Advice on upgrading insulation of vaulted ceiling

[FozzieBear]

I have a vaulted ceiling in two of the bedrooms of my bungalow. When rebuilt in around 2000 the spec was for 125mm rafters and 75mm celotex cut between the rafters with a pvc vapour barrier stapled to the rafters and then overboarded with 12mm plasterboard as below.

I have two aims. Firstly to get rid of the artex and secondly to increase the insulation of the ceiling. Can I attach foam backed plasterboard directly over the exuting plasterboard and skim or should I put 50 x 50mm battens secured to the existing rafters and then cut 50mm celotex between them with a new 12.5mm plasterboard and skim ceiling. with the existing vapour barrier in place is this trapping moisture in the new battens and insulation? Alternatively do I need to remove the existing plasterboard and vapour barrier and plant battens directly on the rafters and then add additional insulation? Any advice would be appreciated.

 

[noseall]

I have a vaulted ceiling in two of the bedrooms of my bungalow. When rebuilt in around 2000 the spec was for 125mm rafters and 75mm celotex cut between the rafters with a pvc vapour barrier stapled to the rafters and then overboarded with 12mm plasterboard as below.
View attachment 369370 View attachment 369369

I have two aims. Firstly to get rid of the artex and secondly to increase the insulation of the ceiling. Can I attach foam backed plasterboard directly over the exuting plasterboard and skim or should I put 50 x 50mm battens secured to the existing rafters and then cut 50mm celotex between them with a new 12.5mm plasterboard and skim ceiling. with the existing vapour barrier in place is this trapping moisture in the new battens and insulation? Alternatively do I need to remove the existing plasterboard and vapour barrier and plant battens directly on the rafters and then add additional insulation? Any advice would be appreciated.

Add more underneath. No need for battens. We do it a separates rather than inso' backed boards. Much easier and you can stagger the joins and foil tape the joints and use Duplex boards.

 

[FozzieBear]

Add more underneath. No need for battens. We do it a separates rather than inso' backed boards. Much easier and you can stagger the joins and foil tape the joints and use Duplex boards.

Thanks @noseall . What do you mean by separates rather than inso" backed boards?

I assume you mean Gyproc Duplex which has foil backing but where does the extra insulation figure?
Many thanks

 

[noseall]

Thanks @noseall . What do you mean by separates rather than inso" backed boards?

I assume you mean Gyproc Duplex which has foil backing but where does the extra insulation figure?
Many thanks

Separates as in sheets of foil PIR insulation and sheets of foil backed plasterboard - rather than insulated plasterboard. I hate the stuff.

 

[FozzieBear]

Ahh so you would mechanically fix or glue the insulation first then overboard with Duplex using long screws through both?

 

[mikeey84]

I find it easiest to stick a couple of screws through the insulation (at an angle to stop them pulling through) so it stays up

 

[Ivor Windybottom]

Get the proper washers, don't put screws in at an angle!

Buy TIMco MID36SS Stainless Steel Metal Insulation Disc Washers 36mm

Large diameter disc insulation washers, for use with screws to hold insulation materials to battens. They can also be used for cracked ceiling repairs. These insulation washers have a 5.5mm diameter hole in the centre. They do not come supplied with screws, meaning you can use the desired length...

www.tcfixings.co.uk

 

[mikeey84]

Think the washers are overkill if you are overboarding within the next 10min myself

 

[noseall]

Ahh so you would mechanically fix or glue the insulation first then overboard with Duplex using long screws through both?

Correct. You don't even need to line the insulation sheet edges up with the rafters either. But it is wise to stagger the joints with the plasterboard.

 

[FozzieBear]

One last question. How do you professionals deal with the joint between wall and ceiling when overboarding. do you:
1) Cut boards as close to wall as possible and justs skim, telling housholder to get decorator to caulk corner with decorators caulk?
2) Tape edges, skim and cut off surplus from wall and brush in?
3) Leave a slightly larger gap and fill with bonding prior to skimming?
Many thanks

 

[stevie888]

Mark the joist locations carefully before you start and be careful to keep the screws straight.

You'll want at least 75 celotex over to comply with current required insulation values

 

[mikeey84]

Regarding the join between ceiling and wall, it's likely to crack at some point in the future, regardless of what you do.

All 3 of your points would work, so I'd just leave it to the plasterer to decide.

 

[FozzieBear]

Mark the joist locations carefully before you start and be careful to keep the screws straight.

You'll want at least 75 celotex over to comply with current required insulation values

There is already 75mm between joists. Do I need another 75mm below or just 50mm?

 

[mikeey84]

You will not regret putting more in. I would put as much as possible- even 100mm would be good

 

[stevie888]

LABC standard for 150 rafters would be 100 between/50 over.

When you run the various build ups though the Celotex online calculator you find that the thicker the top layer is, the lower the overall thickness needs to be. It's not a masive difference and it's a result of the celotex being a better insulator then the wood itself.

My comment should have said "about" rather than "at least"