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Community and Q&A

Siding remodel in Climate Zone 2A (Houston)

Kiya Lee | Posted in Plans Review on

My research has lead me to (inside-outside):
Drywall -> Roxul (rockwool) -> QuietBrace (sheathing) -> perforated radiant barrier -> 1×4 cedar furring -> Hardie lap siding

I picked QuietBrace (fiberboard) due to its permeability factor. I am worried about foam (polyisocyanurate) in such a humid environment such as Houston. Another option is just straight up plywood. I’m not worried about structural integrity of the material since my house doesn’t currently have wall sheathing on it.

Also a question regarding the furring strips. I settled on cedar the other day after racking my brain about this. I didn’t want untreated pine since it would be outside the housewrap (radiant barrier), and I didn’t want PT (pressure treated) due to off-gassing. I was thinking along the lines of fence pickets (5/8″ x 3 1/2″), but am concerned over the air gap for the radiant barrier. I can’t find a concise air space necessity for radiant barrier.

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

    First of all, you are less likely to have any condensation problems with exterior polyisocyanurate than with a perforated radiant barrier. Even with perforations, a radiant barrier can easily trap moisture. Polyiso, on the other hand, has inherent R-value -- and is therefore less likely to be cold enough to provide a condensing surface. In your climate, exterior polyiso is perfectly safe. Just make sure that your wall can dry to the interior. (Don't install interior polyethylene or vinyl wallpaper.)

    If you want the benefit of a radiant barrier, just choose foil-faced polyiso.

    Second, you don't need to buy cedar furring strips. The air gap between your sheathing and your siding will stay very dry, so you don't need to worry about moisture. Ordinary 1x4s are fine. Wood furring strips in such a dry environment should last for hundreds of years.

  2. User avatar
    Armando Cobo | | #2

    In Houston, one option I would recommend is 1/2"-1" Dow SIS structural sheathing. ½” has R3 and 1” has R5; quite better than QuietBrace at R1.3. When the SIS boards are taped and sealed, they become a WRB, saving money and installation time. Roxul is a good choice, but good ol dense packed cellulose is great too, as it is hygroscopic and should cost less; your wall assembly should dry well from the SIS panels to the inside.

  3. Kiya Lee | | #3

    @Martin: If I'm understanding correctly then, a layer of Rmax (polyiso foam with foil facing) in place of the quietbrace is the ideal path, simply taping the seams with foil faced tape? I was using radiant barrier as a housewrap, will the foil faced foam (taped seams) provide the same air barrier?

    @Armando: I can't find information regarding the outer shell layer of SIS, is the facing a radiant barrier? I see dow sells Tuff-R (which is equivalent to Rmax), do I need a structural sheathing like SIS?

    I also forgot to ask about fasteners. I will be using 11ga SS roofing nails to apply the hardie to the furring strips, what is the best method to apply the furring strips (and which ever sheathing) to the wall studs?

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Does your project have a designer or a builder? Or are you doing everything yourself?

    Your questions are rather basic, and I'm a little worried that you may be getting in over your head.

    Every wall needs bracing. Do you have a bracing plan? QuietBrace sheathing can be used for bracing, but polyiso cannot.

    It's possible to install polyiso over structural sheathing (plywood, OSB, or QuietBrace). If you are using polyiso without structural sheathing, you had better have a good bracing plan.

    Every wall needs a water-resistive barrier (WRB). This can be housewrap, some types of rigid foam, and many other products. Whatever you use, you need to be sure that your WRB is integrated with your flashing.

    You might want to read this article to get you started: How to Install Rigid Foam Sheathing.

  5. Kiya Lee | | #5

    I am planning on doing it myself (with the help of family members). I ask about the sheathing because my house currently doesn't have any. Current construction (inside->outside) is drywall, fiberglass batts, 15# felt, masonite lap siding. IIRC from what I saw in my attic, we have corner (let-in) bracing. The sheathing was more so my want to build the house better while I had the siding off.

  6. User avatar
    Armando Cobo | | #6

    I tend to be more practical and design with simpler methods, especially if you are doing the work w/o professionals in the industry. I have no use for radiant barriers in general, since it’s a lot easier and cheaper to concentrate your efforts and money in a tight wall assembly. I believe that using the SIS panels (taped and sealed), you achieve great structural integrity and insulation value at the same time, better than any radiant heating would do. If you were to install radiant barrier on the wall assembly, you would need 2x4 rain screens (1 ½” air gap) to attach the Hardie Siding plus plywood for sheathing plus a moisture/air barrier.
    In the case of installing Radiant Barriers in the roof assembly, I think is a waste of money too. I would rather seal the attic floor and install more blown cellulose on top; that is more material, labor and cost effective.

  7. User avatar GBA Editor
    Martin Holladay | | #7

    There is no way to tell from your description whether your house has enough let-in bracing to satisfy an engineer or building inspector. You can either install conventional, well-fastened sheathing (to address the bracing issue) or hire an engineer to evaluate the let-in bracing (if there is any way to inspect it).

  8. Kiya Lee | | #8

    I neglected to mention that one of the family members assisting with the project has a master's in architectural engineering (c/o 2010). I asked him to evaluate my house before we proceed with purchase of the sheathing.

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