Attic Conversion and Air-Tightness

tipping

Soon to be embarking on attic conversion in detacted double hipped roof bungalow. House was built (not by us) with conversion in mind, it has full head height, is floored, knee walls in. About 1000sq ft of space available up there so 3 rooms going in. Planning is done and have an arch working on spec at the minute but doing my own research as I only want to do this once and have read a lot about cold attic conversions. I've read through the NSAI docs on retro fit but a few things I can't figure out especially around airtightness.

Building make up is as follows.
Outer pebble dash finish, outer leaf 100mm block, 100mm unfilled cavity, inner leaf 100mm block (un-plastered), 50mm insulated plasterboard (polystyrene).
9 x 2 joists sitting on wall plate on inner leaf.
Cold roof structure, 150mm rafters, then non breathable membrame, battens and slates.
Existing house is pretty warm and not bad to heat. Definitely colder on a windy day with noticeable drafts from a few of the windows.

Was at the self build show at the weekend and talked to lots of suppliers.
Insulation Options seem to be:
1. 100mm rigid insulation between rafters, airtight membrame, counterbattened service cavity then insulated plasterboard to give desired u value.

2. Add 2 x 2s to the rafters and use 150mm fibre insulation (Metac or similar), then airtightness, counterbatten, insulated plasterboard. There's enough room up there to happily lose the few inches off each room.

I would have a preference for option 2 as I believe option 1 is a pain to fit and you can end up with lots of little gaps etc. Also option 2 is breathable

It seems likely that we will fill the existing unfilled cavity if the interstitial condensation calculations say it will be ok. Arch did mention external insulation but budget probably doesn't stretch to that and if existing house is warm enough then I don't see it being required.

Airtightness:
I'm assuming airtightness in the converted attic area is a good idea even if the overall house is not airtight but I had different opinions on this at the self build show at the weekend. I am unlikely to tackle airtightness downstairs in the near future.

Assuming it is a good idea where should the insulation and airtight layer terminate.

Should be run down to the eaves (leaving ventilation gap) or terminate at the knee walls?
If you terminated insulation at knee walls leaving the storage area cold where could you terminate the airtightness layer to?

If running down to the eaves would you terminate to inner or outer leaf?

Also we asked the architect to detail how to prevent air flow from the eaves between the joists, he mentioned about removing bottom rows of tiles and doing this from the outside but any pointers to what we should be looking for would be appreciated.

Any advice appreciated.

blueskys

option 2 your best bet by far. definitely airtight membrane as it if cold air gets in it will drop like a stone through the rest of the house, same for keeping hot air in.
would recommend the siga stuff for membrane.bring the insulation to the eaves but you need to be careful to leave a breathable gap between the felt and your insulation of 50mm. Also install roof vents to help ventilation. not sure where you have you knee walls but basically with membrane you are looking to create a sealed room within a room so you need to go down to the floor and seal it to floor.
Have a look at this attic insulation video, its in german but is excellent guide to attic conversion/membrane installation:

https://www.youtube.com/watch?v=23mes1qr_Ms&t=2672s

tipping

Thanks for this. Bullet point summary from arch.

100mm Rigid insulation between the rafters with insulated plasterboard to acheive 0.16 u value but I'm going to ask him to reconsider for the mineral wool insulation option.

350mm rockwool on the flat ceiling to acheive 0.13 uvalue

Insulation and airtightness membrame specified down to eaves (allowing the gap) with air tightness terminating on inner leaf.

Airtightness layer bedded in the internal plastering layer which seems a bit strange as you'd be knocking holes in it for all services afterwards so I think I'll ask him to adjust allowing for service cavity.

Pump the existing cavity and to remove bottom rows of slates on outside and use OSB board to prevent airflow between the joists.