Best recipe for straw in-fill insulation

Been reading a lot on the www about straw to update my knowledge.  I read a lot of "stuff" and would like to get some kind of consensus here.  I am looking to insulate a pole barn type building and considering straw as the material.  How do I get the highest r value out of straw?  Is there one kind of straw that produces a higher insulation value?  Is there a specific dryness that makes it more effective?  Does the degree of compaction raise or lower the r value?  How about using clay slipe?   Heaven forbid, what about mixing in repurposed styrofoam, putting it into a burlap sack and then pressing to the desired degree of compression and shape?  The building will mostly be used for utility purposes - will work in them but not live, but am hoping to get an r value of 10 in the wall to keep it comfy.

Hi Tom,

A book written a few years ago by the California Straw Building Association, Straw Bale Building Details: An Illustrated Guide for Design and Construction answers all your questions about bale building, and then some.  

In short, bales on-edge insulate better per inch than bales laid-flat, R-1.85 per inch compared with 1.55 per inch.  In N. America two-string or 3-string bales will measure 15", 18", or 23 wide depending on orientation, so R-values between R-26 and R-36 are possible.  

Denser is better up to a point--say around 9 - 10 lbs./cubic foot--but minimum dry density is 6.5 lbs./cubic foot.  The International Residential Code Appendix AS Strawbale Construction (available for free download from www.strawbuilding.org) is a model code available for adoption by code jurisdictions in the U.S.  It states that bales must be drier than 20% moisture content.  In the arid west you'll probably be working with bales of around 8% to 12% MC.

Less dense bales can settle and also develop convective loops, which reduce R-value.  There are no studies that I know of that differentiate between types of straw and R-value.  Most of us work with what we can get--in my case, wheat, barley, and rice (S. Oregon).  You'll want to use long straw bales.  I avoid bales made with flail-chopped straw or straw partially shredded by rotary combine harvesters--it can be made to work but more difficult to notch and re-tie.

Bale walls are quite heavy.  When plastered on both sides they can weigh between 45 lbs. and 55 lbs. per square foot.  This is four or five times heavier than most conventional wall assemblies, so the footing/foundation needs to be sized accordingly.  The CASBA book also describes the many shear wall options you have with a bale wall assembly.

Jim Reiland
Many Hands Builders

Tom Connolly wrote:Been reading a lot on the www about straw to update my knowledge.  I read a lot of "stuff" and would like to get some kind of consensus here.  I am looking to insulate a pole barn type building and considering straw as the material.  How do I get the highest r value out of straw?  Is there one kind of straw that produces a higher insulation value?  Is there a specific dryness that makes it more effective?  Does the degree of compaction raise or lower the r value?  How about using clay slipe?   Heaven forbid, what about mixing in repurposed styrofoam, putting it into a burlap sack and then pressing to the desired degree of compression and shape?  The building will mostly be used for utility purposes - will work in them but not live, but am hoping to get an r value of 10 in the wall to keep it comfy.

I'll just speak to light clay straw since that's what I have experience with. Less compaction results in higher r-values. The balance is if the straw is too loose it's difficult to plaster and you have to reinforce with cob which reduces your r-value. I've read that light clay straw can achieve a R-value of 2 but I more realistically it's around 1.5. So, for R-10 you need a 7-8 inch thick wall. I don't know if there is really much difference between straw types as long as the basic structure is a hollow tube. Just make sure you get straw and not hay. Regarding dryness, dry straw works better. Damp straw will work but you just want to cull rotted portions. Once in the wall you want it to dry completely before applying plaster or siding. I've never done it but I think mixing some in Styrofoam with light clay straw would be fine. It takes the foam out of the waste stream and would create some air pockets which is key for insulation.

Lydia Doleman has a good book about building with light clay straw and its various properties. Here's a video of one of her presentations:
     

I'll second part of what Aaron said about less compaction (or a straw-clay mix with somewhat less clay) yielding higher R-values, but probably not more than R-2, and more likely somewhere around R-1.5 per inch.  That's one reason some straw-clay building designers have moved towards wrapping a 12" wall (not quite R-21) with a wood fiber board (an additional R-7+), then applying a suitable exterior plaster over that.

I agree that Lydia Doleman's book is a really good resource on this topic, too!  I have worked with Lydia for over ten years (we both live and build in S. Oregon), and even contributed a handful of pictures for it.  And if you're using round poles for your building's frame, you might find it's easier to pack straw-clay around them than it is to notch a straw bale for a round hole.  

Where I'd be more concerned is moisture in the walls. The rule-of-thumb for straw-clay wall dry time is 1" per week of warm, dry weather.  So in theory, if you pack a 12" wall form at the end of May with straw-clay, it should be ready to plaster by the end of August (in the northern hemisphere).  If your building schedule can accommodate this you can work on ceiling insulation, setting windows and doors, etc.  But waiting up to three months is a deal-breaker for those who have a shorter construction schedule or have a narrow building season window.  That's probably a huge reason that straw-clay hasn't become as popular as straw bale construction, where the building constraints aren't quite as narrow.  

The last project where Lydia and I worked together before I retired in 2022 was to plaster a straw-clay building. The 12" walls were placed in May of 2021, and when we were scheduled to plaster it in October of that year we learned that the walls were still quite wet--in some places between 25% and 35% moisture content.  The upshot here is that the walls were still damp after five month of drying here in S. Oregon's dry, warm summer--not the expected three months.  The straw-clay was placed by people who have done this hundreds of times before, so its not likely that the material was too wet going in.  And we had an above-average summer in terms of hot, dry weather, so that wasn't the problem.  It might have been quite different were we building in a place with very humid summers, but our part of Oregon has very dry summers.  It's possible that I used the wrong tool to measure moisture content--the same one we use for measuring the MC of a straw bale.  It may be that contact with the clay in the wall gave a consistent misread since most moisture meters are made for use in hay and straw. The material may actually have been plenty dry, although it felt damp to the touch in places.  Still, I don't think there's a tool specifically designed for measuring moisture in a straw-clay wall, and since the Oregon building code and best practice in construction requires us to wait until substrates and framing are below 20% before covering with a material that will further slow the dry time (drywall, siding, plaster, etc), we held off.  The home-owner ran dehumidifiers and heaters through that winter (in part so it was comfortable to do other interior work), and by March of the following year--nearly ten months after the straw clay was placed in the wall--the MC readings I had been taking monthly since October were finally well below 20% so we could plaster.

I love straw-clay walls.  I love plastering them.  And, just know that you want to give yourself plenty of dry time.  Your part of Nevada may be even warmer and drier than my part of Oregon.  Great!  You may be able to rely on the three-month dry time for a 12" wall.  But if not, give yourself a time buffer.  

Jim Reiland
Many Hands Builders