Domestic and non-domestic energy performance certificates review: supplementary notes

4. Rooms in Roof

Do assessors need to measure rooms in the roof in detail? It would appear that depends. The RdSAP guidance has become more confusing on these issues. Maybe the question should be rephrased and split into two: 'what do you need to measure in rooms in the roof?' and 'when do you need to measure them?'.

4.1 What is a room in the roof?

Appendix S of the SAP 2012 methodology^[15] sets out that attic rooms and roof rooms are to be included within an RdSAP assessment of a property when they are accessed via a permanent fixed staircase such that one is able to walk downwards facing forwards. They do not need necessarily to contain habitable rooms – which is defined in RdSAP as a room with a window. To assist with the identification of rooms in the roof space, Appendix S and the RdSAP conventions contain illustrations of typical rooms (see Figure RIR1).

Figure RIR1: Illustrations of rooms in the roof^[16]

RdSAP has been designed so that the areas of the room in the roof components may be calculated using one of two default formulae:

"Where the roof room is not connected to another part of the dwelling:

A_rw = 11 √(F_rr/1.5)

where A_rw is the area of roof wall and F_rr the total floor area of the roof room

Where the roof room is connected to another part of the dwelling:

A_rw = 8.25 √(F_rr/1.5)

where A_rw is the area of roof wall and F_rr the total floor area of the roof room"

These formulae calculate the areas of the stud walls (i.e. verticals), the slopes and gable ends that make up the room in the roof (see Figure RIR2). The flat ceiling area of the room in the roof is taken to be the same as the room in the roof area in RdSAP^[17] so in reality the room in the roof as calculated by RdSAP would look more like the one in Figure RIR3 rather than the example set out in Figure RIR2, and is in keeping with the right hand illustration in Figure RIR1 above.

Figure RIR2: Illustration of the components that make up a room in the roof^[18]

Figure RIR3: Re-marked illustration of the components that make up of the room as derived from Room in Roof algorithms

Under the Convention 2.06 of the December 2017 RdSAP Conventions (i.e. Conventions v10),

"Detailed measurements of all elements are required only if evidence exists that the flat roof/slope/stud wall/gable wall have different levels of insulation or their U-values are known"^[19].

A subtle change was made to this December 2017 statement compared to previous versions of the RdSAP conventions with the insertion of 'flat roof' into the list components where evidence of different levels of insulation triggers the need for detailed measurements. Convention 2.06 before December 2017 did not include 'flat roof' into the list:

"Detailed measurements are required only if evidence exists that the slope/stud wall/gable wall have differing levels of insulation or their U-values are known."

The revised December 2017 convention also inserted the words 'of all elements', so the convention reads to the effect that if any part of the roof room components has different levels of insulation then all elements now have to be measured, where previously, detailed measurements were not required if the flat ceiling had a different level of insulation from the other room in the roof components.

As many dwellings have had the area above the flat ceiling insulated as part of one of the many loft insulation programmes or grant schemes, this convention would appear to now require the need for all the components of the room in the roof to be measured. Was this the intended implication of this change in the wording – to increase the number of roof rooms where detailed measurements were required? If so, the unintended consequences may be negative if assessors stop evidencing any insulation in the flat ceiling area so they are not then held to account for not measuring all the components in the roof room which can be quite time consuming.

Another potential source of different levels of insulation, is where the gable ends of the roof room are the gables of the dwelling, and these gables have been cavity filled.

Further confusion about when to measure the components of the room in the roof is introduced by the wording of another part of RdSAP Convention 2.06:

"Where detailed measurements are made and the floor area of the parts of the dormer windows protruding beyond the roof-line is less than 20% of the floor area of the roof room, measure the elements of the roof room as if the dormers were not there. Otherwise total the vertical elements of all dormers in that building part and enter as stud wall and the flat ceiling elements as flat ceiling."^[20]

So, if the floor area protruding beyond the roof-line is less than 20% of the total roof room floor area, you do not have to separately account for the dormer elements. If the floor area protruding beyond the roof-line is more 20% or more of the total roof room, does the assessor have to measure all the components of the roof room and account for the dormer? This is where the wording of this part of the convention is confusing, as it states, 'where detailed measurements are made', so if the assessor is not making detailed measurements or chooses not to make detailed measurements (because there are no different insulation levels within the roof room elements) then it would appear that the assessor would not have an account for the dormer. Nowhere in this convention statement does it state specifically or require that the assessor measure all the roof room components where the floor area protruding beyond the roof-line is 20% or more.

