System Total R-value Calculations (CIBSE Guide A)

The Speckel Floor System R-Value calculation approach has been derived from CIBSE Guide A Section 3.5 Ground Floors and Basements.

Following the guidelines of CIBSE Guide A, there are four modes of floor:

  • Ground Contact with edge insulation

  • Ground Contact without edge insulation

  • Suspended Floor (insulated)

  • Suspended Floor (uninsulated)


  • The floor is initialised as if it were in contact with the ground.

  • Relevant components for each type of floor are collected.

  • Determine if the floor is edge insulated or suspended above the ground and if so, collect other necessary components.

  • Using the complete collected components, these are combined into the final calculation to produce an R-Value of that floor type.

Ground Contact Collection

All floor types require several inputs:

  • floor area

  • floor perimeter

  • ground type (and its conductivity)

  • surrounding wall width

  • the thermal resistance of the floor itself.

NOTE: The thermal resistance of the floor is derived solely from ‘any all-over insulation layers above, below, or within the floor slab’, with some pre-described surface resistances added. In addition, the floor perimeter is solely the perimeter of the floor that is exposed to unconditioned space.

Edge Insulation Collection

There are two approaches for edge insulation - direct edge insulation applied to the floor and surrounding wall or when the surrounding wall has a lower conductivity than the ground. The first option is currently unavailable on Speckel, however, the check for the second option is always performed if the floor is not suspended.

If the floor is edge-insulated, components derived from the surrounding wall thermal resistance, thickness, and depth are collected.

Suspended Floor Collection

A floor is suspended when the user specifies a height above ground greater than zero metres. There are a few more inputs required when this condition is me;

  • wind speed at ten metres above ground level

  • the wind shielding factor

  • area of ventilation openings per unit perimeter of the surrounding wall.

There are two types of suspended floors, which then determines which calculation methodology is used: uninsulated and insulated, with the latter relying on the calculation of the former.

NOTE: The thermal resistance of the floor itself is treated differently here again. It is treated as a regular centre pane R-value calculation for uninsulated floors, adding R 0.17 twice for internal and external surface resistances. Insulated floors are treated as a thermally-bridged (if the design is deemed so) thermal resistance, omitting the surface resistances. Here, we use AS/NZS 4859.2:2018 to calculate and omit the surface resistances.


As mentioned above, the calculation methodology depends on the user's inputs. The base method is that of a simple ground contact floor, which is then used by all other methods (but is changed slightly when suspended).

  • If edge insulation is detected in the design, then the edge insulation factor is calculated, which will alter the final result.

  • If the floor is suspended, then the calculation is always for the uninsulated suspended floor.

  • If the floor is found to be insulated, the previous calculation (where the floor thermal resistance is considered without insulation) is then used in conjunction with its prescribed thermal resistance.

Last updated