Retrofit insulation into cold roof

[0Floz0]

Hi all

Currently refurbishing my dormer extension that was done by previous owners about 25 years ago. The roof is just an empty uninsulated space. Essentially from top to bottom layer it is:
Roof felt
osb
Joist (oddly it’s a joist with a wood batten on top so it’s 185mm thick)
Plasterboard

I’m looking to insulate this if it’s cost effective as my house gets very cold in the winter. We are probably going to move next year so don’t want to spend loads doing it but as I’m installing an en suite in the same room I’m ripping a few bits out anyway.

My hope was just sticking some mineral wool there. The joists go the right way in that I can cut a hole in one side of the ceiling and poke it all through.

I know ventilation is an issue but when it’s windy you can feel the breeze coming through a hole I’ve cut in the ceiling to do some plumbing. So I’m assuming that’s sufficient (the soffit on the front is vented but not sure about the other side).

It’s not a huge space so was thinking 150mm of mineral wool to leave a 35mm gap. Given part of it is an en suite I’m a bit worried about the absence of a vcl…

Would appreciate any thoughts.

Thank you.

 

[George_engineer]

If you moving soon then buyers will probably want to see building control sign off for any works you do now. In which case just whacking some mineral wool in would not be enough. I'd just do the en-suite properly and not touch anything else ... 'it was like that when we got here'.

Indeed, why do anything if moving soon? You will not add any significant value by making these sort of changes.

 

[0Floz0]

If you moving soon then buyers will probably want to see building control sign off for any works you do now. In which case just whacking some mineral wool in would not be enough. I'd just do the en-suite properly and not touch anything else ... 'it was like that when we got here'.

Indeed, why do anything if moving soon? You will not add any significant value by making these sort of changes.

Yeah I’m getting building control sign off for the en suite as well as several other things.

It’s a fair question. I keep asking myself it. But heating cost a fortune last winter and I’m wary about that. If it costs me £50 in mineral wool and helps save near to that then it’s worth doing. I’m getting new carpets in the dormer room so just thinking about the next occupants and the mess of doing it afterwards but I’m probably being too nice.