Insulating cold spots around bay window

[Tom Pickles]

Our living room is by far the hardest room to heat in our house built in 2001 and it loses heat the quickest out of all the rooms and its also where our heating thermostat is, so it's probaly bumping up our energy bills somewhat.

I got a thermal camera that plugs into my android phone (very handy piece of kit) and found there are two very cold spots at the side of our bay window where it meets the wall where I'm guessing there's no insulation behind the plasterboard.

You can see in the photos.

How can I go about insulating behind the plasterboard here without potentially causing any damp issues?

Thanks!

[Theo]

It's a little hard to tell what's going on just from the thermal pics. What is the construction? Dot and dab PB on top of what?

Is it possible there's a thermal bridge: ie the edge of the window is a wooden stud/brick/block/etc that's joining the outside to the inside? And so heat is conducting directly through that, rather than having insulation in the way? Or is it a void the insulation didn't quite fill out?

If you tap it, does it sound solid or hollow?

Theo

[Tom Pickles]

Dot and dab on top of breeze type of blocks. The blocks downstairs behind the PB are definitely more dense than the blocks upstairs.

It sounds hollow.

[Theo]

Is there a thermal bridge from the outside through to the blocks? ie the PB is dot and dab (and so sounds hollow), but there's a block across the end of the wall joining outside to inside. And so the insulation does not extend up to the edge of the wall?

I think the cavity should normally extend right up to the window (and so the thermally broken window is the only thing across the gap), but maybe there's something different about a bay window?

Theo

[Andrew]

This is classic problem with bay windows and the way they are fitted.

Ideally they need to be mechanically fixed to the outer leaf and an insulating cavity closer used.

Normally heat loss issues with bay windows is via the roof or down under the window seat/board if the bay is just a projecting window that does not go down to the foundations.

If you are happy with messy DIY then remove the plaster on either side of the window itslef and replace with insulated PB, then over- insulate the walls on either side of the bay.

[Animal]

The general procedure for insulating a cavity yourself is drill a 2" hole & fill with polystyrene beads. A handheld vac with a tube is good for doing a room, for a small area a cardboard funnel & scoop ought to get it done. Fill hole.

[Theo]

There are lining products for internally insulating the reveal, but you would need a continuous layer of them so there's no cold bridge remaining. That would mean either overlaying the internal PB or splicing them into it.

That link is interesting, led me to:

which is good at describing the various detailing issues that often get missed.

Theo

[RJH]

OOI, what sort of camera have you got? Thise pics look pretty good.

You could spray expanding foam if there's a cavity/space behind. That'd help with draughts, as well as insulate the frame/wall itself. I did this to my bay window and it has given a measurable improvement - the whole frame is hollow. I'd recommend a proper gun to dispense the foam, and take care that excess foam has somewhere to go. So drill at least 2 holes - or risk splitting the entire frame apart :-)

Depending on the windows, obviously seal gaps, especially around the openings (mine are sash and pretty rubbish). I think a third level of glazing would help (acrylic sheet probably), but I've yet to try it - put off in part by the cost. And heavy well fitting curtains.

The bay on mine is solid brick faced with stone - that's as bad as it gets for thermal insulation, so needs lining at some point.

On damp, I can't see how it these measures would make things worse. You've already got clear lines of thermal bridging. But obviously monitor carefully after you do anything.

[Andrew]

Ta. I didn't browse that far. Downloaded my own copy.

"Cold air blowing behind or through insulation. e.g. insulation floating off substrate"

Yet the Energy Savings Trust still thinks that you can just slap extruded poly insulation on the top of the deck of a flat roof that was built to cold-roof specification, i.e. plenty of cross ventilation through the void and insulation inside the void (hopefully) on top of the PB ceiling.

[Andrew]

His house is 2001 with dot'n'dab PB fixed to lightweight blockwork.

If there are just the teenyest of gaps where the bay is attached to the blockwork then he will get cold air leaking into the ?10mm gap between inner leaf and PB which is awful. If this is the case just making a series of holes in the window reveals (tap to avoid plaster dabs) and squirting in proper foaming PU plasterboard adhesive or low expansion expanding foam to create a solid layer of foam and plaster dabs all around the window frame could give a noticible improvement.

The biggest improvement would of course be to rip off all the plasterboard and skirting from the entire wall where the bay is and re-doing with insulated PB. With the existing PB removed he will soon see and feel where any cold air is leaking in.

If he was careful in the deconstructing it might be possible to get the original boards of the wall then hack off the dabs, fit 'celotex' to the wall with battens, infill between the battens with more 'celotex' and re-use the PB.

[Animal]

PB can go straight onto celotext or EPS, battens not needed.