Good points, and the U- value calculator is useful, it also will answer your question about interstitial condensation risk.
Well, it shows a little red asterisk for different parts of my wall depending on which construction I pick, but doesn't really explain exactly what that means. Condensation on front face?, back face?, actually inside the brick (aren't bricks reasonably waterproof)?, under what conditions? I assume it is only a problem if it stays wet for ages and never dries out? Cambridge is very nearly a desert, and this is a sheltered spot.
I'll try asking a load of people at the homebuilders/renovators show in London next week, and the point about suppliers' tech depts is a good one. One thing I wondered about - does one need to worry about 1st floor wooden joists sticking through insulation (and thus vapour barrier)?
I have a friend with an exceedingly cold sandstone house (listed so no chance of external insulation) who is also very interested in this stuff, but has been warned to be _very_ careful about condensation.
Well the Web based calculator is a simplification. Which is why its worth getting a free calculation from people like Kingspan. When you have decided on the level of insulation you need. The asterisk mean there may be condensation at the levels of humidity set at the top of the calculator. In a swimming pool the internal humidity could be near 100% then there will be interstial condesation in many constructions. The manufacturers programs check out the quantity of vapour likely to be present over time and will allow for the brickwork to dry out over a heating season. Showing that in practice there is unlikely to be a problem. As for floor joists thats a good point, not only the vapour barrier but also the heat loss at this point. I have insulated between the joists against the wall with Kingspan and foamed up the gaps. In therory I suppose ther could be some minor condesation, but again thats no worse than in any solid wall, as long as it can still dry out to the outside over time.
There should never be any kind of vapour barrier on the cold side of insulation. Breathable membranes such as Tyvec used for roofs should also not be used in walls unless there is a vented air gap on their cold side. Difficult to achieve, and unnecessary, in most solid walls that are reasonably sound, and well pointed with a lime based mortar. If the external wall is already damp, fix the dampness first never hid it with drylining. Or an internally applied material. Old walls need to breath outwards.
Jack asked about suppliers, ring Kingspan, I got mine from my local family builders merchant, 92.5 overall 80 +12.5 plasterboard, they ordered it at a price that was less that some of the specialist laminated board suppliers, over 2.4 m and you will have to cut boards, how about a dado rail? if your taping and jointings not so good!!!!
The discussions in this thread I'm afraid, show why it's worthwhile involving building control, unless you really know what you are doing. Although we've talked about general principles, solid walls vary in traditional construction through out the country, some will be softer, harder, more windswept, and good local knowlege can be a great help.
