As you have maximum floor insulation, there will be little heat coming into the loft area from below, and in summer there will be a large solar gain from the uninsulated roof.

I decided to put Kingspan between my rafters on my pitch roof for following reasons.

I am an Engineer so tackled the problem using thermodynamics.

The pitch roof has a thermal transfer, or heat loss/gain value.

I measured my loft temperature, winter 4C, summer 40C.

The insulation has an R rating, this gives you the rate of thermal transfer. Having a loft at 4C and the room below at 20C, there will be a heat transfer pressure of 16C.

Insulation in the rafters will slow down the heat transfer loss/gain from the loft, and reduce the temperature swings, in my case the summer high was reduced dramatically but the winter low remained the same.

Thermodynamics are hard to comprehend but if you think about 3 rooms in line. Room 1 is heated to 20C, room 2 next door 19C, Room 3 (the loft) is 4C.

The thermal gradient between the rooms is 20/19 and 19/4.

with no insulation, ie an open door, both rooms attain similar temperatures, ignoring thermal gain from a south facing window, with the door closed, the heat required in room 1 will be less. If room 2 is now a cold draughty loft, room 1 will transfer heat faster into room 2.

you said your DHW is in the loft, it will benefit from a better insulated loft.

One thing to be aware is to ensure the eaves are not blocked, to avoid mould, etc. Better, check how your roof design ensures ventilation.

Posted by: @batpred

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One thing to be aware is to ensure the eaves are not blocked, to avoid mould, etc. Better, check how your roof design ensures ventilation.

I suspect @agentgeorge is already aware of that, @batpred, but it's a sensible observation to make nonetheless, particularly for anyone else who may happen across this thread.

Yes, the builder reminded me to allow a good air gap, have 50mm gap up to the top of roof pitch.

always good to state the obvious incase some newbies read the thread.

Like @agentgeorge I too have a warm-roof construction.

It's retrofitted onto a 1930's farmhouse for which I was extending the roof in any case.
I have 75mm PIR insulation board between the rafters, and 50mm outside of that, running on top of the rafters.

The outer 50mm layer is then counter-battened (vertically), followed by a breathable membrane held in place by the horizontal slate battens.

Posted by: @trbob

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I wonder whether it's worth adding kingspan boards underneath the felt and tiles to give a bit more insulation

I'm slightly more concerned about that strategy.

It's not a warm-roof construction because the external top-side of each rafter isn't covered by a final outer layer of insulation.

There will be a temperature gradient across the rafter, making its inside face 'colder' than the adjacent PIR insulation board.
I wonder whether condensation will accumulate in winter, making the rafters susceptible to rot or fungal growth.

If so, then installing mechanical ventilation (with heat recovery) into the loft-space would most likely solve the problem.

Posted by: @batpred

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One thing to be aware is to ensure the eaves are not blocked, to avoid mould, etc.

Well, that's the tricky issue, isn't it?

For a warm-roof construction the air-flow is between the counter-battens - on the outside of the upper insulation board and beneath the breathable membrane.

But if it's a cold-roof construction then you must have airflow from the eaves into the main loft space.
Typically that will use ventilation trays to hold the ceiling-level insulation away from the sloping roof above it.

@transparent

The warm deck roof 150mm thick, eventually a second layer of wool insulation will go up with the ceiling boards

The front extension has kingspan between the rafters and wool insulation above the ceiling, i am still finishing the lighting for this area so have 2 access panels still open

Thank you everyone for all of the information, it looks like something best done if replacing the roof perhaps given the complexity.