Have to get some loft insulation for between the rafters for our attic conversion. The architect has specced 100mm for under the floor but the joiner suggested getting thicker than that as it would provide more sound insulation. Will it make much difference? Will it be worth the extra cost?

Loft insulation isn’t really useful as sound insulation, making it thicker will make almost no difference for this purpose. It will of course, make your house warmer so I say get the thicker stuff if you’re doing it.

you need to make sure there’s still space for air to circulate around the entire inside of the roof void.

I’ve used rockwool(as opposed to fibreglass) which is intended for sound insulation and it works quite well. I’ve used 100mm which is just about adequate but wouldn’t be if anything noisy went on up there.

Get 100mm acustic stuff if you concerned about noise, the roof should be insulated with celotex or similar anyway between the spars if you doing a conversion. At least the ones I’ve done have been

What flooring are you having?

Just had a look at the plans and they were changed from 100mm to 150mm. There will be Kingspan between the roof joists. Was looking at Wickes website for prices and they are cheaper than B&Q but it is their own barnd stuff rather than Knauf which is what is on the plans. Will the own brand do the job just as well?

The own brand stuff is more than likely the same stuff, b&q stuff was made by knauf last time I got any.

The insulation specified in the floor is to be 100mm to meet the building regs, 150 will be fine, just don’t put in so much that it ends up compressed

Avoid Kingspan rigid insulation as youll pay more for it, try ringing apex insulation for a price to go by then hammer your local merchants. Also when placing it between the rafters cut it tight (so it holds in place) and keep it flush with the underside so as to allow the airflow above.

Under Part E (Sound) the min requirement for absorbent material within a new timber floor construction is 100mm mineral wool – min density 10kg/m3

See if you can find rockwool at BandQ. They start to roll it out at £3 for 4.8m2 100mm at about this time of year.

If your ceiling on 1st floor is not 30min fire resistant, you will need to put chicken wire between the floor joists to hold the rockwool up in the event of a fire.

Between the rafters, I’d put 50mm celotex or kingspan, and overlay with multifoil (tlx silver or similar).
Linky for installation detail

Good prices to be found here Seconds and Co. I got kingspan and foil from them. Call them and ask for cheaper alternatives to the expensive branded multifoil, and then check that your building regs will approve the make. Saved me a fortune.

Multi foil, oh dear, that’s a contentious one. As far as I know only one multi foil has an agrement certificate and the figment details are battened out to a 25 mm void either side of the foil so it takes up a lot of space. The certificate also says that on its own it will be insufficient uvalue to meet the regs. Personally I’d avoid it like the plague

Some independent building control surveyors will accept it no trouble at all, one manufacturer even gives a list of independent surveyors who will accept their product. Make of that what you will