extending window sills for exterior wall insulation

[stewartcampbell]

Hi, we will be getting exterior wall insulation done in the next few weeks months and as it is free on a government initiative as far as i am aware there is no choice on the sills apart from a white plastic oversill, i am not keen on these so was thinking about extending the 3 front sills in concrete and just wanted to know thoughts on this from more experienced construction diyers or professionals.My plan is to drill and insert bolt anchors into the existing sills and using wire attach a 1m theaded steel bar between them resting on the bolts,sills are about 2m so will need 2 each sill then build a form and i think extend the sill downwards too so it is thicker as i think this will look better, can anyone see any problem with this? thanks.

[V8RX7]

Surely it will crack on the joint ?

If I was looking at it in concrete I'd either remove the old one or at least cut it back flush to the window so I could hide the joint.

Unless you form some kind of detail at the joint but I'd be concerned about freezing water cracking it.

[wolfracesonic]

I agree with VX. I've done a few of these on 30's properties where the old metal reinforcing has blown the cills open. Chisel most of the old cill away with a small breaker (far enough back so the new concrete goes under the window) form your shuttering, not forgetting a drip groove, then epoxy/bolt some kind of bar into the remainder of the old cill to tie the new concrete to the old; I'm not sure how wide your new cill is, if it's not too wide the stainless wall starter ties are ok for this. Pour your concrete, leave it a couple of weeks then seal between the bottom of the new cill and window with mastic or silicone. Or have some new cast stone cills made if your feeling flush!

[dazm]

Sorry to hijack the thread but what free government initiative is this? I've been looking to get this done to a house we've just bought but they want £12 - 15k!

[V8RX7]

dazm said:

Sorry to hijack the thread but what free government initiative is this? I've been looking to get this done to a house we've just bought but they want £12 - 15k!

British Gas were going to do mine but I decided against it.

[stewartcampbell]

E on are doing it as far as im aware to meet targets on carbon reduction or something to avoid big fines, its in scotland by the way.