Insulating between roof rafters

Read the OP, his house is was built in the 1930's and soffit and ridge vents didn't exist then, unless it's been re-roofed, which I seriously doubt due to the lack of underfelt.

They don't need them in that case because the loft is habitable, which makes the bedroom ceiling joists just like the downstairs ceiling joists, IE ventilation above and below with people coming and going, he's only thinking of using it for storage, it would probably only get opened every few months...temperature has little to do with timber rotting, it's ventilation that stops that happening.

That makes sense. There's a good page on wikipedia:

Phew! :-)

We're talking about insulating the sloping part of the roof so that heat coming up from the house warms the loft. You're thinking of the case of insulating the loft floor which can indeed create a cold loft. We had that problem where previous owner had added an extra layer of insulation to the floor making the loft space much colder. There was still enough water vapour escaping from the house that the colder conditions caused condensation on the roof.

MBQ

It has nothing to do with "habitable".

Exactly the same argument applies to a loft conversion that is closed off by fire doors instead of a loft hatch and only used by guests every few months.

No, you still don't understand. By insulating between the rafters and creating a warm loft you prevent condensation in the loft, thus preventing the timbers rotting. It has everything to do with temperature but it's the condensation caused by too low a temperature that is the problem, not the temperature per se.

MBQ

That way you make *most* of the loft warm but the external sides of the rafters will be cold Any moisture in the loft will condense in the rafters somewhere between the cold external side and the warmed internal side.

A real wam loft has the insulation on the outside of the rafters.

Actualy I agree with everything you say but if it was me I'd want add a vapour barrier eg foil backed plaster board to prevent condensation in the rafters.

Which is why you have to ventilate between the insulation and the roof, as we've been saying all along, to stop the condensation forming.

MBQ

Loft conversions usually have windows, so no need for airbricks.

Why do you think that old houses with floorboards downstairs have airbricks allowing cross-flow ventilation? - by your argument it's un neccecary with the temperature of the rooms above etc, except that if you block them off, (and you only need to block one elevation off to do this) the joist ends will rot through within a year or two.

I see, and how does dry rot start? - I'll tell you, it's caused by a lack of ventilation, as are most other kinds of wood rot, moisture and condensation has little to do with it....if he insulates over the tiles and seals the loft from all draughts, his roof timbers will rot within a few years

Phil L wrote:

Well there are various ways to achieve it, BUT the key is that you put a vapour barrier inside the insulation, and ventilate outside it.

With e.g. celotex sheets between the rafters, the foil backing of the insulation does the barrier, and you should use aluminium tape OVER the rafters. and an air gap ABOVE the celotex with ridge and soffit vents feeding it.

This is the standard way to retrofit insulation to a 'cold' roof.

Insulating above the rafters is fine, and can remove the need for vents entirely, but is hard to retrofit without stripping the roof first.