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Confused on air and vapor barrier with vaulted ceiling

Hello, I've done a ton of reading here and in just general and I'm thoroughly confused on what I should do. I'm in the middle of building a two story addition off the back of my house in central Pennsylvania. I've used scissors trusses and have fully vented soffit with a ridge vent. I plan on using knotty pine 1x6 on the walls up to a chair rail and drywall to the ceiling and then the ceiling will be 1x6 knotty pine.

Before I started doing any reading, I thought i was ok putting batt insulation between studs/trusses with a 6 mil poly vapor barrier over that and then my 1x6 tongue and groove knotty pine. After reading, it appears as though that is not a good idea as the 1x6 boards leak way too much air. I don't really want to put up drywall first and then the 1x6. Is this a proper way of doing it: batt insulation, 6 mil poly vapor barrier on the bottom of the truss, rigid foam taped to completely seal and then the 1x6 tongue and groove?

Also, how should the walls be done for proper air sealing? Any help would be greatly appreciated as I'm very confused at this point. thank you

Asked by SCOTT KOONS
Posted Sun, 06/29/2014 - 10:00

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8 Answers

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1.
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Scott,
In your climate, a polyethylene vapor barrier is usually a bad idea. To learn more, see:

Do I Need a Vapor Retarder?

Vapor Retarders and Vapor Barriers

You want a very durable air barrier on the underside of your rafters. Drywall is generally the least expensive, most durable material for this layer. You should tape the seam with drywall tape and mud, but you don't have to finish the drywall seams carefully if you will be installing pine boards.

Once you have a good air barrier, you are ready to install the pine boards.

To learn more about cathedral ceiling details, I recommend this article: How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 06/29/2014 - 16:16

2.
Helpful? 0

Thank you for your response, I've read that article several times which has led to my confusion. I thought I had to install a vapor barrier in my area per code. I really don't want to install drywall if it can be avoided. Is there any other material I can use that would provide a good air barrier? Thank you

Answered by SCOTT KOONS
Posted Sun, 06/29/2014 - 18:05

3.
Helpful? 0

Scott,
You could use a so-called "smart" vapor retarder like MemBrain or IntelloPlus. These are preferable to polyethylene, because they allow the assembly to dry to the interior. However, I don't know how airtight they will be once you have all the fastener penetrations that come from nailing up the pine boards.

I recommend that you stick with drywall -- it's cheap, and it's tried and true. Any local contractor knows how to install it.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 06/30/2014 - 06:37

4.
Helpful? 0

Martin,
Thank you for all the info. I'm doing all the work myself with some help and direction from people who are either in the trades or have been in the past. It appears as though my county (York) is one of the only section 4 Counties in the climate map, which means I wouldn't need a vapor barrier. It looks like I'll be installing drywall over everything for the air barrier. I'm assuming on sections that were going to be finished with drywall (original plan was tongue and groove pine on walls to a chair rail and then drywall from chair rail to ceiling) i would now just add a second layer of drywall to bring the thickness out even with the tongue and groove pine, would that be correct? Thank you for your help, I appreciate it.

Answered by SCOTT KOONS
Posted Mon, 06/30/2014 - 18:14

5.
Helpful? 0

Scott,
Q. "I'm assuming on sections that were going to be finished with drywall (original plan was tongue-and-groove pine on walls to a chair rail and then drywall from chair rail to ceiling), I would now just add a second layer of drywall to bring the thickness out even with the tongue-and-groove pine, would that be correct?"

A. The boards on the lower sections of wall are called wainscot. When installing wainscot and a chair rail on a wall that has been drywalled, the chair rail disguises the fact that the wainscot is not in the same plane as the drywall above the chair rail. Most homeowners don't expect the wainscot to be co-planar with the drywall.

If I were you, I wouldn't worry about the fact that the two surfaces aren't co-planar, and I wouldn't bother to install a second layer of drywall above the chair rail.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 07/01/2014 - 08:24
Edited Tue, 07/01/2014 - 08:25.

6.
Helpful? 0

Martin,
There are 7 windows and a french door where the knotty pine and drywall will intersect about 8-10 inches up on the windows. If I didn't bring the drywall out to the thickness of the knotty pine, I'm not sure how the window casing would fit? There would be a 3/4 inch (the thickness of the knotty pine) gap from the window casing to the drywall. So, I can either add another piece of drywall to bring it close to even, or now that I think about it, I guess I could cut 3/4 inch slats that would go under the casing in the gap. I'm not sure what would work best. Any other suggestions?

Answered by SCOTT KOONS
Posted Tue, 07/01/2014 - 16:58

7.
Helpful? 0

Scott,
You have listed your two options. Either way will work.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 07/02/2014 - 05:47

8.
Helpful? 0

Martin,
Thank you for all your help and information. I appreciate it.

Answered by SCOTT KOONS
Posted Wed, 07/02/2014 - 16:37

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