We have loft insulation about 6-8 inches think of the itchy stuff but in a pink bag at the moment. Just aquired a load of B&Q vouchers and noticed they are doing massive discounts on loft insulation. Is there any benefit in putting another layer down or is the money better wasted elsehwere?

Based on my latest calcs for my new house:

K Depth mm R
Rsi 0.1
Rso 0.04
Ra 0.18
R1 Roof tiles 0.85 20 0.02
R2 Felt 0.50 3 0.01
R3 Insulation 0.044 300 6.82
R4 Pasterboard 0.16 15 0.09
Sum 7.26
U Value 0.14

roof has a target U value of 0.16
300mm of rockwool just beats the target.

EDIT: Sorry, tables dont work in phorum.
Anyway, 7" gives you a U value of only 0.22

I've 8" (snigger) with some loftboards on top, I think any more than that and you're not going to see a lot of benefit

B&Q have their insulation this cheap every year. They have 15% off if you spend over £50 tomorrow onwards (but they do that quite often too).

this

http://www.energysavingtrust.org.uk/Home-improvements-and-products/Home-insulation-glazing/Loft-insulation

suggests going 5cm-25 isn't a huge benefit, so I recon 20-30cm is even less?

I'd waste your money on some spades, and build a nice trail. Or, some loft boards, board it, stick the wife up there and turn the dining room into a bike workshop

Most of it is already floorboarded. Double garage with beer fridge and sauna for work on the bike. I live a gifted life.

😉

keep an eye out on Money saving expert (or Npower) as Npower do a thing of £1 per roll of rockwool insulation.