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Old Apr 10th 2017, 02:57 AM   #1
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Question Insulating efficiency

I have a conservatory with an solid (insulated) roof with two large lantern lights (see image attached). These are elongated rectangular pyramids which sit above wells cut through the roof (approx 35cm deep) and project above the roof by about 30cm at the apex. The glass is standard UK 24mm gap double gazing - I think/hope they are gas filled.

I am considering installing a second layer of double glazing horizontally across either the top or bottom of the opening to improve the thermal efficiency.
I'd appreciate advice on the following:
1) Would I be likely to see an effect from installing a second layer of double glazing?
2) Should I install it across the bottom of the opening or the top: it seems to me that the bottom would be the better option because- a) it would tend to prevent warm air gathering in the cavity adjacent to the outside air and b) it would create a bigger air pocket between the two sets of double glazing, which I assume would be better.

Many thanks in advance ... Andy
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Old Apr 10th 2017, 04:40 AM   #2
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Good morning Andy and welcome to the physicshelpforums.

I note to want to guard against thermal loss rather than thermal gain from the greenhouse effect of your glazing.

Thermal loss is driven by temperature difference and opposed by thermal resistance.
the opposition offered by thermal resistance is of a time dependent nature, whilst the driving force of temperature difference is absolute.

Thermal resistance slows down the passage of heat, it does not prevent it.
So long as there is a temperature difference heat will always tend to transfer.

So why not consider placing your additional glazing on the outside?
This would reduce the temperature difference between the outer surface of your existing glazing and the interior temperature.

This is how triple (multiple) glazing works.

A word about double (or more) glazing.
It is true that the air gap offers increased thermal resistance compared to conduction through a solid pane of glazing, but within limits.
A gap of less than around 1/4 of an inch is ineffective due primarily to radiation between the panes.
A gap of greater than around 1 inch leads to internal convection within the airspace and again greater heat transfer.

So your idea of a low horizontal panel across the bottom of the light will simply generate very efficient heat transfer convection cells within the enclosed airspace.
Further how will you clean inside or maintain the framing? And you will loose the attractive lantern light architecture.

Note also that too many glazing designs pay insufficient attention to the framing.
This degrades performance significantly if the glazing panels are super insulating but loads of heat is lost through conduction in the framing.
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Old Apr 10th 2017, 04:49 AM   #3
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Thanks studiot

Thanks for that, so you don't feel there would be any benefit placing an additional double glazed layer (24mm) lower down? I assumed the transfer through the heat lower glazing would be less because the air in the roof well would be warmer than outside but colder than the rest of the room. Also, that gap would presumably be heated each day by sunlight and provide a buffer?
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Old Apr 10th 2017, 05:23 AM   #4
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It may have got lost in my previous text but I meant to say

Adding an extra glazing panel inside or outside will improve the insulation so long as the air gap is not too large.

Outside addition may be more architecturally acceptable.

Attention should also be paid to the framing material.
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Old Apr 10th 2017, 05:29 AM   #5
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Putting a layer of double-glazed glass at the level of the roof, creating a large void between this new glass and the existing glass, will give some improvement. Heat loss due to conduction through that area of the roof should be cut about in half. However - large glass ceilings allow for a tremendous amount of heat loss through radiation, and adding a second layer of glass won't help that very much. Radiation heat loss goes as the 4th power of the difference between the inside temp and the temp of the sky. On a clear night the room radiates heat into space (which is obviously very cold) at a tremendous rate. So my advice would be to install some sort of a shade that you can close at night. It doesn't have to be thick; it just has to be opaque.
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Old Apr 11th 2017, 01:47 AM   #6
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your help

Thanks for your advice studiot and ChipB, I think I'll try some blinds first, probably be the cheaper option. Cheers ... Andy
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