Loft conversion insulation - help for an ignorant person please!

[crazyhead]

We're about to convert our loft - our excellent builder has a slot to start quite soon so we need to resolve some issues quite quickly including insulation.

The roof space of the loft is quite high (our house is a fairly tall-ceilinged 3-bed 1930s terraced house) The floorspace of the floors below is 610sq ft, so I guess the size available will be that, minus all the restricted head height parts and the insulation. We'll be doing a dormer conversion and we want a bedroom (maybe two, but prob one) and a bathroom. DH and I want the conversion to be a nice area for us to have a little privacy away from the kids, much as we love them, so it needs to be well insulated for all year use.

The dormer will be south-west facing, and we want to put in French windows.

Can anyone give me some advice on what insulation is best? I understand that it takes up loads of room - so what do you need to make it really temperature controlled in summer and winter - and are there high-quality space-saving options? Should we replace all the house tiles and put insulation under that as well as internally?

Also, how about windows? I'm assuming Velux with build-in blackout blinds, but do we need triple-glazing/anti-glare glass finish/blinds in the French windows given it will be south west facing?

Thank you for any help - I don't know where to start!

your architect or draughtsman will have this covered. It is very common. To get Building regulations approval, you will almost certainly have slabs of rigid foam insulation such as Celotex or Kingspan between the rafters, with another layer pinned to them on the room side. It is possible to get a bonded product of plasterboard and insulation together. There will be a moisture barrier to prevent warm moist air from the room getting into or behind the insulation. There will be a ventilation gap on the cold side of the insulation and under the felt or membrane. If it is not a breathable membrane under the tiles, you may need some tile vents or ridge vents to keep the gap ventilated.

The foam slabs have about double the insulating power as the same thickness of mineral wool, so can be thinner (but they are more expensive)

Your knee walls need to be insulated to the same standard.

Brilliant - thanks so much PigletJohn. Presumably the architect will tell us what to do with the windows too?

We're not bothered if it costs a bit more if that is the difference between the loft conversion working well or not - what I find really hard with building work is knowing when you should take the more expensive option and when it makes little difference.

If you have an 80-year old roof, this would be a good time to have it replaced. It will make the insulation and ventilation easier. If you can afford clay tiles they last longer and IMO look better.

It will be depressing to have a roof leak after you have done the loft conversion.

If you have any gable walls, tell the roofer you would prefer a dry verge. The alternative is poking mortar into the gap, which falls out.

You will need ventilation of the loft room on at least two sides so you can get a throughflow of air. Water vapour is lighter than air so will rise through the house into the loft. Have trickle vents on all windows and a powerful ducted fan for the shower.

Thank you - that's massively helpful and I'll follow your advice