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Cathedral ceiling retrofit

I have a section of my house with a cathedral ceiling.

SW Michigan - Zone 5A
~50 lb snow load area
4/12 pitch.
One side of the cathedral runs up to the house.

The current ceiling is (from inside the outside):

Roof is 17' wide and 20' run, gutter to ridge.
Timber Rafters - ~6' on center, about 4'x10'
T&G boards on top of the rafters.
About R-20 (guessing it is 6-8" thick) insulation.
Roof deck and black asphalt shingles.

I have 2 problems.

Bad ice dams
Noticeable temperature change to the next room. (hotter in summer and colder in winter).

The basic outline of my plan is to take off the existing shingle layer, then add 6" of foam board and then add a cold roof. This should take the total R value up to R50 or so. (I'll need to inspect the existing roof deck and fix any issues there.)

The question is how to I site build the insulation and take care of the structural component of the roof?

Two basic options:
A)
Air barrier made with peal and stick ice guard (no can lights, make sure there are no air leaks).
Dimensional lumber "ring" around the edges.
Lay down foam sheets with staggered and taped seams 6" layers thick. (foamed gaps.)
Lay down roof paper.
Lay down 2x4s to make up the cold roof air gaps - screwed through the whole thing to the OSB.
OSB nailed to the 2x4s.
Ice and water, felt and shingles
(taking care to do ridge and eave vents and dress gaps in the foam sheet with spray foam.)

B)
The alternative I've seen does basically the same thing, but with each layer of foam sheet layed at 90* with 2x4s between them and staggered so the thermal bridges are minimized. Seams are taped and foamed to control air leaks.

Thinking through this, it seams that the difference is that using 2x4s on a bias trades more wood for fewer long (8" or so) screws.

When you build up the roof with 6" of foam board, how to you manage sheer loads?
Is "B" overkill?
What is the fastener pattern for the 2x4 air gap and OSB roof deck?

Asked by Ian Osborn
Posted Tue, 09/02/2014 - 14:47
Edited Wed, 09/03/2014 - 04:52

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8 Answers

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1.
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B offers no advantages beyond the shorter screws.

Option A needs to have the 2x4 furring through-screwed to your rafter timbers or (presumably) 1/5" thick t & g decking, not just OSB. Use pancake head timber screws 24" o.c for securing the furring, then ring-shank nail the OSB to the furring.

It's also possible to skip the 2x4 furring and through-screw the OSB nailer deck to something structural 24" o.c. with pancake-head timber screws and just assume you will have to replace some of that OSB when you re-roof if the roof leaks.

In a zone 5A climate you get more performance out of a 6" stackup if the inner 3-4" is polyiso, and the outer 2-3" is EPS, due to the non-linear performance derating of polyiso with temperature. (6" of XPS would deliver R30 too, but would slowly degrade to R25 as it loses it's HFC blowing agents, blowing agents that pack a powerful global warming punch compared to those used for polyiso & EPS..) That would give you a nominal R30 of foam on the exterior, which is sufficient for dew point control with up to R45 in fiber on the interior side of the roof deck, for a center-cavity R of R75. As long as 40% or more of the total center-cavity R is on the exterior of the structural roof deck you'd be good to go in your climate without need of interior side vapor retarder tighter than latex paint on gypsum.

You don't say what that 6-8" of pre-existing insulation is, or whether it has foil or kraft facers on it. It's not likely to be more than R30, but if it leaks copious air exchanges with the interior you may still have some amount of moisture risk at the roof deck.

Answered by Dana Dorsett
Posted Tue, 09/02/2014 - 15:09
Edited Tue, 09/02/2014 - 15:15.

2.
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I do not know what is on the inside of the existing roof. I have a different problem related to the same roof that will be addressed more conventionally. There is no step flashing against the house-roof interface and not step or counter flashing against a chimney on the exposed gable end.

We're planning on doing this next spring, but need to build a budget. I'll need to pull the roof off and call an audible on what to do. I may have to pull off the existing roof deck and get down to the T&G that makes up he interior finish. In that case i'll need to put down an air barrier there. Given when and how it was build, my guess is that it is fiber insulation, so it will all come off. If that is so, then i may put down spray foam.

Do i need to put down roof paper (or peal and stick) on top of the insulation sandwich and under the 2x4 air gap furring?

I appreciate your advice!

Answered by Ian Osborn
Posted Tue, 09/02/2014 - 18:04
Edited Tue, 09/02/2014 - 18:08.

3.
Helpful? 0

Ian,
Q. "Do I need to put down roof paper (or peel-and-stick) on top of the insulation sandwich and under the 2x4 air gap furring?"

