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Community and Q&A

OSB for rainscreen furring?

Mateo K. | Posted in Energy Efficiency and Durability on

Hey all – I’m going to be residing my single story house this summer (climate Marine 4c – Seattle). The walls look like this (inside to out):

Gypsum > 2×4 cavity (to be filled with dense-pack cellulose) > 1×8-1×10 shiplap sheathing > 15/30 lb Felt, taped > 1″ polyiso > OSB furring (with core-a-vent sv-5) > hardie plank/cedar shingles.

My main concern is what material to use for furring strips – I realize that 1×4 stock is commonly recommended, but I wanted to go with something a) cheaper, b) more dimensionally stable than 1×4, and c) wider furring than 4.5″, especially in the corners. I had settled on OSB strips due to price, and thickness (because there are shingles and clapboards, I am going to double to furring – all vertical where the clapboards are, vertical and horizontal where the shingles are).

Are there are any concerns with using ripped OSB for furring?

This article merely mentions if you’re using OSB furring to add 0.5lbs to the weight of your siding, and I imagine if there was a great concern with OSB as furring there would be recommendations against it, but I haven’t found any – because the walls will be insulated, there won’t be condensation forming because of heat – but what if the walls are below the dew point?

From what I’ve read, the main concern with modern OSB is that edges swell when wet and this can telegraph through the siding material – should I just paint the ripped edges to “seal” them? Or should I just go with CDX (about 2x as much)? Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    If this were my house, I would use plywood. But there is no reason to believe that OSB won't work. Rainscreen gaps are very dry -- one of the driest regions of most wall assemblies -- so the OSB will probably be OK.

    When it comes to fastening the siding, however, you'll be nailing or screwing into OSB. Personally, I prefer plywood to OSB for holding fasteners, even though tests have shown that OSB holds nails and screws as well as plywood.

    You really don't need that many sheets of material to rip rainscreen strips -- so the upcharge for plywood may not be as much as you think.

    By the way, you can't tape asphalt felt. Either leave the asphalt felt untaped, or -- if you are depending on that layer for airtightness -- switch to plastic housewrap. Housewrap can be taped.

  2. Mateo K. | | #2

    Martin, thank you for the response.

    That was my assumption, that OSB will be Ok in this assembly. I'll be using ring-shank nails, so I'm not too concerned with holding power of OSB vs plywood for fasteners (as I know they are very similar).

    Thank you for the tip on felt, I will be taping my polyiso, so I think I'll be Ok on the airtightness front. I'm still researching on whether it will be worth the effort to pull the WRB to the face of the polyiso and skip the felt, and put the polyiso directly over the sheathing. On most of the existing windows it would be relatively straightforward - as they are in the "middle" of the wall, and extending the flashing outwards is simple enough - it's the 2 existing windows that are remodel (with a nailing flange) that concern me.

  3. GBA Editor
    Martin Holladay | | #3

    I can't think of any siding manufacturer that allows siding to be fastened to 3/8 inch plywood. Check with the siding manufacturer for requirements. The default strapping used when siding attachment is expected is 1x4 spruce/pine/fir.

    For more information on choosing the right furring strip material, see All About Rainscreens.

  4. Mike M | | #4

    Is 3/8" plywood cut into 4" strips enough for siding to hold onto over 4" of xps? Would I just be better off going with 1x4s?

  5. Mike M | | #5

    I was aware that most manufacturer's want 1 1/4" fasteners, even with 1 by, there is only 3/4", so I wasn't sure this was followed. It appears that it's followed when done with standard sheathing and different with furring over foam.

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