How To Insulate Suspended Timber Floors

The ground floor of a house does not lose as much heat as, say, the walls or roof, but in a thorough insulation scheme the floor should not be forgotten. Ground floors are usually made of concrete laid direct on to the earth or of wooden boards nailed to joists with a ventilated air space beneath (i.e., a suspended floor). Occasionally floor boards are nailed to pieces of wood set into a concrete slab. If you jump on the floor you will be able to tell if it is of boards on joists by its slightly springy feel. Another sign of a suspended floor is the air vents near the ground in the external walls of the building.

Suspended timber floors

Heat losses occur through the structure of a suspended timber floor and through the draughts that it may create The problem is that the space under the floor must be well ventilated to prevent the build up of moisture and subsequent dry rot, but if it is well ventilated a powerful draught will often come into the house through the gaps between the floorboards.


The most thorough solution to the draught and insulation problem is to take up the floorboards and insulate between the joists. The best material for the purpose is expanded polystyrene standard duty, with flame retardent additive (Type SD FRA). If you plan to re-use the floorboards you will have to remove them very carefully; and remember to turn the electricity off at the mains before you start.

The first step is to find if the boards are square edged or tongued and grooved. Push a penknife blade between two boards in several places: if the knife goes through easily the boards are square edged. Lift one end of a square edged board carefully with a bolster until the claw of a hammer can be slid underneath to keep the gap open. Then slide the bolster along the gap, carefully levering up the board. Once you have lifted the first board use the claw hammer to lever the others free of the joists, using the bolster to ease them up as necessary. Do not be tempted to rush in with a crowbar as you will split the boards and they will be useless.

If the boards are tongued and grooved cut the tongue off one board with a padsaw so that you can lever it up with the bolster. This is a long job so try to choose a short piece of board to do it on. Once the cut is started with the saw held vertically, tilt it back to a shallow angle and cut very slowly, using the tip of the saw. After every few strokes of the saw, turn it over and feel about with the back of the blade for pipes, joists and electric cables. Once you have cut off the tongue of the first board the others can be levered up as before, but you will need extra care to avoid splitting off the tongues. All this assumes that, on inspecting the floor, you have been unable to find a board previously removed and screwed back after some rewiring or some plumbing job.

When the boards are off the insulation can be put in. Clean off the tops of the joists with a scraper, and give them three coats of timber preservative. Next nail 25mm x 25mm pieces of Tanalised (or preservative painted) softwood to the sides of the joists, 50mm below the top for 50mm insulation, 75mm below for 75mm insulation and so on. Now cut the expanded

polystyrene to fit between the joists using a normal hand-saw. Make the pieces of polystyrene a fairly tight fit so that there will be no draughts through the floor. When electric cables run across the tops of the joists, use a Stanley knife to cut narrow V-shaped grooves in the top of the insulation, to accommodate them.

Once the insulation is in place, fitted tightly up to the walls and between the joists, you can replace the boards. To do this properly wedges should be used; they should be cut from wood 25mm thick and should be 500mm long by 50mm at the wide end. You will need a pair of these wedges to fit every metre. Lay a few floorboards loosely without nailing them to the joists, and place a pair of wedges against the edge of the boards at one metre intervals. Then nail a scrap board against each pair of wedges leaving the nail head protruding so that it can be removed easily. Use two hammers to tap the wedges inwards to force the floorboards together, working on the sets of wedges alternately to keep the floorboards parallel.How To Insulate Suspended Timber Floors

When the boards are good and tight nail them down, starting at the board near the wedges and using floorboard or oval wire nails about 75mm long. When all the boards are nailed down take up the scrap board and the wedges; lay another five boards and wedge them together. When all the boards have been laid, lever the final board firmly against the others with a chisel and nail it down.

You may feel that taking up and relaying the floorboards is rather a large job. In this case a simpler solution is to seal the gaps in the floor to stop the draughts. The easiest way is to cover the floor with sheets of hardboard: lay it rough side up if you plan to put carpet or tiles on top of it, or smooth side up if it will be visible. The hardboard should be stored in the room where it is to be used for seventy-two hours before fixing it down to prevent it buckling because of changes in its moisture content. Nail the sheets of hardboard (which should measure 1200mm x 1200mm for ease of handling) to the floor at 150mm centres all over and at 100mm centres round the edges, using hardboard pins. Normal floor coverings can be laid on the hardboard if it has been laid rough side up, or the smooth side of the hardboard can be sealed to act as a floor surface itself.

Another way to seal the gaps is to lay a fitted carpet plus underlay or a sheet flooring on top of several layers of old newspapers. However, if you wish to have a floor of exposed timber boarding, you will have to fill the gaps between the boards to cut down the draughts. The filling can be done with stopping, a flexible filler available in a number of colours to match the wood of the floor. The stopping comes ready mixed in a tin and can be pressed into cracks with a scraper. Stopping is preferable to plastic wood as it is more flexible.

An alternative to stopping is to make papier mache by tearing up scrap paper into pieces about 20mm across and mashing it to a thick pulp with boiling water added a little at a time. When the pulp is cool pour in cellulose wallpaper paste and stir to make a very thick mixture. If the papier mache is to be visible the paper should be white (newspaper dries to a grey colour). If you want the mixture to match the floor you can colour it with liquid dye before using it. Press it firmly into the gaps with a scraper, and sand it down after about three days.

If the floor is draughty, sealing the cracks will make a considerable difference to the heat lost from the house. However, fitting insulation under the floorboards is more effective since it will cut structural as well as ventilation heat losses.

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