When converting a loft to a storage or living area, it is not essential to insulate the floor. In fact in some cases it might be a disadvantage. But if you are using the space for storage only, insulating the floor will certainly save on heating bills below, even if the walls or rafters are insulated as well. So before you plank or floor the joists, give some serious thought to the insulation of the floor. The most popular methods for floor insulation are the laying of polystyrene granules or fibreglass blanket in between the joists. You might like to consider a third alternative-installing a false ceiling below.
This creates a layer of still air, which is the best insulation of all. A false ceiling, although it entails more work, is particularly attractive if you have high ceilings as it will lower the height of the room. A false ceiling can be constructed quite simply with aluminium angle strip and insulating boards. In any case, if the roof insulation is to be at all efficient, part of the floor will have to be insulated. This is at the edges where the joists and the rafters meet at the wall plate. Cold air entering up through the cavity wall or blowing in through the eaves can ruin an otherwise perfect job. In this area it is best to lay several layers of glass fibre blankets, covered if possible with sheet polythene or aluminium foil, to stop the draught and seal off the cold zone.
Whether you intend using the loft for storage or living, you will have to insulate and weatherproof the walls (the spaces between rafters). It must be noted that in all probability you will have to do some restructuring of roof timbers, and strengthen the joists, and it would be pointless to insulate the walls before this has been done. Wall insulation has been included here for convenience. The rafters of most modern houses are covered with roofing felt before the roof tiles are laid.
The felt not only prevents rain and snow from getting into the loft, but also provides better insulation by preventing draughts. If, however, your rafters are not covered, and the tiles or slates are visible from inside your loft, then this is your first job. It is obviously impractical for a home handyman to remove all his roof tiles in order to cover the rafters with felt; anyway this is not necessary. The felt – membrane can be placed in position from the inside. The felt is held in position with tacks or drawing pins, and if a little plastic compound is applied to the side that is secured to the rafter, the seal should be perfect. Always start at the top and work down. Apart from the convenience of pinning the felt at the top and allowing it to hang down while you continue fixing, any overlap will mean that if rain should enter through a loose tile then it will run down the felt and not be funnelled into the loft space.
There are many methods of completing the insulation of the walls. One is to completely line the rafters with a good lightweight building board such as fibreboard or, even better, a plasterboard with an aluminium foil facing. The foil side should face outwards, to deflect back heat in the summer and the cold in the winter. Insulating blanket can also be used. This is attached with battening and, if you decide to board the walls later, the boarding can be nailed to the battening, providing a double layer of ‘space’ sandwiching the blanket and giving excellent insulation. Between the rafter spaces, insulating material such as fibreglass blanket, solid blocks of mineral wool or cork 50mm thick, or flame retarded foam polystyrene can be fitted. With this method, some means of holding the insulation material in place must be devised. It is ideal if you are going to board the rafters over, and do both jobs together.