DIY Doctor

Replacement Window Cill Details for A-Rated Aluminium Units

Postby qthisup » Sat Nov 21, 2015 1:09 pm

I am replacing my old UPVC windows with A rated Aluminium into a cavity wall construction (90mm outer leaf, 70 mm cavity, 100 mm internal blocks). I need to decide which cill to order and your expertise would be appreciated;
Option 1 Preferred: 85mm stub cill which would protrude roughly halfway onto the outer leaf therefore dripping onto the existing cill bricks.
Option 2: Maintain similar existing full cill (see picture) and drip clear of the outer leaf. Visually I prefer not to have the cill sticking so far out, but if this is the "better" choice I'd go for it.

With regards to the cavity, there appears to be a concrete type closure in place which I think may be difficult to remove in turn making fitting new insulated cavity closures difficult, so I'm currently planning to fit 32.5mm insulated plasterboard (20mm coolthrem plus 12.5mm plasterboard) or Marmox type board on the internal reveal to limit cold bridging (the insides will be getting re skimmed).

Thoughts greatly appreciated!
window cill detail existing.JPG
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Simply Build It

Postby thedoctor » Tue Dec 22, 2015 1:35 pm

It seems to us that if these are proprietary cill bricks there is no need to have an ali cill at all, The windows can be fitted towards the outher skin we are assuming, especially with a completely colsed cavity so this would allow the new windo to be fitted directly down nto the brickwork cill.
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