The thermal performance of timber floors may be improved by fitting installation between the joists, immediately beneath the deck. This construction may be used to upgrade existing floors or to create new floors. Timber floors in extensions can be easily matched to the levels of existing floors.

The properties of Ballytherm Insulation match those required for this application: it is robust and self-supporting, whilst its low thermal conductivity minimises the thickness of insulation between the joists.

The Ballytherm boards are installed between the joists, resting on battens fixed to the sides of the joists or on nails. The boards may be set either directly under the flooring with no air gap or, close to the base of the joists, leaving voids between the boards and the flooring.

In the latter construction the reflective surface of the boards will result in a series of low emissivity air spaces which will cut the rate of radiation heat loss across the cavity and improve the thermal performance of the floor.

The thickness of Ballytherm Insulation required to meet regulations may be determined from table 7.

Design guidance
Any Radon barrier should be installed within the concrete covering of the void beneath the floor and should extend across the entire footprint of the building.

To ensure the insulation extends to the perimeter of the floor Ballytherm Insulation should be packed between the inner surface of a wall and the joist which runs next to it (figure 15).

Service penetrations should be designed out wherever possible: at any unavoidable penetrations the Ballytherm boards should be cut neatly to accept the pipework or cable and the remaining gap sealed. An alternative method is to fit the Ballytherm boards at the base of the joists to form a series of service voids (figure 16).

Sitework
Installation sequence

1. Install joists at specified centres.
2. Fix timber battens to the sides of the joists.*
3. Cut Ballytherm boards to fit between the joists and set them on the battens.
   
4. Lay and fix flooring.

* The distance from the top of the battens to the top of the joists will depend upon the thickness of Ballytherm board specified and whether a service void is specified.

Points to watch

• Cut Ballytherm boards carefully to give a snug fit.
• Pack insulation between the perimeter joists and the wall to avoid thermal bridging.
• Seal service penetrations with flexible sealant.