PIR used as ceiling insulation.

[Annker]

Due to a IWI spec change I now have a lot of 40mm and 60mm PIR sheets that need either reselling or using elsewhere in my renovation.

 

There are three potential situations I think they maybe be used as additional ceiling insulation fixed directly to the underside of GF joists (cellar ceiling), 1st floor joist (GF rooms ceilings) and 1st floor ceiling joists (1st floor rooms ceilings).

 

The original plan in all these instances was to solely fit rockwool between the floor/ceiling joist, given its characteristics  (easy of friction fit, acoustic and vapour open) and I still do that at the very least.

 

Overboarding with PIR prior to plasterboarding will obviously increase thermal insulation performance but are there any drawbacks? 

Obviously ceiling height will be reduced but I have high ceilings and regardless lowering them in bedrooms and cellar is not a big loss.

 

I have never seen PIR used in this situation so would appreciate others opinions. 

 

And just to add I don't plan/want to batten behind the PIR prior to plaster boarding unless necessary.

Or go to the faff of taping the PIR joints to provide a VCL layer, which I don't think is required here. 

   

 

 

Edited February 2 by Annker
Additional text

[nod]

Your going to be making work 

Put it on market place 

[Canski]

I can’t see any harm in using them on the basement ceiling as long as you have fitted the flexi insulation between the joists. This was the design for our integral garage to insulate the room above. Works well here. For other rooms it seems like a waste of effort as there is no need to provide a thermal break between gf and upper floors. External cold walls maybe but not ceilings. 
 

or for your not yet built garden room ? 

Edited February 2 by Canski

[Annker]

22 hours ago, Canski said:

I can’t see any harm in using them on the basement ceiling as long as you have fitted the flexi insulation between the joists. This was the design for our integral garage to insulate the room above. Works well here. For other rooms it seems like a waste of effort as there is no need to provide a thermal break between gf and upper floors. External cold walls maybe but not ceilings. 
 

or for your not yet built garden room ? 

Only difference I have with my cellar vs your garage is the cellar is damp, not very but not bone dry either.

Although I guess the PIR wont be able to trap any dampness within the floor build up, that is the main concern I have.

[Annker]

23 hours ago, nod said:

Your going to be making work 

Put it on market place 

It is work but, work with a return (although it may not be a necessary return)

 

I'd add dealing with tyre kickers on market place is more work, certainly more painful!