Modernising porch

1 Nov 2021
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United Kingdom

My wife and I have moved into a 60's fixer upper which has barely been touched in 30+ years.

With the cold setting in, our porch is starting to develop a lot of condensation and damp (photos are from when we moved in). It's UPVC with no insulation and there's just a very thin stud wall with a single pane glass and an internal glass door to the house.

The porch has been very draughty since we moved in and as I understand it, the condensation is due to the warm, moist air from the house leaking into the cold porch which has no ventilation.

So the rough plan is to properly isolate the porch from the rest of the house by taking down the internal wall and adding a well insulated wall with an external door, insulating the roof space using rockwool (it extends into the warm part of the house) and adding ventilation in the porch. Is this the right approach? Assuming it is, I have a few questions...
  1. How thick should the internal wall be? I was thinking of using 140mm CLS timber with one layer of 100mm Celotex, is that enough? Overkill? Would 2x50mm with an air gap be better?
  2. There is a side window which is on the "warm" side of the house but it's still part of the porch construction if that makes sense? It's double glazed but there's an area underneath it which I've circled above. Without taking the porch apart I'm not sure exactly how it is constructed but I'm assuming it's just wood with UPVC cladding? Should I remove the cladding on the side and add insulation? Presumably it will be quite a cold wall once I've done the other work if I don't?
  3. What type of ventilation should I add in the porch? Is there a particular type of vent I should look at?
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