Helpful? 0

How to insulate stud walls and roof with OSB-wrapped XPS on outside already in place

We have a new construction garage with living space upstairs (room in attic trusses). We are in zone 4, about 10 miles south of zone 5.

The entire interior will be considered "conditioned space". The roof is metal over underlayment over 7/16 OSB over 2" XPS over 7/16 OSB on trusses. The walls are Hardie Plank over Hardi Wrap over 1/2 plywood over 2" XPS over 7/16 OSB over 2x6 - 24" o.c. studs.

Our insulating contractor has proposed the following:
Exterior walls within living space: R-21 kraft fiberglass.
Exterior walls outside living space: 2" closed-cell foam = R-21.
Roof deck: 3" closed-cell foam.

My only concern is the potential for moisture on the OSB between the XPS and the spray foam. From what I can find in my research since I am in zone 4 this should be ok???

BTW per the contractor, this was the recommendation from the NCIF (North Carolina Foam Institute). He was concerned about getting a proper air and vapor barrier throughout the structure.

Comments and advice would be greatly appreciated.

Asked by Ed Barnett
Posted Mon, 10/01/2012 - 12:13
Edited Mon, 10/01/2012 - 13:17

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10 Answers

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1.
Helpful? 1

Ed,
First of all, I'm not sure why you chose to insulate the walls of your living space with fiberglass batts, while the walls of your garage get spray foam.

In general, I don't think it's a good idea to sandwich OSB sheathing between two layers of low-permeance foam. Now that you have installed 2 inches of XPS on the exterior side of your OSB, it's not a very good idea to install closed-cell spray foam on the interior side of the OSB. It's better if your OSB can dry out in at least one direction.

If you want to use spray foam on the inside of your OSB, it would be better to use open-cell spray foam, not closed-cell spray foam.

Frankly, you are building a complicated sandwich.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 10/01/2012 - 12:47
Edited Mon, 10/01/2012 - 12:48.

2.
Helpful? 0

Sorry for the confusion in my explanation. The walls of the garage will also be fiberglass batts and drywall. We will be living in the garage while the new house is being built.
The contractor wanted to use spray foam in between the trusses because they are of uneven dimensions and the webbing will be a pain to work around. He wanted to use closed cell because he wanted an air and moisture barrier. I thought that the xps served that purpose and that we wanted the interior of the wall to be able to breath and dry out. I was origionally going to put fiberglass batts throughout but the trusses will make that a difficult task and he was afraid of air leakage around the edges. YES I know this is a complicated mess but unfortunately now we have to try to make it work. Any suggestions?

Answered by Ed Barnett
Posted Mon, 10/01/2012 - 13:16

3.
Helpful? 0

Ed,
Please explain what you mean by "Exterior walls outside living space." (The walls that will get spray foam.)

You wrote, "The contractor wanted to use spray foam in between the trusses." Which trusses? Will this foam be sprayed against the underside of the roof sheathing? Or perhaps the underside of the subfloor above your garage? Or somewhere else?

Answered by Martin Holladay, GBA Advisor
Posted Mon, 10/01/2012 - 13:22

4.
Helpful? 0

All foam directly on the osb. 3" spray foam on the underside of the entire roof eave to peak. 2" spray foam on the gable end walls above the flat ceiling upstairs (room in attic truss design). 2" spray foam on the gable ends outside the knee walls. I could try to send you some pictures if this is still confusing. Thank You for taking time with this. I was nervous with his recommendation and wanted to make sure whatever was done would not come back to haunt me later. Some postings that I have found explain that since zone 4 doesn't get the extreme winter temperatures that the condensation is not significant?

Answered by Ed Barnett
Posted Mon, 10/01/2012 - 14:02

5.
Helpful? 1

Ed,
My answer is the same: "Now that you have installed 2 inches of XPS on the exterior side of your OSB, it's not a very good idea to install closed-cell spray foam on the interior side of the OSB. It's better if your OSB can dry out in at least one direction. If you want to use spray foam on the inside of your OSB, it would be better to use open-cell spray foam, not closed-cell spray foam."

Another option: cellulose.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 10/01/2012 - 14:42

6.
Helpful? 0

How would you keep the cellulose in place on a trussed roof? wouldn't fiberglass batts be easier to keep in place with something like wire mesh?

Answered by Ed Barnett
Posted Mon, 10/01/2012 - 15:14

7.
Helpful? 1

Ed,
You're right; there's no easy way to install cellulose against roof sheathing if you have roof trusses.

Back to the open-cell spray foam idea...

Answered by Martin Holladay, GBA Advisor
Posted Mon, 10/01/2012 - 15:23

8.
Helpful? 0

From all that has been discussed, I like the open cell option best. I did at one point ask the insulation contractor about open cell and he was strongly oposed to it stating that I needed an air barrier and open cell wouldn't provide that. In actuallity don't I want some air movement to the interior sheathing to allow for some drying effect? Am I thinking correctly? If using open cell, would you use it throughout the structure or go with dense pack cellulose on the finished walls?

Answered by Ed Barnett
Posted Mon, 10/01/2012 - 15:33

9.
Helpful? 1

Ed,
As long as it is at least 4 inches thick, open-cell spray foam is an air barrier. If your spray foam installer can't install open-cell spray foam in an airtight manner, you should choose a different spray foam contractor.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 10/01/2012 - 15:49

10.
Helpful? 0

Martin,
Checking out other installers already. Thank You for all of your help
Ed

Answered by Ed Barnett
Posted Mon, 10/01/2012 - 19:21

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