(MCNZH concrete floor being bathed by sun through a 9’x6’ window)
The most important design considerations for cold climate building are insulation, building envelope, and passive solar design. Given our lofty goals for the Mill Creek NetZero Home (MCNZH), we pushed hard to maximize our return on every one of these fronts.
The MCNZH collects 54% of its annual space heat through passive solar design – that’s 8747 kWh or 31.5 Gigajoules. It does so by:
- having huge south windows that are specially manufactured to maximize solar heat gain
- containing a large amount of thermal mass to absorb the solar heat when the sun shines
- having movable solar awnings that allow 100% of the sunlight to hit the windows during the heating season (the awnings are strictly speaking not a passive part of the solution).
Passive solar design is a very simple concept that is encapsulated well in this picture (source):
The idea is to add a lot of south-facing windows to your home, allow sun to shine through them during the heating season and capture the solar heat in some form of thermal mass. During the summer and fall, it’s imperative that you block most of the sunshine to avoid overheating. The beauty of passive solar design is that you get cheap solar energy with a system that has no moving parts.
The MCNZH was designed for solar energy collection. With an east-west width of only 25 feet, we had to cover every square centimetre with windows, PV modules, or hot water collectors:
(MCNZH, as designed)
(MCNZH being built)
The site is wonderful for solar – there is almost complete solar exposure throughout the year.
To maximize passive solar production, we made the south-facing windows as big as possible. They main floor windows are 9’x6’ each and the second floor windows are 10.3’x6’ each. That’s a total of 232 square feet. The windows are triple-paned and they have fibreglass frames. They have insulating spaces where the panes of glass are separated along the frames, and the cavities between the panes are filled with Argon gas.
Most importantly from a passive solar standpoint, the Low-E coatings on the glass have a solar heat gain coefficient (SHGC) of about 0.57. That means that the windows allow more solar heat to pass through them than the east, west and north windows. It does so at the expense of insulating (R) value (the Centre of Glass R-value for the south windows is 5.7 vs. 8.3 for east, west, and north windows), but the extra heat loss is more than compensated by extra solar heat coming in when the sun is shining. One always needs to make a compromise when choosing south-facing windows. We could have had a higher SHGC with dual glazing and a lower R value and still come out ahead on an annual heat loss basis, but the house would have been less comfortable on winter nights and have more condensation on the windows in cold weather
Thermal Mass (Concrete Floors)
On a sunny day during the heating season a super-insulated house like the MCNZH will be entirely heated by the sun. However, in some instances it will be overheated. Those huge windows let more solar energy in than is needed. It would be great to store the excess and to reduce overheating, so we added thermal mass to the home. We did so by pouring concrete on the main and second floors of the MCNZH. The concrete weighs 10-12 metric tonnes on each floor, so we had to design a beam down the middle of the house to support the weight.
As I mentioned above, the passive solar design accounts for 54% of our heating needs (8747 kWh or 31.5 Gigajoules). However, if the design incorporated the big special windows alone, with no extra thermal mass, solar energy would only provide 41% of our heating needs (6583 kWh or 23.7 Gigajoules). So the concrete floors will provide a net benefit of 2164 kWh or 7.8 Gigajoules annually.
(installing concrete floors - MCNZH)
Technology does not have to be complex. In fact, the simpler it is, the more resilient and cheaper it is. Passive solar design is as simple as it gets – as long as the MCNZH stands, it will collect almost eight Gigajoules of solar heat every year with nothing to break and no parts to replace.
The mantra for solar homes in the warmer United States used to be “mass and glass” (lots of south facing windows and thermal mass). Up in Canada we realized at some point that a performant home should be “light and tight” (insulate and seal a house, forgetting about passive solar design). Now we are proving that the optimal solution is “mass, glass, tight, and thick” – passive solar design, a very well sealed building envelope and super-insulated walls.
(cross posted at raisingspaces.com)