in·so·late [in-soh-leyt] verb: to expose to the sun's rays
When planning a cold-climate eco-home, you first insulate and seal, then you insolate. That is, you design the house to capture as much energy from the sun's rays as possible.
The Mill Creek NetZero Home (MCNZH) will be situated on a lot that is 33' wide from East to West. Local bylaws dictate that the sideyards of a house must be a least 4' wide. That leaves 25' of width for our all-important south wall. As you can see from the above picture, we have maximized the opportunity.
Not including the basement, the MCNZH has 274 square feet of south glazing. That's a lotta windows. According to our computer model (using HOT2000 software, which I'll discuss in a future post), they will provide about 3600 kWh of heat energy per year.
It won't only be the windows providing that heat. Our modeling indicates that, given the big windows, the MCNZH will use 1000 KWh energy per year less if it contains a large amount of mass (concrete floors, for example) versus a medium amount of mass (wood frame with extra drywall, for example).
So we're going to pour concrete floors on the main and second stories of this house.
In a regular house with this many south-facing windows, it would get way too hot during sunny days. The living space would rise to 29 or 30 degrees Celsius, and the extra heat would quickly dissipate once the sun went down.
With concrete floors, the mass in the concrete will absorb extra heat and radiation from the sun during sunny days. The living space temperature will go up to, say 24 or 25 degrees on a sunny November day, and the concrete slab will gain an extra degree or two in temperature. Once the sun goes down, the concrete will slowly release its heat back into the living space as things cool down. And presto, solar energy captured and utilized!
Up until you reach 8% of a home's floor area in south glazing (that would be 80 sq. ft. of glazing for 1000 sq. ft. of floor area), you don't need to add extra mass to stay comfortable and capture the energy of the sun. However, "when the ratio of south-facing glass to floor area exceeds 8% [, it is] necessary to include more thermal mass in a conventional wood-frame house" (p. 16, Tap The Sun: Passive Solar Techniques and Home Designs).
You increase the mass to increase comfort (the mass absorbs heat during the hottest times of the day) and to capture solar energy for use in the evening. The MCNZH has about 2000 square feet of living area. With 274 square feet of south windows, that's 13.7% of the home's floor area in south glazing.
We need to add mass to make this home comfortable and to maximize its solar effectiveness. The concrete will also make the home extremely soundproof (good if we have tenants downstairs) and the dyed, etched concrete floors will be beautiful and unique.
The only problem is the very high embodied energy in the concrete. That's something we'll have to live with.
I think that the benefits are well worth the cost. Especially given the fact that the concrete floors and windows are a completely passive solution - they provide heat and comfort year-round with no moving parts and no external energy use.
Note: I had a long (ongoing, actually) discussion with Peter the MCNZH builder about whether adding concrete floors was the right thing to do. In explaining my rationale, I wrote a position paper on the subject. For the keeners out there, I've attached it to this post.
(cross posted at raisingspaces.com)