Wow, I go away for a few days and come back to all this information. Really interesting conversations going on here.

Is there any reason why, to support the window/door, it couldn’t sit on a 4x2 or 4x3 CLS timber plinth that is screwed back into the existing cavity wall leaf, with some steel brackets up the sides?

Air tightness tape etc. all round and then encapsulate it all in the EWI, which in my case will be 200mm thick.

Wouldn’t this remove all the thermal bridging while making it very simple to deploy? Maybe I’m missing something or not appreciating the weight of the windows and doors sitting on the timber.

I also need to look at ordering windows, is Russel Tech Timbre still good value for money as I had a quote from 21degrees and can’t really afford them - more money was spent in the ground and rebuilding some walls ect than anticipated.

Posted By: englishjohnIs there any reason why, to support the window/door, it couldn’t sit on a 4x2 or 4x3 CLS timber plinth that is screwed back into the existing cavity wall leaf, with some steel brackets up the sides?

The steel brackets are a large thermal bridge; and a timber plinth is a smaller thermal bridge. A ply box does away with both of them.

Thanks, I have a different question so will start a new thread.

Posted By: VictorianecoWhat if the window is going to be inward opening and we decide to make the window bigger than the opening which has been suggested previously. How would you then fix them?

The opening sash needs to be smaller than the inside of the opening, to allow it to open. So you can't make the window too much bigger. Make the rough opening big enough to fit the ply box, which is in turn big enough to fit the window. Use insulation and plaster (or whatever) to fill gaps or reduce the opening.

Posted By: VictorianecoWould it support the weight of say a 2m x 2m 3g unit?

Yes, all our windows are in boxes, and one is 3 m wide.

Or a 3m x 2.4m sliding door set?

Probably but I think you'd need to consider the weight of people and whatever else you'd be passing through the door as well. Make the box thicker, maybe?

But what does that fix to? That'll be a lot of weight surely?

The more I'm thinking, why doesn't one just mount them on the outer masonry skin. With 2/3 resting on the original sill and 1/3 in the EWI layer? With a small overlap?

The thermal bridge can't be that bad surely as the window frames are made of timber, so surely a better u value than plywood anyway?

That's not correct

If you use a box the reveals are as deep as you make them. See discussion above. Having only used 60mm reveal on mine I would now use 100 (as suggested also by FT).

By 'feature bands' I guess you mean a planted-on 'portal frame'. How would that interact with the cill? (If you want to replicate a 'stone cill' you can just use planted-on EPS).

What was the last bit of the last sentence meant to say? '(''t the''...)

I haven't seen mentioned here before, so.. I recently came across a product called Triotherm+ which the manufacturer says is designed for supporting/cantilevering windows out in the insulation layer.

It seems to be a very high density EPS and is supplied in planks of varying sizes 70/80/100/120/140mm x 85mm x 1175mm. It's supposed to be affixed to the masonry wall with 7.5mm concrete screws and adhesive.

I haven't personally used this for windows, but did use it to keep the door thresholds in my house cosy. Observations: it blunts circular saw blades very quickly, and if I had to guess on behaviour and smell alone I'd say that the adhesive they supply is an MS-Polymer of some sort.

Andrew_Doran wrote: ''I haven't personally used this for windows, but did use it to keep the door thresholds in my house cosy. Observations: it blunts circular saw blades very quickly, and if I had to guess on behaviour and smell alone I'd say that the adhesive they supply is an MS-Polymer of some sort.''

Sounds similar to Compacfoam, as previously discussed, which is effectively load-bearing EPS. 21 Degrees sell Compacfoam. @Andrew_Doran, if you don't mind looking at 21 degrees' website, does it sound like similar stuff to you?

I have not used it for windows - I have no idea what it's like for strength around screw-holes, for example, but like @Andrew_Doran I have used it for thresholds. Then again, if the manufacturer says it is designed for cantilevering windows then it seems to fit the bill well.

Whew! All date from just after we built or later, so I'm not embarrassed.

BTW, when we installed our front door, there was an empty gap under the threshold, until I found some spare EPS to fill it with.