Garage and shed roof insulation question

[tozerman]

I have a flat roof detached garage (and shed) which simply consists of the usual joists with plywood on top for the roof.

The garage is just damn cold and the shed roof (new in the summer just gone, so first winter) has mould starting to grow.

I'm planning to insulate both roofs using 25mm celotex or similar.

Should I simply cut the sheets to size and simply insert in the gaps between joists and push up snug to the actual roof boards or should I leave a gap between the board and the roof ? and should I lave a gap on the ends ? if so how big ?

I have searched online and there is conflicting advice as to which method is best ?

Thanks in advance

[SoliD]

If neither have a heat source, the celotex won't be keeping any heat in, so seems a bit pointless, if you've got mould forming you need more airflow unless it's MDF in which case it'll carry on growing until you give it a lick of paint to protect it.

[trickywoo]

In the scenario you describe I think the wool type insulation would be better but don’t expect miracles.