Opaque Constructions - Tabs & Ground-Contact Adjustment

Surfaces Tab

Outside surface:

Emissivity: the emissivity of the outside surface of the construction. Values are provided in Table 23 in the Apache Tables document. Most materials have an emissivity of about 0.9. Lower values apply to unpainted metals.

Resistance: the thermal resistance between the outside surface and its environment. This is the reciprocal of the outside heat transfer coefficient, which is made up of convective and radiative components. Ticking the default box displays a standard value determined from the construction category, together with the Wind exposure in the case of external adjacency. The default value is used in the ApacheCalc programs. In ApacheSim it is replaced by algorithms that take account of the changing heat transfer conditions at every time step. If the default box is not ticked, the entered value is used by all programs.

Solar absorptance: the fraction of incident solar radiation absorbed by the surface. This is a function of the colour and surface finish. Typical values are given in Table 14 in the Apache Tables document or CIBSE Guide A.

Inside surface:

Emissivity: the emissivity of the inside surface of the construction.

Resistance: the thermal resistance between the inside surface and its environment. This is the reciprocal of the inside heat transfer coefficient, which is made up of convective and radiative components. Ticking the default box displays a standard value determined from the construction category. The displayed default value is used in the ApacheCalc programs. In ApacheSim it is replaced by algorithms dependent on simulation options. If the default box is not ticked, the entered value is used by all programs.

Functional Settings Tab

This tab is only displayed if the construction belongs to the Ground Floor, External Wall or TSC Wall categories.

Ground-contact Floor or Ground-contact Wall: by ticking this box, a construction can be identified as a ground-contact floor or wall.

Adjust: When the Ground-contact floor/Wall box is ticked the Adjust button may be used to adjust the construction properties to make allowance for the building geometry.

CIBSE uninsulated U-value: For the purposes of Part L2 of the UK Building Regulations, the U-value of an uninsulated floor calculated using the parameters set in the Ground-contact U-value adjustment dialog (accessed via the Adjust button). These parameters must be correctly set for Part L2 analyses. The rules state that where the U-value of an uninsulated floor of the same area and exposed perimeter is less than 0.25 W/m²K, a floor with this U-value must be used in the notional building. If the uninsulated U-value is less than 0.25 W/m²K it will be used to create a bespoke floor construction that will be automatically assigned to the relevant floors in the notional building.

Ground-contact U-value adjustment dialog (Adjust button)

This dialog is accessed via the Adjust button on the Functional Settings dialog for exposed floors, external walls and TSC walls which becomes active when the Ground-contact Floor/Wall box is ticked.

The dialog provides a means for adjusting the properties of the construction to make allowance for 3-dimensional heat transfer through the building element and the adjacent soil. The adjustment is a function of building geometry and a set of parameters interfaced on the dialog.

Analysis method: Either the EN-ISO 13370 standard or the ASHRAE F-factor (Heat Loss Coefficient) method may be as the basis of the adjustments. This choice affects the input parameters presented on the dialog.

Parameters and options applying to both methods:

Set ground-contact floor plans: This button invokes the Ground-contact U-value adjustment dialog, which allows the following parameters relating to the building geometry to be set either manually or automatically for each level where a floor is in contact with the ground (use Add level, Insert Level and Delete level):

Level: Assigned automatically to 0, -1, ...

Depth below ground: The depth of the floor surface below ground level

Floor plan area: The total area of ground-contact floor at this level

Floorplan exposed perimeter: The exposed perimeter length of the floorplan at this level

Characteristic dimension: A property of the floor geometry with the dimensions of length, derived automatically as

2 x Floorplan area / Floorplan exposed perimeter

Update from building geometry: This button automatically updates the above parameters from the current building geometry.

Level: Select the level (0, -1, …) to which the construction is to be applied.

Characteristic dimension, Floor depth, Exposed floor perimeter, Floor area: The values of these parameters are copied from the dialog Set ground-contact floor plans. In the case of the first two the values may be overridden after ticking the adjacent box Override for U-value calc?

Note on F-factor method. In the case of floors, the adjusted U-value is calculated to give the correct overall heat transfer coefficient when applied to the entire ground-contact floor area. Users applying the F-factor method may wish to apportion the heat transfer to perimeter rooms only. In this case the Characteristic dimension for the perimeter room floor construction should be calculated as

2 x Perimeter room area / Floorplan exposed perimeter

and a construction with a low U-value should be assigned to non-perimeter rooms.

Apply adjustment?: Tick this box to apply the adjustment to the construction layers on exit from the dialog. An insulating layer will then be added to the construction beneath the soil layer (or if such a layer is already present it will be modified) to achieve the appropriate adjusted U-value.

Parameters applying to EN-ISO 13370 method:

Ground conductivity: The conductivity of the soil. If Copy from layer data? is ticked the value is taken from the soil layer of the construction (when present).

Floor resistance: The combined thermal resistance of the floor layers, excluding the U-value adjustment layer (when present) and any other layers with a conductivity greater than 0.2 W/mK. If Copy from layer data? is ticked the value is derived automatically from the properties of the construction layers.

External wall thickness: The thickness of the external walls of ground-contact spaces.

Edge insulation type: The type of edge insulation (Horizontal, Vertical or None).

Edge insulation width, thickness & conductivity: The width, thickness and conductivity of the edge insulation (where present).

Parameters applying to ASHRAE F-factor (Heat Loss Coefficient) method:

F-factor: The F-factor appropriate to the type of external wall construction. Suitable values are provided in the ASHRAE Handbook – Fundamentals, Chapter 18.