"RichardS" wrote in message
...

Well, I'm running a sadistic hardware experiment at the moment, running 2
pcs and an ultrasparc in the loft. Despite my long-standing must-do job
of
putting insulation in the loft it remains unstarted, so I think now is as
good a time as any. (If I have a burst of energy tonight I will probably
cave in and bring the poor machines back down into the office).

It's a victorian terraced house with standard roof construction (the name
escapes me at the moment, but they're not trusses).

My plan is to install as thick kingspan or celotex between the rafters as
I
can get away with, whilst leaving a ventilation gap of 50mm between the
insulation and the sarking/tiles. At the apex I will sacrifice a little
headroom and fix some insulation horizontally so that there is a
ventilation
route connecting both sides and all gaps betwen the rafters. At the edges
I'll leave a small gap & install vertical insulation so that there is no
airgap underneath & into the loft, but ventilation is maintained from any
gaps in the soffits up to the rafter ventilation space. Everything will
be
sealed as far as is possible.

There are 2 air vents per roof face, about 1m from the apex.

Does anyone see any fault with this plan, and do I need to install any
additional vents (if so, how - I really don't fancy climbing on the roof)?

The best way is to install vent ridge stiles at the apex. About 4 of them.
Install Kingspan in the space between the rafters leaving a 25mm minimum gap
from insulation and the underside of the tiles. Put insulation over the
rafters, then boarding over the insulation right down to the ceiling of the
bedrooms. Seal all this up, so it is air tight. Inside put a small flat
ceiling at the apex about 12" wide with insulation on top of this.

What you have is an air vent from the eves to the ridge tiles. The void at
the apex will allow air between rafters that have no ridge tile above to
flow into the space and out the nearest ridge tile.

Lay a vapour barrier on the loft floor, to prevent water vapour getting to
the loft. At least put a vapour barrier over the wet rooms. Make sure the
hatch is sealed. And have as much insulation on the loft floor as you can.
Board over.

You now have an air tight vented roof, with warm loft.