The most cost effective way to reduce the energy used in a home both during construction and while it’s lived in, is to make it smaller. The average size of a home in the United States has almost doubled since 1970. I don’t think those of us who were around then felt deprived by the size of our home.
There are some strategies to get more space from the same amount of materials. One is the row house. Part of my childhood was spent in intercity Philadelphia. Our 4 bedroom home was only 20 feet wide though it was quite deep front to back. Much of the side or what was called a party wall was heated on the other side so there was no heat loss through more than half of the exterior walls. There was also a surprising amount of privacy with no side windows. Since the wall and foundation was shared it saved cost, materials, labor and energy.
Another way to increase the usable space with less material is to build a square house instead of a rectangular house, thus improving the surface area / volume ratio. A 30 foot square house contains 900 square feet and 120 feet of exterior wall. A 15 foot by 60 foot house also has 900 square feet of living space but has 30 more feet of exterior wall. This is 25 % more material, labor, and heat loss.
Once we determine the amount of space we need, designing the shell of the house to minimize heat loss is the next priority. A thermos can keep something warm for a long time. High insulation levels including the windows and doors and air tightness are essential in an energy efficient home.
A well insulated floor can be done in 2 basic ways, a wood floor on joists and a slab on grade. Most homes in this area use a slab on grade. The thing to consider is to eliminate thermal leaks. Usually the foundation and load bearing walls sit directly on undisturbed soil. This means the heat flows continuously from the warm floor to the cooler earth. The difference in the temperatures is what determines how much heat flows. A typical floor temperature in a 70 degree house is about 65 degrees and the earth below is 55.
This 10 degree delta T (temperature differential) can transfer a substantial amount of heat to the earth but not near as much as a slab with radiant heat. When the fluid temperature in the slab is 105 degrees the delta T becomes 50 degrees. This means that 5 times as much energy is being lost below the floor than in a non-radiant heated floor.
A well insulated wood floor needs a continuous air barrier under the insulation. Cool air leaking through the insulation would limit the effectiveness of the insulation. There are several places that air can get through, the perimeter, electrical and plumbing penetrations; and other structural supports. This air barrier should be made continuous with the walls and ceiling. It’s surprising how much air leaks into a house and is often the major heat loss of the building.
As I mentioned before minimizing the length of wall by designing a more compact house can save dollars and energy. Adobe walls are high mass and store heat but are much more expensive today than they used to be, so looking for an inexpensive way to store the suns heat is important in designing a cost effective solar home.
It used to be considered that a bubble of heat was stored in the mass of the earth below the floor.
This would be true if there was an insulated layer a few feet down to stop the continuous migration of heat away from the house. I’ve designed a few houses that use this insulated deep mass floor and it is an inexpensive way to get solar heat storage inside the insulated envelope. The important piece is “no thermal leaks” especially if there is a radiant floor.
The Father of earth sheltered architecture Malcolm Wells wrote a book in 1981. He referred to the heat loss through the floor of his passive solar, earth covered home, built in 1966. He said he feels the heat continually ebbing away to the earth through his un-insulated mass floor. Think of a thermos with some inexpensive mass.
Wood frame construction is the standard of the industry, but is usually not done in an energy smart way. The normal 2x6 wall with 6 inches of insulation is called R-19, but is actually R-14.5. The fiberglass or cellulose insulation is rated at R-19 but the studs that interrupt the insulation are a heat leak. Also the air that leaks through a typical stud wall further reduces its thermal performance.
Nailing a layer of foam insulation to the outside of a stud wall is a simple and effective way to increase the R value. Building a double stud wall can increase the insulation by making a thicker cavity for the insulation. This also creates a thermal break with the space between the inner and outer studs.
In both these systems the air leak also needs to be addressed. A layer of spray foam on the outside of a single stud wall can stop the air leak and provide the added insulation. Air barrier information can be found in the German passive house design. It requires attention to many details but the comfort and ease of heating is worth the effort.
Structural Insulated panels or SIPS are another choice for a highly insulated wall. They are made from OSB (a plywood like material) and EPS foam. They use much less wood than a frame house and are air tight. If a house is designed on a computer the CAD file is sent to the manufacturer and then panels are shipped to the site with precisely located window and door openings. These SIPS are also used for floor and roof with no other framing required.
There are a lot of choices for the roof but basically all are made from wood. If the insulation that is being used requires a cavity then a typical attic can provide the space for 18-20 inches of blown in insulation. At the edges where the wall meets the roof structure, a provision needs to be made to allow the thickness of insulation to be maintained. Also this is a critical location for air leaks into the heated space. Top plates are full of penetrations for wiring and plumbing that allow cold air through. These can be sealed w/ foam by a patient hand.
Light fixtures are another place to look for air leaks in a frame vented roof. Spray foam and SIPS have no cavity to be vented and by design are easy to make air tight.
To achieve the air tightness of the German Passive House may be out of reach to many and a cost saving compromise may be more appropriate. Most houses built today have drywall for ceilings and walls. This can become an effective air barrier. Putting a layer of caulk in strategic locations during the drywall installation can minimize air leakage into the heated space. Along top and bottom plates; and windows and doors, also in the corners are the primary locations to calk. Included on the KLDK blog is a diagram showing this air tightening technique. Plumbing drain lines pass through the drywall air barrier below the sink and behind the washer. These are vented and go through the top plates. Foaming both locations is a go idea. Electrical penetrations in the drywall need to be gasketed to complete the drywall air seal.
Once we have a small compact house designed that is tight and has high insulation levels then we can add heat hot water electricity and fresh air.
The heat and hot water for such a house can come from a fairly small solar system.
The collection area can be 10% or less of the floor area, depending on how much hot water is used. If half of this area is in reflectors, the cost can be reduced by a third. More details on this system are available from last week’s program, as seen on the Dixon Dallies.
Solar electricity is much more cost effective today, primarily because of the 40% rebate available from US and state energy credits. Conservation is the first thing to consider when designing a solar electrified house. Using less electricity without giving up anything but leaving the lights on when we are not in the room, can be done with good choices in appliances and lighting. Appliances with electric heating elements should be avoided. The toaster only runs for 3 minutes and a hair dryer not much longer so they don’t in total use much electricity. Things like a hot water heater that can be on several hours a day are inappropriate for use with a solar electric system. The next biggest user of electricity is the frig. Our Sun Frost frig uses 92 watts when it’s running and has twice as much insulation as a typical refrigerator which uses 500 watts or more and runs more often because it has less insulation. The Sun Frost is not frost free (which uses electricity) and has to be defrosted 2 a year. The less the door is left open the better.
Last and maybe most important is providing fresh air to an air tight house. An energy recovery ventilator is essential. A small fan circulates outside air through a heat and moisture exchanger before it enters the heated space. The exhaust air is usually sucked from the bath and kitchen and heats the incoming air before leaving the building. The fresh air is delivered to the other rooms in small ducts that are inside the insulated envelope.
The amount of energy used is far less than what leaks through a typical exterior wall. In our dry climate it’s important to use a energy recovery ventilator not a heat recovery ventilator because the HRV does not retain the moisture and dries out the house.
Quote of the week
"I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that."----------------Thomas Edison (1847--1931)