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Community and Q&A

Permeability of retrofit insulation in zone 4C

Douglas_Furr | Posted in Energy Efficiency and Durability on

How important is maintaining vapor permeability in a cool humid climate in the case of an insulation retrofit that neither replaces the interior or exterior finish, but merely adds insulation? We’ll presume hypothetically equivalent sealing and leak prevention, and equivalent r values. If most moisture comes in through leaks, how much difference will it make to an old zone 4C home whether the in-fill is semi-impermeable, semi-permeable, or quite permeable?

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  1. GBA Editor
    Martin Holladay | | #1

    I assume that you are trying to choose between two different types of insulation. Unfortunately, you are vague, and you haven't mentioned which two types of insulation you are considering.

    The permeance of insulation materials is only one of many characteristics of the insulation, and in the case you are describing, is not the most important characteristic to guide your decision.

    You are worried about water entering your wall assembly from the exterior -- we're talking about rain -- and you should be, since rain is by far the most common source of water in walls. To address water entry from the exterior, your two most important lines of defense are (a) good flashing details, and (b) a ventilated rainscreen gap between the siding and the sheathing. Other factors (like the permeance of the insulation) are almost irrelevant. After all, these types of leaks (rain that gets past your siding or gets past your flashing) almost always need to be able to dry to the exterior, not dry to the interior.

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