snow modeling

malimog · Post by **malimog** » Thu Sep 15, 2011 3:09 am -1100

Hello
regarding simulation of snow on the roof by WUFI, i am trying to have a 0 degree Celsius for temperature and 100% relative humidity.
after creating a kli climate file(export surface T and RH )and assign 0 for T and 100% for RH for the period we expect have snow and also 0 for heat resistance, i checked the result and noticed that surface temperature doesn't have problem and it is 0 for the time that we have snow but relative humidity is not 100 however we define it 100% in our climate file.
so can you please tell me what is the problem because i want to see hygrothermal behavior of roof when we have 0 for T and 100% for RH.

Daniel · Post by **Daniel** » Thu Sep 15, 2011 4:16 am -1100

Hi Malimog,

What is your RH? Normally this should work...

Temperature remains constant at 0 °C - that means you don't consider radiation etc.?

From our experience the hygrothermal conditions in roofs are more severe if you neglect the snow cover because of the stronger overcooling during night time compared to the addtional "insulation" of the snow.

best regards
Daniel

malimog · Post by **malimog** » Thu Sep 15, 2011 4:48 am -1100

i am trying to put 100% for exterior surface RH, i already implement effect of radiation and etc in my first calculation and then export surface RH& T.
how about cold climates like Montreal that we have 3-4 month of snow on the roof?are you sure that over cooling is stronger than snow even in cold climates like Montreal that we have 3-4 months of snow cover?

Daniel · Post by **Daniel** » Thu Sep 15, 2011 4:55 am -1100

We compared the conditions here in Holzkirchen - but in winter time the long wave radtiaition emission is dominant versus the solar radiation absorption due to the low level of solar raditation gains (long night - short day - low altitude of the sun).

During night the temperature on a flat roof can fall far below 0 °C while it remains more or less at this level with snow cover.

So the assumption should become even more true in colder climates...

Daniel

malimog · Post by **malimog** » Thu Sep 15, 2011 5:02 am -1100

how about high exterior RH while we have snow?did you do any simulation for effects of snow?I am studying hygrothermal behavior of cool roofs and now I am working on snow simulation to find out that is there any difference between dark roofs and cool roofs when we have snow? (in terms of moisture accumulation)

Daniel · Post by **Daniel** » Thu Sep 15, 2011 5:15 am -1100

The influence of the RH above the roofing membrane should be negligable in relation to the vapor diffusion tightness of most membranes.

Depending on the material properties you normally get only a permeablity of a few g/m² during the whole year.

What are the properties of your membrane...

Daniel

malimog · Post by **malimog** » Thu Sep 15, 2011 5:18 am -1100

for membrane i am using Oriented Strand Board (density 595 kg/m³) with sd 100m.

Daniel · Post by **Daniel** » Thu Sep 15, 2011 5:25 am -1100

I dont mean the sheating below the membrane but the roofing membrane above (plastic film however...).

The OSB 595 from the database has an sd-Value of about 2.5 m sd...

During the snow cover period you have no difference between a white and a black roofing membrane in the simulation. The worst case for a white membrane is always without snow!

Daniel

malimog · Post by **malimog** » Thu Sep 15, 2011 5:47 am -1100

but this is a file which i received from Mr.Christian Bludau .and i am working on it.

Daniel · Post by **Daniel** » Thu Sep 15, 2011 5:53 am -1100

maybe you have replaced the membrane by the surface transfer coefficients? sd value - no rain water absorption - radiation absorption according to the color or the membrane - but OSB is the last material of your construction?

Then the sd value from the surface transfer coefficients is the one of the membrane... so with 100 m there is only little vapor transport through the membrane. You can quantifiy it by the difference between sd 1500 (more or less vapor tight) and the sd of your case.

best regards
Daniel

malimog · Post by **malimog** » Thu Sep 15, 2011 6:06 am -1100

yeah it is my last material for exterior surface.