Seeking advice on waterproof strawbale foundation for a yurt

Hi all, we are having a yurt built, for which we are thinking of making a circular straw foundation on gravel. We live in temperate northern Europe in Lithuania where we get wet cold winters and hot summers. We can't build a solid foundation like concrete where we plan the yurt and building a platform on screws seems more complex than using strawbales, which are easier to come by, more eco-friendly and affordable. I have attached a drawing of how I currently see the layerings (or see image below).

We are not sure if this is a safe option to protect the straw bales from both mice and moisture, and three questions arose: First, would a layer of rodent mesh on the ground alone be enough, or should the sides be protected as well if we were to add wood siding, if so, how far up? Second, does anyone know of alternative ways to protect the straw from moisture from something other than a tarpaulin or using what we already have: wooden beams, paneling, and lots of thick meter-long boards of various thicknesses and heights (see photo)?


8meter² tarpaulins are expensive and considering that they don't sell them in round or cylindrical forms I'm thinking there must be a simpler and possibly cheaper alternative.

Third question, I am thinking to have the wooden part of the platform sit on the straw bales instead of sandwiching something into the bales to screw into, thinking that the weight of the boards and plywood would be heavy enough so as not to move. Is this ok, or am I oblivious to a vital flaw in having them separate?

Straw bales in walls are under compressive load and this keeps them stable. Maybe you could poke some steel rods with threaded ends all the way through the bales. Then put plates to spread the load on the outside and tighten down nuts to provide compression. That should tie it all together...you could also run a couple of bands (big ratchet straps?) around the perimeter.

Hi Phil, my initial thought was to tie the perimeter with ratchet straps, and lastly to tie it down with some heavy-duty rope and to tighten it like a wine cap before removing the belts. We're praying we'll be able to get a circular platform from rectangular hay bales, we're just worried about mice and humidity.

Hi Maruf,

Thanks for your interesting proposal—thinking outside the box pushes the envelope of how we can make better buildings.  And, my instincts tell me that this isn’t a long-lasting foundation.  If you just need to insulate the yurt floor for a season, go for it.  Plenty of people have used abundant straw bales and other found materials to craft temporary emergency shelter.  But I believe that the building science is against your design providing long lasting effective insulation.  I wouldn’t do this unless it was for temporary shelter only, or if not, that I could afford to do it over.

This design creates a “bathtub” with the bales (supposedly) sealed from below and the sides.  Unless designed for puncture resistance, most tarps I have worked with don’t hold up well when compressed against gravel, especially if the gravel has sharp edges that can tear the material and admit ground moisture. And if the membrane remains water proof (not punctured) it may cause water vapor to condense in the bales.  I’m not sure what would drive warm, moist air from inside towards the cold exterior membrane, but it’s a risk I wouldn’t take.  If liquid moisture should get into the bales from a spill in the yurt, it’s probably going to stay there for a long time—long enough to begin decomposition.

Unless there’s another structure bearing the yurt’s weight, as the bales settle so will the yurt.  Straw bales in a load-bearing wall assembly do well in compression, but the roof assembly is a comparatively light weight part of framed building.  Yurts are much lighter of course, but you’ll need to compare the respective weights of a framed roof assembly with a yurt, people, and all their belongings—it may be more than they can bear.

Finally, the gap between the floor deck and top of the bales will likely develop convective loops which will greatly reduce the insulation value.  These loops form when air can move between warm and cold surfaces, carrying heat away from the warmer surface.

As for pest intrusion, depends on the kinds of pests you have.  Where I live, I’d place galvanized hardware cloth under the entire platform and up the sides to the roof deck to keep mice and ground squirrels out.  I have seen mice and rats scale 3 meter tall plastered straw bale walls to access holes where the roof eve meets the wall!

I live in a temperate part of North America—wet, cold winters, hot summers. I wouldn’t attempt this design.  If I had a client who insisted on using straw bales to insulate a yurt floor I’d need to point out that having an R-20+ floor insulation in a structure with R-10 walls (at best!) doesn’t make much sense. Heat loss and gain is primarily through ceilings, windows, doors, and walls.  Floors account for a relatively small amount of heat loss or gain.  If they still insisted, I’d propose a platform supported by floor joists spaced so that straw bales could be tightly packed between the joists.  I’d keep the bottom of the bales at least 30 cm above grade, and staple hardware cloth to the underside of the joists to exclude rodents.  If the design fails because the bales deteriorate I'd be prepared to drop them out and replace with an insulation that can tolerate damp conditions.

Jim

Jim, what foundation would you use for the floor joists in this case?
The PO gave definite resrictions that limited any ideas I have.

Good question John.  If a "solid" concrete foundation--which I interpret to mean a slab--isn't an option, and screws are too complicated, I'd used piers, assuming the site soil can bear  the weight on a dozen or more footings.  It's still concrete, but a lot less.  And it could be stone if that's locally available and abundant, or urbanite (re-purposed concrete).  The floor joists would need to be deep enough for the bales to fit into, and some engineering might be involved to design for equal load distribution of the deck assembly and yurt weight on the posts, as well as anchors that hold the posts to the piers.  

Jim Reiland
Many Hands Builders