4.2 Half wall / 1.5 storey dwellings

So far, this note has confined the discussion to rooms built into the attic / loft space.

Within RdSAP, another type of roof room defined, and that is where the common wall of the dwelling extends upwards into an upper level of the dwelling but the height of this wall is less than 1.8m high where the plane of the roof meets the wall head for at least 50% of the common wall (excluding gable ends or party walls) (see Figure RIR4 below). This type of room in the roof construction was known as a 'half wall' room in the roof under the previous NHER methodology, but is referred to more colloquially in Scotland as a 1.5 storey cottage (or it would if the illustration included dormers).

Figure RIR4: Illustration of Common Wall Room in the Roof

In RdSAP, once this structure is 'defined' as a roof room, the program will assume the common walls of the upper level are a timber stud wall construction and, without any other information, will assume the default U-values for a timber stud wall for the relevant age band, rather than assume the 'stud' wall is actually a continuation of the main wall construction of the dwelling with the same U-value. In a pre-1919, stone built 1.5 storey detached cottage with a 600mm sandstone wall with drylining and no other insulation, the default external wall U-value would be 1.38 W/m²K in RdSAP; on the upper level, i.e. the roof room, RdSAP would assume this same wall has a U-value of 2.3 W/m²K – a rather significant difference^[21].

In such a dwelling, under RdSAP Convention 2.06, an assessor would be expected to measure all of the room in the roof components if the external wall had been insulated. As RdSAP does not take note of the fact that the roof room is a continuation of the half wall, it also does not automatically insulate the this upper level wall of the room in the roof (i.e. the continuation of the common wall) if the external walls of the property are insulated. Nor does it automatically insulate the gable wall of the room in the roof.

Would it not be better to separate out this type of a room in the roof construction into a separate category, or to allow for a half storey to be entered in the dimensional calculations? Assessors already measure the roof room total floor area so will have the basic dimensional data to calculate the perimeter. They may have measured the height of the common wall to demonstrate to their membership scheme that the common wall is less than 1.8m high. Why not use the perimeter length of the common wall, and the storey height of the common wall on the upper level, and overwrite the stud wall area calculated by the room in the roof algorithm, and use the U-value of the main external wall of the dwelling? If the main external wall was insulated, then so would be this vertical extension of the common wall.

It also seems such an easy question to ask if the gable walls of a room in the roof are of the same construction as the main external walls of the dwelling and if so, to use the external wall U-values instead of assuming the walls are timber stud walls with very different U-values.

Using the pre-1919, 1.5 storey detached sandstone cottage from above, adjusting the default RdSAP U-values of 2.3 W/m²K for the stud and gable wall to match those for the external wall (i.e. 1.38 W/m²K) improved the energy performance indicators across the board (see Table RIR1 below).

Table RIR1: RdSAP results for changing roof room default U-values to match external wall U-values

Roof room Dwelling	SAP score	EI score	CO2 emissions (tonnes / year)	Space heating fuel costs (£/year)
1.5 storey pre-1919 detached cottage with RdSAP roof room default U-values of 2.3 W/m2K for flat ceiling, stud walls, slopes, and gable walls	51	43	5.9	£1049
1.5 storey pre-1919 detached cottage with RdSAP roof room default U-values of 2.3 W/m2K for flat ceiling and slopes. Overwritten U-values of 1.38 for stud walls and gable walls	54	46	5.6	£982

Without requiring that all the components of the roof rooms are measured for the purposes of RdSAP, there seems to be straightforward modelling solutions that could be adopted within the RdSAP software to improve the calculation of the energy performance of such room in the roof dwellings. It is not that 1.5 storey cottages are rare in Scotland

4.3 Roof Room with Large Dormer Windows (chalet style)

Up to and including RdSAP Convention v9, the three roof room illustrations set out in Figure RIR1 above included a fourth illustration – the Roof Room with Large Dormer Windows (chalet style) see Figure RIR5 below.

Figure RIR5: Roof Room with Large Dormer Windows (chalet style)

However, in RdSAP Convention V10, this diagram has been removed, with no explanatory text of why it is no longer included. Is this structure no longer deemed to be a room in the roof? Is this type of structure now to be treated as a vertical extension?