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In a 2x4 wall in the northeast, R13 is recommended..however, R15 is the same exact depth ( 3 1/2"). Can you then use the R15 fiberglass insulation because the depth is the same and technically wont be any more compressed?
 

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Either way, you have the thermal short circuit from the studs The R13 and R15 are just laboratory numbers for the insulation part of the wall. R15 should be better than R13 by a little bit in the wall. If you have steel studs instead of wood, then the R15 and R13 walls would be about the same because of high conductivity of the steel.

You will never get an R15 wall with R15 insulation if it was tested as a composite system. R19 can be as low as R9 in the wall depending on the stud material and spacing.

That is why XPS foam performs so well in the wall.

High R-values for bulk insulation are good peddling/marketing tools.
 

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I've built a lot of 2x4 walls with r-13 and sheathed with 1 xps, t&g. Then a brick veneer. The inspectors gave me r-19 for this wall because of the air space between the brick and the xps added 1.

So it was equal to a 2x6 wall with r-19 insulation and vinyl siding or whatever.

But I'd be interested in knowing the difference between my aforementioned wall and a 2x6 wall with osb sheathing and vinyl siding.

As concretemasonry mentioned, I think the xps sheathing adds much more than its stated r value because you are over the studs with it.
 

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In a 2x4 wall in the northeast, R13 is recommended..however, R15 is the same exact depth ( 3 1/2"). Can you then use the R15 fiberglass insulation because the depth is the same and technically wont be any more compressed?
Yes. They make R-15 fiberglass insulation for 2x4 walls. I think the other responses are a little overboard...I don't imagine you plan on removing the wall and rebuilding it using an insanely different material.
 

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I've been told the reason they make R15 is for real 2x4 walls (old homes), and the R13 is for nominal 2x4.
 

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R15 is better. Simple answer. Being higher density, it tends to fill the void more completely as well.

They are requiring it in Tacoma, which I think is bs. The payback period is too long.

In my other locale, fairbanks, Alaska, r30 walls are code, and that is a good thing.
 
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