What type of insulation do you need and how much?

Type. Isonat [hemp] and Thermafleece [wool] come in 'batts' or blocks 1200mm long and varying widths and thicknesses. Isonat is slightly more rigid than Thermafleece and therefore may be more appropriate where the insulation might need support - up against the roof tiles or under a floor for example. But Thermafleece is more pliable and so is easier to squeeze into smaller widths between joists should there be any variation, which there can be in older properties. Both can be used for loft floors [between the joists], against the tiles [between the rafters] or in walls [stud] however. Thermafleece is also marginally easier to cut. Between the rafters, both will need a breathable membrane seperating them from the tiles. [Klober on this site, in the Thermafleece section]

Warmcel 100 is a loose fill insulation. That is, it comprises lots of small fragments that you pour out of bags. This means that it 'forms' to fill every nook and cranny but, of course, it's only suitable for laying on the loft floor.

Thickness. What thickness you would require is partly dependent upon a range of other issues [see below]. Ideally put as much as you can into your loft. But each subsequent layer of insulation has progressivly less effect. However, for Isonat, Thermafleece or Warmcel 100, 200mm should be OK but 250mm would be better. Some conventional insulation would need about 20% more than this to achieve the same insulating abilities as natural insulation, which are in line with building regulations. How this works is this....

Why. You may be aware that there is a whole lot of legislation emerging, particularly in June 2007, that obliges home owners and builders to make houses far more energy efficient than they are at present. You've probably heard of HIPs [the Home Information Packs that every house seller will have to produce to sell their house]. Well, an energy efficiency certificate is going to be an essential part of this and will probably be compulsory. Certification will be arrived at through an inspection which will allocate points for the energy efficiency or otherwise of different components of a property. More points will be gained for double glazing as opposed to single glazing for example. Or for cavity wall insulation or draught protection or different kinds of heating system or, of course roof and wall insulation. In this way, a property will acquire an efficiency grading.

The effectiveness of a given barrier to heat loss is called it's U value. The lower the U value, the lower the heat loss. So a double glazed window will have a lower U value than a single glazed window. All the points are added up and an overall efficiency rating is arrived at for the building...rather like the A, B, C, D ratings that appear on fridges and suchlike. So, where a given aspect of a property gets a low score, it might be made up for somewhere else. If you have 250mm of loft insulation, you may not have to worry so much about your glazing, for example, to get a good rating. On the other hand, if you have triple glazing, you may get away with less loft insulation. However, loft insulation probably contibutes most to making a building energy efficient and, given its cost relative to, say, windows, it can also be the most cost effective.

The target U value for lofts given in the formal specification is 0.16 for between the joists [the loft floor] or 0.20 between the rafters. This would be achieved by 250mm of either Isonat or Thermafleece or 220mm of Warmcel 100 for the former and 200mmfor the latter. There are other tricks to make the most of insulation. As you place the insulation between joists for example, the joists themselves act as whats called a thermal bridge between the space under the insulation [the room space] and the space above it [the loft space]. By laying a second layer at 90 degrees to the first, across the top of the joists, this effect is eliminated. And so your 250mm now has the effect of more than 250mm.

The upshot is that there isn't a fixed answer to the question of how much insulation is needed. The more, the better. But generally speaking, 200mm should be sufficient, 250mm would be better.