Additional insulation in single-storey extension

Have just removed some of the ceiling from a single storey extension (am replacing the ceiling) and expected to see no insulation between rafters ... In fact, there is 100mm thickness or so of yellow (possibly that nasty glass fibre stuff) insulation.

I'm not sure whether it's a warm room construction or cold roof. Is there any way of working it out on inspection?

I suspect it's the latter type (built in the 1980s ish), which means I have a need for ventilation - though there is none evident in the construction so far and we haven't had condensation problems.

Assuming it is cold, the joists are 250mm so I'm thinking some more insulation could be added.

What would people advise? Removing what's there and replacing with something else or adding to it?

NB: We're adding 60mm kingspan k17 to other 3 external walls of room and were planing on doing the same to the ceiling when we starting thinking it we would look a lot of ceiling height...

Thank in advance.

Warm or cold deck roofs refers to the positioning of the insulation relative to the rafters.

A warm deck is specifically where the insulation sits above the rafters and is unvented.

A cold deck could have insulation between and/or under and has a ventilation gap.

In your case it is neither as the insulation is fitted at joist level and you will require 250mm Fibreglass insulation (100mm between the joists with 150mm going across the joists).

This stype of roof requires ventilation, via breathable membrane, soffit vents, overfascia vents and/or tile vents or abutment vents.

http://www.dorset-technical-committee.org.uk/reports/U-values-of-elements-Sept-2006.pdf