Best Loft Insulation Option - Long Term

[NRM1979]

Right. We have a house. The loft is partly boarded with degraded rock wool as joist level insulation ranging from 10cm to 2cm in places.

It is pouring heat out the roof. Within the next 6 years we have plans to convert the loft to create another bedroom. Thinking long term I can't quite figure out whether I'm best to:

1 - Kingspan/Cemotex between rafters (leaving a 50mm clearance off the sarking for ventilation) but saving the drylining for another day.

2 - Kingspan/Cemotex between the joists as it'll all be floored eventually and reinsulate the water tank is it will get a lot colder in the loft above the ceiling! My thoughts were to simply cut to width, insert between joists and leave it at that after reaffixing the existing floorboards.

3 - Rockwool it all for a pittance now, only to rip it all out within a decade?

My preference is for option 1 or 2 but would welcome the thoughts of the learned souls on here as googling the subject provides a dazzling array of ways to go about it.

[Bob Minchin]

Lots of local 'green' schemes are giving rockwool away free whic might sway your decision?

[Muddymike]

If you plan to convert it the go for the cheapest option. You will have to insulate the ceilings of the new loft room when you convert anyway.

Mike

[John Rumm]

The gap alone would not be enough without also ensuring there are ridge and soffit vents. Depending on the type of conversion you ultimately plan, you may find it easier to opt for reroofing with a breather membrane in place of the existing sarking. That would allow full fill insulation on the inside which may also give more headroom.

Note also that 50mm of PIR foam between the rafters will not by itself meet building regs for a habitable room - you would need a second layer of it under the rafters.

I would probably be less keen on this given you plans.

There would be no requirement to remove it later, and also its worth noting that it would add extra fire resistance to the existing ceiling. You may in fact find installing rockwool (or similar) to be a requirement when you convert the loft. (it also adds a bit of noise insulation as well)

I think I prefer 1 or 3 myself.

[Alexander Lamaison]

I'd suggest 3 for now. It's dirt cheap and we all know only too well that plans for the future often remain just that. Plan for now unless it *prevents* the future plans.

P.S. I have trouble following my own advice.

Alex

[Dave Liquorice]

That it well worth looking into but the conditions attached might not be meetable by the OP.

Even so glass wool is cheap (are the power company subsidies still about?) and as others have said what is planned, what happens and when are completely different things.