skylight insulation value

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skylight insulation value

Postby pgarch » Wed May 05, 2010 1:48 am

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
pgarch
 
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Re: skylight insulation value

Postby Albrecht Stoecklein » Sun May 09, 2010 12:27 am

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
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Re: skylight insulation value

Postby pgarch » Mon May 10, 2010 2:03 am

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.
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Re: skylight insulation value

Postby Albrecht Stoecklein » Mon May 10, 2010 10:55 pm

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
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Re: skylight insulation value

Postby David144 » Tue Dec 23, 2014 11:42 pm

Not sure I am using the calculator correctly. I am adding a small bedroom to an upper floor (9.5m2). Reading the other messages it seems I ignore the floor and internal wall, however the 0 reading for the floor does not allow a tick. The note beside the floor suggests inserting 1.3 in the floor section, which I did and it now complies. Is this correct and if yes does it need to be justified/described. The ceiling below has been lowered so the ceiling to floor height is 600mm with both floor and ceiling joists - not touching. Will that have a significant effect on the R value.


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Last edited by David144 on Sat Jan 10, 2015 1:09 am, edited 1 time in total.
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Re: skylight insulation value

Postby Albrecht Stoecklein » Wed Dec 24, 2014 8:10 am

Hi David,

Have a look at the posts in this topic: http://www.designnavigator.co.nz/phpBB3/viewtopic.php?f=4&t=75. That should explain how to tackle this project.

You do not need to worry about the R-values of any internal elements such as the ceiling under the new floor above.

I hope that helps. If you are still stuck you could e-mail me the project number and I'll have a look.

Sunny regards

Albrecht
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Re: skylight insulation value

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