Insulation: R-values, U-values, and Other Such Mind-numbing Stuff

In this article I'm going to try and explain the principles of home insulation and ... through the use of hundreds of mind-numbing equations and calculations ... make simple the wonderful world of R-values and U-values. (exciting, isn't it?).

Remember, Insulation is step two in the process. The first should really be air sealing. (click on my name "Tom Bacc" to get to my page and check out "attics" for more info on air sealing)

First off ... how does insulation work? Well, If I tried to spew out all that physics I learned in school, my brain would hurt. So I'll keep it simple.

It goes like this... air that can't move is a lousy conductor of heat. If you can keep air from moving, it makes a pretty good insulator. "But", you say ... "I INSULATE WITH FIBERGLASS!?!?"

Well, no ... not exactly. Actually you insulate with the little pockets of air that are trapped between the fibers of fiberglass. If you flattened it out, fiberglass would conduct heat like ... well ... glass. Yup, it's actually made of glass. Think of it as glass cotton candy. It is just fluffed up so much that it traps (or really really slows down) large amounts of air. And like I said before ... still air is a lousy conductor and therefore a good insulator.

This brings up a good point about insulation. (I'm just learning how to segue) It is important that you put the right thickness of insulation into the cavity your filling. The R-value was figured based on the insulation being fluffed up to its design thickness. If you squeeze it into a tight space, you squeeze out the air and with it ... all the good insulating properties. "Stuffing" ... is bad.

Foam insulation, both the ridged board and the spray stuff that never comes out of your hair, takes it one step further. The "air" is trapped completely into millions of tiny bubbles. Since there is no air movement at all, Foam usually has a higher R-value than fiberglass (FG), inch for inch. It tends to cost more ... but it has properties that make it a better choice for some applications.

Now then ... what about that whole R-value-U-value thing...

R-value is ... how well it insulates.

U-value is ... how well it conducts.