skylight insulation value

[pgarch]

I am looking at providing a skylight comprising a double layer of clear Durolite- clear Fiberglass roofing, which will be spaced 20mm apart from the other layer. This will be done to the manufacturers standard detail which is primarily used to prevent condensation. In addition to this I am looking to provide a 400mm deep air space with polycarbonate to the underside of the ceiling, this space will be framed around and relatively airtight although not sealed. Could you please give me some idea of the relative R value of this skylight opening. There will be 300 deep DHS steel purlins at 1200 centres but I will line these with 50mm XPS insulation.

Thanks in advance

[Albrecht Stoecklein]

Hi,

The fibreglass would have a very small R-value. So I think you can ignore that.

The polycarbonate has an R-value of about R-0.032 per mm thickness.

On top of that you have the surface resistances (on the outside: R-0.03, on the inside R-0.09).

A 20mm still airgap has an R-value of R-0.13, the 400mm gap will have an R-value of R-0.17.

So assuming the polycarbonate is 4mm thick you will have:

R-0.03 (external surface)
R-0.13 (20mm air)
R-0.17 (40mm air)
4 x R-0.03 (poly)
R-0.09 (internal surface)

Total: R-0.54

Sunny Regards

Albrecht

[pgarch]

Thanks again Albrecht, Is the R.17 correct for the 400mmm of air?, it seems only marginally better than the 20mm of air at R.13.

[Albrecht Stoecklein]

Yes, that's correct.

Most of the R-value comes from the surface layers because there the air is still.

Once that cavity gets too large you have more and more convection (especially in the roof space because the air is rising unlike in the subfloor). So therefore the R-value does not increase a lot more above a certain gap width.

Cheers

Albrecht