Confusion about diffusion – variable-perm materials
Hello everyone;
So, I am currently struggling in understanding the relevance of variable permeance materials, such as faced batt insulation.
What is their relevance in the construction sector? I am not talking about the importance of controlling air, just the ability to adapt the permanence.
Imagine I have an exterior wall with drywall and faced batt insulation in the cavity. Drywall is pretty much vapour permeable (let’s assume that is unpainted). The permeance of faced batts can be around 10 perms when RH is 60%.
In this position, where the inner humidity is (let’s assume a colder climate, not extreme, but with more heating needs than cooling), faced batts are not much of a vapour retarder, are they? I am feeling that faced batts only work as vapor retarders when there is no vapour to retard is the first place.
If I have exterior insulation I am less worried about diffusion. However, only with insulation in the cavity, I should control some of that vapour diffusing through the wall. But again, faced batts will open when I most need a vapour retarder.
The same thing with other materials such as Membrain smart vapour retard. These materials will open up almost exactly when I need a vapour retarder.
So, why is this vapour variable permanence a thing?
Kind Regards,
Peter
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Replies
Hi Peter.
In cold climates interior vapor retarders are used to keep vapor from diffusing from the warm interior winter air towards the exterior where is can meet a condensing surface like the cold sheathing and result in wetting inside the walls. This is why when you have enough exterior insulation to eliminate the potential for the dew point temperature to occur inside a wall assembly, you do not need much interior vapor control. Without a condensing surface, vapor drive is not a problem.
In warmer months, or warmer climates, vapor drive happens from the exterior towards the interior. So, if you have a wet assembly and, say a class I interior vapor retarder, the assembly will not be able to dry in warmer times because the vapor cannot move towards the interior. This is why in warmer climates, interior poly sheeting is dangerous. Not only does it prevent drying, but when running A/C, the poly can become a condensing surface inside the wall.
To oversimplify a bit, a variable perm membrane is more vapor closed in common winter conditions and will prevent outward vapor drive and it becomes more vapor-open in common summer conditions to allow inward drying.
Most wall failures are the result of water intrusion, which is why water management is the most important thing for building durability. Air carries more potential for condensation and moisture problems than vapor drive, which is one of the reasons why air sealing is so important, second to water management. Vapor is third on the list, then thermal, which beyond dew point control is all about comfort and efficiency. I suggest you read this: The Four Control Layers of a Wall.
Hi Brian.
Thanks for the detailed answer.
I guess it really depends on the indoor and outdoor conditions, during the day, month and season.
If for instances, my indoor RH during the winter is 50%, which may be plausible without raising mould issues, faced batt presents a perm rating of about 7/8 (see image below). Lstiburek classifies a perm rating of 10 as vapour permeable.
So, when I most need a Class II vapor retarder, I get a vapour semi-permeable material or Class III vapour retarder.
I understand the importance of vapour permeance variability for bulk water drying (e.g. intrusion). However, I still feel that in terms of vapour control, these materials only offer a modest contribution.
I accept their importance in providing a vapour "throttle" (is better to have an "open phase" faced batt than an unfaced batt), only.
For instances, I have recently hear a contractor saying (I will apply some faced batt in this kitchen because dryboard is vapour permeable and I want a class II vapour control on this). In my opinion, this happens to be semi-correct. If a lot of vapour diffuse trough the drywall, the faced batt will open up to also allow to escape that moisture. At a smaller rate than unfaced batt. And if that moisture finds a cold surface ...
Kind Regards
Brian's response hit the nail on the head. The goal is slowing (but not stopping) vapor movement in winter, while allowing inward drying in the spring.
That said, 60% RH is quite high for winter...Other smart vapor retarders are made to stay more 'closed' up to about 80% like Intello or DB+.