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Exterior foam and residing…with Roxul?

peaceonearth | Posted in Energy Efficiency and Durability on

This is a follow up to an earlier post, from which I got great input, but I think this alternative idea needed another post. After reading about Roxul (and not yet being locked into anything about my previous plan), I wondered if this scenario would be workable, and/or what problems might be most in play.

I would need 2″ of Roxul over 5/8* plywood sheathing. I’m looking at using vinyl siding over that. I would not need furring strips necessarily, which is how roxul installation is typically done (vinyl siding does not need a rain screen, and in fact if furring strips were used one would need some filler in between to have the backing required by the vinyl manufacturers). I’d need to hold up the Roxul temporarily with cap nails, but the vinyl siding fasteners would hold the roxul thereafter. So, could the vinyl siding go directly onto the Roxul, with the Roxul being the flat backing?

The fastening could be much the same as what was suggested for 2″ of EPS, such as 4″ roofing nails into 24″ oc studs, and 3-1/8″ GRK screws in between studs and into the sheathing. This should hold the light vinyl up no problem, I think., but would also be holding up the roxul. The Roxul would also be a bit more squishy than the type II EPS, but would this be a problem? How could the WRB be applied?

Thanks for any input.

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  1. GBA Editor
    Martin Holladay | | #1

    I don't recommend this type of installation. If you happen to lean your shoulder against the outside of your house, your wall would feel like a pillow. I don't think you want that.

  2. peaceonearth | | #2

    I hear ya, Martin, although vinyl siding seems to feel like that anyway. I guess I'm not clear on just how squishy the Roxul is, since some of the posts and articles speak of it as being more firm than one might think.

  3. user-4524083 | | #3

    HOWARD - As you probably know, Roxul comes in different desities, with the batts being around 2 lb./ cu. ft., and then the various "comfort boards", up to the IS which is 8 lbs./cu.ft. You're basically paying more, the more it weighs. The 8 lb. stuff you can evidently use under slabs, and can walk on it without leaving a dent. While I really prefer the idea of Roxul much better than foam, since the vinyl siding needs backing, it sounds like recycled EPS or polyiso might be a better choice for your application. Good luck with your project and choices.

  4. peaceonearth | | #4

    Thanks Kevin. I guess I'm back to EPS. I'm wanting to avoid polyiso both for the total lack of permeance and the poor cold weather R performance (I'm in zone 6), but thought Roxul might be an alternative. The 8 lb is probably quite a bit more expensive, but it also might be that the weight makes it a non-starter in this application, since it would be essentially supported by cantilevered nails and screws. It seemed to have some advantages with its permeance, no need to be taped, and insect proof (at least as far as eating it).

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