Moisture accumulation

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Moisture accumulation

Post by agnes.ahlund » Wed Sep 06, 2023 1:24 am -1100

I have a question regarding moisture accumulation in a construction.
(I → O): Plywood, insulation with inbedded glulam pillar, aluminum

I have simulated a wall that is completely diffusion-open on the inside (plywood against the inner surface) and completely diffusion-closed on the outer part of the construction (aluminum against the outer surface), meaning that moisture from the inside cannot vent out, and moisture from the outside cannot penetrate.
When I simulate this, moisture seems to "get stuck" in the insulation closest to the aluminum sheet. How is it possible that the moisture content (M%) can be as high as 800-1500% insulation but doesn't migrate inward into the construction and make the glulam pillar wetter?

Do you have any proposal on how to make this simulation the right way and measure the water content in the glulam pillar?

Best regards,

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Re: Moisture accumulation

Post by Thomas » Wed Sep 06, 2023 10:02 pm -1100

Hi Agnes,

this may not be a question of moisture transport but of the moisture sorption equilibrium in a temperature gradient.

The insulation material you are using is probably vapour-permeable but has no (or only little) liquid transport capability. Under these circumstances, vapour diffusion is the only (or at least the dominant) moisture transport mechanism in the insulation. Vapour diffusion is driven by vapour pressure differences, so it tries to equalize the vapour pressure in the component. Therefore, if your material is sufficiently vapour-permeable, and if you induce only limited vapour pressure fluctuations via the boundary conditions, a more or less constant vapour pressure distribution will result in your component.

However, the relative humidity which corresponds to a given vapour pressure depends on the temperature. If (on average) the aluminum side of your component is the colder side, the relative humidity there will be higher than elsewhere in the component, even if the vapour pressure is about the same everywhere. As dictated by the moisture storage function, a higher relative humidity corresponds to a higher moisture content.

So the moisture accumulation on one side is probably not caused by some moisture transport mechanism driving the moisture to that side, or by the refusal of a moisture transport mechanism to re-distribute the accumulated moisture, but simply to the fact that (A) if vapour diffusion is the dominant moisture transport mechanism and (B) there is on average a temperature gradient, the equilibrated vapour pressure corresponds to a higher relative humidity on the colder side, which in turn corresponds to a higher water content.

Kind regards,

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