Insulating Concrete floors

Many houses have ground floors which consist of a concrete slab laid either on hardcore or directly on the earth. It is not possible to take up a floor like this and lay insulation underneath it as you can with floorboards, but you can put a layer of insulation on top. First remove the skirting boards from all walls. If the house is old and you suspect there is no damp proof course in the walls and no damp proof membrane (dpm) to stop moisture rising into the concrete from the ground, have a free survey and quote from a damp proofing firm. If the walls need damp proofing it must be done before the floor can be insulated.

If the floor is damp, you should treat it with a waterproofing material to prevent damp rising into the room. The easiest way to do this is to use a liquid such as Synthaprufe, a black bituminous emulsion. Paint it on to the concrete in three coats, using a very old brush which you can throw away afterwards, and wearing rubber gloves and boots. Whatever material is used the manufacturer’s instructions must be followed regarding preparation and priming of the floor. Carry the damp proofing material about 300mm up the walls before replastering or insulating them, so that it meets the new damp proof course.

If the concrete floor is not damp, you can omit the black emulsion and the process for damp or dry floors is now the same. Lay sheets of expanded polystyrene, of whatever thickness you think from your calculations will be appropriate, on the floor. The polystyrene should be standard duty grade, preferably with flame retardent additive. The insulation should be laid with as few joints as possible so use 1200mm x2400mm sheets. Butt the polystyrene sheets tightly together so there are no gaps and try to stagger the joints. If you have to walk on the insulation lay pieces of plywood or hardboard as stepping stones to spread the load and prevent crushing the relatively soft foam.

On top of the insulation lay sheets of tongued and grooved flooring grade chipboard, with the joints running the opposite way to the joints in the polystyrene. The chipboard will probably have ‘this side down’ written on one side and the tongues and grooves will not usually work if one sheet is upside down. Check all your sheets before laying, as occasionally only some of a batch will have ‘this side down’ printed on them. Lay the first sheet with its groove against the wall and the tongue protruding into the room. This makes it easier to lay the last sheet. Lay the pieces of chipboard like bricks with staggered joints, and squeeze PVA glue into the groove of the sheet before fitting it on to the tongue of the previous one. The aim is to build a ‘raft’ of chipboard which will sit on top of the insulation. If you plan to seal the surface of the chipboard to use it as a floor covering be sure not to leave glue on it as this will affect the colour; spills of PVA glue can be wiped off with a slightly damp cloth before they are dry.

To allow for expansion there should be a gap of about 10mm between the edges of the chipboard and the plastered walls. This gap, which will be covered by the skirting board, also allows a little room in which to manoeuvre the pieces of chipboard to make them fit together. The only bit likely to be difficult is the last piece and if you cannot make it fit by levering against the wall with an old chisel you can nibble off the bottom of the groove with pincers and glue the chipboard down to the tongue of the next sheet.

When all the sheets are in place leave them for twenty-four hours for the glue to dry. Then fix a skirting board to cover the expansion gap and hold down the edges of the raft. Finally seal or cover the floor with conventional flooring materials such as cork tiles, carpet, or sheet vinyl.

The new floor will be somewhat higher than the old one, so you will have to cut off the bottoms of doors to suit. This means that the technique is really only suitable for old buildings which have real wood doors. The average modern door, which consists of two sheets of hardboard separated by. A paper honeycomb core, has only a thin edging of wood. If you decide to insulate your floor with 50mnn of polystyrene plus 19mm of chipboard you will need to cut about 75mm off the bottom of the door and there may well not be any wood left at the bottom. You could cut the door, make a new piece of wood to fit the bottom, and glue it into place.

This method of insulation will also reduce the ceiling height of the room and may therefore upset the building inspector. The relevant regulation, in fact the only really silly one in the building regulations, lays down that a room must be 2300mm high from floor to ceiling, so the inspector may not be anxious to let you remove a precious 69mm from this magic figure. However, most building inspectors are reasonable and practical people so you should be able to reach a working compromise with them. Our experience is that in old buildings they only start to worry if the ceiling heights come down to about 2100mm.

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