Insulating Flat Roof - Extension to Old Bungalow

Trying to insulate roof in 1970s extension to an old bungalow. Wondered if anyone else has come across a similar challenge / what you did please?

The extension is for a quite small kitchen and bathroom - very cold in winter.

As a refurb project, I’m resigned to not being to be able to meet best practise for new builds. Approaches I’ve considered so far, with main snags AFAIK:

Warm Roof – The position of the fascia/soffit on main house allows no more than 30-50mm (probably) of PIR insulation on the flat roof. Feel this just isn’t enough.

Cold Roof – Could manage 60mm PIR between joists at shallowest end and 30-40mm under. The soffit is only a couple of inches wide. There’s a clear run between joists except for alternate noggins at the ends (hard against fascia with no gap). Tricky to install vents.

Hybrid Roof – 30-50mm PIR on top with up to 60mm between joists, hard up against roof deck. Plastic VCL fixed to underside of joints and sealed around edge with false ceiling below as service void. Certainly not best practise and would need a ‘condensation risk analysis’.

Btw, the joists in the extension run parallel to the house (hence potential for through ventilation but with snag described above). The current roof has 50mm joists at 400mm centres which fall to a depth of about 110mm at the shallow end. OSB on top of this (prob 18mm) with built-up felt covering

Hope someone can help. Pictures might be useful:



Cheers, Simon.

Just fill it with as much of the most efficient insulation board as you can, ensuring no air voids anywhere. A layer on the underside of the joists will be beneficial too.

And then put a vapour check membrane on the underside.

Thanks Woody. The space tapers due to flat roof slope - any idea if it'd be safe to fill awkward gaps (after packing with PIR) with expanding foam? Overkill?



In practise I think the expanding foam 'wedges' in the pic would taper from about an inch to next to nothing.