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How can we stop heat loss from our bay window?

Posted by Tim Pullen on 13 January 2012 at 9:31 am

Q: Hi we have a semi-detached 1950s bay fronted house and the only part not insulated is the upstairs bay (which sounds like it's made of wattle and daub!) and the flat roof above the bay. The flat roof part is perhaps 3 inches deep only, and covered with lead. Despite the radiator being in the bay, that part of the room is cold and its the hardest room to keep warm. Is there an established method of insulating the flat roof and / or the the bay wall itself? I'm not sure where to begin looking or how to gauge anything i might find! thanks!. 

A: The problem with insulation is that when you leave a gap, it is really noticeable. It is a bit like a puncture in a tyre; 99% of the tyre is fine but all the air gets out of that one tiny hole. And so with heat. NASA did a survey of their buildings in 2004 (or thereabouts) and found that 5% gap in insulation accounted for 50% of the heat lost. 

Bay windows are notoriously difficult to insulate – often tightly space constrained, odd shapes and with a radiator in them. Conventional insulation materials tend to be just too thick to be usable. Thinner materials, like multi-foils, can work but are often difficult to install. The one option I have found that works is aerogel, marketed as Spacetherm by The Proctor Group.

It is available as either a blanket or laminated to plywood and plaster board. Thicknesses available vary from 5mm to 40mm in 5mm increments and it is at least twice as effective as an equivalent thickness of polyurethane. You don’t need any special tools or equipment to install it, and it is handled in the same way as mineral wool – but less itchy. However, it is ruinously expensive so limiting its use to just the bay walls and roof would be prudent.

In all probability the laminated option would be best as this can be simply screwed to the existing roof and walls. And you might still put a reflector behind the radiator. However good the insulation it will never be perfect.

And if that does not suit, think about curtains. A good set of ceiling-to-floor curtains, lined with fleece (your local fabric shop is likely to stock fleece similar to that they make jackets from) will help get the room warm in the evenings and cut down draughts. 

Photo VCU libraries

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

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1 comments - read them below or add one


cswdComment left on: 5 December 2012 at 8:27 pm

The 'inverted warm roof' method of insulation works well for flat, bay window roofs, as shown in the diagram here:

Most bay window roofs are strong enough to stand on, so will have no problems with a layer of insulation and some ballast (e.g. gravel or pieces of slate). You may need to pick your insulation carefully if you want to continue to be able to stand on the bay roof - e.g. most foam insulants will crush when you stand on them so go for the sort that's used for under-floor heating. A reasonably straight forward DIY job.

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