There are some special considerations, but it can be done easily enough. As far as bathrooms go, I would recommend a lime plaster to protect the straw from moisture. I have a wet sauna with lime-plaster over light straw walls and there have been no problems so far with moisture.

Q: I'm working with Builders Without Borders on housing designs for Afghanistan. One possibility is building hemispheric domes out of straw-clay blocks. One or more coats of straw-clay would help tie it together. Lime plaster seems like the best finish. I know you have some experience in this area, so maybe you could answer a few questions. Does this sound like a feasible idea? Do you know the compressive strength of straw-clay? Has anyone done this before? Any ideas? By the way, I really enjoy your website and have spent a number of hours reading your articles. You have what most websites lack - usable information. Keep it up.

A: (Kelly) I certainly believe that you could make a dome out of straw-clay blocks and that it would be inherently well insulated; the question is how long would it last? Straw-clay is vulnerable to moisture penetration, and domes are especially open to all of the weather. Straw-clay is typically used as infill in timber-framed structures with substantial eaves to protect the wall from rain. This means that some way of sealing the dome would be essential. I'm afraid that lime plaster would not do the trick, since it is quite breathable and does absorb moisture readily. The other concern is that even if you managed to seal the exterior completely, then you have the potential problem of condensation forming on the inside and not being able to escape through the wall. All in all, I would look to some other method of building that eliminates these problems. Domes are great because they are so resource efficient...you just need to build them with materials that can handle the weather, naturally, such as lightweight cement, stabilized adobe or cob, earthbags filled with mineral material (is there volcanic rock in Afghanistan?) and covered with a stabilized plaster.