IWI Window Cill and reveals

7 Oct 2017
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United Kingdom
Hello All,
I am about to start insulating our dining room. We live in a 1930s Yorkshire stone and red brick cavity wall end terrace house. Cavity wall insulation is not possible nor is External Wall Insulation so Internal Wall Insulation it is.

I have already used Gyproc Thermaline Super 90mm on Gyplyner metal stud work in our bedroom and this has made a huge difference to the heat retention and comfort level of the room. So now it is the turn of the Dining Room (coldest room in the house).

When I did the bedroom I didn't know as much as I do now about minimising cold bridging, airtightness and vapour control. So when I did the window reveals I retained the original angled wooden revels and stuffed insulation behind them (a layer of thin solid insulation that I sealed as best I could with expanding foam and then stuffed rockwool to fill the area behind the wood). I left the original window cill as this was well fitted and solid (but obviously attached directly to the brick). As I had the original sash windows replaced with uPVC units before the project there was a gap between the original revels and cill. So I attached plywood to the original wood to extend back to the window and forward to the face of the insulated plasterboard I had installed.

For the dining room I am thinking I will remove all the original wood work around the window and properly insulate the reveals with 27mm solid insulation. I can then put new wood around the reveals and a new cill.

Eventually, here is the the question I have...
The window cill will only be about 30-40cm form the ground and will be very deep and so would make an ideal place to sit but that means it has to be quite strong and well supported. I don't think I will be able to get away with putting a wooden cill on top of some thin solid insulation as it won't be able to take the weight. Also the materials I use will be attached to the outer stone behind the window and to the inner red brick wall but will not be supported across the cavity gap (except for some foam filling the gap) and will only have minimal support from the internal insulation and the Gyproc frame at the front. I also want to minimise the number of fixings from the cill into the brick work to minimise bridging.

Any suggestions or experiences?
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I'd have thought the insulation would be strong enough if you put a decent board on it. Otherwise you can use timber as although timber is a thermal bridge it isn't as bad as stone or brick.
Thanks for the reply, I think (unless anyone has any better ideas) maybe I should put a few timber supports and attach the cill to those and put insulation under the rest of it. I could screw the supports to the stone / brick and then independently screw the cill to those to reduce cold bridging.
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Should be fine for normal levels of insulation ie not super insulation(y)

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