GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Furring strips and Roxul

pbout | Posted in Green Building Techniques on

I just applied for our building permit for a major remodel, climate zone 3 coastal, headed for a ‘very good house’. I’ll be the owner builder.

The 2×4 wall is sheathed in CDX, blueskin, 1.5” Roxul, 1×4 furring strips, cladding (stucco downstairs, banding divider, then fiber cement siding upstairs).

When fastening furring strips over the Roxul, a builder in town has said that we should buck our windows out 1/4” less than calculated, because of how the Roxul compresses under the furring strips. In our case, instead of a 2.25” buck out (Roxul thickness plus furring strips), we should just buck the frames out 2”. Any comparible experience?

Related: I’ve heard of various materials being used for the furring strips, at least locally, including primed fir, unprimed/raw cedar, CDX plywood, and something pressure treated. What do people recommend and why?


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Michael Maines | | #1

    Phil, I don't have first hand experience doing this, but based on projects I've seen and designed, your builder's suggestion makes sense. It is a bit of a trick to get all of the furring into plane, but can be done. Get your outer pieces parallel to your framing, then string lines and use spacer blocks to get the rest of the strips in line. Or use a long straightedge. Before committing to anything, I would do a mock-up so you know what works for you.

    Furring strips don't see much water, and when they do they dry readily, so they don't need to be rot-resistant. They need to have some structural integrity and to not split when you attach them, or when you attach your cladding. Solid stock is stronger and stiffer than sheet stock in this application. Here in the northeast we have spruce and white pine readily available; both work, but pine is less prone to splitting. If fir and cedar (I assume red, but yellow performs similarly, and white is an east-coast material) are easier for you to get, either will work. Fir would be a bit stronger, but also a little more prone to splitting, but probably not enough to matter as long as you pre-drill your holes.

  2. pbout | | #2

    Michael, thanks for this response. I think we'll stick with fir, for the price and strength (stronger than cedar I think). I'll pre-drill holes. Thoughts on priming the strips?

  3. billingsdave | | #3

    I'm planning on using exterior Roxul Comfortboard with furring strips under steel siding on a cabin project this year. I spoke with a builder in Missoula, MT who has used the same ICF system I'm using and he said the same thing Michael Maines did above. The strips dry quickly and moisture generally isn't an issue. However, he did tell me if I wanted to error on the side of caution, I could treat the furring strips with Lifetime wood treatment (, which he described as non-toxic and enviro-friendly.

  4. Jon_R | | #4

    "Pressure treated strapping is only required in high rain areas, though it is recommended in all jurisdictions. "

  5. Expert Member
    Michael Maines | | #5

    Phil, absolutely no need to protect the fir, as long as you take some care when installing siding, and you provide ventilation at the top and bottom of the gaps. But it's your house; if you want additional protection, it won't hurt anything.

  6. billingsdave | | #6

    I was also advised to avoid using pressure treated lumber for furring when using metal siding as some of the chemicals used in PT lumber can be corrosive to metal siding. I'm sure the risk varies based on the preservatives used and the metal (galvanized, aluminum, steel, etc.), but it just seems like one complication worth avoiding.

  7. Expert Member
    Michael Maines | | #7

    Good point, David. Pressure treated lumber is usually treated with a copper compound, which will react with most metals other than copper.

  8. Expert Member


    The problem is between metals on opposite ends of the galvanic scale, typically copper and aluminium. It is particularly severe when using ACQ treated wood and Galvalum siding or roofing.

    MPS treatment, which is becoming the norm here, doesn't leach copper, and most panels that have a painted finish are fine even with ACQ as long as the wood isn't continually wet.

    Having said all that, I'm with you. I wouldn't risk ACQ treatment on furring that would be exposed to most metal siding.

  9. jberks | | #9


    I'm in the middle of placing furring strips over Rockwool comfortboard 80. I personally don't like how it compresses, I would have liked to try out comfortboard 110 to see if it makes a difference. I specd Rockwool for fire protection purposes, but I'm now learning how XPS could have made a much easier install as it doesn't compress as much.

    For my particular matter, all my furring strips are painted black and will be exposed to create 1/2 reveal between the fibre cement panels. One thing I've learned is that each strip will sink into the roxul differently and it's quite the skill to get them all planar and the butt joints flush with each other.

    So to answer your question, that sounds like a good trick to recess your window bucks back a bit to allow for compression, but it will still be difficult to get it right as the depth of compression will vary based on the individual driving the furring strips in.

  10. pbout | | #10

    Thanks everyone for the comments, this has been a great thread. I think my plan as of now is to use 1x4 fir with primer. As for compression, I need to make a mock up before I buck out the windows.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |