Hidden ceiling insulation

That's not a huge change in temperature, which suggests that those bays may not be insulated as well as the others, but it unlikely they were "forgotten" as such.

What you can do will depend a bit on the type of roof covering. In places where there is a breathable membrane (aka "sarking") under the tiles, then you can full fill the space between tiles and plasterboard. However if you have a traditional roof sarking, then you need to leave a ventilated air gap above the insulation so that no moisture can get trapped between the insulation and the tiles.

It might be that you have some air leaks around the edges of the insulation, where the boards were not cut snugly or the gaps sealed.

As you say, not easy - and you would need to detile, and rebatten and re felt the roof just to get the insulation in there. So unless the whole roof requires replacing I would not go that route.

That's doable. Unlikely to be asbestos in most parts of a roof - the exception being the soffit boards on some '50s onwards places that have asbestolux sofits.

(asbestos cement boards - low risk if left alone)

You normally need a vapour control layer that can stop warm moist air reaching cold "interstitial" spaces. The foil cover on PIR foam boards like celotex does this. Then foil tape over any joints etc.

You can get insulated plasterboard that includes vapour control membranes as well.

Probably not a good choice (see comment above about maintaining an air gap between insulation and tiles). You also don't want to bond the tiles to the roof - which injected foam would tend to do.

You probably have two realistic options. Underboard the lot with an insulated plasterboard. It need not be particularly thick - say 30mm of insulation - but that would make a reasonable difference and also reduce any thermal bridging at each stud.

or, strip the existing plasterboard and then do as above - taking the opportunity to inject expanding foam into any board gaps, and tape up joins if required.

You could always chop out a bit of PB and investigate the construction before committing to a course of action.

All these things are DIYable if you are prepared to have a go, but it does depend on how much time/effort/expense you want to go to. There is a bit of detail on roof insulation on my loft conversion page:

...

This one is the best compromise between cost and function ...

You can get special clips that would allow tightly-fitting celotex to be mechanically attached between the rafters, leaving a gap between insulation and sarking felt. Then overlay the whole lot with celotex sheets going the other way. even 25mm thick will help and won't affect headroom. Then finsih with pasterboard. foil tape all the insulation joints plus the perimeters.

and don't cut holes for silly downlighters.

It's about comparable with the patches of ceiling I see elsewhere with no loft insulation behind it - ie the loft is warm because of heat flux from other rooms, which means the temperature differentials aren't so obvious. For exmaple, on the loft side of one of the (uninsulated) hatches the thermometer reads 15C, when the outside temperature is a few degrees. Being a cold loft, if it was a perfectly insulated ceiling I'd expect the temperature to match outside.

There is indeed sarking. It looks shiny, so I'm not sure how breathable it is (I don't know if the roof has been retiled since building in the 60s but it looks in good condition).

I thought some ceilings were asbestolux in 60s/70s?

Ah. So assuming it's sealed that would suffice? And if the board isn't butted right up against the tiles there's some airflow for the rafters?

Yes, I saw the wiki article warning about spray foam insulation on roofs. (and lucky I don't have a canoe)

I should probably invest in one of those USB endoscope cameras and take a look...

Thanks... some useful food for thought. I should do some more investigations - after I've fixed this week's task of a flat roof leak :(

Theo

Only way to be sure will be to look...

Possibly, but not much advantage under rafters (between floors might make more sense to suppress the travel of fire).

Normally you would have ridge and soffit vents to provide airflow, then battens fixed to the sides of the rafters to prevent the insulation boards seating too far back in the gap. That way you get a complete thermal envelope in the loft, but with the outside of the rafters, and tile battens still ventilated.

Even chopping out a 4" square of PB for a better look leaves an easy to patch hole:

Yup once you know what you are dealing with you can decide what the best option will be.

Having done some more investigations, I've managed to burrow my way through the insulation from the loft behind the wall. It turns out the 'cold' bays are mostly uninsulated with some rockwool pushed right the way up to the apex, while the 'warm' bays are full up with rockwool, with no air gaps.

Some of them will be tricky due to conflicts with other things, but the plan for the accessible ones is to hook out as much rockwool as possible and ram up some 50-60mm celotex (120mm rafters). I might try and make some celotex spacers to keep an air gap underneath the sarking.

I wonder what's the best way to stick small things to celotex, without tearing the foam...

Theo

I just ripped down some narrow strips of wood about 10mm square, and nailed them to the sides of the rafters. That way they stopped the PIR foam pushing right back against the sarking.

impact adhesive, and a bit of polyurethane glue, or "board fix" expanding foam.

hot glue