Technical Bulletin
Domestic/Existing
Dwellings
Technical Bulletin: A Guide to Measurements for DEAs
Aug 2015
Not a member? Switch to Elmhurst Energy Today!
Energy Assessors regularly visit properties where they are required to measure bay windows, encounter
vaulted or uneven ceiling heights, or find that rooms within the same building part have height levels which
need to be averaged, not to mention discovering the widths of walls have different thicknesses. This guide has
been compiled to assist the DEA with the various scenarios which present themselves in the course of a
working day.

Bay Windows
Bay windows should be measured and included in the floor area of the
dwelling (except those that project externally from cill height only, as
shown on the left).
We appreciate that the methods for curved bays illustrated below may
not always give a completely accurate result, but particularly if the
dwelling has multiple bay windows the overall floor area of the
property will be more accurate using one the methods shown – never
ignore bay windows which extend to the floor as this can have a major
impact on the area of the dwelling, which appears on the finalised EPC.
Bay windows are found in two basic shapes:
Shape 1 – a trapezium. To calculate the
area of a splayed bay, measure at the
two points shown by the red arrows
above. Multiplying one measurement by
the other will give the area of a rectangle
which is equal to the trapezium shape.
To obtain an accurate HLP, measure the
two splays and the length of external
wall in between, add these together and
add them to the dwelling’s HLP. Finally,
subtract the length across the opening to
the room (shown by the green arrow
above) to indicate an accurate HLP.
RdSAP Technical Bulletin
TB60 A Guide to Measurements for DEAs
Version 1 August 2015
Shape 2 – a semi circle. To calculate the area of a
semi circle measure the radius (shown by the arrow
above). In this example, let’s assume the radius is
0.42 of a metre. Multiply this figure by itself then
by 3.142 as shown.
0.42 x 0.42 = 0.1764 then calculate
0.1764 x 3.142 = 0.5542
Divide the 0.5542 by 2 = area of half the circle.
As with angled bays, the HLP will be greater than a
property without bay windows. Adding the depth
of the bay to the HLP as it would be if there was no
bay window will create a sufficient increase.
Technical Bulletin
Domestic/Existing
Dwellings
Technical Bulletin: A Guide to Measurements for DEAs
However, most bay windows do not comprise a complete a semi circle – read on....
Shape 3 – a portion of a semi circle. It is
likely that the bay’s depth and shape will
not include the grey shaded area. In this
scenario, you will need to calculate the
half circle area, then deduct the area of
the rectangle. Once again, adding the
depth of the bay (shown by an arrow,
above) to the HLP as it would have been
for a ‘normal’ window set in line with the
external wall will give an acceptable
increase.
Shape 4 – angled turret shapes. These may be
best approached as more than one shape
joined together. Plot the shapes and sizes and
add the two resulting areas together.
LH example = triangle + rectangle
RH example = 2 x trapezia added together
RdSAP Technical Bulletin
TB60 A Guide to Measurements for DEAs
Version 1 August 2015
Shape 5 – a Triangle. The area of a
triangle is calculated by multiplying the
length of the base (green arrow) by the
height (red arrow) and dividing by two.
Aug 2015
Technical Bulletin
Domestic/Existing
Dwellings
Technical Bulletin: A Guide to Measurements for DEAs

Aug 2015
Uneven Ceiling Heights
The photo shows the rear
elevation of a typical terraced
house. The roof of the rear
projection slopes and the first
floor room inside is usually
shaped like the diagram shown,
right, with a portion of flat ceiling
which then changes to follow the
roof shape.
1.83
high
2.41
high
Once you have taken the usual measurements for the floor area of the rear portion of the dwelling, you will
need to take three more measurements, as indicated by the blue, green and red arrows before being able to
average the ceiling height. Then:



Split the room in two where the flat ceiling ends.
Divide the floor area into two parts, (1) the area using the width of the flat ceiling multiplied by the
length of the room and (2) the area covered by the sloping ceiling multiplied by the length of the room
Calculate the average height of the sloping portion of the ceiling by adding the measurement taken at
the red point and the measurement taken at the blue point and dividing by 2 = 2.12m
Using an example where the room with the sloping ceiling measures 3 metres by 2 metres, (a total of 6 metres
square) and the flat ceiling extends for 0.90 metres record the following information:
Multiply 0.90 by 3 metres (the length of the room) =
2.7 m2 Insert this figure on the left hand side of the
table. Multiply 2.7 by the measurement taken where
the red arrow is located = 6.51 and enter this figure in
cubic side of the table
Repeat the process for the sloping side – the entire
room is 2 metres wide. Deduct the flat ceiling length
(0.90), and multiply the remainder (1.1 m) by the
length of the room (3 m) = 3.30 metres. Insert this
figure on the left hand side of the table. Multiply 3.30
by the average height of the sloping ceiling (2.12) and
enter this (6.99) in the cubic side of the table.
Total enter a total for both columns and divide the total of B by the total of A. This result is the average ceiling
height – in this example 13.50 divided by 6.00 = 2.25 metres.
RdSAP Technical Bulletin
TB60 A Guide to Measurements for DEAs
Version 1 August 2015
Technical Bulletin
Domestic/Existing
Dwellings
Technical Bulletin: A Guide to Measurements for DEAs
Aug 2015
This method works equally well for rooms of differing heights, but without any slope to the ceiling......
Living Room
8 x 4.5 metres
Dining Room 3 x
3.70 metres
Ceiling 2.53
metres high

