High water content in mineral wool that is in contact with internal air

[Antoine]

Hello all,

I used WUFI a long time ago, and almost did not touch it last few years. I am doing a pretty "basic" modelling of an insulated timber frame wall with a ventilated cladding. The question I have is not related to the ventilated gap and the drying, but more to the internal side and a surprinsingly high water content in the insulation material.

The insulation is some mineral wool (13cm) and behind it (internal side) there is a perforated (15% open) metallic sheet. A ventilated gap and a cladding are considered but in this case there is no air renewal in the ventilated gap.

In order to take into account the resistance to the water diffusion from the metallic sheet, I add a 0,1 Sd to the internal boundary condition.
For the internal boundary condition, I chose the EN 13788 with a class 3. The HR for the internal air is the following. Note that it does not go over 65%.

The mineral wool that is chosen is from the WUFI database. The water content is typical of an "inert" insulation material, being very low under 80% RH and then "climbing up".

The simulation shows a very high water content in the mineral wool. It varies between a 3 kg/m3 to a 30-40 kg/m3. (The density of the mineral panel is 115kg/m3).
Also, it is shown clearly that the water content is higher on the inner side of the insulation as you can see in the graph.

First do you see any mistakes in this modeling ?

If not, interpreting the results, are those high water content or RH values "plausible" ? Or are we at the limit of the modelling that offers WUFI ?
Without knowing that I guess we can draw as a conclusion that it is highly recommended to place a vapour barrier on the internal side.

Basically, I am looking for external outlooks as those values bother me.

Thanks
Regards
Antoine

[Christian Bludau]

Hi Antoine,

what climate is on the exterior side? I guess it is an external climate with cold periods? In this case due to the non existing vcl there occurs condensation of the internal air in the insulation.

Christian

[Antoine]

Hello
thanks for the reply.
To answer : on the exterior side, we use the weather file of Grenoble. There is no rain absorption (due to the cladding).

Clearly there are condensations risks (even shown by Glaser method), but those high values were striking me.

Antoine

[Christian Bludau]

Dear Antoine,

depending on the thickness you are assessing, the value seems not that high.
Please have a look on our tutorial "Condensation Assessment":
https://wufi.de/en/service/downloads#co ... assessment

Maybe try to compare your dew water amounts with them from glaser.

Christian