No benefit to too much insulation

September 8, 2009

I'm debating whether to put more insulation in the attic to save energy. How do I know when I have enough?

Insulation helps reduce a home's heat loss, but there is a point when adding more provides minimal benefits.

Insulation is rated by R value. The higher the number, the more effective it is in stopping heat from escaping into unconditioned spaces.

The real payback occurs when you first add insulation. For example, you see a 90 percent improvement in reducing heat loss when you go from no floor insulation to R-19, but only a 36 percent improvement when upgrading from R-19 to R-30. The Washington energy code requires new homes to have R-38 insulation in the ceiling, which is about 15 to 18 inches of blown insulation. R-30 is required over crawl spaces, which equates to a 10-inch batt.

"These levels may be hard to achieve in older homes, given their construction," said Rick Cantonwine, an energy counselor with Clark Public Utilities. "We recommend adding attic insulation if there is no existing insulation or if it is less than R-30."

Cantonwine said if an attic has six inches or less of insulation, the home probably would benefit from adding more. "Every home is different though," he cautions. "Plus how well the insulation is installed and how well the home is sealed makes a huge difference in how effective any insulation will be."

Blown-in vs. batts

Blown-in insulation, which is typically found in attics, will compress over time, especially if grandma's treasures have been stored on top of it. In this case, we recommend blowing more insulation on top, rather than laying batt insulation, which will only compress it further. If the insulation does not top the floor joist, you could simply lay batt insulation across the joists. That's an easy job, but our preference is additional blown-in insulation as batts will inevitably have gaps that allow more heat loss.

As we mentioned a few weeks ago, before adding more insulation in the attic, install insulation where there is none, such as under the floor, and seal air leaks in the home.

"Attic insulation is just a filter for warm air, so if you have an air leak it will rise through the insulation and still leave the house," said Bob West, an energy counselor at Clark Public Utilities. "You have to insulate and air seal to really make insulation do the job."

Clark Public Utilities currently offers incentives for weatherization such as air sealing and insulation to owners of electrically heated homes, if the work is completed by an approved contractor. We also offer loans of up to $15,000 for weatherization to qualifying customers.