Hi all,

Two of my extensions are 175mm SIP panels. These will be battened (to create 25mm air cavity) and have WRC cladding fitted horizontally.

I'm just trying to determine where to position the 88mm frames within the 175mm panel. As there won't be an outerskin of blockwork/masonry, will there be much of a difference thermally whether I place them inboard/middle /outboard? On a side note, I was going to have a 20mm gap between the window frame and SIPs so that I could add some slim PIR in there to reduce thermal bridging as much as possible.

There is an optional 'cill extension' that fits underneath the main window frame and is used to fit the cill onto. One option however would be to do away with this and have the window oversail the SIPs panel by 10mm (see photo) and fix the window cill underneath to the SIPs panel instead . Aluminium cills are to be fitted long enough to take the water away from the face of the cladding.

But then I'm also thinking to take the window inwards as much as possible to take it away from the elements outside.

Advice anyone?

Thanks!

This shows the window frame with the optional 'extension cill' underneath (in red box). Ignore the adjacent bit of insulation.

Posted By: davidfreeboroughthe interior window sill we are used to in the UK, although with inward opening windows you won't be able to use it for much!

This turns out to be not really a problem as long as there's an MVHR system. Windows are hardly ever opened, and even less by turning rather than tilting. So the window boards are quite usable for many purposes and the cats have established their favourites!

Posted By: davidfreeboroughYou get best thermal performance by centering the window within the SIP insulation.

Thanks David. My SIP panels aren't the ones where you have the insulation exposed at the window reveals. There's a nice big chunk of timber that edges the SIP all the way round...loverly bridging...

So the window frame can't directly sit/touch the SIP panel insulation. But I can still ensure that the front face of the frame is no further forward than the front face of the insulation within the SIP...even though there's the timber between the two. I plan to add some PIR or similar as well on the reveal, internally and externally.

Posted By: djh

Posted By: davidfreeboroughthe interior window sill we are used to in the UK, although with inward opening windows you won't be able to use it for much!

This turns out to be not really a problem as long as there's an MVHR system. Windows are hardly ever opened, and even less by turning rather than tilting. So the window boards are quite usable for many purposes and the cats have established their favourites!

Yes, these are indeed inward opening/tilting...so where I place the windows also determines how wide I can swing them open. But my main concern is jumping out of the room in event of fire/emergency etc...in which case a 90deg opening should be fine. And apart from that, I would be tempted to lock all the windows (and hide the keys )as I'll ave the MHVR on.

Posted By: Ed DaviesNaïvely, I'd think as far outboard as possible, within the limits David Freeborough mentions, would be best to limit the opportunities for anything to go wrong allowing the weather to get behind the outer skin of the SIPS.

Pros and cons with either option. Outboard = less water risk, more interior cill, but more clutter potential for the wife/daughter!

inboard = more exterior cill = more water risk, more aluminium cill (£££££), more space for me to down tools/coffee when I'm working outside = more scratches!!

I may just play it safe and place the window in the middle.