For building a long lasting, relatively low cost, high thermal capacity and long lasting earth sheltered home, concrete is my preferred material of choice. I have seen many concrete designs from small earth sheltered homes made of 12 ft diameter precast concrete drain pipe designed by Michael Janzen to the very sculptural earth sheltered homes designed by Peter Vetsch. These designs look great, but probably wouldn’t perform very well in a norther climate where their solid concrete walls would make it very difficult to maintain a thermal differential between inside and outside. In other homes where the designers have striven to separate the inside from the outside thermally, there may still be some direct connection points that allow heat to conduct outward. This path for heat loss is known as a thermal bridge and it is a serious problem in concrete construction…
I like how Joseph Lstiburek puts it at the start of his building science.com article on the subject;
If an alien from another planet looked at our construction practices he would conclude that we have too much heat in buildings and we want to reject that heat to the outside. We expose our concrete slab edges and our concrete frames. We build our structures like heat exchangers with protruding fins that transfer every last available BTU across them—like huge concrete “Harleys” with air-cooled structural frames ~ Joseph Lstiburek
In this somewhat humorous article, Joseph is talking about high rise buildings where the slab for each floor also forms the balconies that cantilever from the building. This is a bit of an extreme case. However, I have seen cases where earth sheltered or earth bermed homes are constructed with some exposed structural concrete or perhaps with a steel stud front wall leaking heat, which I discussed in an earlier post.
He ends with this;
Who says we have to live with those thermal bridges? You want to get serious about energy efficiency? Get serious about thermal bridges. That means exterior insulation on steel studs and structural frames, off-set relieving angles for brick veneers and some serious structural-thermal thinking for balconies and projecting structural members. We mechanical engineers are going to have to get to know those structural engineers better. And then we both have to have a chat with the architect. Some interesting times are coming . . . ~ Joseph Lstiburek
The point is that, whatever you build, you need to be very careful about thermal bridging. This is even more true if you are building in the earth with the purpose of energy conservation. It would be a shame to get it 99% right, and then lose much of your heat thru that one thermal bridge you didn’t worry about…
I saw pictures of a concrete dome home in Colorado (not earth sheltered, but could have been). The owners had constructed the dome out of cement, then added several inches of insulation over “almost” all of it, and then covered that with a second layer of steel reinforcement and concrete. There was one spot at the peak of the dome where a steel connection plate connected the steel reinforcing between the inner and outer concrete shells. This one square ft of steel prevented any insulation from being placed between the inner and outer domes at that location. The insulation covered 99.9% of the dome, but that one spot was a steel-reinforced concrete thermal bridge. Even worse, because it was the structural connection to the reinforcement on both sides, it was essentially the highly conductive hub of a heat collection and distribution array. It allowed heat to efficiently travel around the carefully placed insulation and radiated enough heat to keep a 5ft radius area on the roof snow free all winter.
Earth sheltered homes have some advantage, at least for the earth sheltered parts. Earth sheltered builders can use a continuous umbrella of insulation to cover the majority of the home without being too concerned with structural attachments etc. The weight of the earth (gravity) will keep the insulation in place (assuming you have been careful to design for stable soil, including looking after erosion, angle of repose, etc.). Gaps in the insulation can fill with dirt which is considerably more conductive, but not nearly as bad as a steel/concrete thermal bridge. This gap problem can be prevented by offsetting the layers of insulation and using sheets of plastic between the layers.
However, most earth sheltered homes do still have some “above ground” portions. Many don’t seem deliberate enough in their efforts to prevent thermal bridging.
If you need to connect concrete on either side of the insulation your engineer may specify rebar. It is not the end of the world if a little energy is leaked thru the rebar thermal bridge, but it would be even better if you could reduce or avoid this. In my case, I had several horizontal concrete structures that would need to be outside of the insulation and would carry spill over earth. I managed these in two different ways.
For the eyebrow sunshades which needed to cantilever out away from my building, I extended the structure backward to more than counterweight the cantilever. The eyebrow concrete is completely isolated from the structural concrete of the home. The majority of the eyebrow structure will be buried under the earth the on the roof. The weight and shape of the eyebrow should be more than enough to keep it in place. If I need to add any rebar, one option would be to use fiberglass rebar which would be much better and preventing heat loss.
Instead of a parapet (which can be a huge thermal bridge if not designed carefully), I have chosen to go with a horizontal sunshade and slope the earth down on to it. This will make the green roof more obvious from the front, instead of hiding it behind a high parapet. But I needed a way to support that sunshade against the front of the house. Instead of cantilevering it out and worrying about such heavy loads hanging from the front wall of the house, I designed separate foundations and columns that carry all the load. The overhang appears against the house, and the columns appear as pilasters of the main home, but thermally and structurally speaking, it is a totally separate structure.
(work in progress, images to come, etc)