Hi I am currently insulating my vaulted celling 

 

the make up it 150mm PIR - VCL - 32.5 mm insulated plasterboard.  Will I loose insulating performance if there is a air gap between the two layer of insulation

 

thanks

As long as the 150mm layer is properly sealed with foam and tape and if necessary a vcl then you won’t loose any performance, if there are ANY air gaps in the 150mm layer then cold air will flow through between the two layers of insulation and then you would only have 32.5mm insulated plasterboard as working insulation…….. so many houses have  this problem where cold air bypasses the insulation and blows around in the service void behind the plaster board. 

 

 

 

 

been using expandable foam and silver tape to tape the edges.  The silver metal tape from Jewson's is not very good though and after a few day looks like it is coming off in places not sure why.

You need to shop around fir good tape, or try and get recommendations from on here. The best stuff I’ve had is from my local travis Perkins and the next best from screwfix.