In an earlier contribution to this discussion,
Mike Hall wrote:

I've been quoted £125 for supply and installation of 350mm loft
insulation in the roof of our 3 bed semi, (includes a grant based
discount). This seems like a remarkably good deal, and it doesn't
seem worth getting my hands dirty for this kind of money, (so much for
DIY!)
Before I proceed though, I've got a couple of questions that I hope
the collective massive can help me with:

The upstairs lighting cable needs replacing, (currently stranded
aluminium with no earth). I've seen this question asked before, but
have not found the answer as to; whether or not lighting cable fixed
to rafters on one side and then covered with loft insulation needs
derating. If so then I guess the derating factor is 0.5. My thoughts
are 'yes' that this counts as being totally enclosed in insulation,
but I have also seen it suggested that having the rafter on one side
means that it is *not* totally enclosed in insulation. - which is it?

Secondly, I will wish to board part of the loft after the insulation
is applied, and the insulation guy suggests putting the boards on the
insulation and screwing *through* the insulation to the joists. Does
this seem OK, or should I clear a path through to the joists for a
wood-on-wood joint? Also I read a lot of talk about vapour barriers
under the insulation, but am still unclear as to whether this is
really necessary or not!


I'll others to comment on the cable issues.

With regard to the insulation, how tall are the joists? If it's an old
house, they're probably no more than 100mm - if that. There's no point
whatsoever putting in 350mm of insulation if you are going to compress it
into 100mm. 350mm of insulation is only effective if it traps a 350mm layer
of air! Admittedly the boarding will add some insulation, but you really
need to pack the joists up as high as possible with something solid to
support the boards.

Having said that, I'm not sure what to suggest for packing - timber would
really be too heavy. Anyone know whether you can get aluminium (or maybe
rigid plastic) I-sections which are 50mm wide and maybe 250mm high?
--
Cheers,
Set Square
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