Energy Efficiency in Historic Buildings
Insulation of suspended timber floors
This guidance note provides advice on the methods, materials and risks involved with insulating suspended timber ground floors. The applications described are also appropriate for timber upper floors where there is an unheated space below. Advice is also provided on how suspended can be draught-proofed where the installation of insulation may be difficult or potentially damaging to the historic character of the building.
Improving the insulation to suspended timber floors will always require access to the space under the floorboards. Unless there is a cellar beneath, or an existing trap door in the floorboards, this will mean lifting at least a few floorboards. If the crawl space is too shallow to allow work under the floor, all the floorboards may need to be lifted.
Extreme care is needed when lifting floorboards as they are not easy to lever up in one piece and there is a great danger that lifting will cause damage. It is a good idea to wear gloves and a breathing mask when working in crawl spaces – these are places where rodents and birds often live (and die).
Before starting work on insulation, use the opportunity offered by access to the crawl space to inspect and repair the floor structure. Where it has been weakened by the inappropriate cutting of holes and notching for central heating or other service pipes, the floor may require strengthening. Also check the sub-floor ventilation. Make sure that there is a good air-flow and that the insulation will not restrict the cross ventilation. Remember that until now the ventilation of the rooms above may have been provided partly by draughts passing through the floorboards, a source of air that will soon be blocked. Where suspended ground floors were constructed without ventilators it is recommended that a specialist surveyor be called to make an assessment of the environmental conditions in the sub-floor space and to design a new system of cross ventilation.
When relaying a floor it is good practise to provide for later inspection, maintenance and upgrading, thus minimising the need for future disturbance of the floorboards. Where space permits, design a trap door that will allow a person get into the void and crawl around the floor structure. Otherwise provide a hatch that will allow a head-and-shoulder's visual inspections of the environmental conditions in the floor void. When a suspended floor is insulated, the trap doors also need to be insulated and draught-sealed.
Energy is lost through floors in two ways: by heat passing through the relatively thin floorboards, and by warm air escaping though cracks and gaps between the boards. They are equally important. Where there is a cellar, or access is available from below, insulation in the form of batts or boards can be added between the joists. The insulation should be cut carefully to fully fill the space between the joists as any gaps in the insulation layer will allow air movement and heat loss. Install the insulation by pushing it between the joists, but take care not to compress it too tightly as the air between the fibres that provides the thermal insulation. However, the insulation will also need to fit tightly enough to stay in place until it can be supported from below.
Where the walls are damp, use building paper to separate them from the insulation. To reduce air movement, install a continuous layer of breather membrane, supported on battens below the joists. This material also serves to support the insulation. If additional thermal insulation is required, insulating boards can be added beneath the joists. If the boards are tongued-and-grooved, or have their joints taped, they can also provide an air barrier, though fitting flat rigid boards under irregular joists can be difficult.
Where the floorboards have had to be lifted because the crawl space is too low to work in, insulation can be installed between the joists from above. Suitable materials are batts, boards and loose- fill materials like cellulose. When working from above, first choose and install a method of support that will hold the full width of insulation between the joists. This could be in the form of rigid boards or breather membrane installed beneath the joists as described above. Alternatively netting can be hung over the joists and used to support the insulation between the joists. Another possibility, if the room can afford to have the height of its floor raised a little, is to lay tongue- and- grooved insulating boards over the joists in addition to or in place of the insulation between the joists. The floorboards can then be replaced on top of the insulating boards.
Many floor voids are used for routing water and heating pipes and electrical cables. Often these are passed through notches in the floor joists or are clipped to the joists, which means they will have to be moved to allow the insulation to be fitted. Lag any cold -water supply pipes to prevent frost damage and any central-heating pipes to prevent wastage of heat. Make sure electric cables are free of insulation to prevent them overheating.
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