How to mitigate cold bridging around bay window dormer with very little space to work with?

[maznaz]

Hi there. I'm slowly renovating a Edwardian house that has a room-in-roof construction with a bay dormer that has been fitted with some ugly but warm UPVC windows. I've rebuilt the bay with 100mm of PIR and put 50mm of PIR between the dormer cheek framing and I was intending to overlay 25mm PIR on the dormer cheeks and window frame area to prevent cold-bridging. I am not certain that there's enough space to actually achieve that once plasterboard and skim are taken into account. I do intend to use architrave at the transition which would mitigate some of the impact. 

 

1. How important is it to minimise cold-bridging in this aspect? The wooden frame portion is basically uninsulated on the cold side, with some light cladding. The dormer cheek is a ventilated roof cavity.

2. Would it make sense to use a thinner laminated insulation material incorporating aerogel or similar? It would potentially be quite costly to do the whole area.

3. Does it matter if the walls come in a little narrower than the window itself. I could taper back out to the full width of the window. Would this look awful?

 

Pictures follow:

 

[Russell griffiths]

Go and get some 50 mm wide upvc trims. 

Cut a little bit 150mm long to use as a bit of a dummy up. 

Place the trim at 90 degrees to the frame and see what you think. 

Then stick it at 45degrees to the frame. 

Have a think what looks best. 

You could always do this in wood, but upvc will be easier to wipe down. 

[Iceverge]

7 minutes ago, maznaz said:

1. How important is it to minimise cold-bridging in this aspect? The wooden frame portion is basically uninsulated on the cold side, with some light cladding. The dormer cheek is a ventilated roof cavity.

 

It all helps. However the window frame probably has a U value of about 1/2 that of the timber as is. More important is getting it air sealed correctly.  

 

BTW you can get 22mm and 30mm insulated plasterboard.

 

9 minutes ago, maznaz said:

. Would it make sense to use a thinner laminated insulation material incorporating aerogel or similar? It would potentially be quite costly to do the whole area

 

No, it's too dear and the house is unlightly to be so good elsewhere that it'll make a material difference, especially on timber which isn't actually that bad. .Some kind of an insulating blinds or shutter would be much better if you were really concerned. 

 

 

12 minutes ago, maznaz said:

Does it matter if the walls come in a little narrower than the window itself. I could taper back out to the full width of the window. Would this look awful

 

It's unlikely anyone but you would ever notice. 

 

 

 

If it was me I would put some PIR there, but not sweat about it. Make a really good job of the airtightness though. 

[maznaz]

11 minutes ago, Russell griffiths said:

Go and get some 50 mm wide upvc trims. 

Cut a little bit 150mm long to use as a bit of a dummy up. 

Place the trim at 90 degrees to the frame and see what you think. 

Then stick it at 45degrees to the frame. 

Have a think what looks best. 

You could always do this in wood, but upvc will be easier to wipe down. 

 This is a nice idea, I'll do that

[maznaz]

7 minutes ago, Iceverge said:

 

It all helps. However the window frame probably has a U value of about 1/2 that of the timber as is. More important is getting it air sealed correctly.  

 

BTW you can get 22mm and 30mm insulated plasterboard.

 

 

No, it's too dear and the house is unlightly to be so good elsewhere that it'll make a material difference, especially on timber which isn't actually that bad. .Some kind of an insulating blinds or shutter would be much better if you were really concerned. 

 

 

 

It's unlikely anyone but you would ever notice. 

 

 

 

If it was me I would put some PIR there, but not sweat about it. Make a really good job of the airtightness though. 

 

Thanks this all makes sense and is reassuring, thank you. I am doing my best with airtightness. I originally didn't gravitate to the insulated plasterboard but I can't remember why. I'll make some different thickness samples and have a test.

[Iceverge]

Another option might be to forgo the PIR on the window jambs and use something like MDF which has a thermal conductivity of about 0.1 W/mK. Not as good as PIR but not terrible. 

 

Screw one layer into the jamb and tape it to the window and inner face of the PIR as your airtight layer. 

 

Then glue another layer over the top and paint. It might be easier to work with and get a good finish than UPVC which always cracks when I try cutting it. (Might be my ham fists! )