Engineered shell + natural interior

I wanted to use a sustainable approach to build a low-energy house. Natural wall systems appealed to me, so I did a lot of research and built straw bale, cob, and cordwood masonry experiments. However:

• Straw makes me sneeze and wheeze. I was also concerned about moisture and the large footprint required.
• Cob (like adobe) is attractive but doesn't have the R-value I wanted for exterior walls. I'm in Indiana and wanted an insulating shell with mass on the inside.
• Cordwood masonry would require endless caulking, as my cordwood duckhouse quickly proved.
• These wall systems aren't respected by most banks. I was concerned that if I needed to sell or borrow against my place, I would have trouble finding a bank to mortgage it.

What to do? Pick an insulating wall system the banks will approve of, then do what I want with the rest of the house.

SIPs shell: high performance, easy approval

I chose an engineered wall system based on oriented strand board (OSB) and expanded polystyrene foam (EPS). The foam is sandwiched between sheets of OSB to form structural insulated panels (SIPs). It's greener than it sounds--good R-value, tight, little offgassing, little waste, recyclable, fewer trees used than in many other forms of construction, etc. More on SIPs.

With good windows and a metal roof, this gives me a low-maintenance shell that

• will require little heating,
• pleases the code folks, and
• was easy to insure, so would probably be easy to finance.

With SIPs as my insulating shell, I could use cob and other natural materials to provide interior mass.

Natural interior, sort of

I originally planned an almost entirely natural interior. I was planning to use cob to create sculpted benches and earth plaster to finish the walls. However, I grew fond of my minimal and easily moved furniture, and the floor clearly provides enough mass, so the cob benches are unlikely to appear. My plaster experiments failed, so I put up drywall.

I'm still hoping to use materials from my woods or slabs from a local sawmill for window and door trim. I'm considering a fat, very strong grapevine for a stair banister. Three cedar trees form support posts downstairs.

The downstairs floor is naturally stained concrete scored to look like big tiles.

I'm hoping the end result will be a cottage with a natural, old-world feel, low energy use, low maintenance, and few hassles on resale, if that becomes necessary.

The basic design

• 20 by 32 footprint on a concrete slab
• cottage with 1.5 stories, gable roof + dormer
• about 1100 square feet
• open floorplan downstairs; bedroom, office, and bath upstairs

I've hired a local contractor to build the shell and rough in the plumbing and electric. I'll do the rest.

The south windows already at work during construction in December. The OSB on the floor is temporary, protecting the stained slab.

My main goals and my attempts to meet them:

Low energy use; silent radiant heat

• tight wall and roof system with a good R-value
• extra south windows with slab to store solar heat
• efficient woodstove
• zoned backup heating with propane radiators
• tankless hot water heating (propane)
• efficient fridge and clothes washer; no dryer, dishwasher, garbage disposal, etc.
• efficient windows placed to get summer breezes
• compact fluorescents
• switches to cut power to always-on appliances (e.g. stereo clock)

Minimal site disturbance (lots of trees)

• small footprint that avoids root zones
• slab instead of basement

Good indoor air quality

• solid walls with no fiberglass or other dusty insulation
• propane appliances are direct vent, sealed combustion
• outside air is ducted to the woodstove, though the need for this is debatable
• low- or non-offgassing materials:
  • the OSB in the walls uses low-formaldehyde resins and will be sealed and covered with drywall or plaster
  • the EPS foam doesn't offgas after manufacture
  • furniture and interior doors mostly salvaged, solid wood
  • concrete and wood flooring; no vinyl
  • maybe clay paint (alís) for drywall
• no basement or crawlspace
• no carpeting
• efficient windows to reduce condensation and mold

Few resale or code hassles

• wall and roof system that's already accepted by code people and banks
• mostly conventional plumbing (mound septic system)
• wiring and plumbing in place for the appliances that more normal people will want but that I don't use