Moisture content green roof

[Theasjo]

Hi!

I am currently investigating whether a ventilated air cavity can improve the evaporation rate in cold and wet countries. I am therefore building a standard green roof model in WUFI (without ventilated air cavity), which I will compare with measured moisture data from a test field in Trondheim (Norway) in order to validate the model. I have therefore added the actual material properties for the substrate that is used at the test field. The substrate is 100 mm. I have also added the bulk density and porosity for the sedum layer (which is the only material properties I have available for that kind of substrate). The other material properties are the same as for the sedum layers in the material database in WUFI. I have also used the climate data for the specific location, which I have added as a WAC file.

I add 40% of the precipitation directly into the lowest 20mm of the substrate. When comparing the simulated results with the measured results it can be seen that the simulated moisture content is higher between the larger precipitation events, but the lowest moisture content is lower for the simulated results compared to the measured results (see attached photo). I am therefore wondering what it is that controls minimum moisture content. Is it any of the boundary conditions?

Compared results

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The largest precipitation events led the substrate to get supersaturated. When comparing the simulated results and the measured results, I have manipulated the measured results so that the substrate only gets saturated. I am therefore wondering if it is possible to simulate a situation where the substrate gets supersaturated in WUFI?

I am also wondering if there are any available research where the percentage of precipitation added directly into the lowest 20mm of the substrate gets investigated (and 40% is found to give the best result). It would be good to read the research in order to understand why exactly 40% in the lower 20mm. I have only found papers saying that that gave the best fit, but not showing where it comes from.

As I am investigating the effect of a ventilated cavity in the green roof I have added an air layer in the model (I will do this after validating the reference green roof model, but I have built up the air layer now and will just adjust the green roof build-up as I get the simulated results to match the measured results). I have added an air layer without additional moisture capacity. In this layer I have added the air change source. On both sides of the air layer without additional moisture capacity I have added two thin air layers (with additional moisture capacity) This in order to capture both the moisture and condensation capacity. When I perform the simulations, the water balance do not add up. What can I do to balance the water balance better?

Is this correct way to build up the air layer when I want to simulate a ventilated air cavity?

Air layer

airlayer.JPG (56.25 KiB) Viewed 3060 times

I highly appreciate your answer,

Best regards,
Thea Johannessen

[Theasjo]

I am also wondering what "Free water saturation" is? How is that different to max water saturation?

[Thomas]

I am also wondering what "Free water saturation" is? How is that different to max water saturation?

Hi Thea,

maximum water saturation is the degree of saturation which is reached when the whole accessible pore space in the material is completely filled with water.

Free water saturation is the degree of saturation which is reached when the material is exposed to liquid water and allowed to absorb the water by capillary forces until it stops to take up more water ("capillary uptake", "suction experiment"). In this case, the water flowing into the material will usually trap small air bubbles in dead-end pores etc., so that at the end of the suction experiment the degree of saturation will be less than maximum saturation.

Maximum saturation can be reached if the capillary uptake takes place in a vacuum (which is done in an industrial setting but doesn't occur in building physics), if the air bubbles have time to dissolve in the water (possible but unlikely in building physics), or if condensation takes place in the material (occurs often in building physics). Condensation starts in the smallest pores and fills the material "from inside out", pushing all the air out of the pores. During capillary uptake, by contrast, the water is rushing "in from outside", trapping the air bubbles.

WUFI therefore distinguishes between free and maximum water content. Capillary water uptake (for example absorbed rain) can fill a material only up to free saturation, condensation can fill a material up to maximum saturation.

Regards,
Thomas