On Oct 16, 7:42 pm, wrote:
On 16 Oct, 16:38, AJH wrote:





On 16 Oct, 00:21, wrote:

Looking to board my attic for storage; it has some scrappy Rockwool
and joists are 4"x2". I was wondering whether to plant 2"x2" timbers
on joists to allow for greater depth of Rockwool, or maybe the
chipboard would provide enough extra insulation? Another thought -
better grade insulation material in place of Rockwool but in the same
4" space? Any ideas welcomed.

It sounds as if you are asking 2 things:
1) boarding the loft
2) insulating the ceiling against heat loss

The best way to insulate your ceiling is at ceiling joist level not at
rafter level, that is for loft conversions.
4x4 joists should be fine for general storage as long as they are at
400mm crs and not overspanned. Heavier loads should be kept over load
bearing walls or near the joist ends.
If you have plasterboard ceilings you will have virtually 100mm depth
available so drop in 90mm Celotex, Xtratherm Ecotherm or similar
ridgid boards cut as a good fit between the joists. To seal any slight
gaps run aluminium tape across the top of the joists overlapping onto
the boards, it's fairly cheap for a 100mm wide roll. The just lay
boards over. Don't use MDF as it is heavy, go for building ply, it's
light and strong and if screwed to the joist tops will effectively act
as a diaphram and provide strength. Buy insulation at ENCON for best
rates.

I agree with AJH who had a better solution than mine
Chris

A bit of a U-turn from "To get storage on this you will need blocks
on the joists close to the bearings and run bigger joists on top -
quite expensive and not a good structural solution and even then
limited to loading

to get a good loading you need a good well-engineered structure"

MBQ