Thanks, Rick, your explanation and the "flat or lumpy" article you posted were very helpful. I think I get it now. (Thanks, too, Allison).
I suppose It's not about the resistance to heat flow, it's more about the TOTAL heat loss. And, because heat flows via the path of least resistance, the effect of a low r-value section of the wall is disproportionate. I'm still trying to come up with an intuitive analogy. I keep coming back to a bucket with holes in it or a sieve of some sort. In that situation, I think u-factor would be comparable to the gallons per minute of water flow out of the bucket. Maybe. I gotta think it through a bit more.
One minor comment: Take another look at your innerR10/outerR20 scenario. I think the total value of the wall you describe is R30. Certainly it's more than R20...adding R10 on top of an existing R20 wall shouldn't diminish the performance of the wall, it should improve it, right?
Thanks, ktkcad, but I still don't get it. Anyone else want to give it a try?
The only analogy I can think of is an old math problem: If you drive 10 mph for 10 miles and then drive 20 mph for another 10 miles, what's your average speed? 13.3333 mph, yes, I know and I know why it's the answer. But is this analogy even relevant to the R-Value problem? How?
I'm not trying to turn this into a "Cartalk" puzzler, really I'm not. I guess I just don't understand the nature of what it means when we say "R-Value."
Yes, that's what I calculated with the formula. Can someone explain why it's not 15? Thanks.
Can someone answer for me a much more basic question? What's the R-value of a wall that is 50% R10 and 50% R20?
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