Insulating a non vented vaulted ceiling in Canada
Hello!
I’ve recently had a 10×14 foot backyard studio built for me by a local company. I asked them not to insulate it because I was hoping to save some money by doing it myself. However, after some research I’m not really sure how to continue.
The studio has a vaulted 2×6 ceiling (with one skylight) with no ventilation. The soffits also have no vents. Initially, I was planning to just shove some fibreglass batts up there, putt up a vapor barrier and then finish it off with some wood paneling. However, after researching it sounds like that’s a recipe for moisture and mold.
So, now I’m wondering what my best option is. Currently I’m thinking of putting foam panels directly under the roof sheathing and then finishing it off with some batts underneath. But I’m not sure if that will get me the r value I need as I live in Alberta Canada and it gets really cold here. Also I was just going to put fibreglass batts in the wall. Is this a bad idea and what kind of other barriers would I need?
If you have a suggestion please share it! I’d like to keep my costs under $500 as I’ve already spent a lot on this studio but I’m realizing it could cost more. Also I don’t have any building codes or permits required where I live.
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Replies
Kirsten,
In Alberta, you are either in Climate Zone 6 (the southern part of the province) or Climate Zone 7 (up north). Here is a link to a climate zone map that includes Canada:
https://www.greenbuildingadvisor.com/sites/default/files/Climate%20Zone%20map%20including%20Canada.jpg
If you want to build an unvented roof -- and considering the fact that the roof has a skylight, that's the best approach -- it would have been a good idea to install a significant amount of rigid foam insulation above your roof sheathing, before you installed your roofing.
But it sounds like you didn't do that. That limits your options. At this point, you have to insulate this roof with spray foam, which isn't very cheap. You'll need a layer of closed-cell spray foam rated at R-25 (if you live in Climate Zone 6) or R-30 (if you live in Climate Zone 7) to keep your roof assembly out of trouble. Once the spray foam is installed, you can install additional insulation -- something like fiberglass batts or mineral wool -- on the interior side of the spray foam. You want enough additional insulation to reach a total R-value of R-49 in your climate. That means that you will need to scab on some additional framing lumber under your 2x6 rafters.
Your suggested approach (using rigid foam insulation) is called the "cut-and-cobble" approach. I don't recommend this approach for unvented cathedral ceilings, because there have been reports of moisture problems and rot when this approach is used. For more information, see Cut-and-Cobble Insulation.
For an article that explains all of the different ways to insulate an unvented cathedral ceiling, see How to Build an Insulated Cathedral Ceiling.
If any GBA readers have read this far, there is a moral to Kirsten's story: In a new construction project, don't wait until the last minute to determine your insulation details. You need to decide on your insulation details before construction begins.
Kirsten: A 10x14 studio will be pretty easy to heat, even with less than great insulation. I wouldn't worry about trying to meet code. I would make sure to install a good air barrier.
We could use more info.
Are there sources of moisture in the studio?
What will you heat it with?
Is it on a slab or over a crawl space or foundation? Any insulation in floor?
What's the roofing, sheathing, underlayment?
What's the make-up of the exterior walls?
How often will you need to heat it? Constantly, or just occasionally?
You don't need R25 closed cell foam under the roof deck to protect the roof assembly unless you are insulating to R49. In a zone 6 climate as long as ~50% or more of the insulation is closed cell foam on the exterior of the fiber-R layer you'll have adequate dew-point control at the foam/fiber boundary.
But it doesn't even take THAT much to be protective of the roof deck. As little as 2" of closed cell foam (~R12-R13) would protect the roof deck but could potentially allow wintertime accumulation of moisture in the fiber layer. Download and read a copy of the BA-1001 moisture study performed by Straube et al for the Building Science Corporation:
http://buildingscience.com/documents/bareports/ba-1001-moisture-safe-unvented-wood-roof-systems/view
It's pretty short- read the whole thing, but the summary tables 3 & 4 indicate that even in zone 7 the roof deck is fully protected when the relative humidity is maintained at normal levels for occupied conditioned homes in winter, even at R49 total-R. In a studio the room humidity is likely to be LOWER than in normal living space unless you are actively humidifying the place, or cooking & showering out there. You'd probably be just fine with 1" of closed cell foam.
The best path forward at your budget range would be to start with 1" of closed cell foam on the underside of the roof deck (R6, costing probably $150-200- if a contractor quotes you more than $300, buy a 200 board-foot DIY foam kit, which runs about $300 at box stores.) Buy 1-2 sheets of 1" rigid polyisocyanurate foam board and cut 1.5-2" wide strips, and attach them to the under side of the rafter edges with dabs of foam-board construction adhesive and cap nails, then friction-fit R23 rock wool batts in to the cavities. Then, detail a sheet of Certianteed MemBrain (about $100 for an 8' x 100' roll online) on the underside of the polyiso & rock wool as an air-barrier in the same manner that you would 6-mil polyethylene vapor barriers before putting up the ceiling gypsum.
