Cut two strips of hardboard slightly narrower than the distance between the joists. Ensure you can slide them down the gap. Make a sandwich of hardboard strip, Rockwall (thickness to suit gap), hardboard strip. Smooth side of hardboard inwards. Feed the sandwich down the gap and then extract the two strips of hardboard to use again.

in architectural jargon the sloping part is called a skeiling. insulation of this is a common problem and has been covered a number of times on this site. as above, you must leave a ventilation gap of 50mm or equal, what i have done is to push down lengths of foam pipe insulation until the space is filled completely as this provides its own ventilation.
you can, of course, also add to the underside of the existing skeiling with battens/cross-battens to you desired thickness, fill with insulation of your choice, and plasterboard over. as the eaves is a point of major heat loss in normal i.e right angled ceilings, one solution is to create a false skeiling in to the right angle and insulate inside this. this forms an attractive coffered ceiling effect (but might be oppressive in small rooms).
another method of filling the skeiling void might be to break up re-covered polystyrene packing into largish chunks (say 50mm uneven) and drop/push into the gap, this would insulate and ventilate but in a less certain way.

its what is called a compromise, sometimes the only way.

Is there a legal requirement for a 50mm air gap. it always seem excessive to me
tom

Polystyrene beads can be bought from Sheffield insulation by the Cu. m. I would not reccomend using them for this application, because of their size (~4mm diam.) and their lightness, they will blow about and constantly find their way out on to the street or front garden.
I have cogitated on this problem too. The big problem is that the top of your lathe and plaster ceiling is as rough as old boots, complete with some nails that have missed their target and are sticking up into the void. I was thinking of using Workaholics technique with a foil type insulation. I think that it will be difficults to extricate the hardboard bits without pulling out the GF. Also you can't hold the end of your GF while someone else pulls the HB away. Hopefully the foil will "slip" out between the hardboard and some can hold the lower end (or tack it some where convenient). Trouble is it'll cost me £200+ to buy one roll, just to try out a 2m strip.
Frank

Nebuchadnezzar - I have exactly the same problem as you except that I have a dormer bungalow so the area of sloping ceiling is much greater.There are 50 mm thick slabs of Celotex between the rafters in the sloping part of the roof already witha 50 mm airgap above; however I am not convinced that these are a good fit, as occasionally a slab will slide down into the attic space either side of the dwarf bedroom walls! I intend to counterbatten across the existing sloping ceiling with 50 mm x 50 mm timber, put in Rockwool batts and put new plasterboard on top. This way avoids having to take down the old ceiling, which strikes me as a particularly messy thing to do. Obviously this method will reduce the headspace a bit but the rooms are so big I don't think it is a concern, especially as they are only bedrooms.

Skywalker - many thanks for your comments. Yes, as I said I suspect that the Celotex in the sloping roof part may not be brilliant, because if it were a good fit it would not be slipping down. However I do have a plasterboard ceiling rather than lath & plaster.

The reason for proposing using Rockwool slabs was because I was going to choose one of their "acoustic" slab range as these apparently have almost as good U values as the straightforward thermal stuff but have the advantage of cutting down noise. When it rains and/or the wind blows here it is very audible in the bedrooms in the dormer, the noise being exacerbated by the two large Velux windows in each of the two upstairs bedrooms and one on the landing! Do you think that the foam backed plasterboard would have the same sound-deadening effect? (BTW: What brand of f/b p/board are you using?).

Would it be a good idea to use the f/b p/board on the dwarf walls too? The back of these currently has 50 mm Celotex wedged between the uprights but of course there is a massive purlin running across the tops of these with probably significant amounts of cold-bridging going on.

I am in the process of making some mock-ups for what will effectively be triple glazing for the Velux windows i.e. clear acrylic sheet in a timber frame which will fit just inside the window reveal. I want to see if this will make much difference to both the rain noise and heat loss!

Jeff B.

I've not looked into sound proofing & have limited knowledge to share on it, I'll leave that to others (I also like the sound of the wind & rain!).

Depending on how your velux windows were installed I would be more concerned about how well they have been thermally insulated. I'm installing a couple as I move round the roof and unless carefull attention is paid to properly insulating around the frame/under the flashing these could represent localised significant heat loss. The noise you get could be transmitting through the flashing with nothing but air between the aluminium and your plasterboard.

If you are insulating the inside of your room you do not need to worry about the purlin. In my case I have desided to make my entire eaves space 'warm' as we use it for storage and it is effectively part of the upstairs rooms anyway (doors in the wall). Also it is the space above a significant part of our downstairs rooms so rather than insulate the eaves floor and the the back of the wall below our purlins (& trying to get all the joins right) I have opted for between & over rafter insulation from the wall to the ceiling of the room. All I'm doing is running the boards above the purlin continuously and joining the plasterboard/insulation laminated boards to the purlin tightly (fast & dirty photoshop pic attached) using foam/plaster for airtigntness & relying on the mass of the purlin to counter bridging.

I'm using normal kingspan for between rafter and the plasterbaord laminate is put together by my local insulation suppliers as Kingspan want you to use their new roofboard stuff which is pricey. All I have found is that I need to do a small cut along the edges to get them properly square for a good join although in practice I have to cut everything as I am fitting into the existing woodwork. If you are using battens you won't have to faff about like this. I'm not sure that there is any material difference between the different brands on the market (others may know more) just make sure it is the same kind of foam (same technical U value).

If you have the headroom rockwool is cheaper I think but bear in mind that with cellotex/kingspan you will only need about 1/2 the thickness for the same level of insulation (& it is much nicer to work with). If you want to be green have a look at thermafleece which is in a different planet in terms of carbon footprint BUT also in terms of price.

Hope this helps (& my picture works!)

S.

Tony - thank you for your comments. The reason for choosing clear plastic rather than glass for the extra glazing is simply weight. The Velux windows are 1200 x 1200 mm square and are built into the sloping ceiling (which is at about 45 degrees). Hence it is extremely difficult to put anything too heavy into the reveal or attached around the edge of the reveal, particularly as I want the extra glazing to be demountable in the summer months. A further complication is that the upper half of the Velux window swings into the room when you open it so nothing can be inserted in the reveal itself (I know that sounds contradictory to the previous sentence but there will be some days in the early spring/late autumn when it would be nice to be able to open the window during the day when the heating is off as it gets "stuffy" upstairs).

The acoustic variety of Rockwool is denser than the thermal which is presumably why it is more effective at noise insulation and is only marginally less effective thermally. Skywalker has suggested that I use foam backed plasterboard instead, so I'm having a think about that now.

Jeff B.

I used Warren insulation as the they have a depot I can collect from just down the road and they were cheaper than other local companies. There must be a similar company in Swansea of Llanelli etc that could do the same thing or supply you with whatever you go for much cheaper than a normal builders yard/B&Q and deliver as well.