Understanding Alternative Walls and Sheltered Walls

When producing an assessment using RdSAP, getting the thermal performance of the external walls right is crucial. 

In many dwellings, especially older or more complex buildings, you may come across parts of the heat loss perimeter that don’t match the main wall construction. That’s where alternative walls come in.

Sheltered walls are also considered an 'alternative wall' but usually recorded in the 'sheltered wall' section for flats. 

Let's learn a little bit more about how they should be recorded.

According to Convention 2.13...

“An alternative wall can be:
• A sheltered wall (to unheated corridor or stairwell), or
• A wall that has a construction type or heat-loss characteristics (U-value) different from the main external wall, or
• Both of the above.”

(RdSAP Conventions v12, Convention 2.13, May 2025)

RdSAP 10 allows for two alternative walls per building part to be input, but only one sheltered wall. Always include the alternative wall if it is sheltered, irrespective of size (Convention 2.13).

There can only be one sheltered wall recorded per dwelling. 

Sheltered walls are commonly used in flats or maisonettes when recording the wall next to an unheated corridor. 

The wall isn't exposed to the outside - but would still be included in the heat loss perimeter - and recorded as a 'sheltered wall'.

This is entered in the unheated corridor/stairwell length that appears when the building is declared as a flat in the software. See below:

Note: For flats with a heated corridor, the wall would be considered a party wall (rather than a sheltered wall).

When a dwelling (flat or maisonette) has a sheltered wall to an unheated corridor on more than one storey or building part, the sheltered length is the total for all storeys with a sheltered wall (example: 2 storeys with sheltered wall on each storey, length of sheltered wall is 5 m on each storey: enter 10 m for the sheltered length). 

Image Credit: Denisbin (2015) CC BY-ND 2.0

You should use an alternative wall where part of the building’s heat loss perimeter is:

• A different wall construction type — for example, a stone gable on an otherwise cavity wall house or in the image above, where you can see two different wall construction types.
• An area with a different U-value — perhaps it has external wall insulation or dry lining.

• When the majority of the house has external wall insulation, and 1 elevation has internal wall insulation. These two insulation types hold different thermal mass parameters so its key to model them separately.

This is recorded in the Alternative Wall section of the software as seen below.

If you take a look at the picture below, you can see the ground floor is of solid brick construction, and the 1st floor is of timber frame construction. 

This is a great example of where you would include a timber framed alternative wall in your assessment.

Note: There also appears to be a room in the roof but we are not concerned with this for the purposes of this article.

Image Credit: Ines (2011) CC BY-ND 2.0

If a non-sheltered alternative wall makes up less than 10% of the total exposed wall area of that building part, you should normally disregard it. 

Disregard non-sheltered alternative wall(s) if less than 10% of the total exposed wall area of the
building part (including windows and doors) (Convention 2.13).

Small differences can be absorbed into the main wall entry unless they would materially affect the SAP calculation.

You can now input up to two alternative walls per building part.

If you find you need more than two alternative walls, the guidance is to handle this by creating an extension in the software instead — splitting off the additional areas to record them separately.

When you enter your window measurements, you will be able to define whether a window belongs to the main wall or the alternative wall. 

This ensures that the window is linked to the correct construction type and U-value for the calculation. So, always check your dimensions carefully and select the right wall type for each opening.

However, you should remove the area of any external doors in an alternative wall in your calculations before adding in the total area.

• Always record significant differences in wall type or thermal performance.
• Enter up to two per building part.
• Link windows correctly to the wall they belong to.
• If you have more than two significant alternative walls, use an extension instead.

Getting this right ensures your EPC is accurate and consistent with SAP methodology.