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Will 1/4″ thick Pactiv fanfold foam work as a vent baffle in combination with mineral wool?

designc | Posted in General Questions on

Sorry the previous question got cut off.
I’m not sure whether it’s a condensation problem, as thin as it is.

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  1. Dana1 | | #1

    Fanfold is thin, but it usually comes with thin polyolefin facers that brings it's vapor permeance down to 0.6-2.0 perms (1 perm or less would be class-II vapor retarder.) Unfaced half-inch XPS would run about 2-2.5 perms, 1" unfaced type-I EPS would be about 5 perms, nearly an order of magnitude higher drying rate than fan-fold with facers.

    Look up the specs for the specific products you have in mind. Some fan-fold perforated on a 1-2" grid to make it highly permeable. The Pactiv fan-fold goods in this spec sheet all have a vapor permeance no greater than 1.7 perms:

    http://www.ujr.ca/EN/PDF_Support_Files/insulation/greenguard/xps/greenguard-fanfold-siding-underlayment-data-sheet.pdf

    That's a bit on the tight to put on the exterior of a batt- ideally you have an equal or lower permeance vapor retarder on the interior side of your rock wool. But it's not terrible- the assembly can still dry into the ventilation gap. Your climate and the construction of the rest of the assembly make a difference.

  2. designc | | #2

    Thank you D, I do plan to add a min.1-1/2" polyisocyanurate to bottom of rafters.
    Also, I'm not sure which exact Fanfold is sold by Lowes. But that is the product in question.
    It looks like a fairly easy way to get a seamless baffle on the cheap.

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