Spray-foam insulation over outsides of earthbag walls?

I have a vague memory of reading somewhere that foam insulation could be sprayed on the outside of an earthbag house.

Does anyone know how (or if) this would work? Could you stucco over the insulation? Would it have to be a natural earthen stucco, or a concrete-type one, or would either work? Would the foam insulation breathe, expand and contract appropriately, etc?

It's my understanding that the spray foam insulation has to be protected from sun and light or it will deteriorate. Maybe check with your local lumber yard or hardware store.

Sarah Tennant wrote:I have a vague memory of reading somewhere that foam insulation could be sprayed on the outside of an earthbag house.

Does anyone know how (or if) this would work? Could you stucco over the insulation? Would it have to be a natural earthen stucco, or a concrete-type one, or would either work? Would the foam insulation breathe, expand and contract appropriately, etc?

I think I saw that in a youtube video and was stunned that it could be done. I'll see if I have it saved somewhere.

I wouldn't do this. Spray foam traps moisture and defeats the entire purpose of earth building.

Michael Helmersson wrote:

I think I saw that in a youtube video and was stunned that it could be done. I'll see if I have it saved somewhere.

It may have been in this video:  

I recall it being an old project that was being revived and the guy was pointing out the unexpected survival of some sort of spray foam on the exterior.

What if I used an earthen plaster on the interior walls, so the earthbags could breathe that way?

I've seen a method of earthbag construction where the earthen plaster is applied at the same time as the earthbags are laid (like, a few people are laying higher courses while other people start throwing mud on the lower courses. The idea is that even though the bags have to cure, leaving the other side of the bags unplastered for a few months allows the moisture to escape that way, so it doesn't matter that the plaster's applied 'too soon' on the first side.

By the same logic, even if the earthbags couldn't breathe against the spray-foam insulation, if they could breathe against the interior earth plaster, wouldn't that be fine? Obviously we would cure them before applying at least one of the layers.

My current thinking is that I like the look, cost, DIY-ability and thermal mass of earthbags, but am worried about their lack of insulative properties. I'm pricing up bulk pumice, but I suspect it'd be too expensive to use to fill the bags. (Haven't priced up scoria yet, but again, I suspect where I live it wouldn't be financially feasible; likewise vermiculite, and I don't think we could even get rice hulls here at all.)

So if I could do earthbag walls and spray insulation over the outside, and then cover that (quickly, so it wouldn't be exposed to heat and light for too long) in plaster/stucco, possible with a stone facade for the bottom few feet... well, that could work well. Assuming it does, uh, work at all.

We built our home with earthbag about a decade ago.  

Sarah Tennant wrote:What if I used an earthen plaster on the interior walls, so the earthbags could breathe that way?

Vapor sealing the outside and leaving the inside open to breath would probably create a good terrarium.   In our climate we would grow mold everywhere.  Especially in unmentionable places.

Sarah Tennant wrote:I've seen a method of earthbag construction where the earthen plaster is applied at the same time as the earthbags are laid

We did that.  Did not notice any problems with it.  Makes timing the build a little more difficult.

Sarah Tennant wrote:thermal mass of earthbags, but am worried about their lack of insulative properties.

We are on the equator.  What we have found is that earthbag is a thermal battery.  It stores up heat throughout day, and releases it during the cooler times.  Same is true for cooling.  Cold absorbed at night is released into the house during the day.  In our location the temp lag is about 8 hours.

Do not discount the effects of mass.  You may not need any insulation.   I built one dome as a prototype first.  We lived in it while we built the big house.  Now I use it to store batteries / solar.   Lessons learned from that little outbuilding made all the difference on the forming of our house.

Sarah Tennant wrote:then cover that ... in plaster/stucco, possible with a stone facade for the bottom few feet

Our earthbag material is a mixture of 3 parts basic river sand to 1 part clay from our rice field.  Then we stabilized that with one part of portland cement.

We left the form sack material on.  We did an earthbag material fill coat onto the sacks to smooth out the dome shape.  Heavily scratched for a mechanical key.  

Then went on top of that with a Lime plaster that is 1:3 lime/river sand.  The lime we get here is "fat" lime.  Limestone primarily from sea shells.  I will stress that for our plaster and washes we used "quick-lime" not hydrated lime.  We had lots of problems with lime until we switched.

Rough finish the plaster so the exterior coating has a good mechanical key.

We tried all sorts of different ratios and mixtures for our exterior lime coating.  So far the best performing one has been a mixture of 1 part freshly slaked quick lime to 1 part exterior latex paint, to 11 parts water.  Super thin with lots of coats on successive days.  Thorough wet down of the area to be painted before, during, and after painting.

The latex cuts down the vapor permeability a bit, but so far it is not detrimental as far as I can see.  


Hope this helps.  Good luck.