Rain Water Absorption Factor value [-]

[Talev]

Hi

There is a challenge to insulate a brick wall form indoor side. I would like to give suggestion of the maximum insulation thickness which we can use on the indoor side. (I do know that external isolation is much better choice but we are no allowed to do that).

The primary goal of WUFI simulation is to predict the initial temperature as well as relative humidity of the brick so that we can avoid ice crystals inside the brick wall and protect the façade from damages. In order to do precise WUFI simulation I am wandering which Rain Water Absorption Factor [-] should I consider within my WUFI calculation?

WUFI help says “With vertical surfaces, it is nearly zero for snow and hail, for other conditions it must be estimated. A value of 0.7 might be a good starting point”.

I tried to do WUFI calculation using Rain Water Absorption Factor 0 as well as 0, 7 but there is a huge variation of the calculated initial temperature as well as relative humidity. Could you please give me some suggestion from your practical experience which value of Rain Water Absorption Factor should I applied to such WUFI calculation? In addition I am also wandering if I should consider short and long wave radiation (absorptivity /emissivity) during such WUFI calculation? I would rather use WUFI suggestion from red brick in this case… what is you suggestion?

[Manfred Kehrer]

Hello Talev,

a "Adhering Fraction of Rain" value (former "Rain absorption factor") of 0.7 is a good start. Note if the outside temperature is below 0 deg C, WUFI assume that all precipitation is snow and no liquid moisture uptake will occur.

Long-wave emissivity will only be taken into account if the Explicit Radiation Balance is switched on.

This is anyway a tough topic and I know that the results in your study will depend a lot on this "Adhering Fraction of Rain" value.