Attic Insulation - Insulation board on ceiling side?

random_

Hi all,

Grateful for any advice on this one.

Moving in to my mother's and we need to maintain as much storage space in the attic as possible, needs to be fully floored and attic has a low pitch.

Attic trusses are just 4".

So we're looking at:
- Adding 50mm insulation board to all ceilings upstairs (which need to be re-slabbed anyway)
- Filling the 4" trusses in the attic with a 100mm layer of insulation, whether that be fibre glass type stuff or foil back board @ 100mm then flooring directly on top of the trusses.
- We might have to add 2 x 1 to the trusses to allow for ventilation.

Question is, will this provide us with a decent level of insulation? I know the ideal is to to go with 2 thick layers of insulation that cross each other, but this would raise the floor so much in what is a fairly shallow attic as to make it more or less unusable.

Grateful for any advice / comments.

BryanF

I'd be checking structural support before storing an great amount
Consider where the vapour barrier will go and how it will be sealed before insulating with slabs - this leads on to where condensation will form.
Also Consider the vapour barrier on the context of air tightness and whether light fitting etc will need to penetrate the layer, and how these might be sealed.

random_

Thanks for coming back to me Bryan.

Builder seems happy structurally that we are ok - there's nothing particularly heavy going up there, just a lot of crap, bags of bags!

In terms of the quality, do you think that a 50mm board ceiling side combined with a 100mm board attic side (or fibre glass), floored, will provide a decent level of insulation? - I mean, I presume it wouldn't be as good as going with two thick layers of insulation crossed as would be the norm?