Construction calculated data

Calculated tab on Constructions Dialog.

This tab provides further information on the heat transmission properties of the construction.

U-values and R-Values are shown including and excluding the effect of surface resistance and are calculated with and without bridging effects.

Note that when working with library constructions, the outer surface resistance is calculated assuming a 'typical' exposure to wind. When working with model constructions the outer surface resistance is calculated using the wind exposure as defined on the Location tab at site level.

Convective heat transfer coefficients

The convective heat transfer coefficients displayed on this tab are only used in EnergyPlus when the 'CIBSE' Inside/Outside convection algorithm is selected. Otherwise EnergyPlus uses its' own convection algorithm as set in the simulation options and the transmission data displayed here is not used.

Km - Internal heat capacity (thermal mass)

The K_m value is the effective heat capacity per unit area and is a measure of the thermal mass storage characteristics of the construction. The higher the number, the more heat will be stored for release later.

The heat capacity or kappa value per unit area, kJ/m²K, for a construction element is calculated following the UK NCM requirements as:

k_m = Σ (t_i d_i c_i) x 10^-6

where:

The calculation is over all layers (i) in the element, starting at the inside surface and stopping at whichever of the following conditions is encountered first, possibly part way through a layer:

The total thickness of the layers exceeds 100mm.
The mid point of the construction is reached.
An insulation layer is reached (defined as thermal conductivity <= 0.08 W/mK).

t_j is the thickness of layer (mm)

d_j is density of layer (kg/m³)

c_j is specific heat capacity of layer (J/kg·K)

Tip: In simple terms, K_m can be thought of as the thermal heat capacity per area of the innermost 100mm of material in a building element before the insulation layer(s). Or, in elements that are less than 200mm thick, of the innermost half of the construction layers that occur before the insulation layer.