Did you know that a house in Michigan can heat and cool itself without a furnace or an air conditioner? By using the sun, the earth, and prevailing summer breezes, we can heat and cool a house using nothing but passive energy flow.
The standards for passive houses were developed in Germany by the PassivHaus Institute. Inspired by passive house in Saskatchewan and another in California that were built after our first energy crisis in the mid-1970’s, Dr. Wolfgang Feist, a German physicist, founded the Institute in 1996.
Passive Houses meet incredibly rigorous requirements for their air infiltration rates, insulation, and total energy use. In fact, a 2000 square foot house can only use 22,200 kilowatt-hours per year. That is equivalent of twenty-five 100-watt lightbulbs being on in a year, and covers all heating and cooling, and all electrical loads. That is without using solar panels, and without using a geothermal heating system, only passive methods.
If you can do it in Germany or Saskatchewan, you can do it in Michigan. Passive houses will have very high R-value walls and roofs, be incredibly airtight, and have “super windows,” efficient windows with a U-value of 0.14 or lower. As tight as a Styrofoam cooler, they must have active heat recovery ventilation (a source of electrical use and some heat loss that will cost many of those light bulbs).
Passive houses rely on the sun for winter heating, and will work better if there is a lot of thermal mass in the house. The thermal mass stores excess energy and radiates it at night, when heating loads begin to climb again. Trombe walls, shown in these diagrams, can help store solar energy and cause thermal currents to distribute it within the house:
Interior Trombe walls can be painted black on the window side to absorb maximum solar energy. The sun can also shine on a dark-colored concrete floor or an interior concrete wall for greater energy gain. Any thermal mass on the inside of the home, however, will help to store and modulate heat content.
Houses can also benefit from thermal mass without Trombe walls. If the sun can shine on floor and wall surfaces with mass, these too will absorb that energy and release it slowly after the sun goes down.
It is important to note that windows must be shaded in the summertime. A Passive house can overheat if too much sun enters the building when the ambient temperature outside is warm. We must keep the sun’s rays from entering the house in warm weather.
Thermal mass can have a strong benefit in the summertime. By designing the house to capture prevailing summer breezes, and an outlet on the high leeward side of the building, we can suck hot air out of the house in the late afternoon and cool the structure of the house with cool nighttime air. The thermal mass of the house can store the cool of the nighttime air and keep the house cool if the windows are closed during a hot day.
When you build a very efficient thermal envelope, normal windows jump from being about 20% of the total heat loss to somewhere around 50% of the total heat loss. Through the use of advanced windows, we can bring that back down in proportion with the low energy use of the home.
These windows must seal well and have an insulated frame as part of the assembly. They can also be triple or quadruple-paned with xenon gas instead of argon gas between the panes. Xenon is a larger inert gas molecule than argon, it will distribute itself in the pane better and will not leak out of the frame nearly as easily as argon.
The glass itself will have high-performance coatings as well. These coatings are tuned to the orientation of the window. Typically we want south-facing windows that let the Sun, and it’s energy, shine on in. East and west windows should reflect the sun’s energy so that we don’t overheat the house in the summertime. East, west and north windows, and those under porches, should keep as much heat in the home as possible.
A typical low-e, argon filled window manufactured in the United States will have a U-factor of about U-0.32. U-value is the reciprocal of R-value, so U-0.32 equals R-3.1. That is the R-value of the entire window assembly, not just the middle of the pane of glass, which can be greater. That’s not a lot of insulation in the part of the wall with a window, and that’s why windows can feel cold to stand next to in the winter.
High-performance windows start at about U-0.15 and can go much lower. That’s starting at an R-value of 6.7 and go up to R-21 (U-0.045). It’s a remarkable improvement, but does come at a cost. When the rest of your house’s envelope is high-performance, however, these windows can make a lot of sense.
In a home that is tight and stores energy well, particularly a passive solar house, tempering our incoming air can help make a big jump in efficiency of the home. There is a perfect place to do that, and we are probably digging in there anyway to construct the home. The earth maintains a steady temperature of about 52° year round at about five feet below the surface. The same principle that makes the earth a good heat sink for geothermal heating and cooling can help us condition the air that is coming into the house.
A perforated and filtered drain tile tube that is placed at least five feet underground and has well-drained soil underneath it, can be a path that incoming air can take before entering the house. Since the difference in temperature outside and the temperature of the ground may be quite large, we need a long obstructed path for the incoming air to travel.
This typically means a serpentine pattern for several hundred feet underground (perhaps going around the house) will create enough opportunity to exchange temperature and moisture in the ground. In the summer, we will cool and dehumidify the air, as its dew point will be reached in the cool of the earth. In the winter, we are pre-heating the air and adding moisture content.
We can make this intake go directly into our Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV), and the difference between the incoming and outgoing air will be much smaller. That means we will use a lot less energy to bring the incoming air up to temperature with the ambient temperature in the house, and can have lots of fresh air in our tight and well-insulated house.
It is important to make sure that the earth tubes are well drained with no standing water, and that the drain tile has a filter sock with peastone and another filter cloth layer above it. Care must be taken to avoid making an assembly where bacteria and mold will thrive, thus introducing unwanted microorganisms into the house. These assemblies can work great and can be an important strategy for a Passive House.
Gaining this interior thermal mass is one reason we like Durisol ICFs, particularly our modified version, which provides a high R-value wall. These passive house methods are what we used to build The Phoenix House. This house will heat up to 63° during the day when it is below freezing outside, and hold it’s temperature for several hours after the sun goes down. Although this house will not achieve “Passive House” status, it is a home that is very close to net-zero energy with the addition of solar panels.
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