Many older properties have attractive bay windows. However there is an issue that tends to go un-noticed by the great buying public. That of insulation in the bay. Basically there isn’t any. The mullions and walls under the window are generally solid walls. This is not too much of an issue as the rest of the house will tend to be the same. However, insulating them is an issue to get right. However, this is the smallest problem. The main issue is that of the roof above the bay.
They tend to look a bit like this:
So there is virtually nothing between you and the outside world. A piece of plaster board / lath and plaster and then a void and the bay roof. So these structures are very draughty and not very insulating. So if improving an older property with a bay, this is one of the first things that needs to the addressed.
It is an easy (if disruptive) fix, as the installation of insulation is relatively simple and then it is a case of just making sure that the details are right. This basically means that you have to get the airtightness correct, so use good quality tapes and sealants to ensure that the final finish is well draught proofed.
Note that this situation can be mirrored under the bay if it is an ‘unsupported’ bay. These types of bays tend to be seen on more modern buildings and / or on replaced bays as they are cheaper. However, they are rarely sealed correctly and hence tend to be very draughty and again poorly insulated. Thankfully they also tend to be timber framed and hence are relatively easy to improve by using conventional insulation, membranes and good tapes.
If you are installing external wall insulation remember that the wall behind the bay also needs to be insulated to stop thermal bridging. Where the roof is tied into the wall will create a thermal bridge as well so think about how this will be dealt with.