Hygrothermal performance of reverted green roofs

[Kristina F. Olsen]

Hi!

I am going to simulate green, inverted compact roofs in connection with the master's thesis, and wondered what experiences people have around this?

It is to be simulated for Oslo, Bergen and Trondheim using the advanced
explicit full radiation balance, to take into account overcooling at night. I see that these places don't have atmospheric counter-radiance, directly, although there is a total total atmospheric counter-radiation in the analysis, which allows me to take this into account anyway.

I have used optigreen's materials and the WUFI catalog "Guideline for the calculation of extensive green roofs", which is based on field measurements and the simulations. I have chosen the solution "Optigreen-System Nature Roof Solution1". The purpose of the simulation is to look at the moisture absorption in the plastic insulation over 10 years.

Addition to the moisture sources in growth medium and the drainage boards (40 % of the precipitation) , I have added a moisture source under the XPS equal to 1 % of the precipitation, to make constant humid area beneath the insulation). Is this still accurate when using drainageboards, how much water will these let through compared to gravel or expanded clay as a drainage material?

Due to many thin layers (separation layer between growth layer and drainage (filter food), separation layer between drainage and XPS (separation food), water film under XPS etc.), these may give me a lot of convergence errors (around 1000). I have managed to reduce them by going up to grid size equal to 500. What else can I do? Can I take away the first two layers, or will this give too big an impact in the simulation? I want almost the same scenario as in a real situation.

Thanks in advance for answers!
Kristina Fjeldstad Olsen

[Daniel]

Hi Kristina,

we did similar investiagation some years ago. However, i was necessary to calibrate the properties of the "storage layer" beneath the XPS boards by field measurements results to well fit with reality.

I think you can leave away the thin layers above the XPS - just use the growth medium and try to adapt the properties of the storage layer beneath the XPS in a way that produces 100 % RH after rain for a longer period, but avoid to come too close or exceed maximum saturation (for example free saturation 50 kg/m³, maximum saturation 950 kg/m³, source limited to 850 kg/m³). That means, you have long periods with 100 % RH to humidify the XPS but dont'r reach maximum saturation which would cause numerical problems.

The determination of the properties was a bit a challenge and is also "secret know how" to a certain extent as we perform such simuliations for approval in Germany. I think you should adapt the detailed properties to fit with field measurements... because it's difficult to estimate the realistic conditions and they also differ a little bit from roof to roof (whith inclination, soiled interface between roofing membrane and XPS etc.).

Hope, that helps a bit further.

best regards
Daniel