Vaulted ceiling insulation

[S Godfrey]

Would be grateful for some advice please. I am currently considering the issue of insulating the vaulted ceiling in my extension. I understand that the U value I need to achieve is 0.18 W/m2k. The specification from my planner states that I should use 120mm of Jablite with a further layer of 50mm Jablite under 12.5mm plasterboard. I would rather use Celotex or Recticel which I believe are better products. It is my understanding that the additional layer of insulation beneath the plasterboard is required to prevent thermal bridging. If for example I was to use 100mm Recticel or Celotex between the joists, could I achieve the required U value by using something like Knauf insulated plasterboard instead of a separate layer beneath the plasterboard? I realise that this may end up being more expensive but it would be much easier to fit.  Many thanks. 

[nod]

The specifications are usually put in by your Architect As he or she would have had to be quite specific when submitting 

What you are suggesting in using insulated plasterboard is pretty standard 

BC will go with what they know 100 mil Recti or similar 

Gypsum or Knauf Will be perfect (62.5)

[S Godfrey]

Thank you for your reply nod. So if I went with 100mm Recticel with 62.5 mm insulated plasterboard, do you know what sort of U value that would give me? Could I get away with say 50mm insulated plasterboard or would that not give me the required U value?

[nod]

From memory 52 is 0.65 U V 

Theres a u value chart on the Gypsum site that you will find helpful 

What ever you spend on over insulating Will soon be recouped on heating costs 

[A_L]

1 hour ago, S Godfrey said:

I would rather use Celotex or Recticel which I believe are better products.

 

Only in the sense that it will be thinner,  not in return for money spent.

 

22 minutes ago, S Godfrey said:

Could I get away with say 50mm insulated plasterboard

 

It will achieve 0.18, just.

 

 

[S Godfrey]

Just checked the Gypsum website and it was very useful. Thank you for your advice nod

[S Godfrey]

Thank you for your reply A_L