ALF help

Design help

On the Design pages you enter information about the particular house design and location to perform an ALF analysis on it.

Ground Floors

Background

On this page you can enter information about the size, type and insulation of suspended timber and concrete slab-on-ground floors. (Suspended concrete floors have similar heat loss characteristics to suspended slab floors. In case no additional information is available they should therefore be treated as suspended timber floors.)

Suspended overhangs should be considered as suspended timber floor areas for calculation purposes.

Suspended floors and slab-on-ground floors have very different thermal characteristics, and so they are calculated using different methods.

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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. generally more complicated, 'longish' floor plans), the wind attack area is increased, and the R-value is reduced.

For a more general discussion of R-values of suspended floors you may refer to BRANZ Bulletin 357, Thermal Insulation of Houses.

Ground Floor Area

In this cell enter the area of the floor which is above the ground. This does not include floor areas of second storeys etc. but only the floor areas which are exposed to the subfloor.

Perimeter Length

Enter the length of the floor perimeter. The perimeter of the floor is the length of the floor outline. For most houses this is the same length as the total length of external walls.

Perimeter Height

Enter the height of the perimeter. The foundation height (or perimeter wall height) is the height of the wall around the subfloor (crawl space). Also 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 moves through the subfloor the more heat is lost through the suspended floor.

Perimeter wall type

The subfloor exposure drop down list allows you to select whether the subfloor is exposed or not, and whether there is a perimeter wall around the subfloor or not. 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.

Underfloor insulation

In this cell the floor construction type is selected. This will give the R-value of the timber floor and the framing (joists) and any under floor insulation. You can select one of the most common floor construction types from the drop down list. If you select 'Custom' you can enter the Floor Insulation R-value directly in the cell. (The R-value of SUSPENDED concrete floors is similar to that of timber floors.)

Floor Covering R-value

In this cell you can enter the R-value of any floor covering. Common floor covering R-values are:

Material R-value
Carpet and underlay

R-0.4

Cork tiles (3mm thick)

R-0.05

Slate or ceramic tiles (25mm thick)

R-0.01

Vinyl tile or PVC sheet

R-0.01

Any high density trowelled or plastered finish

R-0.01

If only part of the floor is carpeted the Floor Covering R-value can be estimated by multiplying the R-value by the proportion of the covered floor area. If for example only 40% of the floor is covered with carpet the Floor Covering R-value is 40% * R-0.4 = R-0.16. This approximation is fairly accurate for cases where the Subfloor R-value is larger than R-1.

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Slab-on-Ground Floors

BRANZ has spent considerable effort over the years researching the thermal performance of slab-on-ground floors. These efforts have led to a revision of the calculation of the thermal performance of slab-on-ground floors. The formula takes into account the geometric outline of the floor, the insulation level of the floor, the thickness of the external walls supported by the floor and the ground water table.
A slab-on-ground floor R-value consists of two parts:

  • the R-value of the floor covering, as for the suspended floor, and
  • the effective R-value of the slab and ground.

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. Therefore the R-value of the ground can be treated as equivalent to the R-value of the ground and slab. The effective R-value of the slab and ground depends on the insulation used, the size of the slab (effective R-value is calculated for the whole area of the slab, while losses mainly occur out through the edges) and the thickness of external walls supported by the floor. 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:
with
and
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 conductivities can range as low as 0.5W/m °C and as high as 2.0W/m °C. In situ measurements of the soil conductivity may therefore lead to quite significant adjustments of the ground R-value. For sites with high water tables, the average values 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 is more appropriate.

Slab Floor Area

In this cell you can enter the slab floor area. This is the area of the slab, which is in contact with the ground. Retaining concrete walls have similar heat losses as slab on ground floors and can therefore be treated as slabs on ground, as well. In this case you must include the Perimeter Length of the walls as well.

Perimeter Length

Enter the length of the floor perimeter. 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

Enter the thickness of the external building walls (NOT the foundation walls!) in meters. The thinner the external walls the more heat escapes through the edges of the floor. Take the whole wall thickness including framing and veneer.

Soil Conductivity

Enter the conductivity of the soil underneath the slab. The conductivity and heat loss of the ground underneath the floor are larger for wetter soils than for drier ones. Common Soil Conductivity values are 0.7 for dry soil, 1.2 for average soil and 2 for wet soil. Larger conductivities can increase the heat loss of slab on ground floors significantly.

Slab Insulation Strategy

Choose the insulation strategy for a slab-on-ground floor from a pre-defined list. If only the edge of the floor is insulated then values for edge width and thickness must be entered.

Edge Insulation Width

If only the edge of the floor is insulated (i.e. with polystyrene sheets) enter the width of the insulation sheets. Common insulation widths are 500mm or 600mm. The effect of the edge insulation is similar for insulation thicknesses between 25mm and 50mm for common insulation products.

Edge Insulation Depth

If only the edge of the floor is insulated (i.e. 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 (i.e. 600 mm to 1000 mm) performs better than a horizontal sheet under the slab, unless it does insulate the footing, as well. However, because the performance depends on the exact details of the installation, ALF3 does not distinguish between the two options.

Floor Covering R-value

In this cell you can enter the R-value of any floor covering. Common floor covering R-values are:

Material R-value
Carpet and underlay

R-0.4

Cork tiles (3mm thick)

R-0.05

Slate or ceramic tiles (25mm thick)

R-0.01

Vinyl tile or PVC sheet

R-0.01

Any high density trowelled or plastered finish

R-0.01

If only part of the floor is carpeted the Floor Covering R-value can be estimated by multiplying the R-value with the proportion of the covered floor area. If, for example, only 40% of the floor is covered with carpet the Floor Covering R-value is 40% * R-0.4 = R-0.16. This approximation is fairly accurate for cases where the Slab and Ground R-value is larger than R-1.

Note that carpet floor coverings will decrease the ability for thermal mass to be used to heat the building. This leads to smaller warm up energy requirements, but it also decreases the usefulness of gains.

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

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