PIR board as loft insulation

[bullethead123]

Hi all, my loft currently has 100mm of fibreglass insulation that has been there for 32 yrs+.
I have heard that 270mm is the new standard depth and was gonna just top it up to that level but wanna ask a couole of questions first:
Should I first replace the old insulation or will it be OK not to?
Someone in the pub told me about using PIR boards instead of fiberglass. He said that it was twice as effective.
Is this so and could I use it in conjunction with the existing fibleglass?
A complication is that the loft hatch measures 70cm × 45cm. Would it be OK to cut the sheets of PIR?
Thanks for any replies People.

 

[Deluks]

It's twice as effective but will be twice the cost, probs more. The other potential issue, being foil backed, it might cause problems with trapped moisture and condensation.

You can just top up the existing stuff, no need to remove. If you want to board the loft for storage, you can either increase the height of the floor with additional cross joists on top, or you can get 'loft storage legs' to create a raised platform.
For the hatch, you can glue PIR to the back of the hatch, and draught proof the edges. I just fixed a slab of polystyrene to mine, held in place with double sided tape.
Cut to size with an old/cheap saw or bread knife.

Be wary about taking building advice from a bloke in the pub.

 

[Deluks]

Come to think of it, be wary about taking building advice from a bloke on the internet!
But if the bloke on the internet is wrong, someone will be along shortly to correct him or start an argument.

 

[hllns]

Having put in a bit of 100mm PIR so I could easily board around the hatch and still have a little headheight It's a right faff, especially if the joist sides aren't completely parallel.

By comparison just rolling some top up loftrolls at 90 degrees to the existing joists is cheap and easy, you can board above this but you do lose headheight and would need something to support the boarding above the extra insulation layer like loft legs/beams etc.