[Bldg-sim] Green Roof

[Cheney]

>
> I totally agree with Mark. The insulation of soil and plants can only be
> considered as a bonus to the green roof. The variable combinations of soil,
> moisture and plants will give you a very big range of R values. For example,
> the thermal conductivity of perfect dry sand can be as low as 0.15W/mK
> whereas it can be up to 4 W/mK for saturated soil (over 26times difference).
> Considering climate, growing media variation, irrigation schedule and plant
> types, it is very difficult to work out a representative R value.
> Moreover,those extensive rooftop gardens with light-weight  growing media
> (such as expanded clay pellet), sometimes, will bring negative thermal
> impact over roof. It has been observed in my previous research. The reason
> is probably the darker color of the pellets absorb more solar radiation
> and trigger higher surface temperature. Eventually the excessive heat has
> been transfered into the indoor space. In conclusion, it is safe to exclude
> the R value of soil in your roof calculation.
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