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Wetting vs drying potential – sweet spot

Robert_Williamson | Posted in General Questions on

Hello everyone;

So, I am really confused about the balance between wetting and drying potential of constructive assemblies. 

Imagine I am a brick wall, insulated on the inside with mineral wool. My interior airtightness layer is the gypsum boards. Now, imagine I do a really bad airtightness, and some vapour can migrate through my vapour open insulation and find the cold condensing surface (the brick). 

Well, because I am using a very vapour open assembly to the inside, do I really have a problem? Can´t the wall assembly dry with the same ease that gets wet? 

Let´s assumed that I am in a mixed-humid climate, without intensive/ constant heating or cooling periods. 

What is the sweet spot between wetting and drying? How can one predict if that wall assembly will exceed its drying/ storage capacity and have moisture problems? 

Thank you in advance,
Robert

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Replies

  1. Jon_R | | #1

    > Can´t the wall assembly dry with the same ease that gets wet?
    Not if the wetting continues and the wetting rate exceeds the drying rate.

    WUFI models such things, but better to just use recommended wall designs and make them airtight.

  2. maine_tyler | | #2

    Robert,

    Generally, you want an air-tight wall. There's not too many situations where air leaks will be a net benefit. Also bear in mind that keeping moisture OUT of the wall is usually step 1 (and air moves a lot more moisture than vapor diffusion), while allowing drying is step 2. (sort of: really you just need more drying than wetting, but it's usually easier to limit wetting so that you need less drying potential).
    The drying path is generally via diffusion, which implies that the wall will only dry when vapor pressure inside the wall becomes higher than the vapor pressure interior or exterior of the wall innards.

    Another variable to consider is temperature and it's effect on relative humidity and condensation. If your brick wall is colder than the air exiting your interior, it can become a condensing surface. If it was kept warm enough via insulation on the exterior (heating season), it is less likely to condense moisture (and RH will be lower, so absorptive materials will not take on as much water). The reverse can happen during cooling season with warm, moist, outdoor air and cool interior walls thanks to AC.

    Hence the specific vapor profile of these layers definitely depends on climate and conditioning variables. I can't really speak to your specific mixed/mild climate, but in some cases, people use 'smart vapor retarders' when seasonality demands different drying/resistance to wetting characteristics.

    This is a nice article by Peter Yost:
    https://www.greenbuildingadvisor.com/article/vapor-profiles-help-predict-whether-a-wall-can-dry

    Another good one: https://sgcuniversity.s3.amazonaws.com/s3fs-public/AIA_BDC_April2016.pdf

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Robert.

    You may find this article helpful as well: The Four Control Layers of a Wall.

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