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.

Dear Peter
You need to address the issue of the strength of the 100 mm joists and
their span wrt their storage capacity. If the storage is light and
next to the bearings (ie small objects or old boxes) then no problem
but there is not likely to be much use to the area with such
limitations. If you wish to store heavier stuff then you need to
provide mid span support or beef up the strenth somehow. Not to do
this will put you at risk of long term deflection of the joists due to
"duration of load" effects.
I concur with the others that your best route is to put in 3" of
Kingspan or the like leaving 25mm gap under the slates in between the
rafters and you can always add inside.

If this does not appeal on cost grounds and you wish to do the ceiling
joists regardless bear in mind the following
you need min 300mm rockwool or the like
you need to lay first layer with the joists and overlapping
next layer at 90 degrees
eaves ventilation facility
a vapour check under the insulation
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

Chris