Roof for small cold-climate A-frame “writing room”
Hi Martin —
I’m helping a friend build a small, simple 10×12 A-frame in northwestern MA (Lanesboro). It will be in the woods a hundred yard from his house, and mostly be used as a “writing room” to escape the house, but occasionally as a bunk house for guests who have overstayed their welcome (like me). Single room for the bottom, half loft on top.
No permit required, so no great concerns about code. Low-voltage lighting from a solar panel and no running water, so generally low humidity. Reclaimed single-pane windows, so our insulation goals aren’t that high. But we’d like something that can be kept warm for a few days even in the middle of winter with a tiny wood stove, is comfortable the rest of the year with minimal heating, and won’t fall down or rot.
We’re building it now in the snow, and would like to get it enclosed in the next week . We have walls and rafters up, and it’s time to put on a roof. I think we have a good handle on the “ideal” from your site and Lstiburek, but we’re trying to understand what corners can be cut given budget and the limited use of this building. The current roof plan (from the top down):
Asphalt shingles
Tar paper
Plywood
4″ Blue foam XPS (2 x 2″, overlapped, taped seams)
2″x6″ rafters @ 18″ filled with fiberglass bat
Some air barrier
Questions:
1) Should we add battens and an air gap between the plywood and the foam, or are we OK presuming everything should dry out seasonally? It’s plywood not OSB, and 4″ of XPS is still slightly permeable.
2) Screwing through 4″ of foam and hitting a rafter is hard. If we put 2″x4″ sleepers on top of and perpendicular to the rafters, life becomes easier, but then we only have 2″ of foam in places. Think we could get away with it?
3) It’s a tiny roof, but steep (slightly greater than 12:12, 51%). This also makes the 2″x4″ cross pieces appealing. If we don’t use the cross pieces, is there a good and safe way of building this? We have scaffolding up on both sides up to the eaves.
4) We could add another layer of plywood on top of the rafters. This would make it slightly easier to work on, but reduces the inward drying we’re hoping will cover up for our cut corners. And it’s more plywood we’d have to buy. Would this layer make much of a difference?
5) In your great cathedral ceiling article (https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling) you say “the biggest air-barrier blunder is to install tongue-and-groove boards as your finish ceiling without first installing taped gypsum drywall”. How about Tyvek, or some other vapor-permeable housewrap?
Thanks, and Happy New Years!
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I hate to sound un green, but I think you are overdoing it on the insulation. A room that small will get blown out by the woodstove. Having it periodically heated makes it less of a good investment. Use the foam and ditch the interior finish and save enough money to buy a half decent window or two.
Hi Keith --
You might be right. But the windows are salvage, we already have them, and there are holes in the walls to fit them. I make them sound a little worse than they are: they do have detachable storm panes. And if they are too weak of a weak point, they are easier to upgrade than the roof.
I agree on the woodstove, and I'd be interested on other suggestions. Right now our thought (to the extent we have one) is that it might balance out with the lousy windows: sweaters and mittens, start the fire, then T-Shirts and an open window. Repeat when the fire goes out. :)
Nathan,
I'm not sure whether you are planning a cut-and-cobble job (inserting the rigid foam between the rafters) or whether you hope to have a continuous 4-inch-thick layer of rigid foam above the rafters.
If the latter, and if you intend to install asphalt shingles as your roofing, then you need two layers of roof sheathing: one under the rigid foam, and one under the shingles. You can't install rigid foam directly to the rafters.
If you want to eliminate one layer of roof sheathing, the easiest way to do that is to switch from asphalt shingles to metal roofing. The metal roofing can be attached to 2x4 purlins that are installed on top of the rigid foam.
If you intend to install tongue-and-groove boards as your finish ceiling, then the boards can be installed as the first layer of roof sheathing, as long as you don't mind looking at your rafters. If you pay attention to air sealing when you install your two layers of rigid foam, then the rigid foam can be your air barrier -- or the plywood installed above your rigid foam can be your air barrier, as long as the plywood seams are taped.
"...install tongue-and-groove boards as your finish ceiling, then the boards can be installed as the first layer of roof sheathing..." Installed like this, each V groove provides a direct air channel to the exterior. Installing it under the drywall means that the whole board - and every termination of each V joint - is in the interior of the house.
Bob,
That's a good point. If the ceiling boards extend past the exterior walls, you have lots of potential leaks. Keeping the ceiling separate is definitely preferable.
It's hard to tell how important it is for Nathan to have a very tight building. It sounds like Nathan is building an inexpensive A-frame with salvaged single-pane windows, and that the building will only be occasionally heated.
I'm not sure whether you are planning a cut-and-cobble job (inserting the rigid foam between the rafters) or whether you hope to have a continuous 4-inch-thick layer of rigid foam above the rafters.
Looking to put 2 layers of 2" rigid XPS on top of the rafters. Afterward, the space between the rafters will have fiberglass batts applied from the inside. Bottom of the rafters will have an air barrier.
You can't install rigid foam directly to the rafters.
