Will rot spread mold? Dew point in cathedral ceiling rafters. Cc spray foam on sides will stop mold sideways, but ‘vertically’, will rot stay by dew point?
Cathedral ceiling with no insulation below or above the rafters. Climate zone 5, north of Toronto, can have 40 Celsius difference within 6″ of Rafter. There’s no vapor barrier below the rafters. Ice-and-water shield is a vapor barrier above part of the roof deck.
If the dew point is in the middle of the 6″ of wood between exterior and interior, and if the wood has “sorption” (absorption and “adsorption”) (like condensation), and if vapor drive outward is less than vapor entering, then: after the middle rots, will the rot spread to the two ends of the rafter, and therefore also affect the roof sheeting above and the drywall below, or will the rot stay around the dew point roughly in the middle of the rafter?
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Replies
Moisture doesn't always behave as it should according to dewpoint calculations. In an example like yours, it typically continues moving outward until it reaches what some call the "condensing surface of interest"--i.e., your roof sheathing. There it will adsorb and absorb into the sheathing and rot will extend from there when temperatures are conducive.
https://buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion