Thickness of light straw clay

I often see 12" quoted as the maximum thickness for light straw-clay construction, and R-1.6/in as an insulation value. That leads to about R-19 for the whole wall. Not terrible, but I'd like a little more!

My understanding is that if you go thicker, the wall will mold inside before it can dry out, but 12" seems to be thrown around as a universal limit with no consideration of climate. I wonder if a sunny, dry, breezy site would allow a thicker wall. I might do some sample walls and experiment with cutting them open to see if mold formed.

But my real question is about stuffing dry material in the middle of the wall to bulk it up. Say 6" of straw-clay, then 6" of dry straw, then another 6" of straw-clay. It would all be packed into the cavity at the same time, just substituting dry straw in the middle third, and more clay slip towards the surface. Can anyone see any issues with this? Would it hold together well enough? It could add significantly to the insulation, with proportionally less work since all that extra straw doesn't need to be mixed with slip.

I don't see any problem with the idea, but it seems like a lot of loose straw work when you can just stack bales for a much higher insulation.  At 6 inches for each layer you are at 18" which is the width of a 2 string bale with the strings facing up, plus you do a lot less work than mixing the slip with the straw. I don't think that the the difference in R value is  30-40 for the double slip wall with loose fill straw versus r 40-50 for a complete bale wall at the same thickness.

I just personally like the aesthetics and the process of straw-clay better than straw bale. I know everyone loves straw bales, and rightly so, but for various reasons I think they are a pain in the butt. I've also seen lots of widely ranging values for straw-bale R-value, and I'm not sold on the fact that it's a slam dunk over straw clay. For the small, tight, solar house I'll be building, it doesn't have to be "super insulated," just reasonably well insulated.

Just curios, butwould you treat the inner layer with borox ,since it would lack the protection of the clay slip?

Ben,Hi
I recently found this information about a new way to mix light clay straw.....first dampen the straw then add powdered clay. More information at this website:

http://endeavourcentre.org/2016/04/light-clay-straw-insulation/

If this method works,as they say it does, it would allow for a wider light clay straw wall that would dry without molding.
I look forward to your ( and others) thoughts on this.
Best wishes
Kate

12 inches is not a firm maximum wall thickness.  You can go thicker - particularly in drier climates.  A common rule of thumb is that the walls dry about one inch per week (6 weeks for a 12" wall).  However, I think our 12" walls dried in 3-4 weeks.  If you have decent drying conditions you could go thicker than 12" if you wanted.

Another thing is that you can change the mix to get more insulation value or more mass.  Typically, people make a lighter mix for the north walls and heavier for the sunny sides.  Also, the light clay straw walls perform better than the R-value would indicate due to the effect of the thermal mass in the walls.

I would recommend against layering the walls with dry straw in the middle.  First, it would be a pain to install with any consistency. Second you would be making planes of weakness parallel to the wall surface.  You could easily get sections that spall off.  One of the beauties of light clay straw is that the material makes cohesive, uniform density walls.  Layering the walls would disrupt the cohesiveness of the walls. You are better off making the walls thicker or using a lighter mix.

To hasten the drying of earthen walls, in one of our recent buildings we used straw-clay bricks that were about 12 x 24 x 8 inches (30 x 60 x 20 cm). I'm not sure these are what you'd call "light" straw clay, but they seem to be insulating very very ewell. The straw-clay is walls of 2-foot (60 cm) thick, around 3 sides of the building, with a trombe wall containing thinner conventional rammed earth on the south face. It's a small building, just two main rooms and two narrow storeroom / corridors in the north side with the entrance. It has been staying by far the warmest of any of our buildings so far, and we've been living in solar-heated rammed earth buildings for some 20 to 25 years.

The advantage of bricks is that you dry them before building the wall, and then when you start building, first of all it's much faster than using wet mud in place, and you only have to dry out the wet mortar between the bricks, not the entire mass of the wall.

That said, I'm very fond of our rammed earth buildings.
Whichever way, go for it!

Rebecca Norman wrote:To hasten the drying of earthen walls, in one of our recent buildings we used straw-clay bricks that were about 12 x 24 x 8 inches (30 x 60 x 20 cm). I'm not sure these are what you'd call "light" straw clay, but they seem to be insulating very very ewell. The straw-clay is walls of 2-foot (60 cm) thick, around 3 sides of the building, with a trombe wall containing thinner conventional rammed earth on the south face. It's a small building, just two main rooms and two narrow storeroom / corridors in the north side with the entrance. It has been staying by far the warmest of any of our buildings so far, and we've been living in solar-heated rammed earth buildings for some 20 to 25 years.

The advantage of bricks is that you dry them before building the wall, and then when you start building, first of all it's much faster than using wet mud in place, and you only have to dry out the wet mortar between the bricks, not the entire mass of the wall.

That said, I'm very fond of our rammed earth buildings.
Whichever way, go for it!

Would these bricks work as the perimeter wall around a timber frame?  Not needing to make a double stud wall, or leland trusts, would be a time and material saver.

Sorry, I was offline for a couple of days. Yes, sure, in my opinion it would work great as the perimeter wall. That's what we used it for, and in fact we put a reinforced concrete tie beam on top and then the load of the roof is on these walls. Not a very big span, but a heavy earthen roof. But we are in an extremely dry climate -- your results may vary.