Looking to insulate the roof. The loft is boarded so any insulation is going under the roof which is has new felt and tiles. Optins seem to be limited. The wool stuff in bags which is sold in strips seems to be in limited widths and not wide enough for me. Whilst using that would considerable increase the insulation the strip left might be liable to condensation. The poly styrene is horribly pricey. if it came in the width to fit between the timbers we need 130 metres of it. Boarding it out with ply and packing the gaps with Rockwool is an option I suppose but costly. The gap between the timber vary from 15 to 17 inches and a bit more where the dormer meets. Any ideas .

Netlon stapled to rafters, glass wool packed in behind it.
Remember you’ll need to vent the roof space though…..

jam some xtratherm rafterloc in between the rafters.

http://www.ashbrookroofing.co.uk/product.asp?id=3

leave air gap between xtratherm and the felt though.

Lift boards, insulate with rock wool, refit boards. Cheap and relatively easy if you are even remotely handy and have a cordless drill

Not a personal recommendation but read about TLX Gold used in a project. May work….

If the loft isn’t properly fitted out, insulating beneath the tiles won’t do a thing. The ventilation at the eaves brings the fresh (cold) air in. You need to lift the flooring and insulate there.

I don’t rate it but you could use a multi foil insulation like YBS super quilt. Otherwise use netlon or polythene sheeting to hold mineral wool in place

any insulation that follows the pitch must have a min of 50mm ventilation space above insulation and the appropriate vent tiles or eaves and ridge vents. Otherwise you will have condensation forming over time which will degrade the roof timbers.

I would do the ceiling joists

be wary of multifoils as a lot don’t comply with current legislation.

INSULATION: ROOF INSULATION IN THE YELLOW HOUSE

75% of the new insulation was inside the roof. Because there was no underlay behind the existing tiles, insulating materials had to be waterproof and so only ‘Celotex’ PIR sheet insulation was appropriate. Building control required a 50mm gap behind the tiles for ventilation, so the maximum thickness possible between the 100mm rafters was 50mm of insulation (or equivalent).

This would have been sufficient to exceed current building regulations, and most conversions go no further. We wanted to double the existing standards so we decided to add further sheets of 35mm thickness laid across the rafters on the inside- a small loss of head space for a large gain in insulation. What is more, the additional sheets prevent the rafters from acting as cold bridges (above).

Another reason for laying an additional sheet across the rafters is as insurance against the loosening of the insulation. It is hard to obtain a perfect tight fit of insulation between rafters, and it is likely to be further loosened as the rafters shift with time. Once loosened, cold air may be able to force its a way around, compromising the insulation.

After fixing 50mm of PIR insulation between the rafters all obvious gaps were filled by polyurethane foam from an aerosol can (not good environmentally, but a justifiable vice). Then sheets of insulation were laid lengthwise across the rafters and similarly sealed. 9mm plasterboard sheets were then laid narrow side on across over the Celotex and nailed in place. The aim was to create a patchwork of layers without adjoining gaps. The final roof had a u-value of less than 0.2, compared with the u-value of 0.35 required by building controls.

It is important to remember that the walls in the loft adjoining the neighbouring attics are also effectively external walls as neither of the attics on either side is occupied. In this case any insulation will abut into the loft space, so it needed to be thinner than we would otherwise have wanted. We opted for a good performance thermal plaster board which projected only 4cm. With the clinker block wall, it gave a pretty miserable u-value of 0.49. Fortunately these side walls only had a quarter of the area of the rest of the roof.

The roof above the extension could not rise significantly above the level of adjoining roofs in the terrace and so, with space again at a premium, expanded PIR sheeting was the best option. The extension is on the cold north side of the house, so we specified 100mm thickness under the turf roof.
Source: http://www.theyellowhouse.org.uk