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Community and Q&A

Exterior rigid foam – horizontal furring?

mothgraphics | Posted in Green Building Techniques on


I am designing a small commercial building for a client in climate zone 5A.
The design has two thermal challenges I am looking to get opinions on. For brevity, I will address only one here.

For detailing purposes and ease of constructibility, my initial thought for the exterior wall system was to use traditional 2×4 metal stud framing with batts. In addition, I was wondering if the group could post their thoughts on running horizontal furring at 24″ on the exterior with 1.5″ rigid in between, which would limit the thermal bridging to the intersections of the studs/furring. Then maybe an insulated sheathing over the whole assembly with drywall inside.

Then I wondered if furring / rigid on the interior would be easier and allow better ventilation on the exterior, but I haven’t seen this detail before, so maybe there is a reason to avoid.


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  1. davidmeiland | | #1

    Why not install continuous rigid on the exterior, with furring over it, attached with long screws? Much easier and better than cutting rigid to fit between the furring. You probably need more than 1.5" to make it work...

  2. GBA Editor
    Martin Holladay | | #2

    If you are designing a building with steel studs, all of the insulation has to go on the exterior of the steel studs. The thermal bridging through the studs is so severe that the fiberglass batts placed between the studs are basically worthless. They are just pretty pink decorations to make the owner feel good.

    Figure out your minimum R-value requirements to meet code, and specify enough rigid foam thickness to meet or exceed that requirement. Install the rigid foam on the exterior side of your wall sheathing. Your will probably need to install vertical 1x4 furring strips as David suggests, but the next details really depend on what type of cladding you intend to install.

  3. mothgraphics | | #3

    I see this detail and again read about it on this website's link I just thought running furring in the opposite direction - even if on top - would create more uniform resistance. Maybe having a horizontal member on the outside is a problem for an drainage layer?

    I always prefer wood studs over metal, but unfortunately the client has balked at this - tradesman are used to metal (blah). My cladding will be a fiber cement clapboard primarily.

  4. GBA Editor
    Martin Holladay | | #4

    In Climate Zone 5, you probably want a minimum of R-20. That means you'll need about 4 inches of polyiso or 5 inches of EPS.

    If you plan to install fiber cement siding, then you will need to install vertical furring strips over the rigid foam.

  5. mothgraphics | | #5


    Thanks for the quick response. My intention was to use sheathing over the furring so I could install siding without worrying about the backup spacing. I've just make a quick little 3D study to illustrate my thought - obviously, not all the layers are showing. The red squares in the image indicate the overlap of framing, versus the entire furring length overlapping the stud for a decreased overall wall value.

    Does this make any kid of sense? Do we need a drainage layer? Would the horizontal furring inhibit drainage? Would an insulated sheathing work? I'd have to check the siding manufacturer's warranty.

    I guess my concern is the additional 4" to the exterior of the structure - however I suppose that would create the ability to run insulation straight through over the face of the slab edge. Then again, Poly-Iso doesn't like to get wet - but that's a whole other discussion topic.

  6. GBA Editor
    Martin Holladay | | #6

    Most designers prefer vertical furring strips, not horizontal furring strips, for three reasons. Vertical furring strips (1) allow water that gets past the siding to drain, (2) provide better ventilation channels for rainscreen gap drying (using the stack effect to move air), and (3) provide more secure nailing for horizontal siding (so that you don't have to depend on the OSB to hold the nails).

    Since your furring strips are outboard of your insulation, you don't really have to worry about thermal bridging.

  7. mothgraphics | | #7

    That was my intuitive thought, but thanks for the confirmation.

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