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Community and Q&A

Roxul ComfortBoard installation with siding

Josh | Posted in General Questions on

We are using 2×6 construction with zip system, 2″ of roxul, furring strips and siding. My question is what to do with the extra 2.5″ cavity that this will create at the bottom. Is the best practice to move the walls in, install a pest scree, etc? Thanks

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Replies

  1. User avatar
    Peter Engle | | #1

    There are two different questions there. Moving the walls in, or not, is more of an aesthetic question. I like leaving the siding overhanging the foundation, just to keep the foundation and soil around it that much dryer.

    Yes, you should wrap insect screening around the bottom of the Roxul and drainage cavity to try to keep bugs out. Let the siding hang down over the insulation and furring by an inch or so for clean sight lines.

  2. Malcolm Taylor | | #2

    User 708 etc,
    Peter gave you good advice. Either way works. The water management at the base of the wall becomes a bit more complicated (and important to get right) if the wall is moved in. If you let the insulation etc. cantilever out, I'd urge you to use something more robust than bug-screen to protect the base. Here we typically use a U-shaped flashing made of perorated material.

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

    User 708 etc.,
    First of all, can you tell us your name?

    Here is a link to an article that includes a photo and a description of how to handle the bottom of a wall with exterior mineral wool: Installing Roxul Mineral Wool on Exterior Walls.

    Below is a photo of the perforated metal flashing they used to protect the bottom of the mineral wool.

    .

  4. Josh | | #4

    It's Josh W. Thank you guys for your responses. I can't find any builders here familiar with this system so this site has been a great help!

    Martin: The link didn't come through. Can you repost? Any idea who supplies this flashing. Thanks!

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Josh,
    The link works for me.

    Most sheet metal shops should sell perforated galvanized sheet metal. Draw up a sketch, and a sheet metal worker should be able to bend some for you.

  6. User avatar
    Peter Engle | | #6

    Make sure the metal has heavy galvanizing, especially if you are in a coastal area. I have seen lightly galvanized metal rust out pretty quickly. There are also vinyl products with similar perforations that would be more corrosion resistant. Also, those perforations will stop mice and large bugs, but not ants. We've had some issues with ants liking the conditions behind exterior insulation. So far, I've only seen ants living in foam insulation locally (New Jersey). That might be related to nobody using Roxul nearby. YMMV

  7. Malcolm Taylor | | #7

    Josh,
    Here is one supplier, although as Martin said, any sheet metal shop should be able to supply it.

  8. Scott Wilson | | #8

    In looking at the article mentioned in the link given by Martin I noticed a couple of things in the pictures included at the bottom.

    In picture 5 the furring strips come down and overlap the perforated metal pest strip that sits under the 2" Roxul Comfortboard but they do not come up to the underside of the window or run up along the sides of the window (should a small gap be left between the side of the window and the furring strip ?).

    Since there is no furring strip running alongside the window how do you attach the exterior window trim and sill (as shown in picture 6)? It seems to be just attached through the Roxul into the sheathing. It would also be difficult to attach the siding between the corner board and the window trim since there is no furring strip there. (as an aside, how much of a gap should be left between the siding material and corner boards, window trims and sills? A bit of a gap, no gap at all, caulk the gap, no caulk?)

    The other thing with picture 6 is that a bottom board has been attached to the surface of the furring strips (with a piece of flashing on top). There would still be open gaps though behind the bottom board between the vertical furring strips (in front of the metal pest strip). Those gaps should also be covered with insect screen.

    One product that works quite well for wall venting comes from Cor-a-vent. I think it has pest protection built into it as well.

  9. User avatar GBA Editor
    Martin Holladay | | #9

    Scott,
    Q. "Since there is no furring strip running alongside the window, how do you attach the exterior window trim and sill (as shown in picture 6)? It seems to be just attached through the Roxul into the sheathing."

    A. Or, more likely, with long fasteners to the underlying framing (the studs and plates).

    Q. "It would also be difficult to attach the siding between the corner board and the window trim since there is no furring strip there."

    A. I agree with you on that point. Perhaps an additional furring strip was added adjacent to the window trim after the photo was taken.

    Q. "How much of a gap should be left between the siding material and corner boards, window trims and sills? A bit of a gap, no gap at all, caulk the gap, no caulk?"

    A. Either no gap or a tiny bit of a gap, in my view; remember that corner trim and vertical window trim often shrinks. And definitely, no caulk. A little air and a drainage path in these locations is much better than a glob of caulk.

    Q. "There would still be open gaps though behind the bottom board [the water table] between the vertical furring strips (in front of the metal pest strip). Those gaps should also be covered with insect screen."

    A. I agree with you on the advisability of insect screen (or equivalent material). And it's impossible to tell from the photo -- perhaps Shannon Cowan and Patrick Walshe included insect screen at that location.

    A final observation: Shannon Cowan and Patrick Walshe shared their photos and methods, and we are grateful for their blog. But these methods aren't set in stone. You can certainly develop your own details if you see room for improvement.

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