Solid Floors

Important to Know

• The importance of ground floors is discussed at the beginning of this section. Although solid floors tend not to suffer from the level of air leakage of suspended floors, they are still a priority for improvement, even if their importance is not widely appreciated.

• Even a small amount of insulation is much better than none.

• The issue of breathability is not as important in this situation, so our guidance is very much like conventional guidance – improve the U-value as much as practical and affordable.

Our Guidance vs Conventional Guidance

The importance of ground floor insulation

The main difference of this guide is that insulating ground floors is much more important than commonly considered because of both the energy efficiency and comfort benefits it can bring. Regarding the details of our proposals, the only difference is our emphasis on airtightness measures. This is because there are no significant issues related to moisture and the pressure to keep insulation thin while providing as good a U-value as possible; we recommend the same sorts of products and materials as others. For this reason, we haven’t produced a table of differences as elsewhere.

Quality of existing floor may result in its left in-situ

An exception is where the existing floor finish is of exceptional quality or conservation significance, in which case it should be left in situ, repaired if necessary and maintained. Depending on location, the issue is that this may become the coldest surface onto which moisture can condense. Hence, this needs to be managed by maintaining lower temperatures or higher ventilation levels. In many Victorian homes, for example, a mosaic-covered entrance hall may happily be left as it is.

In this case, the key is treating the inner door rather than the outer one as the thermal envelope.

Preparation

Doors and skirting

• Depending on the thickness of the final floor makeup, doors may need to be re-cut to allow them to swing freely over the new finish, while it may also be necessary to raise all of the skirting boards. Alternatively, the skirting may remain in place, and the new floor finish comes up against it with an additional, usually very small, skirting bead used to cover the gap. Because of the value of sealing the gap between the insulation and the wall, we recommend raising and replacing the skirting, but this obviously means greater work, disruption and cost. An alternative, if possible, is to cut the lower section of the skirting and slip the new floor finish beneath.

The whole floor must be insulated

• It is tempting to avoid extending any new insulation and floor finish beneath fixed items like kitchen units and baths. However, this then risks these areas becoming the coldest surface, and warm, moist air can condense, raising the risk of mould. In some cases, it is possible to use adjacent kitchen units to temporarily support unit carcasses, adjust the ‘feet’ upwards and extend the insulation and floor finish to the wall before re-adjusting the ‘feet’ back down to the new floor finish. It is usually possible to insulate beneath a bath if the side panel is removed. In contrast, a floor-mounted WC and wash basin will need to be disconnected by a plumber and re-connected afterwards.

Installation

Plastic-based insulation boards are probably best

• The material with the lowest lambda value currently is a vacuum-insulated panel (VIP), but these are both expensive and vulnerable, so they are rarely used. Aerogel products have a lambda value of almost half the nearest high-performing insulation but are expensive. Plastic boards such as PIR, PUR and Phenolic foam boards probably represent the best option unless head height and thickness issues are critical.

Levelling compounds

• If the solid floor is level, smooth and clean, it is possible to continue directly over it. However, if not, a number of levelling compounds may be used to provide both a level and suitable surface.

Use lapped or t&g boards

• Some insulation materials come in simple boards that must be laid neatly and without gaps. Some come with lapped or tongue and groove edges, which are to be preferred. All junctions with the wall and each other should be taped to keep them in place while working and reduce air movement between gaps that could open up.

Consider using a vapour control layer

• Depending on the materials chosen, it is advisable to install a vapour control layer, which also acts as an air barrier and ‘slip layer’ above the insulation, but this is something that the manufacturer or supplier would advise of either the insulation or the finish above.

Seal wall junctions when using compound insulation-decking boards

• Some insulation products come already bonded to a top floor deck, allowing them to be laid in one go. It is important to seal all joins between panels and to the wallat is to go over this depending on wh.

Install final floor finish

• The final floor finish can then be installed along with all the fixings/skirtings/doors and so on replaced in situ.

Alternatives

Existing timber over solid floors

• In many properties, there is a sold floor with battens fixed to it and then a timber floor or similar laid over. In these situations, it may be possible to install insulation and boards over without raising the level of the floor. Alternatively, it is possible to either leave the existing battens or install new ones and infill the gaps with suitable insulation. The significant disadvantage of this solution is that the battens present a thermal bridge, losing more heat and raising the risk of cold surfaces. For this reason, the continuously insulated floor described above is preferable.

• Some people will prefer a traditionally nailed timber floor, so a compromise solution is possible where some insulation is laid first (or affixed to the underside of the battens). Then, the timber floor was fixed to the battens as before.

Health & Safety

There are few Health and Safety concerns with this sort of work in the normal course of events. There may be dust associated with working on old concrete screed or removing more aged floor finishes, and removing old skirting boards can result in nails protruding from the back, which should be dealt with immediately.

How Much is Enough?

• Unless there is a particular U-value target, it is simply a case of more insulation / better lambda values, and better airtightness will improve performance.

• In most cases, there is usually a practical and cost limit to what can be achieved. It is worth noting that the first millimetre of insulation is the most effective, the second slightly less so, etc., so even a small amount of insulation is much better than none.

• Even 10mm of Aerogel or 20mm of a PIR / PUR / PF product is usually sufficient to make a difference and provide better comfort levels.