Unvented, uninsulated cathedral ceiling in conditioned space – what to do?
Hello everyone,
I’m sure others have come across this situation, but I’ve searched the site and can’t seem to find this particular arrangement discussed.
I have a 1928 brick veneer over frame house with original asbestos “slate” roof tiles, which I’m in the process of replacing with Davinci composite “slates”. I’m getting ready to dry in the roof above the vaulted ceiling over the 25×18 ft great room and pondering ways of improving the energy efficiency for the room. This is an uninsulated, unvented, cathedral ceiling with the ridge elevation 17 ft. above floor level. The roof slope is 14;12. The room is equipped with two large steam radiators and a wood burning fireplace. The room has three exterior walls with single pane, leaded glass, casement windows and exterior storms. The one “warm” wall has an opening to the rest of the house protected by a pair of arched single pane divided light doors. Below is an unheated basement. The room is at the farthest point from the central boiler. The room runs about 10 – 15 degrees lower than the rest of the house in winter. I typically keep the doors closed in winter months unless we plan on using the room for entertaining, which takes a lot of preheating, meaning opening the doors, raising the thermostat and heating the rest of the house hotter than we like. In addition we may add a couple electric space heaters, but he room is still drafty and not so comfy. I’m considering giving up on wood fires and installing a vented, high efficiency, gas fired insert for the fireplace to provide heat to the space, plus two ceiling fans to help direct heated air down to floor level. But what about roof insulation…. here lies my dilemma.
The open rafters and rafter ties are encased in chestnut trim. There are two layers of sheathing above the rafters. The first layer (ceiling) is finished chestnut boards of varying widths covered by 1/2 in. of perlite board and then the exterior sheathing. A 50 lb. felt paper was above the sheathing and then the asbestos tiles. We were extremely happy to note the sheathing is in impeccable condition from eaves to ridge showing no signs of mold or rot or ice damming.
So, the uninsulated, unvented roof has held up remarkably over the life of the old roof. I suppose due to drafts and the presence of the voids under the roof tile array has allowed moisture in the air to escape naturally through the 1/16 -1/4 in. gaps between ceiling boards. and not condense on the sheathing.
In an effort to increase the comfort level in the room and reduce heat loss through the roof, I was considering adding a layer of rigid insulation over the roof sheathing. My research tells me that code requires r-49 and that the rigid boards over the sheathing need to be under at least 6 inches in order to warm the sheathing properly from within. I have no intention of changing the aesthetics inside the room, so adding soft insulation and vapor barriers are not an option. In order to maintain the rake line in keeping with the other four gables, I feel I cannot add more than 2 inches of insulation over the sheathing. So, the question is, in an effort to increase the comfort level in the room would putting one layer of 2 in rigid foam do good or harm to my situation. I would likely use a 2 in Zip panel and tape seams. and then apply the new roof over this.
Does this make for a better system than what was originally designed or am I asking for trouble by potentially creating an exterior vapor barrier trapping moisture between the deck and insulation board and future problems down the road.
The alternative would be top keep rood as is and apply the roofing over 2 layers of felt with ice shied at bottom and seal the room as best as possible at the soffits. I’m rebuilding them as part of the roof job and have the opportunity to perhaps place some rigid foam with spray foam in each rafter bay end at soffit to help reduce draft.
Any advise would be greatly appreciated.
Thanks In advance.
D
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Replies
2" of polyiso rigid foam would be a significant improvement.
David,
1. Interior moisture does not need to "escape." It's fine if interior moisture stays right where it is. You don't want to encourage moisture flow through an insulated ceiling, whether by air leakage or diffusion.
2. Any amount of rigid foam insulation is an improvement -- there is no "minimum" R-value from a building science perspective. The only minimum requirement is governed by your local building code. Two inches is as big improvement, as Jon R noted.
3. You should read this article: "How to Install Rigid Foam On Top of Roof Sheathing."
Thanks for the reply Jon R. and Martin.
Martin,
1, By virtue of the current roof/ceiling not being insulated and having a heated space below, I'.m curious why no signs of condensation exist, or is it that any moist air that escaped from the warm room to outside simply did so through gaps in the sheathing and through the old felt paper to the underside of the slate? Just trying to understand the physics. (?).
2. Now, by applying the foam board on top do I potentially trap moisture where in the past it was free to escape?
3. If I were to apply a layer of 30 lb. felt over the existing sheathing to dry in the roof while waiting for the foam board application, would it be safe to leave that down and apply foam board (ZIP r sheathing) on top? Or would that additional layer somehow create issues?
4. My original question proposed 2 in of insulation, but I was actually considering the Zip system R-sheathing which is nominal 2 in. with 7/16 OSB and 1.5 in. foam (r-9). Aesthetically, the nominal 1.5 in Zip r-sheathing works better for my rake line crown molding, but that's a 1 in. insulation board on 7/16 in. OSB (r-6). I would be sacrificing some r value, but based on your original answer it seems 1.5 or even the 1.0 in. of insulation would be an improvement over the current nothing. Do you concur?
Just talked with HuberWood rep who informed me the Zip r sheathing is not approved for roof installations. I know I can build up rigid foam and sheathing in place but really wanted to avoid this. Now thinking of going with the option of re roofing like it was but replacing the felt paper with a peel and stick membrane. Would it be wise to install such an air barrier (ie. ice and water shield) over entire roof sheathing in place of the felt paper? Seems like it would help eliminate drafts, but would it trap moisture in deck making matters worse?
Roofs (other than tile or slate on skip sheathing) in cold climates don't generally dry toward the exterior. Your prior stackup definitely did not have appreciable drying capacity toward the exterior. Putting an impermeable waterproof membrane over the exterior does not make things worse.
Thanks Dana,
So the benefit to an impermeable membrane vs the felt would simply be to eliminate or at least greatly reduce the draft through the sheathing thus conserving heat in the space and
improved confort.
David, there are manufacturers of polyiso pre-adhered to OSB panels besides Huber, and they are made for roofs. (I don't know of any reason why Huber's panels won't work on a roof; they just haven't tested them for that application.) This type of panel is called Nailbase. Hunter Panel and Foard Panel are two manufacturers that I know of.
Thanks Michael, I'll look into those. In your opinion, would I still install either a felt or ice shield type barrier under such a panel over existing sheathing? If so which type and what are advantages and disadvantages to each?
Sounds like you are now considering a partition that is:
a) leaky (to air) on the inside, tight on the outside
b) vapor permeable on the inside, impermeable on the outside
c) has all the insulation on the inside of the sheathing
d) is subject to stack effect pressure pushing moisture outwards
This is a high risk partition in a cold climate.