how thick should loft insulation be?

[hojazz]

as above

[raysparx1]

Reccomended 270mm which is about 10 &1/2 inches

Think that is correct.

[gasman]

The current recommended level of insulation to meet part L (Energy Efficiency in Homes )of the building regs should be 10" (250mm). Thermal insulation legislation requires that Insulation (when not underfloor) is cross layered, the first layer is laid between joists at specific widths, with a second layer laid cross wise to minimise cold bridging from timber joists. You will save approximately �100 per year when you have a fully insulated loft. Compared to homes with no insulation.

[kwicky]

I do not disagree with the answers above but to achieve a really warm house you need cavity wall insulation.

[spamtractor]

It should have an IQ of no more than 95. Anything above this would make the loft insulation too intelligent.