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Closed Cell Foam on Dormer Section of Roof and Fiberglass on Remaining Section?

lazukars | Posted in General Questions on

I’m building a small 12′ x 26′ Coffee Shop near Cleveland, OH in Climate Zone 5. We’re in the building design phase and are trying to figure out the most cost effective way to insulate the Cathedral Ceiling in our shop. I’ve done extensive research and this article on ( “Five Cathedral Ceilings That Work” : )  has been extremely helpful. However, since the Coffee Shop has a dormer we won’t be able to go the cheapest route in the article. That being, “Assembly #1: Vented assembly with fiberglass or mineral wool insulation”.

I’m trying to come up with solutions to keep costs down. Here is one of them. Since there’s a dormer on the building, can I apply r49 worth of closed cell foam insulation in the dormer rafter and the rafter all the way up to the roof ridge ( see the outlined green section in the attached photo )? The rest of roof would be a vented roof with fiberglass like “Assembly #1: Vented assembly with fiberglass or mineral wool insulation” suggests.

Is this a good solution?

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  1. Expert Member


    Why can't you vent the dormer?

    1. lazukars | | #2

      Because the article on GreenBuildingAdvisor says you shouldn’t

      1. Expert Member
        MALCOLM TAYLOR | | #3


        The problem with dormers is when they interrupt the air-flow from eaves to peak - usually because they start further up the roof. Luckily your shed dormer is ideally placed at the outside wall so it can be effectively vented just like the rest of the roof.

        The only possible impediment is if the beam supporting the dormer framing would block the vent channel, but that's easily surmountable. You can do one of the following:
        - Size the beam 2" shallower that the dormer framing.
        - Add 2"x4" parallel strapping to the top of the dormer framing to provide a vent channel.
        -Drop the beam and frame in a horizontal ceiling on the dormer.

        Looks like a fun project.

        1. lazukars | | #4

          Really appreciate your description and help. I understand now!

        2. lazukars | | #7

          Again, thank your explanation of having a vented dormer. We're probably going to go that route. I'm going to swing it buy the architect today.

          One things though, I'm just curious. Per my original question, could you fill up the dormer rafters with closed cell foam ( or flash-and-batt ) all the way up to the roof ridge and make just that section unvented. Then, make the rest of the roof a cathedral vented roof with fiberglass?

          1. Expert Member
            MALCOLM TAYLOR | | #8


            I'd treat the whole roof the same way. I use vent channels and high density batts everywhere - and size the rafters deep enough to accommodate them.

          2. lazukars | | #9

            MARTIN. Yes I plan on doing that. I'd just like to know if it can be done with a hybrid approach.

          3. Expert Member
            MALCOLM TAYLOR | | #10


            Sorry - I completely misunderstood what you were asking. Yes you can mix and match. The main complications are that you typically don't end up needing much foam, so the cost for a small area can be proportionally higher to get done, and it can interfere with the normal sequence of construction.

  2. plumb_bob | | #5

    Another solution would be to run the dormer rafters right up to the ridge, this would create a single pitch and make ventilating the rafters simple. A bit of a different look though.

    1. Expert Member
      Akos | | #6

      Exactly what I was about to suggest. It simplifies structure, venting and roofing, lot of benefits if you can make it work with the look of the house.

      You can even simplify further by moving the dormer towards the front of the house so one of the dormer walls becomes an extension of the front wall. You can then have a bolt on overhang on running down the side to give you the 1 1/2 story look.

      I would also check your local code as lot have R value exemptions for compact roof sections. This generally means you can insulate to R38, in most climates there is almost no energy usage difference VS an R49 roof and would make it much easier to insulate and vent and can be built with dimensional lumber or smaller I-joist.

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