I'm really sorry to hijack this thread but would it be OK if we widened the scope to include not just lofts and cavity walls but any component (floors, walls etc)?

Some of the threads from this discussion appear to have been removed. Tony - yours seem to have gone!!

I have since seen the CAT email so would be interested to hear your take on this issue on Monday (morning if possible) although the essay is pretty well finished.

I added loft insulation when we moved to get up from 75mm to ~300mm. I also made a 250mm thick loft hatch to match, using 200mm polystyrene in a hardboard shell. It has airtightness sealing around the top with foam and brushes. Last winter I found that the frame of this was sopping wet due to condensation from air moving into (and through) the structure, then condensing when it got to the top surface next to the cold loft. I took it apart and gooped up all the joins with acrylic sealant so that no air could get through the hatch.

Having just had a look I find that the hatch is still wet, but now only on the outside where warm air get up near the seals at the top and then condenses. This seems to do no harm on the painted parts, but soaks into the unpainted parts. So I guess I'll paint it all in summer.

A second air seal at the bottom surface (ceiling level) would probably help a great deal with this problem, but as I didn't design that in originally it's difficult to add.

Here is a picture of the hatch, which may help visualise the above explanation. http://www.navitron.org.uk/forum/index.php/topic,5110.msg54952.html#msg54952

I upgraded the loft insulation with ecowool and ended up covering the gaps down between the lath/plaster and thick stone walls of approx. year 1900 stone bungalow. Roof space lost a lot of venting and became damp with some mould on timbers. Having pulled back the insulation the loft is much drier and 'recovering', but the house is colder. The cold air (N.Scotland) comes in through the vents in walls , under the house and makes its way up the inside walls, sucking heat out of the plasterwork on its way up to the loft. Also added ecowool underfloor insulation between joists. Replacing old boiler and adding the eco wool under floor and in loft has cut oil consumption by about 30% I'd say and its been a much colder winter. I'd love to insulate the walls fully. I did this to the large north-facing wall in our bedroom, pulled off the plaster from lath, inserted 50mm kingspan between battens installed over the old lath then plasterboard, which has made a huge difference to just that room. Expensive and messy job. If only aerogel would come down in price.

Interesting thread. I'm definitely eager to hear more about condensation problems arising from adding insulation to the loft.

We've got maybe 200mm of mineral wool insulation in our loft. In one part of the loft, I had left a sheet of laminated MDF resting on the insulation (I was using it to kneel on while I installed the insulation). I recently went up into the loft and found the bottom of the MDF was damp. It hasn't caused any problems but it's an interesting illustration of the problem.

The Energy Saving Trust recently advised that one way to "top up" loft insulation is to put 100mm foil-faced rigid foam insulation over the joists (the thinking being that it's timeconsuming and expensive to install new joists in order to allow 280mm of glass fibre insulation to be installed whilst also allowing the loft space to be used for storing stuff). I'm a bit worried that the rigid foam insulation will reduce ventilation.

related to your point about the bit of laminated MDF, I recently added an over the beams layer of earthwool (the recycled plastic wool insulation) to a huge loft up near inverness during the cold snap when it was significantly below zero even in the daytime. Whoever had added the 200mm existing mineral wool insulation between the beams previously had left the plastic wrappers randomly strewn around the loft, some on top and some underneath the insulation. The plastic wrappers under the insulation were sopping wet on the underside, and the wrappers on top of the insulation had actually frozen solid to the top of the insulation.

I know that there's a school of thought that says to put a damp proof layer under loft insulation. Given the above, I can't see how that can make sense in any situations other than the most airtight of loft spaces, or possibly where a room is being built in the loft with a sealed floor such as a bathroom. If this is done, I reckon it should always be compensated for by installing additional mechanical heat recovery ventilation otherwise you're basically going to be causing increased damp problems in the rest of the house structure, and as well as the ceiling beams if the vapour barrier is only between them, or even between and over them as I've seen advised elsewhere on the we, rather than as a continuous layer across the ceiling underneath.

As an aside as well, that loft gave me an interesting demonstration of cold bridging in action, as all the roofing nails that stuck thorough on the north facing roof had icicles hanging off them