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Community and Q&A

Seeking Feedback on this Shed Roof Plan

rgbtv | Posted in General Questions on

In Zone 6, northern New England, I have a 104 (8ft x 13 ft) sq ft pier/beam addition on my house (approx 11 years old addition, I’m the second owner to it). Oddly enough, heating wasn’t tied in, yet there was R30 fiberglass in the ceilings, R19 in the walls and some R of fiberglass under the floors. Drywall had encapsulated all of this, which I’ve removed. There are also continuous soffit vents on the long exterior wall, and baffles that run inside of each rafter (2×10’s) bay. The baffles start at the top plate, after the soffit vents and run until about 5″ from the shared wall with the house. Not sure why these didn’t extend all the way. There is no other venting. There is also romex that runs in this ceiling about 4″ down from the roof. I’m new to this, but do not understand this set up. Longstory short, we need to make this a conditioned, finished room (if we can) due to a new kiddo on the way.

My thought is that I can remove the baffles, have a contractor do 3.5″ of spray foam directly to the underside of the sheathing, getting me an R value of 24.5. Then, I can put R-30 rockwool (edit: I mean R-23). This would actually allow me to exceed code (edit: nearly meet code). I assume I wouldn’t then use a vapor barrier/retarder after the rockwool. I would prefer to put on T&G ceiling, but am thinking about re-doing the drywall. Do I spray foam under the sheathing over the soffits? Should I then fill up the rest of that cavity with the rockwool or ask the spray contractors to fill that cavity up? Can/should I leave the soffits vented? Does it matter? If I should block or remove them, can you recommend an airtight approach? Do I need to ensure the spray foam doesn’t encapsulate the romex, or is it OK if I decide to do more than teh 3.5″ spray foam and instead do the full 7″ or so?

For the walls, I’ll be air sealing with caulk or great stuff, using R-23 rockwool and then applying Certainteed as the air seal/vapor retarder.

I’m mostly concerned right now with the ceiling and would appreciate any ground-truthing or nudges to what I’ve suggested above.


edit: best as I can tell there is no insulation on the exterior of this roof set up.

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  1. Expert Member
    Akos | | #1

    The existing setup was actually not too bad, all it was missing a set of vents at the roof and wall intersection (many out there, coravent has a Roof-2-Wall one you can check out).

    You can install the vents, keep the baffles and replace the R30 batts with R38 MW and call it a day.

    The SPF approach would also work. You do have to seal off the roof so SPF in the soffits as well. In zone 6 you need 50% of the assembly R value to be SPF for condensation control, the rest can be fluffy, this assumes painted drywall ceiling. If you want to go with T&G ceiling, I would bump that up to 60%. There is no issues with SPF over wiring, residential wiring is sized to be installed in insulation.

    Generally the full SPF is not worth it as you get a lot of thermal bridging from the rafters. The 7" all SPF would actually have a lower effective assembly R value than the 3.5" SPF plus R24 batts because of the thermal bridging. Not the most environmentally friendly reason, but sometimes it is simpler to do full SPF as it saves an extra step.

  2. rgbtv | | #2

    Thank you for this reply! I've got some followups, in the hopes you have more feedback (or others if they see this)

    Can you simply spray into the soffit to fill it with foam, or do I need to seal them first since they're vented and ostensibly would leak before the foam cures. If so, I know this is a basic question, but what would you recommend to sealing them?

    If I wanted to do 4.5" of SPF to get to 60%, I'm assuming I can compress my R30 (edit: R23) rockwool down 0.5-1" to fit compress inside the rafters?

    Am I right in thinking not to use a vapor retarder or barrier below my rockwool?

    I've never installed vents so I'm reluctant since I'm pressed for time and I have a contractor willing to come in with the SPF this week. I would also have to add furring strips since I only have about 8.25 inches or so after the baffles. Not a major problem, though.

    One other concern about the wiring I guess I would have is not being able to access it, but I guess I don't have the need really for that.

    Thanks again!

    1. Expert Member
      Akos | | #3

      With the SPF roof, you can remove the baffles. There is no need for them in an unvented roof unless you are in deep snow country.

      You can squish mineral wool batts a bit but they are pretty stiff. SPF install is usually not smooth and flat, so I would plan for at least a bit of gap instead of squishing batts. Either go for FG batts (which are easily compressed) or go down one size on the mineral wool batts.

      To seal the soffits, you make a set of foam baffles at the end of your rafters above your wall top plate and spray foam against these. This seals off the soffits and saves a fair bit of unnecessary spray foam there. Make sure the installer carries the SPF from the roof along the baffle all the way down to the wall top plate.

      With the extra SPF, you don't need any vapor retarder, you can install T&G directly. If you do have extra smart vapor barrier left over, it doesn't hurt to also install it on the ceiling. An extra warm side air barrier is always better than none.

      1. rgbtv | | #4

        Thanks again, I do appreciate your replies.

      2. rgbtv | | #5

        Actually one follow up. I am in northern new england, about 90 inches of snow a year. While this doesn't strike me as a lot considering places I've lived, I wonder if this qualifies as "deep in snow country" and if you're then implying the unvented approach doesn't work?

        1. Expert Member
          Akos | | #6

          Snow is a pretty good insulator. If you have a lot of snow on the roof, even with a well insulated roof, the layer under the snow can get warm enough to cause the snow to melt creating ice dams. By having a vent space under the roof deck ,you introduce outside air there and prevents the snow from melting.

          The unvented roof will still work well in terms of moisture performance but you might get some ice on the roof.

          If you often have extended periods with 2' of snow the roof, vented is better. I'm in Zone 5 here (Toronto) and we rarely see much more than 6" of snow at a time, unvented roofs with SPF work quite well.

          There is nothing wrong with installing SPF over the vent baffles. Just make sure you don't block the intake at the soffits, install exhaust vent at the top and still carry the SPF down to the top plate of the wall to air seal. This would let you have a vented assembly without needing drywall on the ceiling.

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