Timeline - Design Evolution - Foundation Walls

Past two years

Uninsulated Foundations
Nothing riles my sustainability sensibilities more than the sheer ubiquity of above-grade block and concrete foundations.  "Walk-out basements", with even more exposure, are prized even if they face northwest.  

Walk-outs facing northwest.

Many newer houses have retrofitted interior insulation, especially for finished spaces,  While better than no insulation, the thermal mass of the concrete ends up on the wrong side of the insulation for maximum performance.

A serendipitous example of heat transfer (conductive heat loss) through an uninsulated

foundation is seen in the photo to the right. Dorothy planted a tropical plant (Christmas cactus) next to the house never expecting it to make it through one winter, much less five. The heat conducted from the basement warms the soil enough for he plant to survive the cold season. In the other picture, notice how the snow melts first near the wall. The tan remnants of the cactus on the ground at the top of the photo indicate dormancy, not death.

Our Problem
Over the past two years, I have spent an inordinate amount of time worrying about the design of the foundation under the stick-built walls.  As our original concept of earth-sheltering morphed from earth contact on the north and west sides of the house into earth sheltering on on the north side only, the need for stick-built walls increased. And more stick-built walls meant more energy-problematic foundation walls.

Protecting the Thermal Mass

A lot of the heat that the AGS System generates and stores under the floor would bleed out through the foundation walls and be lost unless the entire foundation is insulated.  But doing this on a low budget is a challenge.  I looked at several ways.

Dry-stacked cider blocks would work structurally and affordably (per Rob Roy in his book, Earth-Sheltered Houses- How to Build an Affordable Underground Home), but we would be back to the waterproofing and insulating conundrum he wrestles with. The Complete Block System would work well and, like the cinder blocks, would be DIY-friendly but too expensive.  A DIY-poured concrete wall in rented forms would not be a bargain, would be lots of work, would be too tall for the frost-protected shallow foundation (see below) and has the same bad choices for insulating and waterproofing as the cinder blocks.

Insulated Concrete Forms

As it turns out, the best value for the foundation walls for us will be concrete poured in insulated concrete forms (ICFs). Setting up the forms, which stay in place after the pour, is surprisingly DIY-friendly.

The ICF walls with 8" of concrete and 5" of Styrofoam will be about 13" thick and will fit nicely with the width of the stick-built wall trusses above them.  And ICFs automatically provide slab edge insulation.  This is a big deal because insulation of the slab edge is important for energy conservation and is one of the things that green building certifications such as LEED and HERS highly value but something that normally is difficult to achieve in the field.

Conventional Foundation

A conventional foundation wall in conjunction with a slab floor in our climate must rest on a footing, the bottom of which must be below the frost line -- 30" below grade. Without proper insulation, the foundation is an energy nightmare, losing heat through the wall and, in the process, sucking energy from the adjoining slab.

Frost Protected Shallow Foundation

In our situation, there will be a footer and no insulation under the slab

However, if the wall is insulated on both sides and insulation is laid horizontally over the footing at the base of the outside of the wall and extended outward for a couple of feet, the footing becomes frost proof and can be raised above the frost line and the wall on it shortened as well. The result is a "frost protected shallow foundation".

For us, ICFs are a reasonable solution.  They provide the insulation for the wall above the footing while the insulation in the insulation-watershed umbrella for the AGS system serves also as the horizontal insulation over the footing that characterizes the frost-protected shallow foundation.