Core Concepts

Dimension Specification

The dimensions of a building can be measured in many ways. The Heat Load Calculator supports three different options for entering the dimensions of all walls, floors and ceilings/roofs:

• Net Internal
• Gross Internal
• Gross External

Net Internal

Net internal dimensions are the default option and would typically be measured whilst onsite surveying the property. They are based on the RICS Net Internal Area (NIA) measurement specification but done on a per room basis and measured vertically as well.

Walls

Wall widths are taken lengthways from the interior face of all adjoining walls, regardless of whether they are internal or external walls.

Wall heights are taken internally from the finished floor level (FFL) to the underside of the ceiling.

Floors

Floor measurements are taken from the interior face of all boundary walls, regardless of whether they are internal or external walls.

Ceilings

Ceiling measurements are taken from the interior face of all boundary walls, regardless of whether they are internal or external walls.

Gross Internal

Gross internal dimensions allow you to account for internal wall thicknesses and intermediate floor heights more precisely. They are based on the RICS Gross Internal Area (GIA) measurement specification.

Walls

Wall widths are taken lengthways from the interior face of all external or exposed walls to the centre line of internal walls.

Wall heights are taken from the floor structural level to the same level on the floor above, incorporating the intermediate floor height.

Floors

Floor measurements are taken from the interior face of boundary external walls to the centre line of boundary internal walls.

Ceilings

Ceiling measurements are taken from the interior face of boundary external walls to the centre line of boundary internal walls.

Gross External

Gross external dimensions would typically only be taken from building plans and include the thickness of external walls. They are based on the RICS Gross External Area (GEA) measurement specification.

Walls

Wall widths are taken from the exterior face of all external walls to the centre line of internal walls.

Wall heights are based on the total storey height of each floor level.

Floors

Floor measurements are taken from the exterior face of boundary external walls to the centre line of boundary internal walls.

Ceilings

Ceiling measurements are taken from the exterior face of boundary external walls to the centre line of boundary internal walls.

NOTE

Refer to RICS Code of Measuring Practice (2015) for more information.

Openings

Windows

Window dimensions should be measured horizontally from the recess surfaces and vertically from the sill to the underside of the lintel surface.

The window area and U-values will therefore incorporate the frame as well as the glazing.

Doors

Door dimensions should be taken such that the internally visible part of door frame is included in the measurements.

Any door with glazing covering 60% or more of the total area would be considered a window for heat loss purposes, e.g. bifold, patio and french doors.

Area Calculations

When entering dimensional areas of walls or roofs, there is no need to subtract the areas of any openings (doors, windows or roof windows). The Heat Load Calculator will deduct the opening areas automatically to compute the correct net surface area for heat loss purposes.

Sloped Ceilings

When measuring the dimension of sloped or vaulted ceilings, it is important to ensure the correct dimensions are entered to calculate the area accurately.

Ceiling measurements are taken according to the dimension specification used but at a angle parallel to the slope of the ceiling.

External Heat Loss Area

When calculating the total heat loss of Net Internal dimensions, all storeys other than the top storey will have 0.25m added to the wall heights to account for intermediate floor voids.

Volume Calculations

Where possible the Heat Load Calculator will try to estimate the internal air volume of rooms based on the ceiling area and wall heights entered.

When using Gross Internal dimension specification, 0.25m will be deducted from the mean wall height to estimate the ceiling height.

When using Gross External dimension specification, the internal air volume must be entered manually as it is not possible to estimate it.