Thermal bridging around window frame

[John Williamson]

n the process of converting a garage to living accomodation. I'm replacing the old garage windows which were just about good enough for a shed with some high performance aluminium windows.

The block walls will be insulated with insulated plasterboard mechanically fixed on to 25 x 47 battens.

For the window reveals, I was wondering if it's sensible to use some 25mm PIR around the blockwork joining to the insulated plasterboard insulation layer and install the frames on top (through the PIR into the blockwork) to minimise thermal bridging.

Does this make sense?

 

[Ivor Windybottom]

I don't see how you could fit the frames on top of insulation, it would be visible around the frame from outside. Or are you planning some sort of trim surround? It could look awful if you're not careful, and possibly weather and fall apart eventually.

Otherwise, the width of the frame dictates the space you have to play with. You normally order a frame about 10mm smaller than the opening, you could stretch this to perhaps 15-20mm to give yourself a little more space on the inside - as long as you're confident you can seal the outside without making it look like it's been wrapped in sausages. Fit with lots of packers, seal the outside, mask the frame and surroundings then fill the gap with foam before trimming off the next day.

Everyone accepts that you always get some thermal bridging from any window. Yours is non-standard in that the outer skin protrudes into the room, but all are compromised in various ways. In the old days they stuck plasterboard over an empty cavity and everyone got black mould in the reveal from condensation. Avoid anything as bad as this and it should be fine.

 

[John Williamson]

I don't see how you could fit the frames on top of insulation, it would be visible around the frame from outside. Or are you planning some sort of trim surround? It could look awful if you're not careful, and possibly weather and fall apart eventually.

Thanks Ivor. I was thinking of something along the lines of this

 

[Ivor Windybottom]

It depends what the outside looks like, but I'd say there's a good chance that could end up looking awful, especially after a bit of weather. I'm guessing you're referring to Cement Board, in which case the outer edge of it will be a raw sawn edge.

If you really want to go down this route then plastic bullnose trim to match the windows would look less bad.

Check with your intended window manufacturer, I'd hope that most windows will have enough frame thickness to get a bit of insulation behind the reveal board while keeping the outside conventional, weatherproof and not looking like a botch job.

 

[Ivor Windybottom]

Or are you rendering the whole outside? If so that could be less bad but I'd still worry it would crack through the joints around and between the cement boards as it would expand/contract differently to the block.

 

[freddiemercurystwin]

I've not red the whole thread but no, fit the frames to the blockwork then insulate up to the frame, you might find 25mm+ PB is too thick and will leave such a tiny slither of window frame exposed so as to look daft, so a thinner insulation/board combo is required or you can buy specific products just for this eg https://insulationgo.co.uk/24mm-15m...-liner-insulated-plasterboard-vapour-barrier/ . Your detail should be a bit like this:

 

[John Williamson]

If you really want to go down this route

I think I may be guilty of over thinking things once again.....

 

[John Williamson]

I've not red the whole thread but no, fit the frames to the blockwork then insulate up to the frame, you might find 25mm+ PB is too thick and will leave such a tiny slither of window frame exposed so as to look daft, so a thinner insulation/board combo is required or you can buy specific products just for this eg https://www.insulationuk.co.uk/prod...ated-plasterboard-20mm-thickness-1200-x-298mm . Your detail should be a bit like this:

View attachment 390957

Nice. I also have a bunch of left over 10mm XPS so could make my own thin insulated plasterboard for this purpose using 9.5mm board. u-values a bit naff but better than a sheet of plasterboard directly against blockwork IMO

 

[freddiemercurystwin]

Link edited for a slightly better product.

 

[Ivor Windybottom]

Don't forget some damp-proofing, that outer skin is potentially damp.

Polystyrene (XPS) is about half as good as PIR/PUR I believe. Check the values yourself, but if you only have limited space then better is better!

Also rigidity. I'd worry polystyrene might be a bit too soft where the load isn't spread over a large area. You can build a floor on the stuff, but that's because the load is spread over a large area.