Dear,
I have a skylight in my living room/kitchen (living) of about 1m70 x 9m long. This consists of wooden beams (section of 130x50mm) on which aluminum profiles are mounted and closed with double-walled polycarbonate plates. The entire skylight has only a gentle slope to divert the rainwater to a gutter, but you could say that the skylight is almost horizontal.
After the recent snowfall, my suspicion that an enormous amount of heat is lost through this skylight has been clearly confirmed: the snow layer had melted away much faster on the PC plates than on the place where there is a “normal” flat roof. The solar incidence is also the same, so that cannot have caused it. So a lot of heat must have escaped from the living room through the PC plates to melt the snow much faster there than in other places. You also always have a cold feeling in your back in the winter when you sit under the skylight.
I would like to provide the entire skylight with a 2nd layer of double-walled PC plates, but then mounted on the underside (inside) of the wooden beams. There would thus be a cavity (space) between the two PC plates of 130mm.
My question is now:
– does this make sense in terms of additional (thermal) insulation value
– and isn’t the distance between the two “layers” too big? I have read on other forums that with “normal” (= vertical) windows it is best to keep a cavity of approx. 2 cm, because otherwise air circulation will occur in the cavity (due to convection currents) and the insulation efficiency will decrease. Reduce.
– if I then apply PC plates, is it more economical to use “coated” plates? (I suspect this is some kind of IR reflection screen)
I look forward to the answer with great interest.
Thank you
Answer
Best,
When I look at the correlations for the Nusselt number (which describes the heat transfer by convection) I deduce that the distance between the plates is not really that important (at least for distances between 2 and 20 cm, beyond this I don’t know) . The further apart you place the plates, the more internal circulation of air will take place, so that more heat could be transported through the air layer between the plates, but at the same time the layer also becomes thicker, which in turn is good for improving heat transport. Reduce. Both effects roughly cancel each other out.
If there is enough space, it is advantageous (from a thermal point of view) to install several plates.
Apart from heat transport through convection, heat will also be lost through radiation, but here too the distance between the plates is not so important. The coating may help for this.
Answered by
prof. Dr. ir. Sander Deridder
Chromatography Heat and Mass Transfer Chemical Engineering
Old Market 13 3000 Leuven
https://www.kuleuven.be/
.