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Community and Q&A

Retrofitting a Vented Roof for Insulation and Air-Sealing

stephenr | Posted in GBA Pro Help on


I have been hired by a client to insulate a 3 season cottage (exposed studs and rafters) in Maine, zone 6.  Basically, it was built and inspected, and now he wants to insulate it.  Here are the details, followed by some questions.

It has been built traditionally with 2×4 walls, plywood sheathing, hydrogap wrap and clapboards. No tape or any modern air sealing measures were taken.  The floor and rafters are 2×10 and the ceiling is cathedral (7/12) with a sleeping loft.   We plan to use rock wool in the bays all the way around and call it good.  The interior is to be finished with horizontal shiplap.

I plan to retrofit a vented roof by ripping one inch boards and installing a sheet product in between the bays creating 1 inch channels.  I am thinking of using 3/8 plywood for this but am open to suggestions.  I plan on caulking this plywood in to create an air seal to prevent moisture from migrating towards the sheathing from the interior.  Would it also be wise to caulk the sheathing and blocking from the inside before I build the channels?  I am pretty sure that the roofer laid ice and water shield on the entire roof before putting down architectural shingles on the 7/12 pitch. I will cut a ridge vent in in the spring and do soffit vents.

I am hoping not to have to hang drywall before I lay shiplap on the interior walls and ceiling.  Would installing a smart membrane, or some other product,  be a way to avoid having to do this?? Would the air sealing that I am doing with my venting channels protect the sheathing from getting wet from vapor from the interior and mean that I was safe to simply lay shiplap??

Thanks for your input.

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  1. Expert Member


    Although it has been inspected and approved, any further work is still subject to inspection and meeting whatever building codes and standards govern.

    So given that, just filling the 2"x4" walls with batts, and not providing an air-barrier probably isn't an option. And similarly I don't think your proposed roof assembly will meet code either.

    Your first step should be to run any ideas past your building inspector, both to see what further permits are required, and whether what you propose meets code.

  2. Expert Member
    BILL WICHERS | | #2

    I would use 1/4" wafer board for building those vent channels, since it's cheaper than 3/8" OSB and you don't need anything strong for this application. There is no need to caulk or otherwise air seal the wafer board or the roof sheathing, because the purpose of the vent channel is to carry away any moisture that manages to make its way up there. Air sealing doesn't really add anything here. Note that you do need soffit vents too, to act as air intakes for the vent channels, a ridge vent alone is only half of the vent system.

    A smart vapor retarder like MemBrain should be fine behind the shiplap. There is no need to provide any fire protection for mineral wool insulation, but you really do need a vapor retarder on the interior of the wall here. I would detail that vapor retarder as an air barrier too. I don't see any need for a layer of drywall here.

    Malcolm has a good point regarding inspections and code approvals though, so you'll want to run this by your local building people. My guess is that if they already approved the open cottage, and you're adding some insulation, you're probably OK. Where you might run into trouble is if the building people think you're planning to occupy the structure year round and fully condition it, in which case they may want you to put in at least code minimum levels of insulation.


    1. Expert Member
      MALCOLM TAYLOR | | #3


      Our code doesn't make the distinction based on how often you intend to occupy a building. If you are insulating, it has to meet code and be inspected before it is covered. Otherwise you end up with some houses which haven't had to meet code because (as in this case) they were approved for occupancy before they decided to insulate, and the rest which did because followed the usual path - which surely makes no sense.

      1. Expert Member
        BILL WICHERS | | #4

        I have found that the more remote places where cabins and cottages are built (think "hunting cabin in the woods" type places) are often more flexible IF you talk to them first. It's at the discretion of the locality though, so they may or may not allow anything outside of "code minimum", but it's worth asking. It's not that the rules in the code are different, it's that the local building people are more flexible when applying those rules in some cases.


        1. Expert Member
          MALCOLM TAYLOR | | #6


          I'm sure there are areas with lax enforcement. The problem is it subverts what we are trying to achieve - which is houses that are well built and efficient. The next owner of the house Stephen is working on should be able to assume that as it is fairly new it was built to at least code minimum standards, and not have an unpleasant surprise down the road.

          1. Expert Member
            BILL WICHERS | | #7

            I agree, and didn't intend to recommend less than proper levels of insulation. My thinking is if this is a cabin-type structure, not usually occupied year round, higher levels of insulation might not really achieve much overall. Some of this is probably just me assuming that it's a very small structure with only a few small rooms, which may be a mistake on my part. If this is more of a house than a small cabin, then I would absolutely advise full/normal insulation in the structure. It was not my intention to recommend less than code insulation levels for a normal house.


  3. Expert Member
    Michael Maines | | #5

    Maine's building code requires all new or heavily renovated structures that are heated, even part-time, to meet the state building code. Enforcement is not required in towns with fewer than 4,000 people, but you are still legally required to do so. That means R-49 in the roof; in the walls, you need R-20 + R-5 continuous; in the floor you need R-30 or whatever the joists will accept.

    To get the required depth in the roof you can add sub-rafters below the existing rafters. A variable permeance membrane, with all seams taped, behind the shiplap will provide air-sealing and vapor control.

  4. walta100 | | #8

    Seems likely the plans were approved without insulation only because the preposed building was to be unheated if the idea is now to heat the building now you should be required to bring everything up to the current code.

    Seems unlikely your plan would meet the R38 code is likely to require.

    One has to wonder if the secret plan wasn’t to build it on the cheap and add heating at the last minute hoping to avoid all the costly code required stuff.


  5. stephenr | | #9

    Thanks for the replies. There's a thing that happens in Maine where people want bunk houses and cottages for their friends and families who visit during the summer months. After a season or two, they want to extend the use of those cottages into the late fall and early spring. They often double as a shop or a writer's retreat, for examples. Many of these structures are technically considered sheds, are permitted as such, and are under 400 square feet. The codes are strange and sometimes allow sinks and bathrooms, including showers, as well as decks, porches, and lofts. The result of this need and the code confusion is a well used end around to expanding your livable square footage. Recently, they have revised the ADU codes, I have heard, in order to address the need for new housing and this may make this shed end around less attractive in the future. Its uncommon to heat and insulate these sheds, but it happens. I will have to have a long talk with this client ( a writer, alas) to see if he is aware of the codes and the risk he is taking in moving forward in this manner. Much obliged for the insight and discussion.

  6. Expert Member
    MALCOLM TAYLOR | | #10


    Sorry if my comments have sidetracked the discussion away for your original question. Probably half my time over the last few years has been spent dealing with issues around approval for the small structures and ADUs that proliferated here. Because enforcement is complaint driven, whole neighborhoods full of them have sprung up, their owners basing their beliefs in their legality on what others tell them, and how many there now are.

    Last week I consulted with a client (and GBA reader) on the nearby Gulf Islands who contacted me because the building inspector wouldn't give her occupancy on her new house because she had a non-conforming tiny house on the property.

    There are grey areas around these types of buildings almost everywhere. I just find it a lot less stressful to deal with the possible problems early on, rather than later.

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