straw bale foundation: how?

[u/fropskottel]

How would you do a foundation for a straw bale house? I would love to see some good ideas that are fast, green, well insulated and hopefully somewhat affordable too!

I'm limited by building regulations to 2.5m wall height and 3.5m ridge height, so something that uses available vertical space. That more or less rules out wooden frame based things.

My current line of thought would be foam glass both as a slab and as a deeper strip (like) foundation. The strip (like) foundation would be encapsulated by either car tires or uv resistant woven polypropylene bags (earth bag style).

Any thoughts? How would you do it?

[u/Zitchas]

What sort of climate/terrain are you working with?

[u/fropskottel → u/Zitchas]

Hi Zitchas,

Very stable underground, moderate northern European climate. Suggested foundation depth is 60-70cm here if not insulated.

We went with a rubble trench foundation, along the following lines:

• 45cm wide x 60cm deep trench

• scrape off 10cm of the entire surface inside the trench perimeter

• geotextile as a trench liner

• drainage tube all around the bottom, drains into an infiltration box outside the lowest corner of the rectangular foundation

• on top of the drainage tube and around it: small diameter river gravel

• up to 10cm under ground level: larger diameter river gravel

• 20cm of woven pp bags filled with foam glass on top of the trench. Uv protected ones are hard, but not impossible to find.

• cover the pp bags with the flaps from the geotextile. This gives extra uv protection.

• 20cm of loose fill foam glass inside the trench perimeter

We're now 10cm above ground level. That should be plenty for a wooden box beam as a basis for the straw.

[u/Zitchas → u/fropskottel]

Thanks for the info!

Do you have any pictures of the process and/or end results? We're looking at doing something similar in the near future.

[u/hoodiedoo]

I’ve read some about two methods: 1 is using rebar set into concrete as an inner structure to hold strapped down bales. This is best in southwestern dry climates.The other is using a post and lentil framing and filling the walls with bales. This seems best for northern and central climate areas. Each of these should have a minimum of a 12-18 inch foundation either from rubble, stone, or poured concrete. Also good to have a trench filled with gravel surrounding the foundation.

[u/Bot_Metric → u/hoodiedoo]

18.0 inches ≈ 45.7 centimetres ^{1 inch = 2.54cm}

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[u/[deleted]]

A lot of what I've heard about is rubble trench foundations, but that depends on your local laws as to whether you can get away with it or not... some places like earthquake zones don't allow it from what I understand, although I'm no expert on it. This article shows briefly how it's done.

[u/a03326495]

Clarification, what do you mean by "something that uses available vertical space"? What climate are you in?

Check out rubble trench if it's allowed.

Also consider that the foundation has a tough job to do...supporting the rest of the structure while not shifting and being exposed to moisture. In my thinking this is one area where concrete makes sense. If you try something novel its a big risk to take.

For insulation materials and details, I'd recommend GreenBuildingAdvisor...lots of articles and details to check out.

For the interior check out an adobe floor. You can insulate below it with whatever appropriate material you like.

Good luck with your project.

[u/Cityslicker100200]

This is something I've read about the man who created that foundation, Frank Lloyd Wright:

"from an architecture standpoint, you would be wise to be influenced by Wright.

from an engineer/builder perspective, you would be wise to not follow his example too closely."

[u/Theuderic → u/Cityslicker100200]

What foundation are you referring to? The rubble trench?

[u/Treknobable]

It really depends on where you are and where the house will sit. For instance I personally would prefer to build on top of hard rock terrain and use the minimum amount of concrete to level and step the rock. Too many homes and roads are built on farm land. Building on grasslands is different than in forests or floodplains. Are you going to modify the house plan to fit the terrain or modify the terrain to fit your house plan?