According to the U.S. Department of Energy, the average homeowner spends about 45% of their energy bill on heating and cooling their house. For those who use Energy Star appliances and have replaced their incandescent bulbs with more efficient light sources, that number will be even higher.
Before ordering photovoltaic cells to reduce your electric bill, the first thing you should do is decrease the amount that you heat the outside during the winter by leaky under-insulated attics and walls.
First, you need to understand the metric used – “R”. An R is roughly equivalent to one pane of regular window glass (not the new technology windows). So R-2 is about the same value as the old thermopane window. R-11, a common value of fiberglass insulation batts would be similar to 11 panes of glass protecting your house from the extremes of summer and winter.
How much insulation do you need? For the DC Metro area, the US Energy Department web site recommends the following: Attic R-49; Walls R-15 + insulative sheathing R-5; Foundation R-12. However, there are very few houses being built in this area that conform to these minimums. There is a new movement in Europe which is slowly making its way to this continent: passive houses. A passive house is built with a lot of insulation – so much that you could survive a winter in the DC Metro without a heater. These houses trap all the heat from appliances, humans and pets, and solar gain. And it would reduce, but not eliminate your need for an air conditioner. Passive houses, and other houses with limited air infiltration, require a heat exchanger to maintain good inside air quality.
There are several types of solid insulation. It should be used when you have a small space to insulate your house, since it is more expensive than other options. Polyisocyuranate has R-7 to R-8.7 per inch when it has a foil face (common products are R-Max and Thermax). This material is also used for spray foam. Extruded polystyrene, sometimes called blueboard (even though it also comes in pink), has R-5 per inch. It can be used below ground to insulate foundations. Expanded polystyrene, the white plastic that shreds off small beads when you cut it, is R-4 per inch, and cannot be used below ground. There is another solid insulation with much greater insulating power (R-10.3 per inch), but it’s quite expensive. Aerogel insulation is just beginning to be used – it was featured in at least one house in the 2009 Solar Decathlon. It’s other value is that it is translucent, so it can be used to create walls that allow in some light, but provides privacy.
For attics and walls with a deep cavity, the most common solution is R-3 per inch fiberglass batt insulation. It’s also is your cheapest option. The downside of using fiberglass insulation is that you must use a mask and long clothing, because it’s itchy and irritating to your lungs. Make sure you place the vapor barrier (kraft paper or foil) facing toward the house interior. If there is already some insulation with a vapor barrier, you will need unfaced insulation. Two vapor barriers will trap moisture and reduce its effectiveness.
There are also loose fill insulation options, which easily fits around irregular areas. These have an average of R-3 per inch. These are mineral wool and fiberglass. In addition there are greener options: 1) Cellulose (shredded newspaper treated to be inflammable and bug-proof), is inexpensive, and fills the cavity, but tend to settle over time; and 2) shredded denim, which is more expensive.
Homes built before about 1970 typically had minimal insulation in the attic and no insulation in the walls. Insulating the walls will get rid of those nasty winter chills and drafts, but it’s not easy. Your options are to gut the interior drywall or plaster (a very intrusive and dusty task) or insulate on the outside of the house (outsulation). Outsulating can be done by framing 2×4’s at standard widths and adding solid insulation, followed by exterior siding, or by spraying on insulation with a stucco exterior. Outsulating over brick provides a thermal mass which minimizes temperature swings.
For new or renovation construction, it is recommended that you install a house wrap, such as Tyvek. A house wrap keeps out the wind, but allows unwanted moisture to escape. For new or renovated construction, there are additional options. Structured insulated panels cost more, but construction goes quicker, resulting in lower labor cost. In addition, they have lower infiltration of outside air. Insulated concrete forms are ideal for foundations and wall, with solid insulation in the center and concrete on the two sides.
Most of these types of insulation can be found in the houses on the solar tour. The solar homeowners are available to answer your questions about their personal experience with these various types of insulation. [written by Steve Gorman, 2011 Solar Tour]