Jay Angler wrote:Thanks for clarifying. It's always been unclear to me whether it is standard for there to be insulation under the floor of a Wofati. I live in a house on a un-insulated slab so instead of it acting as thermal mass, it just slowly leaks the heat. I know two people locally who actually jack-hammered their slabs inside their walls, put a thermal break against the wall, insulated under the floor level, and re-poured the floor. They both noticed a dramatic improvement in comfort level, but talk about mess and cost!
Myself, I do not see a WOFATI having its best attribute as being efficient to heat. I say that because my old house is a 2000 sq feet, super insulated with geothermal heat. Because that home is unoccupied, I shut off the heat and found even in temps down to -10 degrees (-35 degrees with the wind chill) it never got below freezing inside the house. That is pretty amazing. That is because the concrete slab it sits on, is protected from losing its cold around the outer parameter via insulation, but exposed in the center where the ground temp is a constant 57 degrees. However, if the temperature outside warms up...it is often cooler in my home. In other words, the house does not shift tempertures quickly.
This is basically what a WOFATI is, only it uses soil to be super insulated.
There the similarities end.
Because the home is underground, it thereby is 57 degrees year around, and has a lot more mass than my conventionally built home. However, like my home, 57 degrees is NOT warm enough for human living. Unfortunately I cannot possibly see how passive solar is going to be able to bring a WOFATI up to temperature without supplemental heat. There is just too much mass that is 57 degrees. There is just not enough passive solar gain to heat that much mass up to keep a home warm; not with limited amounts of sunlight, and limited amounts of glazing. Then there is the upper threshold of humans; that is we just cannot stand being too hot, which is really what is required in order to heat up the mass of the WOFATI; elevated temperature for long periods of time to bring the mass of the WOFATI to temperature (70 degrees).
But that does not mean I do not like the WOFATI design, I just see its best attribute being something different, something a conventional built, super insulated house cannot compete with. That is a location with spectacular views, but subjected to high winds. I have a location like that, and really it is a horrible location to put a home upon, except for...it has a stunning view!
But how to deal with heating it?
Since a WOFATI will need supplemental heat most of the year, and sunlight is available limited times, and glazing area is limited, I see active solar as the answer. By building a solar furnace outside the structure, and collecting and bringing the heat into the building, it could thus adequately heat the structure. The amount of collection surface would thus not be limited, and the WOFATI would have a lot more leeway in its overall design. But the building would be protected from high winds! That is what my wife and I want; to sleep like a snug bug in a rug without feeling like we are going to get blown off this hill. Living underground can give us that.
A sincere thank you to all of Permies Forums for making Christmas special to Katie and I, and our four daughters. Thank you!