Floor Insulation - A Quick Run-Through

Moving into a new house with a floor that is not insulated can be a problem if the house has solid floors. The difficulty might be in the materials used because floor insulation for each type differs in work and costs.

Generally, ground floors are done as either solid, usually concrete slabs, or suspended. (These are timber floorboards on joists.) When these are not insulated, the U-value between is 0.45/m2k and 0.70W/m2k. Our target value is between 0.20W/m2K and 0.25W/m2K (depending on floor geometry).

It can be more trouble (and costs more) insulating a solid floor than insulating a timber floor. The payback for the former is 8 to 10 years or more, but the payback for timber floors is between 3 and 5 years. (This is recouping your investment-expense on the insulating work.)

Initial precaution

Sloppy insulation work can result in thermal bridging. Thermal bridging happens where the solid building elements continue from the inside to the outside of the structure, conducting the cold out or heat in. this results in severely compromising the value of work done and leads to cold spots and condensations.

During work, you need to consider the airtightness layer beneath the floor next to walls and round the edges. The airtightness prevents unwanted infiltration of draughts.

Wooden floor insulation

It will be easier to stuff insulation between the joists if there is access from below. Simply press the insulation against the floorboards without having to over-compress them.

You can add more insulation than the depth of the joists would permit in two stages. One is stuffing the spaces between the joists and adding battens to the underneath of the joists. You can then fasten the second layer of retaining boards or netting to them.

The holes in the floorboards (where pipes and wires go through) need to be plugged with mastic and tapes to prevent draughts. If there is a possibility that the room could flood, a damp proof layer should not be put beneath the insulation. (Draught-resistant fabrics should be used).

Concrete floor insulation

Floor Insulation can be added over the existing concrete slab or under a new one. If the insulation is placed above the slab, the room will warm up more quickly when the heating is one. It also cools down easily. (This applies to all internal insulation.)

It affects the floor level which can impact on door openings, stairs and other house fixtures. This is the reason why phenolic foam is the most common solution in cases like this because it can still achieve greater insulation with less depth.

Insulation below the slab is generally better especially in warm south-facing rooms since the concrete helps absorb the heat and can limit the overheating. The damp proof membrane is placed on a layer of sand to prevent being punctured by sharp stones.

Since it also serves as the airtightness membrane, it laps up the sides of the walls to link to the plaster or plasterboard. Floor insulation is usually a more efficient heating than the conventional systems.