Ceiling 2.34
metres high
Taking Internal Measurements:
When measuring a property internally, unless you have a specific need to do so, do not measure each room
individually. Where possible measure across from one external wall to the other. In some houses, there are
only one or two places where this is possible – see the arrows below:
In the example shown it was possible to measure the
length of the dwelling only where the red arrows have
been inserted.
The green arrows show the positions at which the width
measurements could be recorded.
It may be necessary to prop door(s) open to achieve this
method, but it will result in an accurate representation
of the floor area of the house. Measuring individual
rooms is more difficult and if the widths of several walls
are not taken into account the floor area of the dwelling
can be under-represented on the finalised certificate.
RdSAP Technical Bulletin
TB60 A Guide to Measurements for DEAs
Version 1 August 2015
Technical Bulletin
Domestic/Existing
Dwellings
Technical Bulletin: A Guide to Measurements for DEAs

Aug 2015
An unseparated conservatory:
In the example shown on the left, the conservatory is
clearly not separated from the main dwelling. Measure
and record the following:
 The floor area
 The length of the glazed external walls
 Inspect and record the glazing; single or double?
 Assess the height – in this example it would
one-and-a-half storeys.
Do not include the area of the conservatory in the floor
area of the main dwelling. This is input separately in
Question 5 of the software.

Area weighted wall widths:
Wall thickness can normally be measured at window or door reveals. However, where this is not possible (e.g.
windows are locked and cannot be opened), it may be necessary to use another method of identifying wall
thicknesses, such as a comparison of internal and external measurements or by counting bricks.
If it is not possible to measure wall thickness the software will revert to the existing wall thickness assumptions
based on Table 3 of Appendix S. However, the reason(s) why the thickness could not be ascertained should be
recorded in the assessor’s site notes (or added as an annotation if using RdSAP GO/GO Capture).
If the wall thickness varies between different elevations or storeys, a weighted average should be calculated.
For example, a mid terrace house with a rear elevation of 250mm and the front elevation is 350mm where the
two walls are the same length, the calculation would be (0.50 x 250) + (0.50 x 350) = 300mm. Wall thicknesses
of up to 1500mm are permitted in RdSAP 9.92. In the example above, the walls are the same length (50/50)
but if the walls are of differing lengths, the proportions would need to be calculated first and allocated
appropriately, as follow:
In the example, left, two of the three solid brick walls are 350 mm (red
lines) with the remaining elevation (green line) measured at 230 mm.
The calculation therefore would two-thirds and one-third – ie (0.65 x 350
mm = 227.50) + (0.35 x 230 mm = 80.50). Add together the two results to
find the weighted average of 308 mm.
RdSAP Technical Bulletin
TB60 A Guide to Measurements for DEAs
Version 1 August 2015
Technical Bulletin
Domestic/Existing
Dwellings
Technical Bulletin: A Guide to Measurements for DEAs

Aug 2015
Stone Walls
When inspecting a stone property, take measurements at various
points to check that the walls are of the same depth, as it is not
unusual for stone walls to vary in thickness within the same
dwelling.
Note 1: When the thickness of stone walls varies by more than 100mm between different elevations
the alternative wall facility should be used accordingly.
Differing Thicknesses: Where there are walls within a building part that are of the same construction
type, age and insulation but of differing thicknesses, they should not be treated as alternative walls
unless they are of stone construction.

Recording Loft Insulation & Wall Depths
When photographing loft insulation two shots are normally needed – one of the ‘loftscape’ so that the extent
of the insulating material is recorded, and one of the actual depth. Maintain three points of contact with the
ladder at all times, and invest in a ‘school style’ rigid ruler which can be propped or wedged against a joist to
verify the measurement when photographed.
For wall depths, we ask you not to use a laser device for measuring, because some models are not accurate
over short distances. Additionally, it is often not possible to relate the measurement to what is being
measured - a photo of a laser measure reading is not in context. A ruler or tape measure shown across a wall
thickness provides strong evidence of your input.
Not a member? Switch to Elmhurst Energy Today!
RdSAP Technical Bulletin
TB60 A Guide to Measurements for DEAs
Version 1 August 2015