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Community and Q&A

Small addition

Jason Schatz | Posted in Energy Efficiency and Durability on

I am in the early design phase of a small (12’x20′) addition for a client in climate zone 5 and am just looking for suggestions as to the best new construction insulation & air sealing details to use.

It will be wood frame construction with a crawl space and a gable roof with vaulted ceilings inside.

I am leaning toward the zip wall sheathing system on walls & roof deck with a rainscreen on top and fiberglass batts in the 2×6 stud bay. Is the zip wall roof sheathing of sufficient thickness to allow me to build an unvented roof without having to spray 2-part foam on the underside?

Open to any/all suggestions

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Fiberglass batts in a 2x6 wall barely meet code (if you choose dense batts). Fiberglass batts are probably the worst form of insulation that is available on the market. It's legal, but it's hard to consider this a "green" approach. Most green builders aim for walls with a higher R-value than code minimum.

    Zip sheathing is a type of OSB. You can use it for roof sheathing if you want, but you still have to comply with code requirements if you are building an unvented roof assembly. If you want an unvented cathedral ceiling, you have two choices: either include an adequate layer of rigid foam above the roof sheathing, or install spray foam insulation on the underside of the roof sheathing.

    For more information, see How to Build an Insulated Cathedral Ceiling.

  2. Jason Schatz | | #2

    Thanks Martin -- I should clarify that I was assuming the zip wall R-sheathing that includes 1 1/2" of foam. If the exterior sheathing is covered by the foam, would dense cellulose be a better choice in the walls?

    For the roof I had thought the zip wall roof sheathing had an available foam but if not, is there a preferable choice between a rigid foam roof sheathing sandwich vs. the spray foam underneath? I have remodeled un-vented roof assemblies using the flash&batt method but since I am starting fresh here I didn't know if rigid foam on top would be a better choice.

  3. User avatar GBA Editor
    Martin Holladay | | #3

    If you are using the R-6.6 version of the Zip-R sheathing, that's a whole other story. That certainly improves the performance of your walls.

    In theory, the rule of thumb for 2x6 walls in Climate Zone 5 is that exterior foam should have a minimum R-value of R-7.5. (For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)

    There is a minor controversy over whether you can get away with less foam if you use Zip-R sheathing, as you are proposing. Some builders feel that Zip-R can work in Zone 5, because the OSB is on the exterior side of the foam. Well, maybe. It's also possible that moisture will collect on the interior face of the foam. Suffice it to say that thicker foam would be safer.

    You can't use Zip-R on a roof, as far as I know. It is only suitable for walls.

    It's far better to install a continuous layer (or several layers) of rigid foam above the roof sheathing than it is to install spray foam under the roof sheathing, because the rigid foam approach addresses thermal bridging through the rafters.

  4. Dan Burgoyne | | #4

    I built a new roof recently and installed 2 layers of 1 1/2" polyiso over the plywood sheathing, and then covered it with Ice and Water shield. Under metal roofing, you would want to use Grace's Ultra HT Ice and Water shield product.

    In my case, I also installed an additional roof membrane above that (Sharkskin Ultra Radiant Barrier) with the reflective side up, and then ran 1x4 redwood battens vertically up roof for ventilation and drainage. My eaves and ridge are vented. I added horizontal battens to attach my roof tile, but you would not need this if installing metal roofing.

    Several years back, I used a similar system and saw a significant drop in my summer air conditioning bills, and the end to all my leak problems.

  5. Jason Schatz | | #5

    Sorry, not sure what I was thinking this moring - I am climate zone 4, not 5... SE Pennsylvania.

    That being said, would my ideal situation be:

    - zip wall R-6.6 on the walls wth dense pack cellulose in the stud bays
    - rigid foam between two layers of deck sheathing on the roof

    If so, two questions:
    1) how many inches of rigid foam on the roof deck is required?
    2) is the best way to air seal the roof by using a quality tape on the rigid foam seams? Presumably if I do more than one layer of rigid foam but staggering seams and taping?

  6. User avatar GBA Editor
    Martin Holladay | | #6

    Q. "How many inches of rigid foam on the roof deck is required?"

    A. The answer is in the article I linked to in my first answer: How to Build an Insulated Cathedral Ceiling.

    I suggest you read the article. It's full of useful information.

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