Timber aids insulation of slab-on-ground floors

By - , Build 100

BRANZ has been researching how to improve the insulation of concrete slab-on-ground floors and has found an alternative method which is practical and improves performance.

Figure 2: Floor slab insulation with timber – single storey.
Figure 1: Floor slab insulation – single storey.
Figure 3: Floor slab insulation with timber – single storey with 140 mm stud depth.

When determining if a concrete slab-on-ground floor requires insulation, the first step is to estimate its thermal resistance when uninsulated. NZS 4214: 2006 Methods of determining the total thermal resistance of parts of buildings includes a formula for estimating the average thermal resistance of slabs.

The formula requires knowledge of the length and width of the floor, the thermal conductivity of the soil beneath the slab, and the width of the external walls of the building. The dimensions of the slab, not just the area, are important because the formula takes into account the ratio of slab area to perimeter length. External wall width is important because it defines for the slab the length of the path of least thermal resistance between the interior and exterior environments. Increasing the width of the wall increases the slab’s thermal resistance. NZS 4214: 2006 Table E6 contains results using the formula, which assumes a typical soil thermal conductivity of 1.2 W/m°C.

Figure 1: Floor slab insulation – single storey.

Thermal resistance difficult to estimate

Accurately estimating a slab’s uninsulated thermal resistance can be difficult in practice as it requires knowledge of the thermal conductivity of the soil the slab is sitting on. The uninsulated concrete slab’s thermal performance is primarily dependent on the thermal conductivity of the soil beneath the floor, not the concrete. The slab itself adds thermal mass but usually provides minimal thermal resistance (typically 0.05 m2°C/W).

The thermal conductivity of soil is typically half that of concrete and the ground beneath a slab provides significant thermal resistance. This resistance is greatest at the centre of the slab, and least at the perimeter, because of the different lengths of the heat flow paths to the exterior. At the perimeter of an uninsulated slab, the heat flow is greatest in the area between ground level and the top surface of the slab. There is no soil to thermally shield the concrete so a relatively good heat transfer path exists from the central area of the slab.

The thermal conductivity of soil can be difficult to calculate, because it is dependent on several factors including the soil composition and moisture content. For example stony material will conduct a lot more heat than clay, which in turn has a higher conductivity than sandy soil. Often, the height of the local water table will determine the need for additional insulation.

Rigid foam insulation

When the thermal performance of a slab is important to overall thermal design, the only practical solution is to add rigid foam insulation. Adding insulation is particularly important:

  • for small slabs
  • when heating elements are incorporated into the slab
  • when the water table beneath the slab is less than 1 m below the surface
  • when the slab is designed to act as a thermal store for solar heat gain.

Table E7 of NZS 4214: 2006 provides multiplier factors to account for the effect of insulation added to the perimeter of a slab. These multipliers assume the layout of the insulation is as shown in Figure 1. For a typical 100 m2 slab, the improvement in overall thermal resistance by adding insulation with an R-value of 1.2 m2°C/W (e.g. 50 mm of rigid foam insulation) is about 10%, which means the R-value increases by 0.13 from 1.33 to 1.46 m2°C/W. The effectiveness of this perimeter insulation is, therefore, only about 10% of the material R-value. Primarily, this is because the location of the added insulation is doing nothing to limit heat flow along the slab and into the foundations.

Timber addition improves insulation

An alternative means for insulating a slab-on-ground floor uses timber in addition to insulation under the slab (see Figure 2). The timber limits the heat flow between the slab and foundations. Foam insulation provides better thermal isolation (thermal break) than timber but is vulnerable to point loads from furniture etc. Insulating a slab-on-ground floor in this way has the added benefit of separating off the thermal mass associated with the foundations, which is much less effective as thermal storage than the thermal mass in the slab.

It is possible to improve the performance even further by locating the insulation under the full width of the slab, rather than just the outer edge (as in Figure 2). This makes the performance less dependent on the relatively unpredictable local soil composition and water table.

Table 1 gives the estimated R-values assuming a typical soil conductivity of 1.2 W/m°C and the addition of rigid insulation with a thermal resistance of 1.25 m2°C/W (either 30 mm of extruded polystyrene (XPS) or 55 mm of ‘S’ grade expanded polystyrene (EPS)). The results are also compared with the traditional edge insulation method which does not include the thermal break between the slab and foundations.

Adding the thermal break along with the insulation doubles the improvement in thermal resistance from adding the insulation without the thermal break. For a 140 mm stud depth (see Figure 3), insulation under the full width of the slab and a thermal break between the slab and foundation almost doubles the thermal resistance relative to the uninsulated floor with a 90 mm stud depth.

Table 1: Estimated R-values for a variety of slab-on-ground insulation options.
Ratio of slab area to perimeter length1.522.533.5
Examples for a square slab (m x m) 6 x 6 8 x 8 10 x 10 12 x 12 14 x 14
Area (m2) 36 64 100 144 196
Examples for a rectangular slab (m x m) 8 x 5 12 x 6 17 x 7 18 x 9 20 x 11
Area (m2) 40 72 119 162 220
      Thermal resistance m2K/W    
90 mm stud depth          
Uninsulated 1.0 1.2 1.4 1.7 1.9
Without thermal break between slab and foundation          
1 m wide insulation 1.1 1.4 1.6 1.9 2.2
Fully insulated slab 1.3 1.6 1.8 2.1 2.4
Including thermal break between slab and foundation          
1 m wide insulation 1.3 1.6 1.9 2.2 2.5
Fully insulated slab 1.5 1.8 2.1 2.5 2.8
Fully insulated slab using R1.7 insulation instead of R1.25 1.6 1.9
140 mm stud depth          
Including thermal break between slab and foundation          
1 m wide insulation 1.6 1.9 2.2 2.6 2.9
Full width insulation 1.9 2.3 2.6 3.0 3.4
Figure 2: Floor slab insulation with timber – single storey.
Figure 3: Floor slab insulation with timber – single storey with 140 mm stud depth.

For more

The ‘Build right’ on page 32–33 details how to insulate a slab-on-ground floor using this method.

This research was funded by the Building Research Levy.

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More articles about these topics

Articles are correct at the time of publication but may have since become outdated.

Figure 2: Floor slab insulation with timber – single storey.
Figure 1: Floor slab insulation – single storey.
Figure 3: Floor slab insulation with timber – single storey with 140 mm stud depth.

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