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Does this plan for a vented over-roof make sense?

jw_adk | Posted in Plans Review on

Hi all, first time asking a question but I’ve found this Q&A very helpful in the past. Thanks in advance for your help.

I’m planning to add a vented over-roof to my zone 6 northern NY house. It has three roof sections: two cathedral and one with an attic. I’ll be dense packing the cathedral ceilings with cellulose and spray foaming the attic before over-roofing, making all three roofs hot and roughly R-25+ before the over-roof. Here’s the thinking on the vented over-roof layers, from bottom up:

1) Decking
2) Air barrier membrane
3) 5.5” of XPS foam with staggered joints–top layer taped at seams
4) 1” furring strips running vertically (spacing for ventilation)
5) Plywood sheathing
6) Ice & water shield / 30lb paper (Grace in two bad valleys and 3ft from all eaves and edges)
7) Asphalt shingles

Does this make sense? Anything I’m missing? The thought is to frame around the roof with two by sixes to give us something to nail new flashing to and make the insulation easier to install and more solid, and the new fascia would overlap the old fascia with an inch of space to act as a soffit vent. All ridges would be vented as well.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jan,
    Your plan will work. You will end up with a very well insulated roof.

    Some green builders might try to convince you to use R-25 EPS instead of XPS, because EPS is more environmentally friendly.

  2. cornercanyon | | #2

    Isn't a full 1-inch air gap required for a hot roof vent? We usually 'sleeper' 2x material when designing this roof detail-

  3. jw_adk | | #3

    Thanks Bruce, I'll switch it to 2x for the sleeper material. In my mind the sleepers would run vertically, and there would be no membrane between the insulation and the sleepers (membrane comes above the vented roof, before the shingles. This is mostly why I'm thinking XPS (for its moisture resistance) over EPS or Polyiso.

    Is this the correct way to do it?

  4. cornercanyon | | #4

    The sleepers should run parallel to the anticipated air flow to reduce any potential obstruction. When we plan to insulate inside (below) the lower deck we call out traditional tar paper for the lower deck with the more robust WRB on the upper deck. With your taped rigid insulation outside (above) the lower deck I agree with Martin, you should be good with the upper deck's WRB-

  5. Kamboji | | #5

    Yes, these Q+A pages are extremely helpful!

    I'm in the planning stages of doing something similar. What are you using for the air barrier membrane? The company that makes the nailbase product I'm planning to use (Hunter panels) recommends a VAPOR barrier (ice & water or 6 mil poly) on the decking just under the first layer of foam board. So vapor barrier or air barrier- which is recommended here?

    In addition, the contractor is planning to apply ice & water to the entire roof on the top sheathing layer (just under the shingles). I just called Hunter panels and they do not recommend that, saying it tends to "ridge up" and "telegraph" (?). Instead they said to use a breathable synthetic that allows moisture out but not in (though possibly with ice&water along the edges + valleys). Any thoughts on these competing ideas?

    Many thanks!
    Hari

  6. GBA Editor
    Martin Holladay | | #6

    Hari,
    For an air barrier under the nailbase, you can use one of the European membranes if you want -- something like Solitex Mento.

    Or if your roof has OSB or plywood roof sheathing, you can create your air barrier by taping the seams of the roof sheathing, followed by any type of roofing underlayment.

    I agree with Hunter that it usually isn't a good idea to install Ice & Water Shield over your entire roof. Just install it at the most vulnerable areas, where you might be worried about ice dams. Installing multiple vapor barriers in these thick roof assemblies generally isn't a good idea.

  7. Kamboji | | #7

    Thanks a lot for the link to the membrane and your input. I forgot to mention that when I questioned it, the prospective contractor framed the ice&water shield as a safety issue- claiming that the fact that it's fully adhered to the deck makes it safer when re-roofing.

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