I see the U value for 20mm double glazing is given at between 1.2 and
1.4 W/m2/degK, is this normally measured just the glass area or does it
account for the PVC frame?

How would I work out the effect of adding a layer of clear 3mm
polycarbonate sheet some 10mm from the inside sheet?

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ajh wrote:

I see the U value for 20mm double glazing is given at between 1.2 and
1.4 W/m2/degK, is this normally measured just the glass area or does it account for the PVC frame?

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 ...

<https://enlightenedwindows.co.uk/windows/window-and-glass-insulation-data-explained#:~:text=precise%20value>

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ajh <news@loampitsfarm.co.uk> wrote:

I see the U value for 20mm double glazing is given at between 1.2 and
1.4 W/m2/degK, is this normally measured just the glass area or does it account for the PVC frame?

How would I work out the effect of adding a layer of clear 3mm
polycarbonate sheet some 10mm from the inside sheet?

Something like:
https://www.ubakus.de/
Demo mode

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:
https://ibb.co/6WHZcjQ

In both cases the condensation problem goes away.


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

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In article <txSdnSsOMYgyBR76nZ2dnZeNn_qdnZ2d@giganews.com>,
Colin Bignell <cpb@bignellREMOVETHIS.me.uk> wrote:

On 11/01/2025 14:16, ajh wrote:

I see the U value for 20mm double glazing is given at between 1.2 and
1.4 W/m2/degK, is this normally measured just the glass area or does it account for the PVC frame?

How would I work out the effect of adding a layer of clear 3mm polycarbonate sheet some 10mm from the inside sheet?

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?

--
from KT24 in Surrey, England - sent from my RISC OS 4t�
"I'd rather die of exhaustion than die of boredom" Thomas Carlyle

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On 11/01/2025 14:16, ajh wrote:

I see the U value for 20mm double glazing is given at between 1.2 and
1.4 W/m2/degK, is this normally measured just the glass area or does it account for the PVC frame?

How would I work out the effect of adding a layer of clear 3mm
polycarbonate sheet some 10mm from the inside sheet?

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.

--
Colin Bignell

--- SoupGate-Win32 v1.05
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On 12/01/2025 10:30, Colin Bignell wrote:

On 11/01/2025 14:16, ajh wrote:

I see the U value for 20mm double glazing is given at between 1.2 and
1.4 W/m2/degK, is this normally measured just the glass area or does
it account for the PVC frame?

How would I work out the effect of adding a layer of clear 3mm
polycarbonate sheet some 10mm from the inside sheet?

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.

Yes, it is more UV resistant.

--
“It is hard to imagine a more stupid decision or more dangerous way of
making decisions than by putting those decisions in the hands of people
who pay no price for being wrong.”

Thomas Sowell

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On 12/01/2025 10:30, Colin Bignell wrote:

On 11/01/2025 14:16, ajh wrote:

I see the U value for 20mm double glazing is given at between 1.2 and
1.4 W/m2/degK, is this normally measured just the glass area or does
it account for the PVC frame?

How would I work out the effect of adding a layer of clear 3mm
polycarbonate sheet some 10mm from the inside sheet?

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.

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.

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ajh wrote:

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.

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

<https://files.bregroup.com/bre-co-uk-file-library-copy/filelibrary/rpts/uvalue/BR_443_(2006_Edition).pdf>

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

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

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On 12/01/2025 17:36, Andy Burns wrote:

ajh wrote:

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.

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

<https://files.bregroup.com/bre-co-uk-file-library-copy/filelibrary/rpts/uvalue/BR_443_(2006_Edition).pdf>

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

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.

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On 12/01/2025 11:15, charles wrote:

In article <txSdnSsOMYgyBR76nZ2dnZeNn_qdnZ2d@giganews.com>,
Colin Bignell <cpb@bignellREMOVETHIS.me.uk> wrote:

On 11/01/2025 14:16, ajh wrote:

I see the U value for 20mm double glazing is given at between 1.2 and
1.4 W/m2/degK, is this normally measured just the glass area or does it
account for the PVC frame?

How would I work out the effect of adding a layer of clear 3mm
polycarbonate sheet some 10mm from the inside sheet?

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?

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.

--
Colin Bignell

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On Sun, 12 Jan 2025 18:21:49 +0000, ajh <news@loampitsfarm.co.uk> wrote:

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

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

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