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Community and Q&A

Attaching siding to houses with exterior foam insulation

GBA Editor | Posted in Green Building Techniques on

How difficult is it to attach siding, trim and windows to a house that has 2″ of rigid foam insulation on the outside? Some people recommend a layer of furring on the outside of the foam, it that adequate to hold siding on? Do building inspectors allow the ommision of a poly vapor barrier on the inside with this system?

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  1. GBA Editor
    Martin Holladay | | #1

    1. How difficult? It's not really difficult, but it is very labor intensive. All of the required details take time.

    2. If the vertical furring strips are screwed through to the studs, they are certainly adequate for holding up siding. Thousands of homes have been built this way.

    3. Believe it or not, no code has ever required the use of interior polyethylene. Codes have required interior vapor retarders, however. An interior vapor retarder can be kraft facing, MemBrain, or vapor-retarder paint. Most building inspectors will understand this if you explain it to them with a kind voice and a patient attitude.

    4. To read more about detailing windows and housewrap on houses with exterior foam, see " 'Innie' Windows or 'Outie' Windows?"

  2. user-669103 | | #2

    I'm going to be doing this in a couple of weeks.

    What's the recommended wood (size and type) for furrings strips?
    Is pressure treated recommended?

    I was thinking of using 1" x 2.75" actual furrings (by ripping 5/4x6 nominal down the center)
    And attaching them with 5" long screws into the sheathing (3/4 tongue & groove), and use stainless if using pressure treated wood. I was going to use 12" o/c which means I cannot even expect to hit the studs.

    On one wall (because I have limited available space and overhang to "grow" the wall, I was planning on using 2" XPS, then 1" XPS then 3/4 XPS with the 1" furrings embedded in the outer layer (thus saving me 1") and creating a 1/4" drainage plain. Potentially I could do that on both walls.
    Or I could just use two 2" and the furring on top where I have the space.

    What do the experts think to this?

  3. user-669103 | | #3

    edit that's 6" long screws

  4. will goodwin | | #4

    Thanks for the answer. Do you think that exterior foam insulation is easier than a double stud wall? I'm starting to lean that direction.

  5. user-669103 | | #5

    Yep which is easier exterior foam or double stud?

    I am wondering the same myself -- please experts chime in (also item 2 above - more related questions by me). Tell me if I should open a separate thread.

    I am planning both: Here are my deciding factors on which to do for retrofit (for new construction the choices are different)...

    1) Interior is crappy decor -- double stud (as I plan upstairs).
    2) Interior is main living space (and want quiet life with wife) -- do exterior foam.
    3) No roof overhang -- either add overhang next reroof or do interior double stud.
    4) Rooms on interior are already small -- do exterior foam.
    5) Mechanical items like split A/C may need to be professionally moved -- do on side that avoids this.
    6) Do you like 'Inny or Outie' windows?
    7) Are you residing anyway -- do exterior foam.
    8) Can you remove the siding without damaging it, and do you have color matched spares? - if not do interior. (I will use the garden shed as a color matched vinyl siding donor if I have to).
    7) There will be wall receptacles to move on the interior.

    For me, and my skill set, doing the interior is a little easier.
    I have some where there is no choice, and others that I'm still wondering.

  6. dankolbert | | #6

    I've done both - I wouldn't bother with exterior foam on new construction, I think double wall is easier and more effective. For retrofits foam is a good option.

  7. GBA Editor
    Martin Holladay | | #7

    Pressure-treated furring is nuts. For hundreds of years, people have been installing non-PT wood behind siding (for example, board sheathing, plywood, etc.). Plenty of old houses have perfectly sound 150-year-old or 200-year-old non-PT board sheathing. Stainless-steel 6-inch screws? Have you counted how many and priced them? Ouch!

    If it was me, I wouldn't do 12-in. o.c. furring. You really want to screw the furring to the studs.

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