We've moved into a 1974 semi in Forres, Moray, (see attached photo) which has - from a cosmetic point of view - a second floor with dormers front and back running all the way along. In reality the 'dormer' storey appears constructed as a second rectangular box on top of the ground floor, with the bits of pitched roof added last and with the lower parts of pitched roof (i.e. below the 'dormer' windows) being quite separate from the upper part, except at the gable end.

The loft ( https://flic.kr/p/2o7tmSb ) contains approximately a thimble full of insulation ( https://flic.kr/p/2o7oKWK ) between c. 175mm joists. Adding more insulation, and probably floorboards for storage, is straightforward enough, but the dormers are an issue as is the ventilation (or otherwise).

The black sheeting between the rafters and wood sarking on the pitched sections ( https://flic.kr/p/2o7tmSb ) is two layers of impermeable and not at all breatheable plastic sheet with a thin layer of polystyrene between. It doesn't quite reach the bottom of the loft ( https://flic.kr/p/2o7stah ) and the view looking up from the lower end, so you can see the polystyrene sandwich, is here https://flic.kr/p/2o7tMto . There appears to be no obvious ventilation in the loft apart from that accidentally provided where an old waste pipe once went through the gable end ( https://flic.kr/p/2o7ssXD ). I don't know what is above the wood sarking or whether it is breathable, but it is likely to be as built in 1974. Due to the 'dormers' there are no eaves here, so no eaves ventilation other than any that might exist at the front of the dormers due to the next issue.

(There is also some pink matting visible above the wooden sarking boards in https://flic.kr/p/2o7tmE2 but I suspect that is connected with the current or previous dormer roof and may not extend very far upwards.)

Due to the 'dormers' not actually being dormers, with the upper floor being really a box shape with pitched sections added afterwards, the spaces above the dormer ceilings are connected to the loft ( https://flic.kr/p/2o7r8ez ). This is the view along one such void https://flic.kr/p/2o7oKPf . From outside we can see that there are soffit vents around the dormers, so there may be a connection from these vents into the loft, but that's speculation/presumption as we can't see what happens at the end of these spaces i.e. the outside upper edge of the dormers.

In addition to the main loft I'd like to insulate the dormer roofs also, as they represent a substantial proportion of roof area. The dormer roofs where recovered in 2014, and based on the paperwork we have, the EPDM rubber sheet is on top of 50mm of Kingspan Thermaroof which is in turn on top of the chipboard which forms the top of those spaces that are visible from the loft i.e. https://flic.kr/p/2o7oKPf .

If the loft is indeed ventilated via these spaces and the soffit vents on the dormers, then I can't really see how the Kingspan above the dormers is doing much - as it lies above these presumably ventilated voids - and neither can I see the point of the (presumably original as it's between rafters and sarking boards) black sheeting/polystyrene combination in the main loft.

Insulating the main loft is easy but the practical conundrum appears how to insulate above the dormers whilst maintaining the speculated ventilation via the dormer eaves? At this point in time disturbing the plasterboard ceilings isn't a practical or feasible option.

Initially I considered pouring loose fill insulation into these voids, leaving a 50mm air gap, but without being able to see clearly what happens at the far ends, I'm concerned I may end up blocking the soffit vents. On the other hand, and since starting to write the above, I had a look at the easiest to access soffit vent, which pulled out and revealed not much ventilation at all, so perhaps I'm worrying about blocking an irrelevancy? See https://flic.kr/p/2o7uNzb.

I also considered coaxing rockwool into the spaces, but there are wooden battens and cables in the way.

Another option is to just insulate the main loft as best I can now, leave the dormers, and at a later date insulate the ceilings from inside the house, though that would loose headroom.

Flickr album at https://www.flickr.com/photos/87684784@N02/albums/72177720304689913/with/52580880272/

Currently the loft is dry and shows no sign of dampness. Despite the northerly latitude it's a relatively dry climate here, being in the rain shadow of the Highlands.

Ta in advance for any suggestions.

Another thought, the triangle of roof below the dormers tends to allow cold drafts in to blow right through the first floor joists across the house. You can lift a floorboard along the edge of the bedrooms to block this path with chunks of PUR foamed in.

The dormer front wall and cheek wall is literally just a stud wall. Needs insulation on the inside faces.

Having done that, the rooms were quite warm!