Rob Horton wrote:

Not sure of the correct terminology, but I refer to joists as the
horizontal wooden beams in the loft that the ceiling is fixed to and
what you actually walk around on. Rafters are the sloping wooden beams
that the sarking and tiles are fixed to.

Yup, that would be my understanding also.

(1)
I want to add more insulation. Current regulations say 270 mm for glass
wool, 250 mm for rock wool, or 220 mm for cellulose. Are there any other
rollable insulation systems that have a higher level of insulation thus
needing less height?

Not that I am aware of. You would normally need to go to the rigid foam
board systems like celotex etc to get more insulation in less space.

(2) If I need to add height to my joists, (it's a small house and need
to use the loft for storage) can I lay wood strips (of the same
dimensions as the existing joists) on top of the the existing joists?
How do you fix them in place, glue and the occasional nail or screw?

If you don't need to store stuff in the space, then insulation just laid
counter to the exiting (i.e. across the joists rather than between)
will actually work better. No need for extra joist depth.

If you do want to floor it, then you can extend the joist height (screws
would be my favoured fixing since there is less chance of damaging a
ceiling with hammering). Alternatively you can use a roll insulation
between the joists, and a foam board over them, with the floor directly
fixed through it. If you only do a section like this is is not as
expensive, and it is quite easy to do.


FFFFFFFFFFFFFFFFFFFFFF --- floor boards
IIIIIIIIIIIIIIIIIIIIII --- say 50mm of foil covered PIR foam
##;;;;;;;;##;;;;;;;;## --- Existing joists with rock/iso wool between
##;;;;;;;;##;;;;;;;;##

(3) I have electrical cables in the roof, although these are for
ordinary electrical lighting. Is it OK to have these covered by the
insulation? The cables are already covered with insulation by the
previous insulator/owner.

Lighting cables usually have sufficient spare capacity that the
derrating effect of the insulation is not a problem (assuming your
lighting circuits are protected with a 5/6A Fuse/MCB). Take care that
other circuit cables are not buried though.

(4) Although it is an unheated and ventilated loft, will I get any
benefit from adding insulation between the rafters? I want to use some
thing that is enclosed in an aluminized bag so that it will (hopefully)
reflect some heat away during the summer.

Depends on whether you want have a "warm loft" - it may make for more
stable environmental conditions for things stored up there.


--
Cheers,

John.

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