What happens here?
What happens here?
Buildings, energy, energy policy, indoor air quality, problems, triumphs, successes, failures and the people and processes that affect them.
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Tuesday, March 26, 2013
Little gaps in insulation can let a lot of heat escape.
OK, that's not fair let's look at some pictures.
Except that's wrong. Q(Btuh) = U X A X ∆T. "Q" is the total energy flow(measured in British Thermal Units per hour), "U" is the rate of heat transmission (in Btuh/square foot). "A" is the total area and "∆T" (Delta T) is the temperature difference across the surface in degrees Fahrenheit. "U" by the way is the the measure of the rate of heat flow. We just lost R though, where did that go?
Engineers and scientists measure the rate at which things happen. R value is how much that rate slows. Getting from "U" to "R" is easy, divide 1 by either and you get the other. R 2 = U 0.5. R 3 = U 0.33. R is handy as a marketing tool, the higher the number the better. That's pretty easy to understand. Also you can add R values, which you can't do with U.
So with our exposed fan above how do we figure the actual R value of the total area? Here's the formula: U(avg) = [(U1A1) + (U2A2) + (U3A3) .../ A(total). (BTW, does anybody know how to get proper math formulas with sub characters out of HTML?)
In the example above we have R 19 at U 0.0526316 and R 0 at OOPS, can't divide by 0, lets call the fan R 0.5 = U 2. We have 74 square ft times 0.053 = 3.922 plus 1 square foot times 2 = 2 which adds up to 5.922 which we divide by total area (75) and end up with an overall U value of 0.07896 which equals R 12.66. That is a big difference for missing one square foot of insulation.
Of course there are those joists in there, 16 inches on center, which have an R value of about 5.5, this is going to make things worse. I won't run the whole formula but in a simple framing system like this about 10% of the area is joists, so 7.4 square feet. Now we have 65.6 square feet at R 19, 7.4 at R 5.5 and 1 at 1/2. Result? R 10.9 at most.
See how fast it slips away? What if things were worse?
That's a big hole. I didn't calculate the effect because I knew it was coming out. The owners were afraid to run it so it just sat there as a hole in the insulation. It leaked a lot of air too.
Want another way to look at it? Let's start with an R 40 attic. Lose one percent of your insulation, lose more than 25% of your R value. Lose eight percent and you only have 25% left. Makes you want to hurry into your attic and neaten your insulation doesn't it? Well, do a good job and don't forget about the attic hatch.