Author Topic: Home insulation math  (Read 618 times)

Financial.Velociraptor

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Home insulation math
« on: November 05, 2020, 12:02:55 PM »
How do you calculate the R value of two layers of insulation? 

I had what I think was R-13 in the attic.  It was very thin.  I just finished topping that off with R-19 and then covered the whole thing with a plywood deck.  I can already feel the difference.

But is it additive (13+19)?  Or some multiplier ((13+19)*coefficient)? or is there diminishing marginal returns where the result is <(13+19)?

Tester

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Re: Home insulation math
« Reply #1 on: November 05, 2020, 12:06:12 PM »
You add them.
At least this is what wikipedia says 😀.

uniwelder

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Re: Home insulation math
« Reply #2 on: November 05, 2020, 12:17:04 PM »
You add them.
At least this is what wikipedia says 😀.

Yup.  Add them together.  Bonus points if you overlap/offset the two layers.  Are your joists super deep to be able to get 9" of insulation in there and be able to put plywood on top without compressing it?  If not, then its not quite as effective.

Dicey

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Re: Home insulation math
« Reply #3 on: November 08, 2020, 07:58:36 AM »
If you're thinking about insulation, @nereo's the person to talk to.

Papa bear

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Re: Home insulation math
« Reply #4 on: November 08, 2020, 08:17:49 AM »
You add them together.  But if the first layer is compressed, it reduces its R value.  So letís say you have r-13 fiberglass batt insulation, meant for 3.5Ē cavity.  You shove that into a 2.5 or 3Ē cavity, and your R value is reduced, maybe to R-10. 

So, if you really want to know, you have to measure the depth. 

And if you want to know the total envelope, you have to factor in your joists, which pine runs about 1.5/inch.  So if you donít have insulation covering that, your total ceiling value is much much lower.  Look up thermal bridging and how that effects your total.


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