Insulating old bay window?

18 Feb 2013
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West Midlands
United Kingdom
The bay window is the old type which has just wood underneath and a few brackets holding it up, the windows are double glased upvc though. Thinking of changing the windows in the summer, but for now is there anything simple-ish that can be done to improve the insulation? In the current weather the window sill on the inside is typically at 8 - 9 degrees C whilst the others in the house are at about 16 - 17c (thermostat is set to 20c).

The local DIY store is recommending a can of spray on foam for the outside but can't say how much this will improve things.
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By other windows I mean the ones over the kitchen sink, half way up the stairs and the bathroom. I'm not counting the ones in the bedrooms which do have radiators under as they would be obviously warmer.
They are all in a warmer enviroment than the front bay and I expect much smaller in area? So you are comparing chalk with cheese.
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Well yes, but there's one big difference: the other window sills are basically inside the brick boundary of the building with nothing on the outside (the outside is the window), where as the bay is about 30 inches long so it has a significant portion of it outside, plus considering it is made of wood it is not going to be as thermally efficient as a double glased UPVC window, correct?
I am just looking at our bay window. A while back I took the wooden panels above the window off. It is easily accessible from the outside. I filled the ceiling space up with rock wool. That made some difference. Our outside wall around the bay is solid wall, with some cladding insulation from the outside though. But that outside wall is one of the coldest in the house still with 13°C in the corners and it regularly attracts some nice condensation.
I guess you mean a bow bay?

You could stick on some 25mm celetex to the underside?

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