ALF calculations and floors.

Slab-on-ground floors and suspended floors have very different thermal characteristics, so ALF calculates them using different methods.

Slab-on-ground floors

A slab-on-ground floor R-value consists of the effective R-values of the slab and the ground and the R-value of any floor covering.

Where there is no insulation under the slab, the effective R-value of the slab itself is negligible in relation to the R-value of the ground. In these cases the R-value of the ground can be treated as equivalent to the R-value of the ground and slab together.

The effective R-value of the slab and ground depends on the insulation, the size of the slab, the thickness of external walls supported by the floor and the soil conductivity. It is given as a function of floor area to perimeter length ratio and external wall thickness. In ALF the effect of slab floor insulation is specified as multiplication factors.

The numeric expression of the ground R-value is:

\begin{equation} R_{Ground} = \frac{\pi A}{k P} \times \frac{1}{ln \left\{ (1+x) \times 1 + 1 / x)^x \right\}}

\text{ with } x = \frac{l d}{t \times (l + d)} = \frac{2A}{t P} \end{equation}


  • d = half width of the floor (m)
  • l = half length of the floor (m)
  • t = external wall thickness (m)
  • k = soil conductivity (W/m°C)
  • A = floor area (m2)
  • P = floor perimeter (m)

Soil conductivity

The conductivity and heat loss of the ground under the slab floor can be significantly greater for wet soils than for dry ones. Soil conductivities can range from as low as 0.5W/m °C for very dry soil, through 1.2W/m °C for average soil, to as high as 2.0W/m °C for wet soil. In-situ measurements of soil conductivity may lead to quite significant adjustments of the ground R-value. For sites with high water tables, using the average value of 1.2 W/m °C may severely under-predict the heat loss.

For a very wet site, edge insulation on its own is not very effective. Adding horizontal extruded polystyrene insulation under the whole slab or using an insulated suspended floor gives better thermal performance.

Slab floor area

Retaining concrete walls have similar heat losses as slab-on-ground floors and can therefore be treated as slabs-on-ground. In this case, include the perimeter length of the walls as well.

Perimeter length

Most of the heat lost through a floor escapes through the edges of the floor. The ratio of the perimeter length and the slab floor area can therefore be used to describe the edge exposure of the floor plan. A more 'square' house has a smaller ratio and edge exposure than a house with a more complicated floor plan. The program calculates the ratio and the loss automatically.

External wall thickness

The thinner the external walls, the more heat escapes through the edges of the floor.

Edge insulation width

Common insulation widths are 500 mm or 600 mm. The effect of the edge insulation is similar for insulation thicknesses between 25 mm and 50 mm for common insulation products.

Edge insulation depth

If only the edge of the floor is insulated (with polystyrene sheets) enter the depth of the insulation sheets.

In terms of thermal performance, a vertical sheet of insulation reaching as low as possible (600–1000 mm) gives better thermal performance than a horizontal sheet under the slab, unless it insulates the footing as well.

For more information, see the following BRANZ resources:

Suspended concrete floors have similar heat loss characteristics to suspended slab floors and therefore the R-value of suspended concrete floors is similar to that of timber floors. Where no specific R-value information is available, suspended concrete floors should be treated as suspended timber floors.

Suspended overhangs should be considered as suspended timber floor areas.

Suspended timber floors

A suspended floor R-value is the sum of three factors:

  • the R-value of the floor structure and floor insulation;
  • the R-value of the floor covering (carpet, tile, etc.);
  • the R-value of the sub-floor component.

The R-value of the subfloor component is the insulating effect of the air contained below the floor. This depends on the foundation type (enclosed perimeter wall or pole house) and exposure to wind. Pole houses do not have an enclosed subfloor and their subfloor R-value is zero.

The ratio of ground floor area and perimeter wall area influences the subfloor R-value. If the ground floor area is large in respect to the perimeter wall area (i.e. a generally more complicated, 'longish' floor plan), the wind attack area is increased and the R-value is reduced.

Ground floor area

This is the area of the floor which is above the ground and exposed to the subfloor. It does not include floor areas of second storeys etc.

Perimeter height

The perimeter height (or perimeter wall height) is the height of the wall around the subfloor (crawl space). Enter the foundation height if there is no actual wall around the perimeter, such as in a pole house. The foundation height determines the amount of air passing underneath the house. The more air that moves through the subfloor space, the more heat is lost through the suspended floor.

Perimeter wall type

A perimeter wall reduces the air change underneath the house and thus increases the R-value of the suspended floor.

Subfloor R-value

The subfloor R-value is calculated using the ratio of the ground floor area and the perimeter area as well as the subfloor exposure. Most of the heat lost through a floor escapes through the edges of the floor. The ratio of the ground floor area and the perimeter area is used to describe this exposure of the floor plan.

Floor covering r-values

Common floor covering R-values are:

Material R-value
Carpet and underlay 0.4
Cork tiles (3 mm thick) 0.05
Slate or ceramic tiles (8–12 mm thick) 0.01
Vinyl tile or PVC sheet 0.01
Any high density trowelled or plastered finish 0.1

If only part of the floor is covered, estimate the floor covering R-value by multiplying the R-value with the proportion of floor area covered. This approximation is fairly accurate where the slab and ground R-value is greater than R1.

With slab-on-ground floors, carpet floor coverings reduce the ability for thermal mass to heat the building. This leads to smaller warm up energy requirements but also decreases the usefulness of gains.

For compliance with the BPI calculation the floor covering R-value is set to zero.