8 Slab-on-ground floors for heated buildings
8.1 Applicability
This clause applies to foundations for which Hf < H0 and to:
a) buildings in which the average internal air temperature throughout the building in each month is at least 17 °C (i.e. θi,m ≥ 17 °C for all m);
b) buildings in which some parts are heated and some parts are unheated, provided that in the heated parts θi,m ≥ 17 °C for all m, and that the unheated parts are treated as described in 8.5;
c) buildings in which 5 °C ≤ θi,m < 17 °C with the modifications described in 8.8.
If θi,m < 5 °C in any month, the frost protection of the foundations should be designed as for unheated buildings (see clause 10).
For data based on a design criterion of 0 °C below the foundations, see annex C.
8.2 General principles
In all cases, provide vertical edge insulation as specified in 8.6.
Heat from the building raises the ground temperature less at corners than along the sides of the building. Therefore additional measures may be needed at corners, either by having deeper foundations at the corners or by having additional insulation there.
This clause provides three options for achieving the necessary frost protection:
- 1) using vertical edge insulation only, with no ground insulation: excavate the foundations to the depth given in 8.7.1 (a greater foundation depth is needed at corners than along the rest of the walls);
- 2) using ground insulation only at the corners, to avoid increasing the foundation depth at the corners: the foundation depth is as for the walls in 1), see 8.7.2;
- 3) using a restricted foundation depth (not less than 0,4 m), with the same foundation depth all round the building: provide ground insulation all round the building, but increased at the corners, see 8.7.3.
- The foundation depth and/or the extent of the ground insulation is determined by the design freezing index, Fd.
Design the floor insulation to give satisfactory floor temperatures and energy economy (i.e. independently of the frost heave problem).
NOTE The use of vertical edge insulation and ground insulation increases floor surface temperatures and decreases heat loss at the edge of the floor.
8.3 Restrictions
8.3.1 Building width
The foundation depths and frost insulation specified in this clause apply to buildings with a width B of at least 4 m.
If B < 4 m the foundations should be designed, either in depth or in provision of ground insulation, according to the procedures given for corners, but applied all round the building.
8.3.2 Floor insulation position
The foundation depths and frost insulation specified in this clause apply to floors for which the floor insulation position h does not exceed 0,6 m.
If h > 0,6 m, either undertake numerical calculations in accordance with annex B or use the procedures for unheated buildings (clause 10).
8.3.3 Thermal resistance of floor slab
The thermal resistance of the floor construction, Rf, is the total thermal resistance between the floor surface and the soil. It includes any insulation layers above, below or within it, together with that of any floor covering.
If the thermal resistance of the floor construction varies over its area, take Rf as the average value over the outer 1 m of floor.
The foundation depths and frost insulation specified in this clause apply to slabs with Rf not exceeding 5 m2·K/W. If Rf > 5 m2·K/W, either undertake numerical calculations in accordance with annex B or use the procedures for unheated buildings (clause 10).
8.4 Ground insulation
Ground insulation shall be protected against risk of mechanical damage. The top surface of any ground insulation should be at least 300 mm below ground level, unless covered by paving in which case the depth may be reduced to 200 mm.
The data given on the width of ground insulation, bg, bgw and bgc, assume that this width is measured from the outermost face of the foundation.
NOTE It may be necessary for the total width of the ground insulation to be greater than bg, if the footing projects beyond the foundation wall, as in Figure 1a.
If ground insulation is used together with internal edge insulation, take care to avoid a thermal bridge by continuing the ground insulation beneath the foundation to meet the vertical edge insulation (see Figure 1c).
Ensure that ground insulation is continuous with no gaps, that it is adequately protected from excessive moisture by roof overhangs, sound guttering arrangements, etc. and that it is placed on a drainage layer.
