Understanding Room-in-Roof Modelling for Non-Domestic EPCs

They can be found in a wide variety of non-domestic buildings, including converted offices, hotels, educational buildings, retail premises and older commercial properties.

Unlike a conventional room with vertical external walls and a horizontal ceiling, a room in a roof can contain several different building elements, each with its own construction, thermal performance and relationship to the external environment.

Correctly identifying and modelling these elements is essential because inaccuracies can affect the calculated heat loss, solar gains, heating and cooling demand, and ultimately the Building Emission Rate (BER) and EPC rating.

This article explains how rooms in roofs should be identified, how their different constructions are treated, and the key considerations when preparing an assessment using SBEM-based software.

What is a room in a roof?

• Sloping roof surfaces forming part of the room's ceiling.
• Short vertical stud walls, sometimes referred to as knee walls.
• Unheated roof voids behind the stud walls.
• Gable-end walls.
• Dormer windows and their associated walls and roofs.
• Roof windows installed directly within the sloping roof.
• A horizontal ceiling beneath the ridge, with a roof void above.

The precise combination depends on the building's construction.

Each of these elements must be considered when establishing the room's thermal envelope.

Why is room-in-roof modelling different?

For a conventional rectangular room, this may be relatively straightforward. The assessor identifies the walls, floor, ceiling, windows and doors, and assigns their relevant properties.

For a room in a roof, the geometry is more complicated.

These distinctions matter because heat transfer depends on both the construction and the conditions on either side of it.

For example, a sloping roof directly exposed to outside conditions will not necessarily have the same thermal behaviour as a stud wall separating the occupied room from a ventilated roof void.

Similarly, a floor separating the room from a heated office below is treated differently from a floor exposed to an unheated space.

The iSBEM User Guide explains that the area of an exposed pitched roof should be based on its internal surface area, rather than simply its horizontal projected area.

Identifying the different constructions

A room in a roof may require several construction types to be defined, depending on its layout.

The U-value measures the rate of heat transfer through a building element and is expressed in W/m²K.

A lower U-value indicates better thermal insulation.

For existing buildings, the assessor should gather the best available evidence to establish the construction and its thermal performance.

This might include site observations, access to roof voids, construction drawings, insulation specifications, photographs or other verifiable documentation.

Where the exact construction cannot be established, the assessor must apply the relevant approved conventions and methodology for selecting the appropriate construction data.

It is important not to assume that a room-in-roof conversion has the same insulation standard as the original building simply because the age of the main structure is known.

A roof conversion may have been carried out many years after the building was originally constructed.

For example, a Victorian commercial property may have been converted to provide attic office accommodation in 2005. The original external walls and the later roof conversion could therefore have significantly different thermal characteristics.

The assessor should record the evidence supporting each construction selection.

Worked Example: 8 Riverside Court

This building has a room in roof. 

It is not a "traditional" type, as there is no roof void behind the knee walls. The knee walls are the same construction as the main construction walls. 

This building has dormer windows, sloping ceilings, and a flat ceiling. 

The floor plan might look like this:

The floor plan to show the features might look like this:

Step One: Identifying the construction types

Knee Wall: In this building, it is the same wall construction as the main building, this will be recorded as Cavity, External Wall

Slopes: Pitched roof insulation at rafters, insulation thickness would be unknown unless there was visual or documentary evidence

Flat Ceiling: Pitched roof horizontal insulation, insulation thickness unknown unless visual or documentary evidence

Gable Wall: Cavity, External Wall

Party Wall: Partition wall to an adjoined conditioned space

Step Two: Measure the different elements

 Knee Walls: Measure from the top of the floor to the where the wall meets the slope to record the height. Measure the width from side to side.

Dormer Cheeks: Measure the height and width, if applicable and work out the area of the shape - in this case it is a triangle. 

Dormer cheeks are considered to be wall envelopes.

Slopes: Measure from the top of the knee wall to the flat ceiling border. Measure the width from side to side. Sometimes the dormer may interrupt the sloping ceiling, so multiple measurements will be needed but the area can be entered as one element. 

Flat ceiling: Measure the length and width of the perimeter of the flat ceiling - it is usually smaller than the total floor area due to the slopes. 

The flat ceiling also includes the dormer roof area.

Note: Knee Walls and/or Slopes that face the same orientation but are broken up into parts due to dormer windows or other features, can be merged to create one envelope. It is not acceptable to merge knee walls and/or slopes from different orientations.

 Continue to measure all surfaces in each zone created. Take careful consideration to the individual element.

Step Three: Create the nomenclature (Zone 3 for example)

Step Four: Check the zone height

A room in roof is only considered a zone if the headroom available is more than 1.5m. Headroom is the space between the floor and the highest part of the ceiling. If the headroom available is under 1.5m, then it is excluded from the assessment.

Convention 10.03: The zone height is measured from the top of the floor to the top of the knee wall. 

Convention 4.09: In some room in roofs the zone height will be less than 1.2m. If this happens, the zone height must be entered as 1.2m regardless of its actual measurement underneath this value. 

When creating a zone which is within a room in roof, you will need to amend the zone height manually after you have added in your envelopes.

First, enter the zone height as the actual height measured.

Add the envelopes as per your workings, ensuring the area is a sum of the true width and height measured on site.

Then after all envelopes are added in that zone, head back to the main zone page and amend the zone height to 1.2m.

Dormer windows are not considered when calculating zone height.

Step Five: Change the slopes

Once all envelopes have been added, edit the slopes in the 'Envelope' tab to change the orientation from Horizontal to the orientation they are facing. 

Before:

After:

Step Six: Turn off automatic daylight zoning

This is because rooms in roofs often have unusual geometry, including sloping ceilings, dormers and rooflights, which may not be represented accurately by the software’s automatic subdivision. 

The NCM guidance confirms that where a zone has an atypical shape or unusual arrangement of windows or rooflights, manual daylight subdivision is recommended.

For vertical glazing, the daylit area normally extends 6 m back from an external wall containing at least 20% glazing. 

For rooflights, the daylight boundary extends 1.5 × the room height beyond the edge of the rooflight array, provided the total rooflight area is at least 10% of the zone floor area.

Example - Vertical glazing (Dormer or wall window)

Example – rooflights and the 1.5 × room height rule

Imagine a room in roof with a room height of 4.5m and two rooflights.

So the daylight zone extends 6.75m beyond the edge of the rooflight array.

Once this has been done, uncheck “Local Manual Switching” so that SBEM does not apply its own automatic daylight control assumption on top of the manual zoning that has already been created. This helps ensure the daylighting is represented in line with the assessor’s own calculated zone layout.

Summary

• identify the true thermal envelope;
• measure sloping surfaces using their actual area, not just the floor plan;
• keep roof planes with different orientations separate;
• apply the correct room-in-roof zone height conventions;
• model dormer elements individually;
• check whether manual daylight zoning is required;
• and make sure your element names, dimensions and supporting evidence are clear enough for someone else to follow.