Is "partial" internal loft insulation (undertile) an absolute no no?

[Mr Gus]

Wind from the north, storming heat from the south facing pitch.

Crawl space only within the loft, bitumen loose slung 70's waterproofing with plenty of wind gap (yup that design but not really sagging)
Aware of past comment that change can = trouble.

Am I able to simply put some 25mm (or a bit thicker) between the southern side that gets the worst of the heat as a partial reflector & heat reducer or is that still a bit much? (north side left as is)

(likely source of legitimate cheap foil faced insulation boards from a builder mate)

Floor already insulated & partially boarded, thickness already laid depends on use / dead space.

One of the gable ends (block & brick has spare, skip dived celotex piled up against it that just needs tidying up a bit, due to dings & dents) to reduce morning summer heat exposure.
cheers for your input guys.

[Moxi]

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[Adokforme]

We found the PV array helped, but perhaps no possible with your construction. Or do you already have it.

[Mr Gus]

Mr Gus,

Can you wedge celotex or similar between the rafters so theirs an air gap on the felt side so the roof can still breath while the main body of the loft is calm air ? Allowing a small breather vent at each ridge line position between the rafters would ensure suitable air changes.


Moxi

That's what I'm wondering, as to set up, the pre existing vent technique was a common one, I will go measure my rafter depth (floor was 60 mm) , it I can wangle a few degrees heat out of summer build up with thin boards I'll give it a go.