DIY Doctor UK DIY Forums

Hello all...

I am looking at having new UPVC windows installed, followed by external insulation. In order to reduce the reveal depth (insulation and render add a nominal 100mm) I am looking to have the new windows installed flush with the external walls and the sills either added afterwards, or temporarily supported, prior to the insulation and render.

The installers I have spoken are apprehensive about fitting the windows and/or sills in this way - does anyone known why either of these are a issue?

The people quoting for the insulation (builders, plasterers) don't see the problem.

Hi dken,
Upvc window frames are normally 75mm so if they are fitted flush to the outside there will be a 25mm strip of external wall in the internal window reveal (assuming you have 100mm outer brickwork. This will be a magnet for condensation if you do not do the external insulating.
Upvc box sections drain moisture through holes in the bottom of the frame, this goes into the sill section and out from there. You either need to fit extra wide sills when the windows are fitted or form masonry sills as part of the insulation but this will have its own problems dealing with cold bridging and frame drainage.
Regards S

Thanks for the reply,

Yes, the windows are to be fitted at the same time (just before) the external wall insulation, so cold bridging the the sill depth (120mm) should not be an issue.

What I can't understand is the hesitation about installing the windows flush to the wall (or as close as possible), and the problem of fitting the sills later, or supporting them temporality until the insulation goes up - the windows have to be installed before the insulation.

Hi dkenn,
Cold bridging all round the window frames is the only reason I can suggest against fitting flush.
I suggest you see if your window manufacturer can supply and fit 225mm sills at the same time they fit the windows. The would give a projection of 225 - 75 = 150mm so would clear the external insulation by 50mm.
Regards S