Sanity check for my insulation plans

[diyquestionsguy]

I live in a 1970s "chalet-style" ex-council house, so all of the 2nd floor is in the roof, and I want to add some insulation. I think there's 3 separate parts:



1. The vertical gable walls: cavity blockwork, currently 50mm studs affixed to the inside with 20mm bits of polystyrene between some of them and plasterboard over the top. I plan to replace the polystyrene with 50mm celotex.

• Are the wooden studs going to be a cold bridge? Should I be insulating over the top of them too?
• I think I need a vapor barrier on the warm side of the celotex - does that go over the top of everything including the wooden studs?
• There's a 10-15mm gap between the wooden studs and the block wall - will it be a problem to have that gap behind the insulation?

2. The walls meeting the roof: Currently there's some soft insulation in the eaves which I plan to top up. Then on the knee wall and sloping roof I want to add 50mm celotex, again between existing wooden studs. That will leave 70mm between the insulation and the roof for airflow, is that enough?

3. There's about 100mm soft insulation in the roof which I plan to double.

Does that sound ok? Any thoughts appreciated, the house is currently freezing but not damp so I'm trying to be cautious about condensation.

 

[motorbiking]

Might have been easier to leave it as it was an then add another layer or insulated plasterboard. While 50mm of celotex is better than 50mm of polystyrene, there is not much in it. Celotex PL4040 would have been easier on top of what you had. But since you have ripped it off, if the Frame is not needed rip it off and use plasterboard foam adhesive to glue the celetox to the wall, foam (standard foam) all the gaps and then plasterboard foam the plasterboard to the Celotex. Its probably worth going for more than 50mm. You may have had some thermal bridging if the existing insulation had gaps.

 

[diyquestionsguy]

Cheers. The plasterboard was needing a bunch of work either way (popped nails, tape peeling) and the polystyrene was only 20mm and missing in some spots so I don't feel too bad about bringing it down. But what you're saying about removing the frame too in order to get a continuous cover of celotex makes sense, I'll look into that.