What's the role of the sheathing on the rafters? European construction often seems to avoid it when using rigid insulation/sarking (http://www.designforhomes.org/wp-content/uploads/2012/03/Pitchedroofing.pdf, PR06 here http://www.knaufinsulation.co.uk/media/786378/2_2-pitched-roofs-rafter-level.pdf) but don't often use XPS. I'd presumed it was mainly for ease of installation, at the cost of greater moisture risk.
If you want to eliminate one layer of roof sheathing, the easiest way to do that is to switch from asphalt shingles to metal roofing. The metal roofing can be attached to 2x4 purlins that are installed on top of the rigid foam.
Yes, this would be ideal, but the friend/owner prefers the look of the shingles.
My discomfort with the plywood-XPS-plywood sandwich is that the top layer of ply will dry very slowly once it gets wet. But considering the other weakpoints of the design, perhaps this is fine. Do you think it's worth adding a 3rd layer of ply over battens so there is ventilation between the shingles and the foam?
Also would be interested in your thoughts on the other questions: Would sleepers between the first layer of foam be reasonable? If not, how should we attach the top layer of sheathing? Is Tyvek reasonable as an internal air barrier below the rafters?
Nathan,
Q. "What's the role of the sheathing on the rafters?"
A. It serves several functions. It is often an air barrier. It is structural, and helps prevent the roof assembly from racking. It is rigid enough that it won't compress under a snow load, the way rigid foam would do.
Q. "Would sleepers between the first layer of foam [and what?] be reasonable?"
A. If you want to install 2x4 purlins on top of your rafters, that approach makes more sense than trying to install a continuous layer of rigid foam on top of your rafters without any roof sheathing under the foam.
Q. "If not, how should we attach the top layer of sheathing?"
A. The top layer of sheathing can be screwed through the foam to the rafters.
Q. "Is Tyvek reasonable as an internal air barrier below the rafters?"
A. I think that gypsum drywall or MemBrain would make more sense.
Great clear answers, thanks!
Q. "Would sleepers between the first layer of foam [and what?] be reasonable?"
I should really figure out how to make an illustration. The sleepers/purlins would be running horizontally across the rafters to prevent racking. The would be laying on their sides, so the top flat surface is in plane with the top of the 2" foam. They would separate adjacent pieces of foam, and then be covered with a contiguous second layer of foam. This article is very close to what I'm describing: http://www.danperkinsroof.com/pdf/metal-roof-venting-and-insulation.pdf
In their case, the contiguous foam is the bottom layer; I was going to invert and put the purlins in the bottom layer so they can be screwed directly to the rafters. But maybe it's easier to speak about their approach: do you like it? It's essentially what I'm aiming for. Thanks again for your time and answers!
Nathan,
2x4s are only 1.5 inch thick, unless you can find a local bandsaw mill to make you some old-fashioned unplaned 2x4s.
2x4 roof purlins are usually installed 24 inches on center, and they can't prevent racking. To prevent racking, you need plywood. However, racking resistance is less important on a roof than on walls; talk to an engineer if you want a definitive answer.
Q. "Do you like it?"
A. Not really. I think that the first layer above the rafters should be plywood.
My terminology problem: I presumed foam was measured in the same nominal inches as the planed 2x4. I agree that true 2" foam will not fit flush with 1 1/2" x 3 1/2" dimensional lumber. In any case, we're putting down plywood sheathing on the rafters now. I'm inside warming up my fingers. Layers of foam will go on top in a manner still to be determined, possibly matching that of the Dan Perkin's PDF quoted above: plywood decking, solid layer of foam, horizontal 2x4 purlins, 1.5" foam, vertical battens, plywood, tar paper, shingles.
Nathan,
Don't put a layer of purlins between your two layers of rigid foam! You don't want any air between the foam layers. Put them right on top of each other, with staggered seams.
IMHO you should do a quick beer bag heat loss.
this will show you that even at a 70 degree delta T the heatloss is so small that the smallest woodstove you can conceive of will blow you right out of the building, and show how much you are overthinking things
If you using expensive fossil fuels to heat it 24/7, it greatly changes the math.
AS a note, it is not that hard to screw into rafters when you are staring at the nails you attached they sheathing with. Chalk line is your friend.
An A frame roof almost cannot leak, the water is off before it can figure out how to get in. A look at my my neighbors missing shingles proves it. I would not worry about moisture getting in.
@Martin: I'm really failing to explain the 'purlins'. An air gap between the foam two layers of foam is not something we're considering. The photos from the Dan Perkin's PDF I linked above explain better than I'm able. Making things simpler, we've followed your advice, skipped them and gone with a simple plywood-foam-plywood sandwich.
@Keith: Steady state, you are right. But for getting the cabin back up to comfortable from a cold state, I'm not sure it's overkill. It was -15 F (-25 C) here this morning, so we decided stay in the warm house and not go outside to finish the cabin roof. That's about as cold as it gets here, but I wouldn't turn down the wood stove for making it habitable in a hurry. As to hitting the rafters through 4" of foam and 2 layers of plywood, yes, we really should have been using a chalk line. Instead we had a person on the inside yelling "No, you missed it again!"