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.

They are inverse, when one goes up the other goes down... or... U=1/R (that's the math part). Higher R-value is good. Lower U-value is good. A Wall with an R-value of 5 has an U-value of .2 (1 divided by 5 is .2)

"WELL" you say "Then why the heck do we need both? Is it just to CONFUSE us?"

Kinda ... but there is a reason. You can add R-values... Stack two pieces of R-11 and you get R-22. But, you can't average them. Half a wall with R-10 and half with R-20 doesn't average out to R-15. (its R-13.3 ... go figure)

U-values (aka U-factors) are just the opposite. you CAN'T add them but you CAN average them. (You'll have to find a guy with a larger brain than mine to explain the math to you, but trust me, it works)

So, when you are working with areas that have windows and doors and walls that have all different values, you work in U-Factor. When you are stacking up all the layers in any one part, you use R-value.

Now that I've explained that, and before your head starts to hurt, let me talk about cavity vs continuous insulation. IF... you put R-11 FG batts between 2x4 wall studs, including the drywall and sheathing you get an R-value for the assembly of about R-9 or so. (the studs themselves are good conductors and drop the average way down). NOW ... add R-3 foam to the outside and you get the whole R-3 for a total of R-12. SO... That silly little sheet of half inch foam they stick under your vinyl siding raises your R-value by a whopping 33%.

So, where would this come into play? For starters, if your attic isn't floored and you plan to add a big stack of insulation, you are better off adding just enough to fill the bays and then adding another layer perpendicular to the joists and butted up tight. That way the joists themselves don't give up your precious heat. And, make sure you use full width baffles in every bay along the eaves. Otherwise, cold breezes coming through the soffit vents will blow right through the FG batts and ruin any R-value they have.

Another place is under your siding. If it isn't too thick for the window and door frames, consider using 1 inch or more instead of the 3/8 fanfold or 1/2 inch sheets.

Back to the "stuffing" I was talking about before. Instead of stuffing FG insulation into cracks and crevasses, use expanding foam. Once you jamb it in, FG doesn't do much insulating.It has to be fluffy to work. Also, it still lets some air flow through it. If you are filling in around a window frame or door that might not be sealed perfectly from outside, Foam will seal much better than fiberglass.

I haven't discussed spray in or cellulose insulations yet. Nor have I mentioned vapor barriers at all. They go hand in hand with all of this. But, my Brain hurts and I haven't even gotten to the hundreds of mind-numbing equations I promised you.

I'll save it for another time. That way you'll have something to put you to sleep another night too.