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Community and Q&A

Detailing WRB/Roxul/siding in retrofit

ohioandy | Posted in Green Building Techniques on

I am putting together a plan for a client to add 2″ rigid mineral wool as a continuous exterior insulation upgrade. One-story 1974 ranch house, huge 4′ overhangs, zone 5 NW Ohio. Existing walls are 2×4 with fiberglass batts and interior poly vapor barrier. Current siding is vertical pine board-and-batten. Already encapsulated crawlspace and insulated foundation walls, air-sealed in attic and blew in more insulation, so doing the walls is Phase 3. Probably should go with at least 3″ mineral wool to approach nominal modern code values, but aesthetic and cost concerns creep in as insulation gets thicker.

1. After removing old siding, can I put housewrap WRB/air barrier over existing “sheathing”, which is 3/4″ fuzzboard interrupted every 2′ by a horizontal firring strip? Can housewrap be a successful air barrier over this? Presumably the fasteners for my new firring strips, on the outside of the mineral wool, will have to find the original studs, so solid sheathing doesn’t seem necessary.

2. Is regular Tyvek or Typar a good choice for air barrier/WRB in this situation? I see frequent references to other wraps, such as Commercial Tyvek.

2. With that interior poly, I know it’s critical that the 3/4″ air gap between new siding and mineral wool be ventilated. How best to do that? Owner is considering vertical steel siding, which means horizontal firring again. Presumably, the shelf below the mineral wool could be perforated, allowing drying up through there. The soffit space above is not ventilated; should the air gap have a screened opening to the outside, just below the soffit?

Thanks for advice.

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  1. GBA Editor
    Martin Holladay | | #1

    I don't know what fuzzboard is.

    Plastic housewrap makes a lousy air barrier. If you insist on using housewrap as an air barrier, choose one of the tougher products like Commercial Tyvek or one of the European membranes sold by 475 High-Performance Building Products.

    You wrote, "With that interior poly, I know it's critical that the 3/4" air gap between new siding and mineral wool be ventilated." That isn't necessarily true, although a ventilated rainscreen gap is always a good idea. Your exterior mineral wool is vapor-permeable, so this assembly isn't particularly risky (depending, of course, on what type of animal "fuzzboard" turns out to be).

    For more information on rainscreens, see All About Rainscreens.

    For more information on retrofitting mineral wool on exterior walls, see Wrapping an Older House with Rock Wool Insulation.

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