Straw bale cottage on thin (CMU) block stemwall / foundation?

I'm trying to design a small cottage and was wondering if it would work to make the stem wall CMU blocks (on a wider poured footing).... but since the blocks aren't wide enough for straw bales, perhaps placing planks of wood on the CMU blocks (anchored down to the blocks) and the straw bale on the wood?

CMU blocks are 8 inches, and bales are 16, so the overhang would be just 4 inches on each side... so it seems like a thick enough piece of wood would be structurally sound (and the CMU blocks under would be solid filled).

Structurally the rest of the cottage is post and beam, so the straw is really just infill for the walls (and the straw would be attached to the posts also as well as the vertical rebar).

But I wonder if it's an issue having such a thin stem wall of just CMU blocks for the bales. Though I guess bales aren't really excessively heavy (especially when compared to a typical masonry wall)....

Anyhow, anyone have thoughts?

I have not built with strawbale, only stud framing, but could you make a double stack of the CMU blocks so you have your 16"? I personally would not risk that much overhang on a stem wall. Hopefully someone with strawbale experience can give suggestions or advice. :)

The problem is the building will be in Maine, which has a 5 foot frostline.... and I was planning on a basement / root cellar (since it has to go down so far anyway). So if I doubled up the CMU walls I'd have to take it all the way down (so a lot of extra work and money).

There are some designs for ultra insulated stud homes that cantilever the insulation out from the side of the home.
It's called a Larson truss, and it can work on retrofits, so I'm sure you could do something similar when building new.
Here's a example :

I believe from experience that amount of overhang, even with timber would be bad news.
The straw bales will have a lot of weight added with the clay coating.
Having the double thickness CMU basement walls will be the price of building the house properly, it should not be seen as an extra cost. Its the basic requirement for doing it correctly.

John C Daley wrote:I believe from experience that amount of overhang, even with timber would be bad news.
The straw bales will have a lot of weight added with the clay coating.

I'm curious what your experience has been with them. Were you having some fail with overhangs?

From what I understand CMU blocks on paper seem like they should be fine.

CMU blocks are rated at 1700 pounds per square inch compression strength at minimum (and I'm reading that they go up more towards 2500 - 3000 psi).

For a wall of 7 bales high, at 60 pounds per bale (420 pounds) across 2 bricks (since bales are double the length), that's only 1.65 pounds per square inch (without the clay render or considering the roof and posts and all that).

Even if the weight of the coating and internal support was another 3000 pounds, that is still only adding another 12 pounds per square inch.

It just seems like such a minor weight on the CMU blocks. The bigger weight seems like it will be from the rest of the structure (the post and beam and roof and 2nd floor).

I agree with John.  Better to support both edges of the bales (outside and inside) than attempt to cantilever that amount of weight.

Straw bale wall assembles are actually quite heavy--we summarize this in the Structural Design Chapter of the California Straw Building Association's book Straw Bale Building Details: An Illustrated Guide for Design and Construction.  On pages 69 - 73 we detail what's involved in calculating the weight of the full wall assembly, which can range from 45 lbs. to 56 lbs. per square foot.  If you multiply that times your wall height, you'll have some idea of what the linear foot weight is; it's usually over 300 lbs.  The weight of the inside wall surface--just the plaster--is close to 15 lbs. per square foot, or 120 lbs. for an 8' tall lineal foot.   For wall weight comparison, a 6" stud wall filled with fiberglass insulation and clad with cement board siding on the exterior and drywall on the interior is around 12 lbs. per square foot, or 96 lbs. per linear foot.

But if you're using the 8" CMUs for a basement wall, just place your bales on the floor above that, with the inside edge supported by floor joists.  There are three illustrations of that on page 76 of the book, Figures 3-10a, 10b., and 10c.  We build straw bale walls on raised floors all the time.  The exterior edge, which usually plays some role in holding up the building's roof in a post-and-beam structure is well supported by the 8" stem wall, and the considerable loads on the interior of the wall are supported by the floor joists bearing under the interior sill.

Jim Reiland
Many Hands Builders

Jim Reiland wrote:

But if you're using the 8" CMUs for a basement wall, just place your bales on the floor above that, with the inside edge supported by floor joists.  There are three illustrations of that on page 76 of the book, Figures 3-10a, 10b., and 10c.  We build straw bale walls on raised floors all the time.  The exterior edge, which usually plays some role in holding up the building's roof in a post-and-beam structure is well supported by the 8" stem wall, and the considerable loads on the interior of the wall are supported by the floor joists bearing under the interior sill.

Jim Reiland
Many Hands Builders

That's a great idea. Avoids the overhang issue completely. Thanks, I should get a copy of that book.

Though I was hoping to have the floor joists be a bit lower than the top of the stem wall (since there is a basement under). But maybe doing a mini wall inside onto the floor joists? Insulation + wood coming out into the room and topped with boards (likely the base boards on top of the CMU blocks before the straw bale is put on top, since I don't believe you can place the straw on CMU directly and still be code?) to create a strong ledge built on the floor joists and offering more insulation.

A little extra work, but seems like it would create the same effect and avoid any overhangs.

Hi Bob,

The CMU wall can support the floor joist in one of two ways.  When the joists rest on top of the stem wall, the exterior joists are known as rim joists.  That's what we do here when we're building above a crawl space and want to maximize headroom for the crawlspace (plumbers and electricians really appreciate that!).  Then there's the approach we use when building above a full basement, and that's to hang the floor joists along the side of the stem wall so the top of the floor joists are even with the top of the mud sill.  

The CASBA book drawings I mentioned don't give you prescriptive guidance like "use Simpson 2 x 12 joist hangers" or "use pressure treated 2 x  6 mudsills" , etc. because codes, best practice, and engineering requirements vary across the United States.   In some places you might be able to use 2 x ledgers bolted to the stem wall to support the appropriately sized floor joists to carry the span and expected loads.   Typically, there's a floor joist or blocking running underneath the interior edge of the straw bale wall.  Both exterior and interior edges of the bale are elevated on a 2 x or 4 x (depends on engineering design) above the floor, and the space between the sills is filled with a rigid material that both drains and insulates.

Whichever method you go with, be sure to insulate the area behind the rim joist or the underside of the floor adjacent to the exterior wall.   I know a lot of people avoid using rigid foam board if they can--definitely not my favorite building material either--but we use it for below-grade applications.  Or you might consider some form of insulated concrete form block.  There are several polystyrene ICFs available that, by themselves, reportedly give you an R-28 wall at 12" oe 13" thick (8" is concrete, 4" or 5" is foam).  And there are two more natural options rated for below-grade applications.  Out here in the west we have access to Faswall blocks, which are made from wood chips, and I believe there's a Canadian or East Coast distributor for Durisol, another wood chip block.

Finally, Chris Magwood at the Endeavor Centre in Ontario and Jacob Deva-Racusin at New FrameWorks in Vermont have adapted straw bale wall assemblies to increase the wall insulation and move the dew point out of the bales themselves.  It's called a straw-cell wall.  By themselves, the best R-value straw bales offer is with 3-string bales laid-flat.  The bales themselves will be about 23" wide, and that'll get you to R-36 or so.  In really cold places, it's not a bad idea to stack the bales against 2 x  framing that has blown-in cellulose insulation, which bumps the R-value into the R-40s and 50s!   There's a detail of a straw-cell wall in CASBA's book, and a very brief description.  If you want to pursue that, you might give Chris or Jacob a shout.  I have seen designs where the cellulose filled-wall is attached to the sill, and also where it's cantilevered over the foundation.

Jim
Many Hands Builders