Rationel Aperture Size Clearance

[SuperPav]

I did a search for this but couldn't find anything. We're about to finalise our window order for Rationel TG window order tonight.

Most windows going into new stonework, but two going into existing. Openings all framed with smooth cut stone quoins/head and stone cill. 

 

House is stone outer/brick inner leaf with rubble infill (350mm total thickness). The new/upstairs will be cavity wall of 140mm stone, 100mm cavity, 100mm inner block. The plan is to install new windows in 18mm plywood boxes, set back just under 100mm from the outer face. Window sections 123mm deep. The boxes will be built in as the walls go up to ensure square/accurate sizing.

 

The supplier is saying that the opening size must be 22mm larger than the window frame (11mm clearance on all sides). This strikes me as ridiculously generous, especially for windows going into new apertures. The plan is to fit with compriband between window and outer leaf, backfilled with foam, and avoid sealant if possible. Windows also supplied without cills (to sit directly on stone cills).

 

We have a big concern with the gap that 11mm clearance will leave visually. The windows are alu clad flush casement, will be finished in a light grey, and against Cotswold stone, so a 11mm black compriband will stand out somewhat. Also at that size of gap, it won't even be easy to fill with colour matched sealant.

 

Anyone on here know why such generous tolerance is being allowed for especially on an exterior without any render/applied finishes? And has anyone got away with smaller as we most likely will chance it here! 

 

Thanks!

[Roundtuit]

11mm isn't really much to play with.  Once the compriband is on, you'll need a bit of slack to get the window in the hole without accidentally pulling it off.  We had one or two of ours that ended up with a bead of silicon underneath rather than compriband.  I suspect if you risk going any tighter you might be restricted to just using sealant, but even that will need some space if you want to get a little finger in to smooth it off!

[SuperPav]

10 hours ago, Roundtuit said:

11mm isn't really much to play with.  Once the compriband is on, you'll need a bit of slack to get the window in the hole without accidentally pulling it off.  We had one or two of ours that ended up with a bead of silicon underneath rather than compriband.  I suspect if you risk going any tighter you might be restricted to just using sealant, but even that will need some space if you want to get a little finger in to smooth it off!

 

11mm across the aperture I can sort of understand (all other manufacturers ask for 10mm).

11mm *on each side* seems excessive when it's going into smooth stone blocks in the outer leaf. I was also under the impression (certainly from Rationel's installation instructions) that the Compriband between the sash and outer leaf goes in after the window is in place through careful poking with a glazing/putty knife?

[craig]

1 hour ago, SuperPav said:

the Compriband between the sash and outer leaf goes in after the window is in place through careful poking with a glazing/putty knife?

 

Correct. However, a lot of installers will apply this before offering the window up to the opening. It becomes an issue if the tape starts to immediately expand rather than a slow expansion (example video of application).

Edited May 25, 2021 by craig

[Mr Punter]

Quite a few composite windows (like Velfac and Ideal Combi) have an external opening sash that is the same dimension as the internal fixed frame, so you need a reasonable gap to prevent the sash fouling on the brickwork or external cladding.

 

I have seen Compriband in gaps of 20mm and it does not look bad.