My guess is that you want to spend as little as possible on heating and cooling in your new home.
One way to minimize those bills is to increase the thickness of your exterior walls. If you build with 2-by-6 wall studs, which are two inches deeper than standard wall studs, you can add more insulation. But is the extra construction cost worth it?
There are several factors you need to consider when calculating the cost of increasing the exterior wall thickness of a house. The wall studs, top and bottom wall plates, insulation, and window and door extension jambs will all cost more money.
Once you have calculated the total added cost, you need to calculate how soon you might receive a payback. The soundness of the investment depends on where you live and how close you are to sea level.
If your home is large, the cost to upgrade from 2-by-4 to 2-by-6 exterior walls can be significant. I calculated the cost for a single-story ranch house 60 feet long by 30 feet deep. The added cost for the wall studs and top and bottom plates amounted to $300. Window and door jamb depths need to be increased with 2-by-6 walls; these extension jambs cost $575. The extra-thickness fiberglass insulation added another $145, making the total up-charge for my hypothetical house $1,020. Larger houses or ones with more exterior wall surface area, windows and doors will cost more.
To calculate the payback, you need to determine how much fuel you will save by increasing the wall thickness. Fortunately, scientists at the University of Illinois's Building Research Council have developed a formula to help you.
First you have to determine how cold it gets where you live and how long it stays cold. There's an accepted method for measuring this -- the degree-day method.
To calculate, add together the recorded high and low temperature for each day of the heating season and divide this sum by two. Take this average daily temperature and subtract it from 65 degrees. The result is the total degree-days for that date. As the weather gets warmer, you obviously have fewer degree-days. When the average daily temperature is 65 degrees, you have none.
The University of Illinois scientists determined that if you live in an area that experiences 5,750 or more degree days per year, then it begins to make economic sense to switch to 2-by-6 exterior walls. The more heating degree days you have above 5,750, the faster your payback will occur. The Chicago area, on average, experiences 6,536 heating degree-days each winter.
Altitude above sea level also makes a difference. Those people who live closer to sea level experience a more rapid conductive heat loss through the air.
Because the air is denser as you get closer to sea level, heat travels from air molecule to air molecule more quickly. For every 1,000-foot gain in altitude, 5 percent less heat is lost through conduction. This is one reason why solar heating works great in mountain areas, but performs poorly at seaside. Houses at or near sea level need to be well insulated.
People who live in the Sun Belt and use air-conditioners much more than furnaces face the same tough choices. The cooler a person wants the house to be, the more sense it makes to upgrade to 2-by-6 walls. A person who prefers an indoor summer temperature of 70 degrees will use considerably more energy than a person who can survive at 76.
Probably the best way to save energy is to increase attic insulation. Heat loss is greatest through the roof. Consider upgrading your blown-in attic insulation to an R-value of 38 or higher.
Write to Tim Carter, c/o The Chicago Tribune, P.O. Box 36352, Cincinnati, Ohio 45236-0352. Or, contact him on the Internet at askthebuilder.com.
You can call him any Saturday from 9 to 11 a.m. at 1-888-737-1450.
For a listing of the degree day totals for over 250 U.S. cities and the step-by-step calculation method for determining 2-by-6 added wall costs, send $3 and your name and address to Tim Carter at the above address. Ask for Builder Bulletin No. 212.