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Insulation system to use for an insulated attic?

user-6622508 | Posted in Energy Efficiency and Durability on

I’m designing and building a house in Houston, TX Zone 2A. I want to insulate the roof line so I have a conditioned attic. For the roof construction, i’m initially planning on using TechShield with an Ice & Water shield and asphalt roof system. I do have some hesitations using spray foam. Interior Air Quality is a priority. Basic questions are as follows.

1. With a vapor barrier on the roof decking should you use a closed cell foam underneath the decking or should you use a batt or blown cellulose for vapor transmission? Will the closed-cell act as another vapor barrier “sandwiching” the roof decking?

2. Can you use a blown cellulose in the roof joists and rigid foam boards to close off the eaves and soffit areas if you avoid spray foam?

3. Is blown cellulose the best insulator for the walls?

4. With all the info available regarding conditioned attics, im a little confused on the best direction in the hot and humid Houston area.

Comments are appreciated.

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  1. user-2310254 | | #1

    You should check with LP to see if it will allow you to install a vapor impermeable product such as Ice & Water. This site contains a lot of helpful advice on how to create a high-performance structure. You are fortunate to find it while still in the design stage.

    I'm curious to know where your ducts will be located? Ideally, you want them inside the conditioned space (and not in the attic if possible). Are you building a code-compliant home or shooting for a higher level of performance?

  2. user-6622508 | | #2

    After researching the TechShield, the underside of the shield has tiny incisions to allow vapor transmission. With the TechShield as a vapor-permeable barrier, a ice & water shield should work with it. Its the closed cell foam under the TechShield is where i start to worry.

    With the roof line as my insulated line, the attic will be a insulated space. I'm trying to achieve a higher level of performance for the house. I have a fairly strong idea on how the wall system will work, i just get a little nervous on trapping moisture at the roof level.

  3. user-2310254 | | #3

    Jeff. I would still check with LP since I couldn't find anything in their installation instructions about installing a vapor impermeable membrane on the exterior. What style roof did you have in mind?

  4. user-6622508 | | #4

    Will do. I guess the roof style will be a combo hip / gable with asphalt shingles on the Ice & Water Shield. After some additional research i think the open cell foam is whats best suited for under the TechShield. I have read some reports from Allison Bailes on the open-cell foam which shed some light.

    I plan on conditioning the attic and adding a dehumidifier. Since most of the construction materials are industry standard materials, i was hoping somebody here has tried this before.

  5. GBA Editor
    Martin Holladay | | #5

    Your decision to install Ice & Water Shield above the roof sheathing is a head-scratcher. That's an expensive and unnecessary layer.

    I always prefer to see solutions that depend on exterior rigid foam rather than spray foam. The choice, however, is up to you. Here are several articles you should read:

    Creating a Conditioned Attic

    Open-Cell Spray Foam and Damp Roof Sheathing

    High Humidity in Unvented Conditioned Attics

    How to Install Rigid Foam On Top of Roof Sheathing

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