Leading the Way in Energy Efficient Building Design & Construction
Building a High Performance Home III - Spray Foam
December 22, 2013
In the last post we talked about sealing the home from air leaks and how to insulate the attic the old-fashioned way.
The better way we like to see attics insulated is to have the underside of the roof sprayed in foam, as pictured. There are pros and cons for every building detail, and the biggest con for this one is cost - it costs more to spray foam than it does to use fiberglass. The pros, however, are manifold.
First, we get to throw all those vent chutes, ridge vents, powered attic fans and gable vents right out the window (if your attic has a window). Spray-foamed attics, or "cold roof" attics, don't require ventilation to the outside. The air in the attic is conditioned air, the same temperature as the rest of the house. The look on people's faces the first time they climb into a "cold roof" attic is priceless - they can't believe it's the middle of summer and their attic is a balmy 70 degrees!
This means those Christmas candles won't melt in storage, and clothes won't get moldy from cold, moist attic air anymore. It also means that with the proper exits, an attic can easily become extra useable space in the home - an office perhaps, or play room for the kids.
Also, with the attic now conditioned space, your attic furnace is much happier. If the ducts leak, they're not leaking directly to the outside. The furnace doesn't need a box to live in, and the water lines to the humidifier won't freeze during a cold snap.
We mentioned that foam costs more initially. The other side of the coin is that a spray-foamed attic will save energy costs over the entire life of the structure. Another benefit is acoustics. If your home is built where there is outside noise, like airplanes, sirens, or nearby roads, foam insulation performs far better acousticly compared to fiberglass - your time indoors is simply more peaceful.
You needn't go all the way with foam either. Combining a suitable layer of foam with fiberglass is a great cost-effective way to get good performance at an affordable price. The one place you should always use foam is in cathedral ceilings. That is, any time you have roof on one side of a rafter and drywall attached to the other side, use foam, and try not to run any ducts in those cavities if possible.
Our bottom line is that the initial extra costs of attic foam are far outweighed by the short- and long-term benefits.