Helpful? 0

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 Sat, 01/26/2013 - 19:02
Edited Mon, 01/28/2013 - 07:05

Tags:

3 Answers

Select your preferred way to display the comments and click "Save settings" to activate your changes.
1.
Helpful? 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 Mon, 01/28/2013 - 07:09

2.
Helpful? 0

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 Mon, 01/28/2013 - 09:14

3.
Helpful? 0

Perry,
Yes.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 01/28/2013 - 09:36

Other Questions in Energy efficiency and durability

insulation dew points, and archaic insulation

In GBA Pro help | Asked by Graham Allan | Oct 20, 14

Rising power plant energy cost

In Energy efficiency and durability | Asked by Terry Lee | Oct 15, 14

Window U-value comparison to wall R-value and window replacement

In General questions | Asked by Keith Miller | Oct 20, 14

Reasonable retrofit efficiency goals

In Energy efficiency and durability | Asked by Keith H | Oct 19, 14

Insulation design

In GBA Pro help | Asked by Robert Brown | Oct 17, 14
Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!