Celotex in old flat roof - air gap underneath question

hi all, first post. need some advice on insulating an old flat roof, came across this forum and although lots of good info via search, couldn't quite find the info i was after - so first question:

I have two long flat roofs that run the length of the house either side of a small triangular pitched section. Each flat roof is about 6' wide and about 4 rooms long.

It appears to be old "cold roof" redsign, with 150mm depth between the joists with barely any/no insulation. The plasterboard underneath has a membrane between it and the joists. The felt is ok, no signs of rot.

My plan is to get some insulation in the flat areas by working from the small pitched section (about 5' traingle) and feeding in cut strips of celotex. It's going to be tight, but i think i can just about do it.

The question concerns existing battons fitted across the joists which support the plasterboard joints. These are 50x50mm batton. These will prevent the strips of Celotex sitting flat on the plasterboard - is this a problem? Should I leave that gap unfilled, or leave some old fibreglass in that space?

As I have 150mm depth, with 50mm taken up by battons in places, I was planning on going with 70mm Celotex, leaving a 30mm air gap (bit less than spec). Do you think this will be ok?

For the are in the triangle section, I plan to use 100mm celotex as no air gap issues above (but will still have the batton underneath issue).

I know the solution will not be ideal (joists not insulated above or below), but alternatives are a big "warm roof" conversion or redoing the ceilings in 4 rooms...

apologies for long first post..

Read this.

If you have further questions after reading the link i will be glad to help.

thanks for the quick repsonse and link.

I had seen that post and others like it previously using Search, but they dont actually cover my query.

I have a large and watertight flat roof, just no insulation in it. Adding an external layer to make it "warm" or stuff underneath in many rooms is not really a cost viable option. So ideally I just want to feed some insulation into the existing gaps. However, there are battons running between the joists in places, so the insulation boards would sit on these and not directly on the membrane&plasterboard. Does this 50mm air gap between Celotex and membrane matter? should i fill it with something (fibreglass)?

thanks for your help.

neil.

You will be left a cold deck situation, thus you will need to ventilate any air that remains between the joists.

The ideal cold deck would be to have the insulation tight against the ceiling and any air gap above. This is so that when you ventilate the gap above the cool air replacing the mist warm air does not interfere with the warmth below the inso'.

Cold air being allowed to circulate will cool any surfaces it moves over.