Helpful? 0

How bad is fiberglass?

1. I am adding a full dormer framed with 2x12's in zone 6. I am planning to put 6" of iso on the decking and 3" on a small addition created walls. Should I reuse the fiberglass or install wet spray cellulose? Money is a concern but not to the point of short-cutting.

2. I want to upgrade the other side, but for trim and OK roofing, want to keep it vented by cobbling reclaimed iso as a baffle, filling 2x8 and adding 4" iso to the bottom of the rafters. Is this a problem as long as it is isolated from the back?

3. Is Tyvek or 3M tape good enough for taping iso? Different answer for reclaimed?

4. When you strap across the rafters, you create a space the thickness of the material, does perimeter sealing of roof plane make this not a problem for moving air?

Thanks, and this a great site, problems or not.

Asked by Robert Mason
Posted Wed, 01/01/2014 - 12:28
Edited Thu, 01/02/2014 - 06:45

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8 Answers

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1.
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Robert,
Q. "How bad is fiberglass?"

A. Fiberglass is the worst-performing type of insulation that is commercially available, but it can perform fairly well if you do a meticulous job installing it. For more information, see Installing Fiberglass Right.

Q. "Should I reuse the fiberglass or install wet spray cellulose?"

A. Either will work, but the cellulose will probably perform better. It's your choice. If you have 6 inches of polyisocyanurate foam above your roof sheathing, however, the bulk of your insulation will be from the polyiso, so an argument can be made that the choice of insulation between your rafters isn't that important.

Q. "I want to upgrade the other side, but for trim and OK roofing, want to keep it vented by cobbling reclaimed iso as a baffle, filling 2x8 and adding 4" iso to the bottom of the rafters. Is this a problem as long as it is isolated from the back?"

A. This question is confusing. I don't know what you mean by "the other side." I also don't know what you mean by "isolated from the back." If you are planning to make a sandwich with polyiso on the top, polyiso on the bottom, and installing fiberglass in between, that's not ideal. Usually it's better to choose one side for the polyiso, not both.

Q. "Is Tyvek or 3M tape good enough for taping iso?"

A. Yes, if the polyiso is foil-faced.

Q. "Different answer for reclaimed?"

A. Reclaimed polyiso usually can't be taped.

Q. "When you strap across the rafters, you create a space the thickness of the material, does perimeter sealing of roof plane make this not a problem for moving air?"

A. Your question is confusing. I think you are talking about strapping (or purlins) installed above the rafters. If that's your plan, you certainly need to know where your air barrier is, and you need to have a plan to maintain continuity between your wall's air barrier and the air barrier on your ceiling or roof.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 01/02/2014 - 06:54
Edited Thu, 01/02/2014 - 06:55.

2.
Helpful? 0

Thanks for your interest and answers,let me try to clarify. On one side of ridge I'am adding the dormer,on the other side I'am leaving 2x8 rafters, cobble filling with iso overlaying with more iso. If tape won't work, is tremco good enough to seal the reclaimed I'm planning to put on the underside of 2x8's. The strapping I was referring to is the support for sheetrock.

Answered by Robert Mason
Posted Thu, 01/02/2014 - 10:39

3.
Helpful? 0

Robert,
The cut-and-cobble method isn't recommended for cathedral ceilings. For more information, see Cut-and-Cobble Insulation.

You can seal the seams of polyiso with canned spray foam if you want, or you can install Tremco sealant (or another brand of caulk) at the perimeter of each rectangle to make the installation airtight.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 01/02/2014 - 10:54

4.
Helpful? 0

Martin,
The last thing I want to be is one who asks for expert advise and not listen, I am, but please explain the difference in making a site built air chase out of iso,fiberglasss in rafter cavity followed by covering rafters in more iso versus cobbling with iso. Regards

Answered by Robert Mason
Posted Thu, 01/02/2014 - 12:49

5.
Helpful? 0

Robert,
Q. "Please explain the difference in making a site-built air chase out of polyiso, fiberglasss in rafter cavity, followed by covering rafters in more polyiso, versus cobbling with polyiso."

A. I don't understand the question. The difference between the first example and the second example is that the first example has fiberglass in the middle, and the second example has polyiso all the way through.

Neither assembly is recommended.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 01/02/2014 - 13:26

6.
Helpful? 0

Martin, I had thought I read that you approved iso being used as vent baffle stock, My mistake. So if I have this right I can use EPS or osb for my baffle, sealed,reuse fiberglass, add caulked recycled iso to rafters followed by strapping and airtight drywall. Glad I asked. Regards

Answered by Robert Mason
Posted Thu, 01/02/2014 - 18:11

7.
Helpful? 0

Robert,
Your latest proposal is the best of your proposals yet.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 01/02/2014 - 18:46

8.
Helpful? 0

Thanks

Answered by Robert Mason
Posted Thu, 01/02/2014 - 20:14

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