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Do I need to remove drywall and vapor barrier when I increase the insulation from 4" to 10"?

I'm expanding a bedroom over my garage to become an in-law apartment. I'll be adding a full dormer on the back. Do I need to remove the drywall and vapor barrier of the existing side wall where the new dormer wall joins to it? The new walls will be 10" double stud. The existing walls are 4" tradition framing. I assume they are insulated with paper-faced fiberglass. The knee wall on the front will also be expanded for more insulation. Do I need to remove that drywall too?

Asked by G. Blake Voss
Posted Jul 5, 2018 11:45 AM ET

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3 Answers

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1.

G. Blake,
Your question is unclear.

Do you intend to build a 2x4 wall on the interior side of your existing 2x4 walls, with a space between the two walls, and then add insulation to create a 10-inch double stud wall?

If that's your plan, it's usually not a good idea to have a polyethylene vapor barrier in the middle of the wall if there is more insulation on the interior side of the poly than there is on the exterior side of the poly. The risk is that the poly will be cold in winter, and will become a condensing surface.

But to really answer your question, we need to know your geographical location or climate zone.

Answered by Martin Holladay
Posted Jul 5, 2018 12:42 PM ET
Edited Jul 6, 2018 5:06 AM ET.

2.

Sorry, I'm in Massachusetts in the northern Connecticut River valley. Yes, I'll be building 2x4 interior walls with space between the 2x4 outer walls. Your answer was clear, even though my question wasn't. Thanks.

Answered by G. Blake Voss
Posted Jul 5, 2018 9:49 PM ET

3.

G. Blake,
I'm going to assume that your 2x4 walls are 3.5 inches deep, and that you have a 3-inch gap between the two parallel walls.

I'm going to assume that the insulation on the interior side of the polyethylene has an R-value of about R-23, and that the insulation on the exterior side of the polyethylene (where the quality of the installation is in doubt) has an R-value of about R-10. So the total R-value of your wall is about R-33, with about 30% of the total insulation on the exterior side of the vapor barrier.

You are in Climate Zone 5. In this climate zone, walls need to have at least 27% of the total R-value of the wall on the exterior side of the vapor barrier to stay out of trouble. (Source: "Combining Exterior Rigid Foam With Fluffy Insulation.")

So the math says this will work. The uncertainty concerns the quality of the insulation job on your existing wall -- and whether my assumption that the old wall has R-10 insulation is accurate or optimistic.

What you do is a judgment call. If you like to sleep well at night, you might want to remove the polyethylene.

Answered by Martin Holladay
Posted Jul 6, 2018 5:43 AM ET

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