Insulated Window Board

It's a small contribution to your overall U-value, but I'd be worried about condensation on this cold spot more than heat losses.

How about insulated timber board:

Theo

If you're prepared to remove the board, see what exists below, refit and make good, you might be able to fit some more PIR or a cavity closer?

Thanks - something to go on there. It has to be thin (<35mm) and highly insulating to be worthwhile - tricky one to pull off.

The board's off. It's a rebuilt cavity wall with a very substantial stone sill on top, on which the window frame rests. So that sill isn't going anywhere. I only have 30mm to work with.

But I think the main problem is the stone sill. Once it gets cold, it stays cold. And so it's the sill that needs insulating, rather than the window frame etc.

Just guessing. Not really sure what's going on TBF.

I don't think you're ever going to win the battle of the U-values in comparison with the rest of the room, it's just too thin, especially if skimming over the top. But what you want to achieve is that any depth permeable by room moisture is above the dew point (for the worst case conditions in that room). If you have an impermeable coating on it (eg vinyl or oil based paint) then you just need to worry about the surface temperature and it may not need so much insulation to achieve that.

I suppose another option is a box made of ply/uPVC and then fill it with closed cell spray foam. Not as good as PIR but perhaps easier if the cill is uneven.

Do the calculations and see (ubakus.de is handy for that).

Theo

If you want something more cosmetically pleasing on top of the insulation what about something like

Anything below the bullnose edge is considered to be your vertical wall surface so it doesn't need to be 35mm deep.

foam is good in a cavity

Ah. Good pic.

Stine will get cold. You may be able to chop some away. Need to put a thermal barrier between stone and cill. Id do for Celotex /expanding foam and glue a new thin wood or MDF cill on top. If it looks too thin, make it deeper at the edge you can see.

That's a good option - I was thinking about butchering up an existing uPVC cill profile but those look better.

Only other thing I might consider is a thermal break at the back of the section, so cold is not conducted from the vertical wall into the uPVC surface. Although if it's butted up against the window frame that's exposed to the room anyway so it doesn't matter.

Theo

When we had some windows replaced the DG firm used fascia boards to create cills you can even get end caps for the parts that hook round onto the wall. There are two types of fascia ones that attach onto the structural roof timbers and ones that are simply used to cover over existing timber fascia’s the latter tend to be thinner. So overlaying 25mm PIR and the thin fascia should be somewhere near your 30-35mm limit.

Gosh, Yankee driver - don't see many of them these days :-)

Looks like PIR on top of the wall up to the window - overlapping any insulation on the face of the bay walls, would do most of what you need. The amount you can get in will be dependant on the thickness of the window board - but at least 25mm looks doable if using a thin window board.

I would not worry too much about the actual U value - it is only a small area. All you need is enough to keep the surface temperature of the sill up so that it does not attract condensation.

You could go for a thin cover sill - that would allow more insulation and also minimise contact area with the bottom of the window frame.

What kind of finish is the current window board, and do you want to retain that style of finish?

If it is painted timber, then I would use 9mm WPB ply, with a real wood lipping on the inside, to create the illusion of more thickness. Then just "glue" it onto the PIR[1].

Real wood, then as above but with veneer applied to the ply and then whatever finish you favour for that.

If just "white", then a UPVC cover sill - just glued directly to the foil of the PIR. (or over a thin layer of ply if you want to make the cover sill a bit stiffer)

Hadn't used it in about 20 years! Pretty effective - *if* you can get your weight behind it.

Painted wood - yes, retain.

Seen those mentioned elsewhere in the thread. That would do the trick - but I'm not that fond of plastic.

For now I'm going to reinstate the window board with a 5cm strip of 25mm PIR between the board and the window frame, and cover the PIR with something yet to be decided.

That'd be quick/cheap/easier - if it doesn't work I'll revisit.

Thanks all for the input.

Rob

I was anticipating the the PIR would run under the whole window board (well up to the vertical wall of the bay anyway). A small siliconed gap (~5mm) at the point where the board meets the frame would be all that is required to break the thermal bridge to the frame. Then the whole window board would not be losing heat into the brickwork below.

Thanks, yes. That was part of the original thinking - 25mm PIR plus up to 10mm surface finishing - like the plastic trims linked, or ply/lipping as you suggest. But I'm not confident I can get a decent finish - it's quite a prominent feature of the room.

I can always return to it.

Rob

Could you take a piece of timber, finish the top side how you want, then route out pockets on the back to fill with non-expanding 'window/door' foam? Or route out a gap for PIR? Means you get the full timber effect but reduce cold bridging through from the cill.

Theo

what about "Space Blanket"? its a multiple stacked sheets of metallised polymer film with closed cell bubbles (like bubbble wrap)? Supposed to have a high U value despite its thinness? could that be used under the window sill?

It only achieves its rated U-value when correctly inflated and with an air gap each side. If it's squashed underneath timber it won't perform to spec. The instructions never use it in anything like that scenario:

Theo

TBH polyisocyanurate with gaps filled by foam is a mechanically very stable. I replaced a soaking wet piece of wood between my bath and the wall with it, replaced the tiles and its rock solid now

Covered in a skin of wood it will do very well.

This might be one of those rare situations where aerogel insulation makes sense! I once held a 10cm cubed block of aerogel in my hand. It was a very strange sensation. It weighed almost nothing and was almost transparent. It had been made as a prototype sample for a Starwars project. The intention was to make very large mirrors with an extremely low moment of inertia for steering laser beams in a satellite. The project was cancelled. John