Insulation - rigid or rolls in loft pitched roof?

[SJRSJR]

Hello

I had to have some additional rafters put in the roof. The roof is clay tile/battens/counterbattens/membrane/closeboarding. It's a converted loft and the insulation behind the plasterboard is 50mm polystyrene board (white with black flecks). I will be re-plasterboarding and my question is whether I should put it the same polystyrene back, improve it, or whether I can use rolls of rockwool. The rafters are 4x2. I'm not sure whether a pitched roof needs rigid insulation what the airflow requirement is and whether closeboarding changes anything. Thanks

 

[noseall]

It's a converted loft

To what standard? A low end loft ladder and some decking or staircase, floor joists and dormer type thing?

 

[SJRSJR]

Proper job, but done in the 90s, 9 x 3 joists have been added, floorboarded but only partly finished internally. Previous occupant finished off the plasterboarding a few years ago.

 

[noseall]

Proper job,

Ad 150mm Celotex or similar. 100mm between the studs and 50mm across the face. Foil tape both layers. Fix Duplex plaster boards.

 

[SJRSJR]

Thank you noseall. So no airgap needed ?

 

[noseall]

Thank you noseall. So no airgap needed ?

On the cold side - yes. 25-50mm.

 

[SJRSJR]

Ok, thanks. So it’s been counter-battened under the tiles so nothing needed internally? What is the dividing point between the warm and cold side?

 

[noseall]

Ok, thanks. So it’s been counter-battened under the tiles so nothing needed internally? What is the dividing point between the warm and cold side?

The insulation.

It's important that you seal any gaps with foil tape or larger gaps with foam and tape.

 

[robinbanks]

edit: did a sketch (attached, but not inlined now) then realised I wasn't sure what you're insulating! If it's loft ceiling, that's what I meant and I'd taken the 9x3 as the rafters (sketched buildup is tile/battens/gap with rafters/insulation with rafters/insulation over rafters, with taped joints/plasteboard)...

If I misunderstood, correct me and I'll sketch something else

 

[SJRSJR]

Okay yes, thanks, that’s really helpful. My roof rafters are 4x2, it’s the floor joists that are 9x3. So should I go for 50mm between the rafters with 50 air gap then 100 across the top? The plasterboard fixings will need to be very long!

 

[noseall]

Okay yes, thanks, that’s really helpful. My roof rafters are 4x2, it’s the floor joists that are 9x3. So should I go for 50mm between the rafters with 50 air gap then 100 across the top? The plasterboard fixings will need to be very long!

Ordinarily, you'd add knock-on thickening battens to the underside of the rafters to deal with the air-gap + PIR thickness.

100mm PIR beneath would mean ridiculous fixings for the plasterboard.

 

[robinbanks]

The plasterboard fixings will need to be very long!

And very expensive.. 150mm phosphates are circa 10p each

Extending the rafters with a 25mm batten may pay off, and then you either get 25mm draught gap to help keep timbers dry or if you want a bigger gap you could go 75/75

Generally if I was fixing 12mm plasterboards I'd want 25mm of screw into the wood