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A few questions on my cathedral ceiling plan

Hi All, I've been reading through the forums for the last month or so as I'm preparing to reinsulate my cathedral ceiling in my master bedroom and home office. I've found tons of great information so far and just had a few questions on my current plan to see if I'm on the right track.

The ceiling in question is in a 22x13 room sectioned into a master bedroom and home office with continuous soffit vent and ridge vent. The rafters are 2x6 and the roof is new. When the new roof went on I had a couple of skylights removed, although the framing is still there, that's where some of my questions come in. I need to finish the interior with the giant skylight holes having nothing but decking and some added 2x4's to support the new sheathing. There are also some can lights that I'm going to remove during the remodel. There's a ceiling fan that's staying that should be the only "hole" in the ceiling. I can't afford to lose more than 4 1/2" of ceiling height without running into the trim for my french doors off of the office. I'm in climate zone 5 and I'd like to stay vented. The current ceiling is from outside to inside

- new asphalt shingles
- 6' of ice and water, followed by 15# roofing felt for the other rows
- 1/2" plywood(osb where the skylights were)
- some venting space with degrading foam baffles(maybe 1" or so)
- 3 1/2" of fiberglass
- 1" foil faced polyiso on the bottom of the rafters(seams not taped)
- 1/2" Sheetrock(not taped)
- tongue and groove pine

Here's my current plan for the new ceiling(outside to inside), to get the depth needed I'm planning to add 2x4s to the current 2x6 framing. Can't afford spray foam on this remodel.

- new asphalt shingles
- 6' of ice and water, followed by 15# roofing felt for the other rows
- 1/2" plywood(osb where the skylights were)
- site built 2" venting space using 1" XPS(3" total) R-5
- 5 1/2" R23 Roxul
- 2 inches of foil faced polyiso with taped seams attached to the bottom of the new 2x4s R-13
- 1/2" sheetrock, joints taped
- new tongue and groove pine(old boards are in tough shape already)

My questions:
- I figure this to be R41 and lose exactly 4 1/2" inches, any other suggestions of insulation combos that are more efficient either cost wise or R value wise?

- I think I'm ok with this foam sandwich as long as the XPS is unfaced, is that correct? Should I use Fiberglass faced polyiso instead?

- How should I make the baffles? Is it ok to tape 2" blocks of XPS to the rafter width pieces of XPS? Can I put in 2" foam blocks with PL-300 and then glue the rafter width pieces to that with it as well? Do I need to attach them to the rafters?

- For the bottom foam layer, do I need polyiso with foil facing on two sides or is one sufficient?

- For the foil faced polyiso, I'm planning on attaching with cap nails, is that the best way?

- Can I just attach the sheetrock with 3" screws and skip any 1x4s on top of the foam? If so how do I attach the tongue and groove?

- If I really need 1x4s on top of the foam, should I run them parallel or perpendicular to the rafters? I would think if I run them perpendicular I have no way to nail the tongue and groove pine to them.

Since two of the bays are blocked by the skylight framing, should I find a way to vent those cavities? Can I notch out the top 2" of the cross pieces top and bottom to allow venting and seal the foam chutes to those?

Ok that worked out to be way more questions than I had hoped. Thanks in advance for any replies and for having such a great forum. I've learned a lot in the last few weeks browsing around and spent a lot of time checking out Dr Joes articles as well.

Thanks!
Ed

Asked by Edward Dziarcak
Posted Fri, 08/22/2014 - 16:08

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4 Answers

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1.
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Edward,
Too many questions -- but I'll tackle a few of them.

Q. "I think I'm OK with this foam sandwich as long as the XPS is unfaced. Is that correct?"

A. Yes. Pay close attention to airtightness on the interior layers, and you should be OK.

Q. "How should I make the baffles? Is it OK to tape 2" blocks of XPS to the rafter width pieces of XPS?"

A. Yes. (I assume that the 2-inch pieces are inserted in the corners of the rafter bays to establish the depth of the ventilation channel.)

Q. "Can I put in 2-inch foam blocks with PL-300 and then glue the rafter width pieces to that with it as well?"

A. Yes.

Q. "Do I need to attach them to the rafters?"

A. No, as long as they are secured in some way that gives you confidence.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 08/22/2014 - 16:38
Edited Fri, 08/22/2014 - 17:16.

2.
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The venting is making this safer than not.

Answered by aj builder, Upstate NY Zone 6a
Posted Fri, 08/22/2014 - 21:07

3.
Helpful? 0

Edward,
I'll try to tackle a few more questions.

Q. "For the bottom foam layer, do I need polyiso with foil facing on two sides or is one sufficient?"

A. Any type of facing is fine. There are two advantages to foil facing: (1) It is easy to tape, and (2) it provides an R-2 boost if it faces an air space (as might be the case if you install 1x4 furring strips below the rigid foam).

Q. "For the foil-faced polyiso, I'm planning on attaching with cap nails. Is that the best way?"

A. That will work. If you are installing furring strips, all you need is enough cap nails to temporarily hold the foam in place, until the furring strips are installed.

Q. "Can I just attach the sheetrock with 3" screws and skip any 1x4s on top of the foam?"

A. You can -- but I wouldn't recommend it. It can be hard to find the rafters with your suggested approach.

Q. "If so, how do I attach the tongue and groove [ceiling boards]?"

A. Exactly. The furring strips are useful.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 08/24/2014 - 05:25

4.
Helpful? 0

Martin,

Thank you so much for the detailed answers, they are extremely helpful. I'm convinced now that it's worth adding in the furring strips, especially if I get another R-2 out of it!

Thanks,
Ed

Answered by Edward Dziarcak
Posted Mon, 08/25/2014 - 07:58

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