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Condensing surface – always the backside of the sheathing?

andyfrog | Posted in Energy Efficiency and Durability on

messy quote from youtube transcript:

          enough thermal mass to handle the change in
phase energy
you need a condensing
surface with sufficient thermal mass to
handle the change in phase energy
for condensation to occur well the first
surface is the back side of the damn

This makes sense intuitively, however, does the backside of the sheathing being the condensing surface still hold true for situations like:

-rigid foam insulation

-peel and stick membranes applied to the exterior of wood fiber insulation

i.e. in these situations do these surfaces have enough thermal mass to produce condensation?


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  1. Expert Member

    Joe explains it much better than I can here:

    To more directly answer your question: Maybe. For condensation to occur it needs both a surface to accumulate on below the dew p0int and the water vapour to be able get there. So rigid foam will keep the sheathing above the dew point and stop it being a condensing surface if it's thick enough. So should sufficient exterior wood fibre insulation, with or without an impermeable WRB.

    1. andyfrog | | #2

      Thanks for the reply, I'll definitely check out that link.

      I think I'm probably lacking in understanding of something, but just to be sure, my original question was not about the sheathing behind either interior surface of the rigid foam or interior surface of the wood fiber exterior adhered membrane--rather it was about those two surfaces themselves.

      Having thought about it briefly after reading your reply (albeit without reading the link--sorry!), I'm guessing the answer is probably not because presumably the sheathing is acting like a sufficient enough vapor throttle + there is enough insulation between the interior surface of the rigid foam/interior surface of the adhered membrane such that no condensation will occur

  2. Jon_R | | #3

    IMO, the thermal mass point doesn't make things clearer. You can get lots of condensation in very low thermal mass assemblies.

    On the other hand, it's an important point that you won't necessarily find water accumulation at the point where the temp = dew point. As Joe writes, "It moves on to a colder and colder spot until it can’t go any farther easily". This is often the sheathing, but fiberboard sheathing can be a counter example - the moisture flows through to the exterior.

    A good mental exercise: We know that even with code levels of exterior insulation, the sheathing is sometimes at the dew point. How much moisture is deposited during these periods?

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