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Window Installation on Cabin With Foam Board-Insulated Walls

NewTinyBuilder | Posted in General Questions on

Evan here with a 8×10 (80 square foot) tiny cabin in Vermont (zone 6). Thank you for being my teachers and guides in this cabin building process!

Ive found affordable foam board insulation locally and plan to install it, seams taped, exterior to my taped OSB sheathing. I have read many articles on GBA, Building Science, and elsewhere – decided to do “outtie” windows screwed, without a buck or frame extension, to framing through 1.5″ of foam with #15 felt paper WRB.  After reading several articles I am left with a few questions and would love your advice:

Question 1: In the Article “Installing Windows in a Foam Sheathed Wall,” Martin lays out the steps for installing an outtie window without a buck. His instructions do not include a backdam or beveled shim on the rough sill, or making an angle cut on the foam to allow for drainage – all things he recommends for an innie window, and that seem essential in the GBA article “Installing Windows the Right Way.” Ive seen elsewhere people add an extra sill trim/extension and want to make sure I do it right. Do outtie windows need any or all of these details? My RO sills are currently not sloped at all in case of water intrusion.

Question 2: In the same article Martin says that flashing details need to be integrated with the WRB (housewrap or foam), but does not elaborate. I found instructions in a JLC article, “Intalling Flanged Windows: Two Strategies Compared,” but the author says the steps are for plastic housewrap and that its different for “black paper” wraps, without explaining those steps. I found Lstiburek’s instructions using “building paper” on Building Science RR-0407, and using the foam as the WRB in BSI-085, but not sure if the steps are the same for felt paper. I also found video instructions for installing window flashing paper here: https://www.youtube.com/watch?v=GoNVLw_7YyY. 
Are the window install details really different for using felt paper – and if so, what is the recommended process for flashing?

Thank you so much for supporting my continued learning and building! Evan

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Replies

  1. DCContrarian | | #1

    To question 1: yes. To question 2: yes. Felt can't take a fold the way Tyvek does. Rather than trying to fold it into the opening you're better off just cutting it flush and sealing with flashing tape.

    Some general principles: know your drainage plane. In your case it's the exterior of the foam, it has to be waterproof for the entire height of the wall. Flash the opening, not the window. This means a pan at the bottom, continuous waterproofing along the sides and a piece of metal flashing at the top with a drip edge to kick any water running down the wall away from the opening.

    Lots of windows have been installed without a backdam or beveled sill without problems. But lots of windows leak. At the time of installation it costs almost nothing to add them before flashing and if you have a minor leak they will keep the water out of the house. The only reason I would omit them is if the rough opening is too tight.

    1. NewTinyBuilder | | #3

      @DCContrarian Thank you for this insight! After a bit more digging I found two instructables here on GBA in the "Detail Library" - they are both "Flanged Window Installation//Water Managed" for moderate to high exposure, one for "housewrap" and the other for "building paper". Is one of these more or less appropriate for using felt as my WRB over foam? It seems that in the housewrap version, the housewrap goes on first, and for building paper the windows are installed before the building paper is wrapped. Neither is specifically for foam sheathed walls, and I was under the impression, from the GBA article "How NOT To Install Windows in a New Home" that the WRB should always be wrapped before windows are installed, but the descriptions make me think I should follow the building paper version and install windows before the felt WRB. Which one of those guides, or a different procedure, would you recommend following?

      Based on all my knowledge so far my flow would be one of the following:

      Option 1 - roughly following the housewrap version (WRB then windows)
      Install metal drip edge or 1.5" thick PT lumber along bottom of rim joist to protect bottom foam edge
      Sheath walls with 1.5" foam and tape seams
      Wrap house with felt from bottom up, secured with 3" cap nails
      Cut felt flush with ROs
      Create beveled sill with back dam
      Angle cut foam at sills sloping 5-6 degrees to exterior
      Flash tape on sill extending from back dam to exterior side of the foam and on top of felt
      Install non-compressible shims on sill with sealant to create level sill
      Cut 45 degree slits in felt at window head and tape up flap
      Flash tape jambs extending from inside RO over foam and on top of felt
      Flash tape window head overlapping jamb tape
      Sealant bead (OSI Quad Max) over felt/flashing tape around window RO jambs and head
      Screw in window nailing flange jambs and head with goal of sealant squeeze out
      Bead of sealant "buttered/tooled" over jamb and head flanges uniformly covering all holes
      Flash tape over jambs
      Sealant and screw window head aluminum flashing through foam into sheathing just above head flange
      Untape felt WRB at head, lower onto drip edge and flash tape over drip edge and head flange
      Flash tape 45 degree slits in felt tape at head
      Seal window from interior

      Or, if its better to install windows before wrapping felt and use tar paper flashing, I could do Option 2 - roughly following the building paper version, windows first then house wrap:

      Install metal drip edge or 1.5" thick PT lumber along bottom of rim joist to protect bottom foam edge
      Sheath walls with 1.5" foam and tape seams
      Create beveled sill with back dam
      Angle cut foam at sills sloping 5-6 degrees to exterior
      Staple strip of tar paper below RO
      Flash tape on sill extending from back dam to exterior side of the foam and on top of felt strip
      Install non-compressible shims on sill with sealant to create level sill
      Staple strips of tar paper at jambs
      Sealant bead (OSI Quad Max) over felt around window RO jambs and head
      Screw in window nailing flange jambs and head with goal of sealant squeeze out
      Bead of sealant "buttered/tooled" over jamb and head flanges uniformly covering all holes
      Flash tape over jamb flanges
      Flash tape over head flange
      Sealant and screw window head aluminum flashing through foam into sheathing just above head flange
      Install 1st wrap of tar paper WRB, lapping below strip of tar paper attached to sill
      Install felt working my way up the wall
      Seal window from interior

      Phew! How does that look?

  2. Deleted | | #2

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