Loft insulator query

[matchmade]

I hate flat-lying loft insulation as if you follow the Government's advice and install 200mm, it goes well over the joists and the loft is unusable for storage. I want to install good-quality insulation between the rafters and board the whole thing, obviously keeping a 50mm gap between the insulation and the slates to allow evaporation.

Can anyone answer this:
- are sheep's wool insulation pads any good compared to Celotex or Kingspan? The cost is a little more but I'm having difficulty comparing installation time, U-values etc.
- can you still get a government grant if you do between-rafters instead of flat-lain?

[KARL]

Without knowing for certain, I would immediately assume that (being a bureacratic beast) the authority deciding grant awards will have utterly inflerxible rules as to what qualifies. As such, any requirement for imagination or lateral thinking means automatic rejection. But only a discussion with the people involved will reveal the truth, and then only once in writing (hopefully irrevokable). Your central problem is easier to resolve. There is the option of having the underside of the roof sprayed with foam which sets rigid. This completely eliminates vapour/condensation/ventilation/draft considerations. Alternatively, if you want to fix the insulation in yourself then either use foam sheets or something like glass wool. The former is impermeable to vapour and no worries about ventilation arise - unless the widespread worship of draughts is invoked (you only need to consider that when you are not insulated plus vapour barred). If the latter type then the best thing is to cover everything on the inside with a vapour barrier (plastic sheet is perfectly OK) and then you need not worry about ventilation - unless your roof leaks and you need to dry the place out. I am unfamiliar with the use of sheep's wool but in practice anything will help - I know a guy who always laid out his old carpets in the attic. Each layer increased the insulation. But the vapour barrier is worth taking into account when sealing the roof as you plan and if you apply one you are completely in the clear. It is warm, moist air condensing on cold surfaces that causes dampness. As described, no surface that the inside air touches will be colder than the air itself and it can't migrate through the barrier to the cold roof.

[buildersmate]

This spray-on stuff. I've never used it but heard plenty of stories about how useless it is. (Rendatherm is one of the brands). Not recommended.

[KARL]

The sprayed foam is exactly the same as used on hot water cylinders (probably one in your house, but have a look in B&Q). There is nothing wrong with the material or the method, but workmanship does matter. The fact that it is an underside that is being sprayed is very significant and to hold against gravity the consistency, etc. need to be right. An experienced, careful installer will do it satisfactorily.