The polyiso strips roughly double the R-value at the rafters, and you'll have a center-R value of about R35. The total performance is approximately that of R38 batts in 2 x12 joists, which isn't terrible. The MemBrain is a "smart" vapor retarder that meets the Canadian code definition for a vapor barrier when the proximate air is below ~40% relative humidity (which it will be all winter), but becomes vapor open at humidity levels high enough to support mold growth. It can dry out the fiber layer more quickly than it takes on moisture, which should be more than sufficient protection in your climate, and with the 1" closed cell foam (which is almost at code-qualified vapor retardency you'll have no problem with the roof deck.
In a couple of decades when it's time to re-roof, you can safely add rigid foam board above the roof deck to improve both thermal and moisture performance It won't create a moisture trap the way 6-mil polyethylene would.
Thanks you all very much. You've given me lots to think about. Had I known what a difficulty insulating the roof was I certainly wouldn't have gone the route I have. I'm quite ignorant about construction so your help is very appreciated.
Stephen here is the additional info:
Are there sources of moisture in the studio? - No. Only myself. I'll be using it for painting, playing cello and doing some reading.
What will you heat it with? - Right now I'm looking a using a oil filled radiator.
Is it on a slab or over a crawl space or foundation? Any insulation in floor? - It is built on skids and sitting on cinder blocks so there is up to half a foot of space underneath. The floor is insulated.
What's the roofing, sheathing, underlayment? - As far as I know it's just shingles and a plywood roof sheathing.
What's the make-up of the exterior walls? - walls are 2x4 construction with plywood sheeting. It has smart panel siding
How often will you need to heat it? Constantly, or just occasionally? - I'll be heating it most days. From a few hours to 12 hours. It won't be heated at night.
A couple of additional thoughts and questions:
Will the cello and paint stay out there overnight while it's not heated? The paint might not like extreme cold, and the cello might also be affected by humidity and temperature fluctuations. Another consideration with some paint might be having sufficient ventilation to avoid breathing paint vapors excessively.
How much headroom do you have? The brainstorming above is along the lines of ways to do it without lowering the ceiling very much below the rafters. If you lower the ceiling enough to create some vent space above the insulation, everything suddenly gets easy again. That could be anything from 2" of vent space to installing a flat ceiling with much larger vent space above.
As far as heat sources, a oil-filled electric heater is rather slow to heat up, and no more efficient than any other electric heat source. Radiant electric heaters will start delivering heat to you immediately when the turn on, and can help you be comfortable even before the space heats up all the way. If you don't mind an industrial look, the quartz tube type with reflectors can direct the heat right at you pretty effectively:
http://www.globalindustrial.com/p/hvac/heaters/portable-electric/cz-heater-ceiling-mount-quartz-czqtv5m-7501500w-5120-btu
http://www.northerntool.com/shop/tools/product_200632848_200632848
For a less obtrusive look, radiant cove heaters can also work well.
http://www.radiantsystemsinc.com/
Or a portable panel heater like this:
http://www.quill.com/heaters/cbs/51621711.html?cm_mmc=SEM_PLA_CB_51621711&mcode=SEM_PLA_CB_51621711
You might want to minimize thermal mass on the interior side of the insulation, so as to allow quickly heating the space up. The extreme would be to use foil-faced foam board insulation with nothing else covering it. If you buy Dow Thermax polyiso insulation it's rated for use exposed with no additional fire protection. It has a white side and a bare aluminum foil side. The bare foil would work well with the radiant heat ... but is probably not the look you are going for so you probably want to use it as intended with the white side facing in.
If you used that approach, you'd still want some kind of fluffy insulation inside the walls, behind the foam layer; and above the foam ceiling.
Thanks for the input Charlie.
No I'll be taking my paints and cello into the house when I'm not using them as it can get down to -40 here sometimes. Ideally, I'd actually love it keep it heated 24/7 however I haven't found a safe and inexpensive way to do that yet.
As far as head space, I have a couple feet. However if I don't need to I would really rather prefer not to lower the ceiling. That's why I got the vaulted ceiling in the first place.
So if I used the Dow Thermax insulation it is meant to stay uncovered or could you put wood paneling over it?
Kirsten,
Q. "If I used the Dow Thermax insulation it is meant to stay uncovered or could you put wood paneling over it?"
A. Most building codes require that any rigid foam in a finished, occupied room be covered with 1/2-inch gypsum wallboard (drywall) or 3/4-inch solid wood paneling. This is a fire safety issue.
The Dow Thermax brand has passed certain fire safety tests and is sometimes allowed to be exposed in a basement or attic. I doubt whether most building inspectors would allow it to be exposed in an occupied, above-grade room, however. When in doubt, ask your local building department.