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I want to add 2 inches of blue board to the addition I'm building. Will it cause moisture to build on the inside wall?

I live in central Maine on the coast. Some have warned the dew point will fall on the inside of the 2x6 wall and not dry out. Others have said it will dry out in the summertime if i use a breathable paint on the drywall so it can dry out that way.

My slab on the 1st floor is heated and insulated w/ blue board 2" on the sides and 4" on bottom. Please advise.
Perry

Asked by PERRY GREEN
Posted Jan 26, 2013 7:02 PM ET
Edited Jan 28, 2013 7:05 AM ET

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3 Answers

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1.

Perry,
I'm going to assume that the "blue board" you are talking about is extruded polystyrene insulation (XPS), and I'm going to assume that you are planning to install it on the exterior side of a 2x6 wall. Is that right?

You are located in climate zone 6. In your climate zone, any exterior rigid foam on a 2x6 wall needs a minimum R-value of R-11.25. Since 2 inches of XPS has an R-value of only R-10, your plan won't work.

You have two choices: switch to polyisocyanurate -- two inches of polyiso will give you the needed R-value -- or make your XPS thicker.

For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

Answered by Martin Holladay, GBA Advisor
Posted Jan 28, 2013 7:09 AM ET

2.

THANK YOU DAN. YOUR ASSUMPTIONS ARE CORRECT. IS THE REASON I NEED R-11.25 TO MAKE THE DEW POINT BE ON THE OUT SIDE OF THE STRUCTURE ? PERRY .

Answered by PERRY GREEN
Posted Jan 28, 2013 9:14 AM ET

3.

Perry,
Yes.

Answered by Martin Holladay, GBA Advisor
Posted Jan 28, 2013 9:36 AM ET

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