Insulating floors of a house extension
New ground floors should be insulated to the Best Practice standards.
The most common types of new ground floors are:
• ground-bearing concrete slabs;
• suspended pre-cast concrete ‘beam-and-block’ floors;
• suspended timber floors.
Ground-bearing concrete floors can be insulated by placing insulation beneath the slab (see Figure 3) or above
the slab, beneath a screed or timber floor.
In the case of insulation beneath the slab, rigid insulation should be used, and if the insulation thickness
exceeds about 75mm it may be necessary to include some steel reinforcement mesh in the slab itself. Depending on
the size and shape of the floor, up to 100mm of high-performance insulating material (i.e. one with low thermal
conductivity, not more than 0.03W/mK) may be required to meet the Best Practice standard. The perimeter of the slab
should also be insulated.
If the insulation is placed above the slab, and the floor is finished with timber, a vapour control layer should
be included beneath the timber finish, as shown in Figure 4. Again rigid insulation should be used. It is not
usually practical for the thickness of the insulation to exceed about 100mm, so a high performance insulating
material (i.e. one with low thermal conductivity) may be required to meet the Best Practice standard.
Suspended pre-cast concrete ‘beam and block’ floors are usually insulated above the floor, as shown in Figure 5.
Again rigid insulation should be used.The thickness of the insulation should not exceed about 100mm, so a high
performance insulating material (i.e. one with low thermal conductivity) may be required to meet the Best Practice
Another way of meeting the Best Practice standard is to use a proprietary type of floor in which the concrete
blocks are replaced by T-shaped blocks of rigid insulation spanning between the pre-cast concrete beams. This type
of construction provides good thermal performance, consistent with Best Practice, which can be further improved by
also insulating above the floor, as described above.
Suspended timber floors can be insulated by placing insulation between the joists, usually to the full depth of
the joists. The insulation may be supported either on timber battens fixed to the sides of the joists or on netting
placed over the joists,