Is weight a problem when insulating between rafters

[allenium]

I have a loft that I use for storage and hence need the floor space. So I want to insulate between the rafters. I have a 1940's build semi with standard high pitched roof.

Because cost is an issue, I have been recommended to use the insulation on a roll (space blanket type) as this will give me the vapour barrier I need on the inside of the insulation. However using enough of this type of insulation will be somewhat weighty. Will weight be an issue? I have researched this on-line and a number of sites recommend plastic covered fiberglass rolls for this purpose, but no site that I have seen that talks about insulating the rafters, mentions anything about the extra weight.

Also having insulated I would probably want to plasterboard, which is even more weight. Will a roof stand up to all this extra weight?

For reference, my roof has 2 horizontal 8x2 inch supports, that the rafters lean on, these are in turn supported a either end in the brick work and at 3 equi-distant point by diagonal 4x4 inch supports to the centre line of the loft floor.

Any help / advice would be greatly appreciated