Are R-values for insulation all they are cracked up to be?

Posted by Chaya Kurtz | G+ | Jul 06, 2010

I met a woman this week who works for a Denver company that re-insulates homes. The company starts by (followed by other contractors around the country) drilling small holes in brick mortar, lift up a siding slat. Then they compress existing fiberglass insulation and spray in loose cellulose insulation. Viola, your insulated house just got another cozy layer of insulation. Or did it?

Something smelled fishy about their method. However, the method checks out, and the results point to the limited usefulness of R-value claims on insulation packaging. This gets a little technical, but the bottom line is that R-values don’t tell the whole story.

The woman claimed a 50-percent drop in energy bills after re-insulation, but I couldn’t figure out how the re-insulation was an improvement. Here is a list of R-values for various insulation materials. Cellulose and fiberglass have basically the same insulation ratings. They each boast average thermal resistance levels, or R-values, of about 3.7 per inch. This is slightly better than cotton and mineral wool, but much lower than closed-cell polyurethane foam, which is perhaps the best home insulator but also among the most expensive per square foot.

Moreover, the air gaps between threads of fiberglass provide most of the insulation in fiberglass batts. That means compressing fiberglass makes it essentially worthless. Therefore, re-insulation would seem to be a replacement of one material with another material that is equally effective. However, R-values can be deceptive.

R-values are determined in a controlled environment at a comfortable 75 degrees by measuring the amount of heat transfer between hot and cold plates on either side of the insulation barrier. At 75 degrees, cellulose and fiberglass perform equally well. However, the insulation value of fiberglass drops with the temperature because, again, it is based on air gaps between fibers.

As the temperature of the air gaps drops, the temperature inside the home also drops. On the other hand, cellulose insulation gets thermal properties from the fibers themselves. These fibers don’t get as cold as air pockets, so cellulose R-values stay more constant in winter temperatures.

If you are looking to add or boost insulation, check with local contractors and experts to estimate real-world R-values and energy savings based on your particular climate. Also, make sure to caulk and weatherstrip around any air gaps, and install the insulation properly. The California Energy Commission found that a 4 percent gap in insulation batts can cut the effective R-value in half.

Blog by: Steve Graham

Learn about soy foam insulation, recycled cellulose insulation, and greener fiberglass insulation. Find out how to insulate a crawl space.  Networx has tons of tips on how to save energy!

blog comments powered by Disqus