Broadback wrote:

As I see it the professional installers will do it the cheapest way for
them, that may not be the best way.

begin rant Very possibly, & some will come out to see how much you
can afford, but with respect, you have been asked for a few very basic
figures about your attic. Having those nos in hand would have made
it far easier to visualize your problem and offer pertinent
observations. Again with the very greatest respect, if you do indeed
find simple measuring difficult (I'm not so nimble with the figures
myself these days) then it might very well be in your best interests to
turn the job over to a commercial firm. end of rant



Though of certain years thankfully
I am pretty fit and still do all my DIY, at the moment I am completely
refurbishing my daughters house. Ideally I would like to decide which
material to use, even what materials are available.

Good. I shall assume your 1960 built bungalow has a plan area 11m x
8m;the ceiling joists are 100x50mm on 400mm (aka 16ins) centres and
the whole ceiling is normal 12mm plasterboard & there are no dormer
lights, and there is no cold water cistern.

Answer first. Snags second. Alternatives third.

By far the most common solution would be 100mm thick
rockwool/fibreglass rolled in to fill between the gap between the
joists. Standard fibreglass rolls 400mm wide are an overfit which is
actually a pro not a con - see below. Insert the roll to make a flat
bottomed U - tight to the joist sides & a slight dip at the middle top,
avoid squashing the material. For other joist centres fibreglass
easily slices longitudinally with a sharp kitchen knife or carver, but
keep to a tight fit.. Then another 100mm or 150mm thickness rolled
across the joists (at right angles to the first layer).

Pros:the method is so boringly common that if & when you come to sell
your home no one will bat an eyelid. It might even get extra ticks by
being 200mm or 250mm thick. Plus the materials are readily available
in almost every shed and builders yard, generally at reasonable prices.
A further plus point is that fibreglass holds up well in fire - in
marked contrast to some other materials (eg polystyrene).

Cons: You have CH & HW pipes and electric cables in the roof space and
the space is partly boarded. Fibreglass type materials are decidedly
itchy and I for one don't trust it. Handling it always leaves me
itching especially around my cuffs and the dust leaves me with a
ticklish cough for days or weeks on end. It is all too reminiscent of
asbestos, which as you will remember, I'm sure, was never going to do
anyone any harm. You are also missing a cheap money saving trick (see
later). On top of that, not having an attic usable for storage might
turn away potential buyers - me for one. You have also hindered
maintenance access.

If you do decide to lay your own fibreglass (rockwoool etc are all just
variants on a theme using different silicate sources) Get one of those
cheap throwaway paper-ish coverall suits; a face mask; all enclosed
safety glasses, gloves (no air gaps) & a balaclava style head covering.
Use parcel tape or duct tape to seal up gaps in the outfit especially
where bare skin shows through. Which gives another con: you'll be
dripping with sweat after a few minutes in the attic, so maybe the job
is best done on a cool day in Autumn, just before your CH comes back
on. Chuck all the gear away when done.

----------------
So time to take a step back and see how things can be improved.

It helps enormously to understand 2 numbers called U (or U-value) and
R. Of the 2, U is the simplest to understand. Using it you can work
out immediately how much heat leaks out through the wall or ceiling &
thus how much of your heating bill is going through that wall or
ceiling. U x area x temperature difference = size of heater required
in watts. The lower you can make U the lower your heating bills.

Example:if your bungalow has just half inch (12mm) plasterboard
ceilings and nothing much else between you and the stars, it would have
a U value around 13. On a temperature difference of 20deg C (eg inside
=18, outside = -2) it will take 1150watts (1.15kW) (ie U=13 x 20deg x
11mm x 8m) to make up for the heat loss through the ceiling alone [ nb
more goes through the outside walls]. An elec fire of 1150 watts on
all the time would put about £250 on a quarterly elec bill. Make that
200mm of fibreglass and U drops to 0.18 and the bill drops to £4 or
less.

You will see R=0.035 (or thereabouts) printed on the wrapping of many
fibreglass rolls. R is thermal conductivity. R measures heat
transmission through a 1m square block of material 1m thick. U can be
calculated from R but it is a little tricky when there's several layers
with different thicknesses and R values.

Some typical values for R from a table I've just consulted : fibreglass
0.040; polystyrene 0.035; timber 0.14; plasterboard 0.16. The lower R
is, the better the material insulates.

How do insuulation materials work? Heat is lost upwards mostly by air
convection and radiation. In most cases insulants work by trapping air
(which is quite a good thermal insulator) in small bubbles and thus
stopping it forming convection currents. They also form a mass block
to heat radiation, but OTOH introduce material which conducts a small
amount of heat. If you look at values of R for the common products on
the market most are in the range 0.030 to 0.040. It's an indication
perhaps that there are basic physical limitations to further
improvement in R from trapped air insulation methods. ie you won't do
much better insulation-wise by using a different material. The
implication is that for more insulation you just have to pile it on
thicker. There are some semi-rigid sheets (mainly meant for
refrigeration use) about which go down to R=0.020, but you'd need to
cut accurately and seal any air gaps. Perhaps a hybrid solution could
be tried, but the sheets tend to be expensive and once you've piled
fibreglass on top, there won't have been much saving in finished
thickness.

