Loft Insulation - airflow allowance

[KDMcM]

I'm hoping to insulate the inside of my pitched roof. My loft is floored, but I don't think there's very much insulation under it (only to the depth of a joist). I think the huose would benefit greatly from having insulation in the roof, too.

I've browsed around the threads looking for specifics about this and decided that I probably want to install polystyrene blocks between the joists. The joists are 50mm deep, and I was hoping to install 50mm thick poly boards.

I'm now completely paranoid about leaving an airgap between the roof cloth and the insulation boards.

How critical is it to leave an airgap?
How much airgap should I leave?

Many thanks.

 

[Wayners]

You don't have breathable felt so I wouldn't.
You may get condensation

 

[KDMcM]

You don't have breathable felt so I wouldn't.
You may get condensation

Confused. I know I may get condensation which is why I asked.
I'm not sure what you wouldn't do. Insulate or leave a gap?
Based, again, on the question, I can't possibly have any idea what breathable felt is. Can I install some?

Is it generally held that with my particular installation, insulation is a bad idea?

 

[Wayners]

Rip felt off and spray foam will give you what you want but its controversial.
If you insulate and leave a gap any moisture in the air will settle on the cold felt.
That's why you need vented soffit and air gap with a flow of fresh air.
If there is no air gap like with foam condensation is not a problem.
Be mindful about using loft for storage as that can cause joists to bow and ceilings to sag. You don't have a floor up there. Its smaller ceilings joists.

Maybe this will help you.