How to lay celotex (insulation board) in loft

[wonderwire]

I want to insulate my loft with Celotex insulation boards

I am planning to insulate between and over the joists.

My joists are 90mm deep, but I am planning to use 50mm Celotex between them, followed by 50mm Celotex across the top.

I have 2 questions:
1. Should I rest the lower celotex against the plasterboard (which would leave a 40mm air gap to the top of the joists), or find a way to make it level with the top of the joists?

2. Will Celotex hold weight if laid directly across the joists, or do I need to support it with chipboard underneath (and then more chipboard on top)?

Thanks

 

[chappers]

method 2 for the insulation , insulation to the top, keep the warm air in the house as far away from the cold air in the loft.
It will be strong enough, once the chipboard is over the top but be careful to know where the joists are if your going to need to get on top of it to work, it wont take your weight in the gaps

 

[wonderwire]

thanks @chappers . i want to be able to walk on the chipboard, so assume I should also add it underneath the celotex. If i do this, does it create an issue for interstitial condensation, or am I likely to be ok?

 

[John D v2.0]

Use wool as the bottom layer, it's cheaper and will stop the air movement between the layers thermal bypass.

 

[chappers]

you will be able to walk on the single layer over the celotex I just meant the celotex wouldn't take your weight whilst the installation was going on.
Also what john says makes sense much easier install too