Vented or non vented roof for shed?
Help me get out of my head on this one. We are in the process of finishing a 10×12 shed kit.
Ultimately it will be conditioned space: not sure if we will do home office or workshop.
Regrettably, it’s 2×4 24 inches OC for both walls and roof, though I am thinking about sistering some 2×6 in for the roof rafters to allow more insulation (could do walls too as well).
Insulation for walls will ideally be leftover Rockwoool batts. I’m not sure about the ceiling: foam panels bug me from an environmental standpoint, though I’m sure I’m being ridiculous.
There is no wrb: I was truly only concerned about air barrier as it’s tiny and we will heat and cool it with a 6k BTU heat pump (unless the Gradient or Midea window units suddenly become available!) powered by excess solar.
We are in northern Colorado at 5k feet: the wind can be tough (and it’s what bothers me when I’m working in our current garage) but the location is wonderfully sunny in the winter and shaded in the summer, so the load won’t be very high even with minimal insulation.
We are doing metal roofing: 26 ga galvanized we’ve reclaimed. Water and ice shield covering the entire deck.
I’m doing a pseudo cathedral ceiling with pegboard as the interior walls and ceiling. The shed walls were wrapped with simple plastic before siding.
I’d prefer not to vent the ceiling and keep it all one very tight structure, though we’re using reclaimed double hung windows, so there will be some air exchange there.
I’ve read the cathedral ceiling article: am I asking for trouble doing non vented? We are incredibly dry in the winter, much drier than most of the country in summer, and it will be a single human in it for about 8 hours four days a week so not a lot produced moisture.
The walls and ceiling insulation will of course be open to the interior given pegboard as the finish. I hope to air seal the bajeezits out of the exterior portion before the insulation goes in.
Feel free to roast me on any aspects or tell me I’m going to be able to grow mushrooms in the insulation in the summer if I don’t vent the roof!
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Replies
foconono,
This being an outbuilding where you will not generate much moisture, and your climate being dry, you may well get away with assemblies that would be risky in other circumstances, but let's just deal with that is definitely safe and what's not.
Assuming from your description that the roof and siding are on:
You can't use permeable insulation in a roof without venting it, and providing an interior side air-barrier. Rather than sister more rafters, I would add 2"x3"s on edge to the underside of the existing ones. Provide a 1 1/2" air-space under the sheathing vented at the eaves and peak, and if you want to use pegboard as the interior finish, cover the underside of the rafters with 6 mil poly or a variable-perm membrane.
The presence of poly on the exterior of the walls really limits your options, as they now can't dry to the outside - and having the siding on means you aren't able to use exterior insulation. If there is no sheathing and the poly is exposed on the inside, I would cut it all out. You don't have a WRB which isn't great, but that would allow you to use batts in the walls, coupled with the same interior air barrier I suggested for the roof. If you can't get at the exterior poly, all I can think of is using foam board in the stud bays carefully sealed at the perimeter.
Ugh thanks. The siding is on and the roof decking is on. We can get to the plastic, so maybe we'll do an interior wrb. We can switch to foam boards though, if that would be compliant.
Thanks again!