Advice on adding insulation to Warm Roof

[suppertime]

Hi all, I have been searching for best ways to add extra insulation between rafters on a pitched warm roof and have so far really struggled to find a solution. I am hoping someone on here could give some insight as to what can/can't be done with using different types of insulation in the roof.

 

Here's my conundrum, I am in the process of redoing the whole ceiling with plasterboards and would like to take the advantage of filling the gaps between the rafters with insulation for better thermal/sound performance. The insulation in there already isn't much, 50mm Celotex on top of rafters with 50mm polystyrene boards cut in between pressed tightly against Celotex leaving around 170mm of exposed rafter. Rafters are 220x47mm spaced 600mm.  Initially I was looking at using Frametherm 35 140mm or Rockwool RWA45 covered over with Glidevale barriair VCL (bought already) and then plasterboard  but after speaking with the Knauf rep, it would apparently contradict the guidance of moisture management. Quote - When using two different types of insulation within a thermal element the insulation with the lower vapour resistance should be positioned on the external (cold side) of the insulation with the higher vapour resistance, to help remove the risk of interstitial condensation.

I did also contact Rockwool but they were unable to help. Preferably I would have gone with the Frametherm route  just for ease of installation but not sure if it's still an option now.

 

 

 

[gaz_moose]

why not change the polystyrene for celotex. 

[EdHat]

PIR overcomes that but won't have the same acoustic performance as the mineral wool. How important is sound performance?

I would probably go PIR and an acoustic plasterboard.

 

 

If you keep the r-value of the inner insulation layer lower then the outer layer (and have a good VCL) you generally overcome the interstitial condensation risk. But that will limit you to about 90mm mineral wool depending on exact lambda.

 

If you are confident you can get the install the VCL completely airtight and their aren't any high humidity rooms directly below, then the risk of interstitial condensation is going to be very low even if you fill it with mineral wool.

 

You could add a layer of multifoil insulations beneath the mineral wool which will improve thermal properties and double up the VCL, but it will be expensive. 

[suppertime]

Thanks for the reply. If I was to replace it with Celotex, It would mean lots of polystyrene to dispose of.  Chances are it would end up in a landfill site and that definitely wouldn't be very good for the environment. I think I just leave it in there as I really want to avoid cutting and fitting PIR boards if I can.  Improving acoustic performance is important as well.

  

[suppertime]

Thank you for your suggestion regards keeping the r value lower on the inner insulation. Would interstitial condensation still be negligible if the Celotex GA4050 and the mineral wool have the same r value? I ask because the Frametherm 40 90mm roll has the same r value as the Celotex GA4050 board - both at  2.25 m2k/w.

Airtightness wise - fortunately the walls already have Barriair VCL  in place so I can overlap and tape to get a nice seal. I like the sound of the multifoil insulation idea and could perhaps(depends on costs) limit this to the areas above the bathrooms/kitchen - would this still be acceptable?  Apologies for the questions.