Andrew Heggie wrote

I am still trying to understand this loft ventilation concept, my,
semi-detached, house is 1862 build, probably re roofed after a fire,
and the bedrooms have a 60cm ceiling sloping to the eaves, there
appears to be no ventilation at the eaves nor any where the joists
join the rafters, visible when I pull back the tatty nominal 2"
fiberglass insulation to expose the area of lath and plaster. Mind
there appears to be no condensation. I have been put off using celotex
between the rafters because of your comments that ventilation of the
cold space above the insulation should be to the outside. Prior to
that I thought an internal circulation would be adequate, albeit
slightly less energy conserving than a warm roof.

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, or do you intend doing any work requiring Building Regs
approval, such as a loft conversion? If it's just upgrading then you do not
need to achieve the very low U values now required by Part L1 and you have more
options.

Unless you are stripping and reroofing completely (in which case insulation
above the rafters is better), Celotex between the rafters is definitely the best
route for you. It is the ONLY way to insulate the sloping ceilings. Also it
will allow you clearer access in the loft and remove the worry of frozen
pipes/tanks etc.

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

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.

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

Peter