If you put insulation on top of the ceiling of the top floor, it follows that the attic will become colder: the amount of heat that rises into the roof from the rooms below will be greatly reduced by the insulation. Any water tanks or pipes in the roof will be much more likely to freeze, and, even if there are no bursts, one freezing winter could lead to at least a loss of water and at worst to a boiler blowing up.
The first way to prevent the cold water tank from freezing is not to put any insulation underneath it when you are insulating the ceiling. This will allow some heat from the house to rise and keep the tank warm. The next step is to carry the insulation round the outside of the tank. Glass fibre and mineral fibre are relatively easy to use: wrap the fibre round the sides of the tank, to a thickness of about 100mm, and secure it in place with string tied round loosely so as not to compress it. Cover the top of the tank with a sheet of plywood, hardboard, or a purpose-bought lid before insulating the sides. You can then also cover the lid with a piece of glass fibre or mineral wool. If the tank has an overflow pipe discharging into it, which is often the case, cut a hole in the lid below the outlet of the pipe and put in a plastic funnel, about 200mm across the wide end, to catch any discharge from the overflow.
A tank wrapped with glass fibre or any similar floppy insulation will keep warm enough.But you may like to go up into the attic occasionally, to check the tanks; and you do not want to go raking about in a lot of itchy insulation every time you want to have a look at the ball valve. In this case, the best solution is to build a box of 75mm thick expanded polystyrene round the tank. You can cut the pieces of polystyrene to size with a panel saw and hold them together
by pushing 150mm nails through the edges. A coating of PVA glue on the joints will make the thing a bit stronger, but do not be tempted to try other sorts of glue such as contact adhesives, as they may well dissolve the polystyrene. Where pipes pass through the insulation a wedge can be cut out and then glued back in place once the insulation is fitted. Make a lid with more expanded polystyrene, held either with nails or an inner block if you want added refinement. Do not forget the funnel for the overflow where necessary. This type of tank cover will be easier to open.
All pipes in the attic should be lagged to prevent them freezing, and to avoid heat being wasted from hot pipes. You can buy rolls or lengths of material for insulating pipes, or you can cut glass fibre or mineral wool into strips to bandage round them. The insulation should be at least 25mm thick, and held on with loosely tied string. Where pipes run at or near ceiling level, lay the roof insulation over them so that the pipe is on the warm side of the building.
It is also important to lag the hot water pipes in the main part of the house, to prevent heat being lost as the water makes its way from the hot water cylinder to the taps. If the pipes are exposed you may not wish to use bits of glass fibre tied on with string; in this case it is preferable to buy pre-formed pipe insulation in the shape of a tube, with an inside diameter made to fit over the more common pipe sizes. The tube is slit along one edge so that it can be pulled open and fitted over the pipe. You can bend most types round corners but you will have to tape them on at bends to prevent the slit coming open — you can use electrician’s PVC insulation tape. At tees and other complicated junctions all you can do is fit the insulation as neatly as possible.
Hot water cylinders
As far as the hot water system is concerned, the most significant amount of heat is lost from the hot water cylinder, which should therefore have an insulating jacket at least 75mm thick. You can buy jackets ready made to fit all standard cylinder types and sizes, but it is better to buy one too big than too small. Alternatively you can tie glass fibre loosely round the cylinder with string to increase the insulation thickness to 100mm or 150mm. Cover the insulation with polythene sheet taped or tied on to reduce the problem of loose fibres causing irritation. You can also box the tank round with hardboard on timber framing, leaving holes for pipes and for access to valves; stuff the gap between tank and casing with offcuts of glass fibre insulation or expanded polystyrene packaging, which you can often get free from shops, broken into bits about 10mm across, or purpose bought granular insulation. Whatever method you choose try to make it at least 75mm and preferably 100mm thick.
As mentioned earlier, savings in the energy used to heat water can also be made by conscious effort rather than by technical means. For example, do not wash dishes, clothes or hands under running hot water, use a bowl or sink instead; mend leaky washers to stop dripping hot water taps; be sure to turn off hot taps properly; and do not rinse clothes in hot water after washing them. These things are all fairly obvious and many other savings will occur to you if you become more conscious of how energy is used.