polycarbonate tertiary glazing

U value is for the whole window, if you see "Ug value" quoted that's just for the glass, similarly "Uf value" is just for the frame ...

Something like:

Wall (the default)

Add layer 1: 'Insulation glass, double glazed, Ug=1.1', thickness 20mm click on hamburger -> place studs or rafters in this layer then in the un-numbered box select Polyvinylchorid (PVC), width = 50mm (width of your glazing beads), distance = 1000mm (width of your window)

gives U=1.389 (and a lot of condensation, unsurprisingly)

- this is a bit of a hack because we're pretending your glazing beads are solid PVC wall studding, and assuming this window is infinitely tall so no frame at the top and bottom

Add layer 2: 'Polycarbonat', thickness 3mm Drag to above layer 1 (6 grey squares on the LHS)

gives U=1.360

Let's add another layer between - 'Stationary air (unventilated)', 5mm between the glass and the polycarbonate

gives U=1.171

If I make the stationary air 10mm, I get U=1.117.

Picture of the stackup:

Have a play and see if you can mock up something that best matches your actual situation. The calculator is not designed to handle glazing but the studding feature is a way to pretend there's a frame.

Theo

I used polycarbonate sheet to create double glazing many years ago. Enough UV still got though the glass to degrade it. Perspex is a much better choice.

how about using glass?

Yes, it is more UV resistant.

Thanks, it seems to be cheaper too.

If I can get my head around the sums and see the savings in heat loss it will be applied to the underside of a 3 light 2.2m by 1.55m inclined rooflight so glass would be too risky.

I had hoped to just sum the reciprocals of U value of the existing 20mm double glazing, an air gap and the 3mm perspex and take the reciprocal of that to arrive at the overall U value. Current losses with a 20C delta T look like 61W and cost excluding labour at £150 ish.

Now you've got another fiddle-factor to add to the sums, for glazing panels the U value is specified for vertical orientation

See section 11.1 of BR443

That will work if you know the individual U values, how well sealed will your extra air layer be?

I only need a ball park figure. I know the biggest improvement here would be to insulate the solid walls but that's a major job and I hear of horror stories from damp/interstitial condensation problems, If I do have anything done it will all have to breathe as it has done for the last 160 years.

Adding a perspex inner leaf (and sealing it with silicone) is something I could manage.

The glazing bars actually have 100mm of depth so the air gap could be up to that but I think internal convection may overcome benefits if it is too deep.

It's not directly for a cost saving but more so I don't burn so many logs or expend the labour on cutting them.

My application required it to be held in place by magnetic strips and to be removable. Perspex is much lighter than glass and is not liable to break if dropped.

I have installed something like that, though vertical, at the request of a friend: perspex/polycarbonate attached by two nails in holes with clearance at the top, to allow for movement and expansion. Then sealed around the edge with clear tape.

The plan was that this would be removed in summer, and that the tape would come off easily and without residue, and reinstalled when the heating season resumed. I didn't hear back how that worked out...

Thomas Prufer