Jump to content

Window fitting in an externally insulated deep refurb


Recommended Posts

Hi, sorry to steam right in with a question, but tbh it's why I joined the forum to pick your collective brains!


About 1/2 way through a deep refurb of a bungalow, all that was left was the walls and I wish I hadn't, but had had such a bellyful of planning in AONB and green belt that I couldnt face going back to argue for a complete rebuild application.


The existing walls are atypical, 220mm thick, 85mm light block, 50mm empty unventilated cavity, 85mm light block. for simplicity I've treated this as a flat laid solid 215mm aircrete block.


The wall build up from outside in will be silicone render, 150mm graphite EPS, 220mm existing wall, 0.5mm aluminium foil membrane (vapour barrier), 25mm PIR, 12.5mm plasterboard and 3mm skim.


I'm planning on Internorm HF410 windows which have a potential U of 0.64 but I have to stick to 3 sashes in 1.8m windows so will be a bit worse worse than that with more frame/mullion and less glass.


I've attached images of the proposed window mounting outrigged in the EPS. I had thought to line out the rough window opening with 50mm celotex or similar on all 4 sides of the rough opening and hang the window out in the external insulation EPS with brackets. I thought that if I ran a chunky sub sill of PIR/PUR under the window I could screw and plate that external to the wall to support the majority of the window weight. The bottom brackets could be L shaped and run down the inner wall face to resist any tendency to "turn out" the bottom course of block?


Suggestions / critique gratefully received.



window outrigger fitting outside.jpg

  • Like 1
Link to comment
Share on other sites

Don't set the window out quite that far.


Our situation is slightly similar, 200mm insulated timber frame with 100mm wood fibre board external insulation and similar render.


The windows were bought with aluminium external cills iirc about 130mm deep. So the windows were set so the cills overlapped over the wood fibre board enough to allow for the render.


Some pictures to explain it:


Here is how the window sits in relation to the wood fibre board, so the windows sits partly in the timber frame (brick wall in your case) and partly in the external insulation (EPS in your case)




The windows are fixed with several galvanised plates pre drilled with holes (someone will tell your the correct name?) first screwed to the windows then screwed to the frame (again that will be screwed to your brick walls)





Link to comment
Share on other sites

We have very similar wall build up & had 150mm EWI

We made plywood boxes to line the rough openings & project into EWI layer about 70mm

For the larger windows we screwed a block of compacfoam to the external wall to add additional support, but not sure it was neccessary.. plywood boxes themselves were very ridged once screwed & foamed in.

Additional benefit of air tightness taping being really straightforward



  • Like 2
Link to comment
Share on other sites

Compac foam do a system fo fixing windows into EWI it consists of very rigid insulation that carries the window load. 


Charlie Luxton used a fibreglass angle section that was screwed to his framing that projected out into the EWI, the window then sat on this angle and the insulation butted up to it completely hiding it. 

Link to comment
Share on other sites

  • 2 years later...
  • 2 weeks later...
  • 7 months later...
  • 5 weeks later...

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...