Help - Insulating Bay Window

[handyman11]

Hi,

The plasterboard under our bedroom window has a bit missing and so I am getting a plasterer to sort this out when he skims the whole room and a few people have suggested taking off the old plaster from the stud wall immediately below the bay and then putting Celotex between the vertical joists. I don't do much DIY and not done anything with insulation before, but I was thinking of getting celotex which is not as deep as the void and putting it in so that it is flush with the internal face of the joists, thus leaving a gap between the panels and external face of the bay i.e. so there is ventilation.

My understanding is that celotex is foil backed so that will act as a damp proof membrane (i.e. have the foil facing the plasterboard / room). But I also read threads talking about sticking foil tape to the wooden joists? Would this mean simply putting the tape over the internal face of the joists, which will then be covered with the plasterboard?

Secondly, is this essential or can I just fit the celotex in between the vertical joists then get the plasterer to plasterboard over i.e. does it matter if the front of the joists are in direct contact with the plasterboard? The joists have been there for 80 odd years without any tape etc and from what I can see they look ok (have pulled some of plasterboard off)... or is it the actual fitting of insulation panels that can cause some type of damp problem between the joists and plasterboard?

Any help appreciated.
Thanks.

 

[^woody^]

thus leaving a gap between the panels and external face of the bay i.e. so there is ventilation.

That is fine if there is ventilation. Typically, there wont be any in these bays, and so you should instead fill the whole depth with insualtion and have no voids anywhere.

You dont really need to tape across the timber, but it would be good practice to do so.

If you just insulate between the timber, you may find that dark stains develop in line with the timber due to it being colder than the insualted areas. Not damp, but dark stains through paint or paper finishes. To avoid this, line some insulation across the whole face of the bay, and not just in between the timbers. The more the merrier, but this will impact on the window boards above, so 10mm thickness should be a minimum.

 

[handyman11]

That is fine if there is ventilation. Typically, there wont be any in these bays, and so you should instead fill the whole depth with insualtion and have no voids anywhere.

You dont really need to tape across the timber, but it would be good practice to do so.

If you just insulate between the timber, you may find that dark stains develop in line with the timber due to it being colder than the insualted areas. Not damp, but dark stains through paint or paper finishes. To avoid this, line some insulation across the whole face of the bay, and not just in between the timbers. The more the merrier, but this will impact on the window boards above, so 10mm thickness should be a minimum.

Thank you for your advice.

When you say 'if there is ventilation' do you mean if there is a proper vent as opposed to air just getting in through cracks (the bay has pebble dashed front).

If I assume that there is no specific ventilation (presuming you mean some type of vent) and if I do leave a gap, does that cause any problems or does it just mean it wont be as efficient?

If I do tape the vertical joists, do I put the tape directly on the wood?

When we had the windows replaced we did not bother with an internal window board, however, if I add extra panels, even 10mm, I would probably then have to get a plumber to move radiator pipes forward, further into room, so probably best if I just put insulation between joists and then plasterboard over.

Thank you.