Insulating the concrete and timber floors

The floors of a domestic dwelling are generally made of either wood (timber) or concrete. If concrete then they can be either solid to the ground or of a suspended beam and block construction similar to timber floors. In our dwelling the floors are either suspended timber or solid concrete.

We already know that one of our priorities is to minimise heat loss, which implies ensuring that the walls, floors and ceilings are well insulated. We are concentrating on the floors first as it is much more difficult to deal with them once you have moved in and covered them with furniture and floor coverings.

Solid concrete floors

Because the method of construction of the concrete floor is unknown we do not know with any certainty what the U value for this floor type is. Hence we will have to assume a generic figure based on construction of the period with no special allowances.

As we have seen, the target U value for a floor is 0.22 to meet new construction regulations and 0.15 for PassivHaus standards. These levels can be achieved for a solid floor, but only by digging it up and starting again incorporating insulation into the build. In a modified form just the screed is removed and replaced, but the improvement here is not as impressive. This does not seem to be an option here.

The alternative is to lay additional insulation over the solid floor which obviously increases the floor’s height. This can cause differences in levels through the building. We have another limitation in that the internal doorways are not particularly tall at 1.96m, but several of our family and friends are. If we were to renovate the floor to meet the Standard then we would need 100mm of foam insulation on top of the floor reducing the door height to 1.86m leading to banged heads. So we cannot afford to lose height here. (See Note 1)

So we will have to go with the current value and use a warm floor covering and then do what we can to minimise or eliminate air leakage and see what we can do with other parts of the rooms.

The nominal U value for such a concrete floor will be assumed to be 0.7 W/m2/oK.

One other factor in our favour is that the main area of solid floor is in the south facing lounge area where we must seek to make best use of solar gain during the cooler months.

Suspended timber floors

Here we have more hope of being able to reduce the heat loss.

The process is very disruptive as practically the whole of the floorboards have to be raised so that the insulation can be fitted below, and then replaced. Doing this means that everything standing on the wooden floor also has to be removed.

There is an additional complication in that active wood boring insect damage has been discovered in the loft. Not a major infestation, but it should be treated before we move in. As the floorboards are to be lifted and insulation inserted below it would seem a sensible precaution to treat the floor area as well. So co-ordination between trades will be necessary to make the best of this ‘opportunity’.

We had hoped to use a natural material to insulate the floor, specifically hemp or wool batts as these are more eco friendly in their manufacture and eventual disposal.  Unfortunately the thermal resistance of this material means that, to get to the target U value, a thickness of 150mm would be needed (Note 2). But the floor joists are only 100mm deep. This type of insulation is installed with supporting netting stapled to the joists and, to accommodate the additional depth (as the joists are only 100mm deep) would have to be hanging below the joists and, consequently would sag meaning that an air gap would develop between the insulation and the floorboards further reducing the insulation’s effectiveness. So, to get a good fit and a good U value it looks like a foil backed manufactured insulating board (Celotex/ EcoTherm/ Kingspan) will have to be used.

The joists under the floor are assumed to be 2” x  4” being the standard construction for the time. Unfortunately we are not yet able to lift the covering and a board or two to check. So that gives 100mm from the underside of the floorboard to the bottom of the joist.

Using the Celotex U value calculator ( we input values for the P/A ratio and the required insulation thickness.

The P/A ration is a ratio calculated by dividing the total length of the exposed perimeter of the floor to be insulated (in metres) divided by the area of the floor (in square metres). As we are insulating the whole area I have taken the total perimeter and divided it by the total area and come up with a little over 0.5.

For the second parameter we are limited by the size of the joists, so select the product that is 100mm thick.

The result is a U value of 0.22 W/m2/oK.

However, the standard method of fixing this type of insulation is to fix small battens close to the lower edge of the joists and then cut the insulation to fit between the top of the batten and the floorboard. Using this method means that about 25mm of the 100 mm of the height of the joist is lost.

Repeating the calculation for 75mm gives a U value of 0.26 W/m2/oK., a reduction of about 20%.

But, checking the Celotex website they can supply the ‘Celotex Insulation clip’ (See datasheet here – . This pushes into the side of the foam batt and has a flat protuberance that holds the block in place to the top of the joist without nails or battens.

Another ‘improvement’ idea that came up arose because the clips hold the top of the board in place. In that case we could use a board that is thicker than the joists which could improve the U value. However, a call to Celotex’s Technical Helpline disabused us of this approach as they do not recommend this practice.

Notes and References

Note 1 – Reference

Note 2 – Black Mountain Insulation Ltd. product information, recommended thickness for Part L, Floor.

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