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Community and Q&A

How to insulate a modified a gambrel roof

darwyn | Posted in GBA Pro Help on

We are looking at changing a gambrel roof (see plan sheets) to a standard 5/12 roof.  
I have a number of questions regarding the new look, height of the new design but mainly the way to properly insulate this new construction.
I would appreciate any input.
This house is located in zone 5.
The plan is to not disturb any of the interior finishes.  We are hoping to work totally from the outside.
Another crucial piece of information is, the current attic area has all of the second floor duct work running through it.

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  1. Expert Member


    The problem is probably mine, but I can't understand how you support the new exterior walls out past the existing overhang.

    1. darwyn | | #3

      The new wall will be in line with the current dormer wall. I am assuming we can tie them into the current floor joist.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Darwyn.

    To be helpful, we need to know what Climate Zone you are in. If you don't know, see this page: Climate Zone Map

    1. darwyn | | #4

      Sorry about that. Zone 5.

      1. GBA Editor
        Brian Pontolilo | | #7

        Hi Darwyn.

        It's an interesting situation. Some others have raised structural questions, but I am going to assume that you have all that worked out. If everything existing is staying in place and you are building over the top of the old roof, I think Matt F's suggestion below to blow new insulation over the old roof and vent the new roof assembly may be the thing to do.

  3. MattJF | | #5

    I am confused by the "keep existing ceiling wall" note. Are you really keeping the current interior walls and not extending them to the new structure? Are you going to have a roof under the new roof? If that's the case, I guess blown insulation over the top of the old shingles would work. I just don't think that is what is going on. What is the intent?

    I am also not seeing what the new wall is sitting on. I think the roof line might be drawn wrong where it intersects with what I assume is the floor joists. Or I am just not seeing it correctly.

    1. MattJF | | #11

      I guess I didn't read the OP very well. The intent is definitely to keep the interior walls/roof as is. I couldn't imagine doing all this work only to still have a gambrel interior on the second floor.

      Thinking about the insulation more, I think you would really want to strip the roof and tape the roof sheathing as the airtight layer. Not doing so might present some risk of asphalt odors getting into the living space.

      You are going to have to tell us more about your goals.

      From an energy perspective, putting in a foam board roof is likely best highly invasive option.

      I guess you could consider a staged approach where you later open the interior up. You would use a raised heel scissor truss, figure out how to get some sort of sheathing or drywall to the underside and put blown in insulation within the trusses. Then you could piecemeal extend rooms up to the new ceiling.

  4. josh_in_mn | | #6

    If you're concerned with the energy performance of this house primarily, why not just install exterior foam, a new ventilated roof deck over that, and then a new roof? Or perhaps you dislike the look and that's the primary factor?

    It seems like it'll cost a lot and delivery primarily cosmetic benefits without improving the function of the house much.

  5. Expert Member
    Akos | | #8

    Structural issues and insulation issues aside, a pretty important detail is the curb appeal of the house.

    The new over roof is really not helping the proportions of the existing building, mushroom top mansard construction is already top heavy, putting the new roof over exaggerates it further. I would look at least changing the window locations/sizes or add some windows above the existing ones.

    If possible, bringing in the walls to line up with the walls on the first floor would go a long way in fixing the look. You can even leave the exising overhangs as a skirt above the 1st floor something like this:

    An architect should be able to hep you get the proportions right.

    1. GBA Editor
      Brian Pontolilo | | #9

      That's a sweet little project Akos. Nice work.

      1. Expert Member
        Akos | | #10

        Would love to take credit for it, but just random google image search. When it comes to design ideas, a picture is always better than words.

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