Helpful? 0

Cut and cobble vented cathedral ceiling

We are finishing our attic on a tight budget. We just had new roofing installed 2 years ago and unfortunately did not add continuous exterior insulation at that time since we had an insulated attic (at the time). The floor-to-ceiling height is so low that every inch counts, so we are looking at cut and cobble to maximize R-value within a very small space. House is 1927 drafty everywhere, so unvented roof seems impossible to retrofit appropriately without spending a lot of time and money. We are in Portland, Oregon (moderate climate). Here's the proposed roof/ceiling assembly going from exterior to interior:

- Existing roof: composite shingles, building paper, roof decking (1x t&g), 2x4 rafters.
- Add new ridge vent and 3.5" rafter cavity vent space with insulation: 1" vent spacers, 1 layer 1.5" rigid insulation, 2nd layer 1" rigid insulation w/ staggered joints.
- Then, to get additional R-value (even though we will still be below code): add 2x2 furring strips crosswise @ 24"oc for drywall fastening, and infill with an additional layer 1.5" continuous rigid insulation over the inside face of studs. Tape all seams - or use drywall as air barrier.
- Finish layer: 1/2" gypsum board.

For the walls separating finished from unfinished attic, we are planning 2x4 studs, 3.5" mineral wool insulation, gypsum board interior finish as air barrier (unfinished on attic side).

Questions:
1. I am confused about the air barrier. Existing house ceiling is far from air tight between 1st floor finished spaces and existing unfinished attic. Does it make sense to try to make the new finished walls & ceiling of the attic air-tight, even though there still be air leaks between the existing finished and existing-to-remain attic spaces?
2. We are planning to use R-Tech EPS foam in the roof for better drying, but would XPS (Foamular) work instead? It has a better R-value.
3. What kind of tape would be best to use for sealing the insulation/wood seams?
4. Does it make sense to add the additional layer of insulation inside the roof rafters, or will this mess up the dewpoint and create moisture problems?
5. Any other comments on the proposed assemblies?

Thanks for your help!

Asked by Miyeko Endy
Posted Wed, 06/18/2014 - 13:58

Tags:

3 Answers

Select your preferred way to display the comments and click "Save settings" to activate your changes.
1.
Helpful? 0

Miyeko,
Your plan will work, as long as you use canned spray foam at the perimeter of each rectangle of rigid foam to make sure than there are no air leaks. For more information on this technique, see Cut-and-Cobble Insulation.

(When insulating a cathedral ceiling, vented cut-and-cobble, as you propose, is less risky than unvented cut-and-cobble.)

Adding a layer of continuous rigid foam on the interior side of your rafters will not cause any problems.

Before proceeding, you should probably check with an engineer to make sure that your 2x4 rafters are safe.

For more information on tape selection, see Return to the Backyard Tape Test.

I don't really understand your air barrier question. The bottom line is, you need an air barrier. Once you have identified your air barrier, make sure that it is continuous with the rest of your air barrier (at your walls).

Answered by Martin Holladay, GBA Advisor
Posted Wed, 06/18/2014 - 14:37

2.
Helpful? 0

Martin, thanks for the quick reply and the links to related posts. There are some great stories to reference.

We're still deciding on which type of foam board to use. Your backyard experiment didn't include testing tape samples on scraps of EPS substrate - but do you know of any recommendations for taping seams between EPS foam boards for air sealing?

Reading the cut and cobble stories about long installation hours, we're now thinking about using only a single layer of 2-inch foam between the rafters (half the installation time of 2 layers) and 2x vent spacers under the roof's board sheathing. Then we'd install a single layer of 1 or 1.5-inch continuous foam boards on the inside of the rafters. Would there be any advantage to hanging drywall from horizontal metal z-furring on the inside of the roof rafters, rather than running 2x2 strapping? We're still trying to maximize head room as much as possible. We'll likely have 6'-6" clear height for a 30-inch width along the length of the ridge, with everything else sloping down at a 6:12 pitch.

To clarify the air barrier question -
Our existing house is one story with a simple vented and drafty attic with cellulose insulation over the ceiling. The existing First Floor ceiling (aka attic floor) assembly is likely not very air tight. When considering our new attic remodel, we assumed we'd want this to be airtight below the vented roof. Does the higher degree of airtightness in the attic's vaulted ceiling and 4-foot knee walls make sense, given the relative drafty construction of the First Floor down below? You could think of this as a "swim cap" - where the sloped roof and knee walls over the finished attic are the swim cap, and the low vented unfinished attic behind the knee walls and the old drafty attic floor are the swimmer's ears and hair spilling out the edges of the (air) tight cap.

We're on a really tight budget and are accustomed to a poorly insulated house. Our priorities are to improve occupant comfort and avoid moisture problems. Does our "swim cap" approach have merit, given the pressure curve of the taller volume and the tendency for air leakage being worse the higher up the house you go?

With this hybrid cut and cob (single 2-inch layer between studs) and additional continuous (1-inch layer inside studs) - is it really critical to seal the gaps in the cut and cob layer? Rather, we could achieve airtightness with a good tape for the seams of the continuous interior layer? We could even run the continuous layer down the inside of the knee walls (with mineral wool batts in the stud cavity) so that all the continuous foam seams would be taped from the ridge down to the attic floor at least.

With our immediate budget and schedule restrictions (3 months and $3k) perhaps we can just consider doing the attic enclosure properly (airtight) now, and then we can fix other potential air leaks at the existing ceiling down below in the future.

Does that all make these questions more clear, or less?
Thanks again for your help.

Answered by Miyeko Endy
Posted Thu, 06/19/2014 - 03:42

3.
Helpful? 0

Miyeko,
Q. "Do you know of any recommendations for taping seams between EPS foam boards for air sealing?"

A. I would use Siga Wigluv.

Q. "We're now thinking about using only a single layer of 2-inch foam between the rafters (half the installation time of 2 layers) and 2x vent spacers under the roof's board sheathing. Then we'd install a single layer of 1 or 1.5-inch continuous foam boards on the inside of the rafters."

A. Your plan will result in an assembly R-value of about R-12 or R-14 (actually, less than that, because of thermal bridging through the rafters). This is a very low R-value -- much less than minimum code requirements.

Q. "Would there be any advantage to hanging drywall from horizontal metal z-furring on the inside of the roof rafters, rather than running 2x2 strapping?"

A. If I were you, I would install 1x4 strapping, 16 inches on center, under the continuous layer of foam, and would screw my drywall to the strapping.

Q. "Does our swim cap approach have merit?"

A. No. Instead of leaving cold, unconditioned attics behind your kneewalls, your insulation should follow the roof slope, all the way down to the eaves. For more information, see Two ways to insulate attic kneewalls.

Q. "With this hybrid cut and cobble approach (single 2-inch layer between studs and additional continuous 1-inch layer inside studs) - is it really critical to seal the gaps in the cut and cobble layer?"

A. Yes.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 06/19/2014 - 06:36

Other Questions in Energy efficiency and durability

Roof insulation

In GBA Pro help | Asked by Steven Clark | Oct 24, 14

Add R-14 Roxul between the garage attic and the house?

In Energy efficiency and durability | Asked by Mike Lucas | Oct 23, 14

Minisplit - One indoor unit for two adjacent rooms?

In Mechanicals | Asked by Steve Robertson | Oct 23, 14

How do deviations in average winter temperature affect energy consumption?

In Energy efficiency and durability | Asked by norm farwell | Oct 24, 14
Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!