Window Installation on Cabin With Foam Board-Insulated Walls
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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