Zone 7, North Georgia Straw Bale home

At the present, working on the plumbing and rubble foundation for a straw bale home. Hope to place a rocket mass heater in it for winter heat. It will have a metal roof with an air gap under it to assist with cooling with a cathedral ceiling with a one foot truss system with cellulose insulation and some radiant heat insulation. With the R45 straw bale walls and near R40 in the roof, should I look at insulating or plastic barrier under the cob floor or just gravel and landscaping fabric??? I have seen both in various places. The building is on top of a hill, so no true issue with water coming up from under, so leaning toward the landscape fabric. Any other pieces of advice would be helpful as well. Regards, Rich.

Steve, you're in building climate zones 2, 3 or 4 depending on what county you are in....click on the map to see the requirements: https://energycode.pnl.gov/EnergyCodeReqs/?state=Georgia

https://law.resource.org/pub/us/code/safety.html

It appears your state has adopted energy code 2009 IECC which dictates alot as you can see to satisfy building permits. Strawbale code is in 2015. Before you get too far along you may want to check with your building and safety office. In code you'll also find vapor barrier and retarder requirements for the walls, roof, floor, that do not work with strawbales, natural floors, roofs, etc....bale code makes it easier.

R-45 is optimistic for SB walls, more like R-30ish depending on density and straw direction. R-45 sounds reasonable for Z-4 roof, but be aware blown in cellulose settles and looses whole r-value. The main purpose of the gap (3/8 min) under roof cladding is vapor control and drive by pressure and temperature differentials, house wraps, water and ice, do not leave enough of a drain plane and can cause heat and condensation build-up, so water gets trapped against sheathing/claddiing that can drive to the interior through holes, rot structure and cause mold health issues. Whether it aids in "cooling" will depend on the time of year, materials and design of your vaulted roof......The interior material coupled to indoor air, and in some cases isolated/insulated from the exterior, sun orientation, has the most profound effect on cooling and heating loads, clay and lime plasters, wood, would be good examples. R-value does not mean as much with them, but unfortunately the inspector that probably does not understand has to be satisfied, or you hire an Engineer to stamp drawing's which may be hard to find too.

Don't mean to rain on your parade, but unfortunately those are the hurdles we natural builders have to overcome.

Hi Steve, Terry makes some very good points about codes and standards in your area. It would behoove you to read up on them a little to see how they can be tailored to your Straw Bale plans.

One point I that caught my eye was your question regarding a barrior under the cob floor. I am an advocate of having a moisture and/or insulating barrior under a poured floor. This allows you to more easily control moisture/humidity levels in the living space, which translates into controlling the temperature as well. If you plan on heating the floor with radiant heat for back up, then you definitely will want insulation board under the floor. Heat is drawn to cold and you don't want to be paying to heat the earth! The old school of thinking was that warming up the earth under the house will offer comfortable returns within the living zone. It just ain't so.