8 X 4 insulation sheets and underfloor insulation.

[BML]

I served an apprenticeship as a Bricklayer in 1950 and moved on from that when I finished my time never to darken the door of another building site so I know little of modern building techniques. I live in a bungalow that I refurbished over fifteen years ago and having used insulation on the pipes that went under the suspended wooden floor and in the loft I have never had a problem with the water pipes until last night. However, a temperature in English of five degrees Farenheight last night changed that record and an under floor pipe froze so it seems as good a time as any to ask the following questions.

First, insulating under the suspended floor. I have seen people drilling holes in the cavity walls of houses and then pumping some form of what they claim to be insulation material into those holes.
Does this stuff work and if so could one have it pumped under a suspended wooden floor to insulate the pipe work or would it compromise the need for ventilation. Having said that is there such a thing as insulation granules which while offering insulation would also allow ventilation.

Secondly, improving the roof insulation. The loft was well insulated with what I think was a form of fibreglass and there have never been any frozen pipes. However, I would like to improve that insulation and improve the appearance of the loft which because there is no roofing felt gets very dusty.
I have seen large 8ft X 4ft sheets of white insulation material about 100mm thick and I wonder if it would help the insulation capabilities of the loft to cut that insulation into sections and stick them between the roof joists.