A. If the top layer of rigid foam is foil-faced polyiso or foil-faced EPS, the answer is no. If the top lay of rigid foam is unfaced EPS (which is hard to tape), you might want to put down a layer of underlayment -- although that will require some very long cap screws.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 09/03/2014 - 04:50
Edited Wed, 09/03/2014 - 04:52.

4.
Helpful? 0

Rigid foam roofs need to be vented above the foam and any existing vent closed. Here in the Adirondacks we have had to demo cold roofs because they build up rot from air leaks condensation and such. Also 6" of rigid is not worth doing as the rest of the home is not built to passive house, 4" is plenty.

Alternative plan;

1- The T&G ceiling has to come down as it leaks air.
2- Then make sure you have venting
3- Then you can have high R batt insulation
4- Then you add rigid foam taped to the inside
5- Then you can add furring
6- Then you can put back the T&G

We build with this alternative plan and it is a good cathedral roof.

Answered by aj builder, Upstate NY Zone 6a
Posted Wed, 09/03/2014 - 08:30

5.
Helpful? 0

BTW: For budgeting purposes, be aware that reclaimed rigid foam from commercial building demolition/re-roofing can be had a huge discount from virgin-stock goods.

There are multiple vendors (large & small) in my area, but there are also who will ship nationally (for a price). eg:

http://www.greeninsulationgroup.com/

http://nationwidefoam.com/

From a design robustness point of view, derate reclaimed XPS to R4.2/inch, polyiso to R5.5/inch, EPS R4/inch.

Answered by Dana Dorsett
Posted Wed, 09/03/2014 - 09:39

6.
Helpful? 0

Thanks again for the advice.

For clarity, the timber beams are exposed to the inside and the T&G is layed on top of the timber. The bottom of the T&G is the cosmetic finish for the room. I do not see evidence of any ridge rot or condensation on the underside of the T&G. So I may get lucky and find that it was detailed properly. I then just need to add insulation to take care of the ice dams and normalize the interior room temps.

Depending on how the roof structure looks when I take off the shingles, I may need to take out the existing insulation.

I do not know what is under the roof deck. Likely 2x6s or 2x8 rafters on top of the T&G and insulation between the rafters. I'd rather avoid taking up the T&G as that would mean basically the entire roof comes off. I may get lucky and find that it is sheet foam under the roof deck - who knows. I do not know when the room was built. Sometime after the initial construction of the house in 1962.

If the roof deck and insulation needs to come off due to water damage, I would be to put down peal and stick ice and water shield on the top side of the T&G to form an air barrier.

My understanding of codes from Table R806.5 from ICC is that I need at least R20, assuming I already have R20 in the roof. So at least 4" of foam (depending on what I end up using). I understand that doing a small part of the whole roof (roughly 20% of the total) won't have an overall impact on the house's energy loss, but I do need to control ice dams. I will put a vented "cold" roof over the top of the insulation sandwich.

Thanks for the recycled foam links. Most assuredly, something I will check into. The budget I've built so far suggests that the foam is around 40-50% of the total cost. So something worth looking into.

Answered by Ian Osborn
Posted Wed, 09/03/2014 - 13:10
Edited Wed, 09/03/2014 - 15:53.

7.
Helpful? 0

It matters how much insulation/space there is between the finish ceiling t & g and the roof deck, since that affects how much exterior-R is required for dew point control. The R20 prescriptive in IRC 2012 chapter 8 is only valid at a total R of R49, which is code min spelled out in IRC 2012 chapter 11. If you have 2x12 rafters with R38 in there you'll need more foam. Drilling a small hole in the ceilng in an inconspicuous place and probing to measure the full depth

If there is a gap between the cavity insulation & roof deck (probably is, since it hasn't rotted away), it's worth blowing the cavities full with either cellulose or fiberglass before building up the membrane & foam layers. If the gap is allowed to persist it becomes a thermal bypass and infiltration channel- stuffing it with fiber insulation makes it very air-retardent, and adds some R-value to boot. The blown fiber can be installed from the exterior just prior to laying down your peel'n'stick membrane. With the inherent air-leakiness of t&g it's probably best to use cellulose, since it's more air retardent than fiberglass, and would buffer some of the moisture burden of any air that does leak in during the winter, protecting the roof deck.

While a vented nailer deck will make the nailer deck last longer, R20+ foam itself provides substantial ice-dam mitigation, since it thermally breaks the rafters. On a thermally-broken R50 roof a vented nailer provides almost no additional benefit from an ice damming point of view.

Answered by Dana Dorsett
Posted Wed, 09/03/2014 - 13:50

8.
Helpful? 0

Ok, so i drilled a small hole to probe the ceiling..... No insulation at all. Not sure how that passed code inspection. Double checked with some measurements inside and out. Lol.

Ok, so i guess i need more than 4" of foam, but the job may end up being simpler in the end.

Answered by Ian Osborn
Posted Wed, 09/03/2014 - 16:58

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