Between rafter insulation advice

[Clc136a]

Hi all, after a bit of expert advice on this please?

1980s house with concrete tiles and what looks to be impermeable felting, 175mm rafters spaced roughly 30-35cm x 32. Insect proof venting at eaves but currently only about 200mm fibreglass ceiling roll insulation.

 

Thinking to install either 400mm omnifit or flexi slab 140mm flush to front rafter face to leave gap behind. U=0.035-8. Venting baffles at eaves. The rafter length is about 4.5m but thinking to install 3 x 1.2m slabs on each and close over below the ridge.

 

Next would be either directly insulated plasterboard across the rafters (moisture barrier?) or more complicated battened superfoil type product. What insulation number could I expect from this approach and would it work to upgrade a cold loft to a better performing one?

 

many thanks, Chris

[Dave Jones]

you have a cold roof so you need to maintain a ventilation gap from the eaves to the ridge.

 

if your not making the loft a habitable room just add more loft roll.

[Mr Punter]

46 minutes ago, Dave Jones said:

you have a cold roof so you need to maintain a ventilation gap from the eaves to the ridge.

 

if your not making the loft a habitable room just add more loft roll.

+1

[Clc136a]

Yes, plan is to bring the loft within the thermal envelope to avoid extremes, even for storage.
 

Not full-fill on the rafters ( only 140/175mm) to allow air circulation above and to ridge.
 

Question was on choice of product and what R number I can realistically aim for?

 

Thanks