I have four roofs, and there is not one jot of insulation in any of them. The first one we're going to tackle is the main one (rather than on an extension), which is a pitched roof with dormer windows, and with two rooms in it which fill the space right to the apex of the roof.

A quick bash through a boxed in section to the side of one dormer reveals that we have 7.5cm rafters at 40cm centres, roofing felt and then concrete tile. Except in the boxed in sections at the lower edge of the roof, the plasterboard goes straight on the rafters. There appears to be a boxed-in steel joist running the width of the house on both front and back sections at the height of the top of the dormer.

Several questions:

1. Since head height is not too low, we were thinking of adding 200mm of rigid board insulation over the rafters. Considering we need to leave 50mm to vent the timbers, cutting 25mm boards to fit between them all seems an unnecessary level of faff. What do you think?

2. This would come down each side of the steel joist , but would obviously leave us with a massive cold bridge along it. Any suggestions for what to do here?

3. We're planning to bring the insulation/plasterboard right down to where the wall meets the roof, and hopefully down the wall a bit, so that when we eventually do the walls there'll be minimal cold bridging. Hopefully there's a picture of this area below. Again, what do you reckon?

4. This is up two flights of stairs, and we don't have a vehicle. In my happy dream world, we would leave the existing plasterboard up to avoid the dust, time and muscle involved in taking it downstairs and getting rid of it. I'm sure this is a bonkers plan, but can't think why, so please enlighten me.

Many thanks, Hannah

1. We were wondering how tricky that would be (as in, holding it above our heads), but also wanted to get as much thermal performance in as possible. I have to admit I'm rubbish with u-values and the like, but had an idea that we'd get better insulation in a smaller space with rigid board. I'm happy to be corrected though. Although I said headspace wasn't an issue, it would be if we used more than 300mm of anything.

2. So better to have the joist on the warm side?

3. Glad that idea works.

4. I can't tell you how happy that makes me! With the seperate VCL, how is it adjoined to the upright walls? I must look online for a diagram of that and the counter battening, as I'm much better with diagrams.

Many thanks for your help, Tony.

"I think in Tony's point 2 the steel ends up, ventilated, on the cold side."

I was trying to wrap the beam with insulation so that it was on the warm side.

I will have a look at the beam to check the details of it, probably tomorrow (far too much on today). I think I understand what needs to happen.

Nick, I realise that I explained the roof really badly. Yes, it must have collars as there is a tiny bit of flat ceiling at the top. We definitely won't be removing these, and if we go over the plasterboard that's there now, won't even have a chance to look at them.

So, there is some debate about whether it's better to have the joist on the warm or the cold side? Hopefully me opening it up and getting some pictures for you will help us to sort it out.

200mm of EPS and two sheets of plasterboard will give us a u-value of 0.17. I'm having trouble finding the regs to establish what the aim for a renovated loft conversion is (if indeed that's what the attic counts as). I found something for 2006 which said 0.16, so I guess we might need to up the EPS a little. Any ideas on what to aim for? (I know it's always the more the merrier with insulation, but there is a height limit which we'll hit eventually).

Thanks massively for your help.

Closer investigation of the rear joist shows that it's a great big timber beam, haven't rechecked the first one in yet (we were assuming steel because of the boxing in of the front one). A quick scout around the room with a tape measure has revealed that the smaller, rear room would lose about 2' of usable standing-up space for a person of average height, but gain storage from us unboxing the lower the parts of the walls.

I realise PU would give us a beter u-value with thinner insulation, but is PU the one with the pentane (or other gas) in it? I remember reading on some threads recently that once the gas seeps out (<10 years) they're no better than EPS, but again, I'm happy to be corrected. As we're planning to be here a long time we'd like to do a good, long-term job.

Off to inspect that front joist :)

So EPS does sound like a better bet in terms of consistent long-term u-values? Hope someone comes along with more thoughts on the matter :)

More measuring occurred this evening, and we have found one annoying spot on the stairs where 200mm will mean anyone over 6 foot has to duck. I'm sure this isn't something a BCO would approve of

Thanks, Jonti. More measuring today then!