From time to time various pour in fills based on pellets have been
marketed (mica and perlite come to mind), but don't seem to stick
around - perhaps due to cost? They do however offer a low irritant
level. There might be a containment problem.

Polystyrene works by trapping carbon dioxide (that well known
greenhouse gas) in small bubbles. It can be fire proofed though not
very well in my opinion, indeed I'd rule it out due to its poor fire
properties. Again you have the trouble of making rigid sheets fit
tightly in the joists. OTOH it does offer low level irritancy.

Look at the R for timber - when you have done the first fill between
joists, it is an indication that the timber joists are now the leakiest
part heat loss-wise. That is why the top layer is rolled across at
right angles.

---------

Next point is overall roof design. There's two basic types of
insulated roof - warm and cold. If you want to use the space for an
attic room then you need a warm roof and insulation must be put up
against the underside of the roof. However, the predominant fashion
now is for a cold roof. This means insulation lying directly on the
ceiling and cold air is allowed to circulate in the atic space. The
claimed advantages is that roof timbers do not dry out and are less
liable to warp and rot. That seems to be what you'd like.

The disadvantage in your case is the presence of electric cable and
water pipes. If you insulate this roof really well then sod's law says
that some part of the exposed pipes will freeze in a cold winter
(because you no longer have heat rising through the ceiling keeping
your attic frost free). Remedy is to specifically insulate all the
pipe runs prior to fibreglass installation with a good pipewrap.
Climaflex (made of a semi-rigid foam - www.nmc.com) is perhaps the
commonest in use at the moment and is easy to use. Paradoxically small
diameter pipes need thicker insulation than thick ones, however don't
stint on the thickness: thick climaflex is cheaper than a burst pipe
and will repay with lower CH bills.

Ideally additional ventilation should be added to the roof - commonly
cold roofs are vented at the eaves and near the apex. I doubt you will
want to go that far, but by not doing so you run a small risk that your
roof will distort.

Your other bugbear is the electric cables. These must NOT be covered
by insulation. On at least one side they should lie on material (eg
timber or plasterboard) with a high R. On other sides they need a
clearance of at least 50mm (2ins) from any insulation material. The
problem is that heat generated inside the cables cannot escape through
the insulation and leads to the PVC insulation hardening. Also NB
contact with Polystyrene rots PVC FTE cable. If you neglect this you
will end up (sooner on an old house) with an expensive rewire needed.
You should also check that cables are kept a distance away from CH & HW
pipes.

Draughts in the roof space will generate dust from the fibreglass,
which in the worst case could blow all your precious fibreglass into
the world outside (and has been known to, though that does take a
decade or 2). There was a suggestion in this news group a few years
ago of spraying the surface with a dilute PVA solution to give a hard
sealing coat. Could you make up the existing joists to 200mm with
timber noggings & then put back the flooring? You'd have to run all
the fibreglass in the same direction, but the up side is that you'd
have an attic which remains usable for storage; and perhaps equally
important you'd retain easy access for maintenance, plus the fibreglass
dust is boxed in.

Almost done. Now, that trick that's been missed. You can cheaply
reflect back some of the heat being radiated through the ceiling by
laying ordinary kitchen aluminium foil on top of the ceiling
plasterboard. Lay it shiny side down, avoid crinkling it, and hold in
place by a few dabs of glue. The final insulation (fibreglass, mica or
whatever) goes on top of the foil

If you can get all of that free (well not really - I pay Gordie Brown
for it, & Gordie passes the money on, after commission, to your
installer), why not try?

After it is done you can check that the pipework & cables have been
correctly dealt with and improve the job if you wish.



Experience tells me
that my joists will not be the distance apart to fit standard rolls
(sod's law)! What about loose insulation material or other than fibre
glass, surely they do not all have the same insulation qualities,
therefore need not all be the same depth.

begin final rant But you asked for advice and are now making
assumptions as to what that advice should be. Your assumptions are
substantially incorrect. I'd wager you have joists on 400mm (16in)
centres as that was more or less standard for decades until trussed
roofs came in, but you do find exceptions. If you had provided the
basic dimensions it might have been possible to suggest ways of mixing
and matching standard rolls to your attic. Oversized fibreglass rolls
tend to trap more air at joist/ceiling corners = better U. There isn't
that much difference in R between materials it is the other physical
characteristics + convenience + cost that decides the issue. For a
given final U, depth of insulation will be much the same..end final
rant

HTH