Andrew Heggie wrote:

On Sat, 31 Jan 2004 13:34:21 -0000, "Peter Taylor"
wrote:


Andrew - the design of your roof, with bricked up eaves and sloping ceilings
sounds just like mine. I understand exactly your difficulty.

I am not clear what it is you are intending. Are you just wanting to upgrade
the insulation,


Yes, I am looking at increasing insulation in general, as the loft has
a smaller "footprint" or net area than the whole house (because the
roof forms part of the bedroom ceilings) it's too small to make more
than a box room. ATM it is piled with junk.

Unless you are stripping and reroofing completely (in which case insulation
above the rafters is better),


Yes I see this is the best solution, from our previous dialogue, but
too expensive and not yet needed.


Celotex between the rafters is definitely the best
route for you.


As I thought.


Only if you want a warm loft space. Otherwise why not use the ceiling as
the iinsulatin barrier. Less area to cover with expensive celotex. AND
you can use cheaper rockwool




It is the ONLY way to insulate the sloping ceilings.


This is what I thought, a sort of dry lined area on the sloping part
of the ceiling?


Thats true for teh sloping bits, bu don' feel you have to carry on on
teh slope.

My house is very similar - sloping ceilings and a flat bit in the middle.

What the BCO came up with was celotex up the slopes, with an air gap
above to allow eaves ventialtion to circulate, and then rockwool over
foil backed plaster board on the flat parts of the ceiling.

This works extremely well, now we have made sure that the rockwool is
stuffed right up to the celotex, and all draughts are sealed.


Its a LOT cheaper than celotex up the sloping parts. About 1/4 the cost.




You are probably not getting too much condensation now because of the low level
of insulation. If you improve this the temperature above the insulation will
drop and more vapour will condense, which could rot the roof timbers. To avoid
this the Building Regs require a clear air path 50mm deep above insulation
between rafters. This means if you have 100mm deep rafters you have space for
50mm insulation.


OK


Also bear in mind the celotex, properly applied, is a vapour barrier.
You will ONLY get condensation if warm moist air percolates up through
the ceiing into the roof void and meets cold timbers.

You can combat this three way - a warmer roof (wastes energy: what you have now)

Ventialtion, as decsribed, to remove the cold moist air
Pervent moisture movement at all, with a vapour barrier.

With insulation, the better the inpermeability of what covers the
ceiling (BEFORE the insulation please) the less condesnation there will
be. Then less venting is required to remove the rest.

With thick insulation, condenastion on the roof timbers is less likely
than condensation on the joists, since these will now be as cold as the
roof timbers were. Laying some membrane beteen them before adding
insulation is very effective at reducing vapour transfer.

Even bits of black palstic bag cut up and sliped between the joists and
taped together will be better than nothig, and there are products
designed for this as well.


Otherwise the rest of the advice is totally sound.




You can quite easily cut 50mm Celotex board to fit neatly
between the rafters and slide it down the sloping ceiling sections to meet the
brickwork filling the eaves.


I had wanted to avoid removing the lath and plaster, I had hoped to
"glue" an insulating board to the old ceiling and feather it in at the
edges.



Do you mean inside the room? You could do that with celotex.. Otherwise
pushing slabs down the top of the slopes is fairly acceptable, if hard
to get perfect fits.




Above this, in the loft, you can continue the
Celotex between the rafters, but also add further thickness underneath if you
wish.


Understood.


Again, I queston teh need to do it this way rather tha insulate the flat
portion of the ceilings.



For ventilation at the eaves you need to allow air to pass between the top of
the brickwork and the underside of the tiles. This will then be above the
Celotex. This can be done simply by fitting plastic over-fascia vent trays on
the top of the brickwork. It will be necessary to take up the tiles. Either
you can fit the trays over the rafters, which will raise up the tiles, or you
can cut out a small part of the brickwork and sit the trays between the rafters.


At this stage the cost outweighs the benefit as my fuel costs are low.
I can still not quite fathom why a ventilation path, to prevent
condensate settling on the roofing timbers, cannot be formed by
fitting the celotex in between the rafters as described but allowing
an internal gap at the bottom and top. a convection current would be
set up from the warm apex down the cold roof (under the tile+felt and
between these and the celotex) and then vented pack at the eaves and
over the ceiling??



I cant see where the gap at the bottom is coming from.

You need eacves vents of some sort, otherwise there is no way for air to
get to the base, unless you are going to let cold air run down the front
of the celotex, which meanas its not insulating the room anymore.

I am sruprsed there are no asoofits. However if ypou do as desribed and
lift the tiles at the eaves line - only by a little - maybe a couple of
cm - and then fit anti rodent mesh, the job is good enbough t comply
with regs.

Ypu shopuld have some high up vents as well - ridge vents mean replacing
a couple of ridge tiles, or vents in the gable ends.


Ventilation at the ridge is best achieved by having a ventilated apex section in
the loft, formed by making a "ceiling" of Celotex fixed to new 50 x 50 noggins
fixed horizontally between the rafters, like a letter A. The apex can then be
ventilated by ridge vents or vent tiles, or by IMM's idea of openings in the
gable walls.

I hope this is helpful. If you don't understand or want further help please
mail me - (remove NO & SPAM) and I can send you a sketch detail. A picture of
your house would be helpful if possible


I'll take a photo tomorrow and put it on a website.