What is a Draft Lobby?

It acts like a small buffer zone between the front door and the main heated space, preventing warm air from escaping too quickly when the entrance is opened.

“A draught lobby is an arrangement of two doors that forms an airlock on the Main Entrance to the dwelling. It may be heated or unheated.”
(Convention 2.09b, RdSAP Conventions v12)

• Cover the main entrance to the dwelling (the one designed as the main front door)
• Have a floor area of at least 2m²
• Includes two doors arranged so that a person (even with a pushchair) can close the outer door before opening the inner one — creating a genuine airlock
• Lead into a circulation area, such as a hallway or corridor, not directly into another room (except a cloakroom or WC, which is acceptable)

Can a porch be both an extension and a draught lobby?

Sometimes, you may need to include a porch as an extension. In this case, as long as the porch is more than 10% of the total floor area it could be an extension, and you would also include it as a draft lobby too.

Note: Unheated porches would not be considered extensions as per convention 2.09a

The image below shows the porch leading into a corridor, however the area of the porch is under 2m² and therefore is not classed as a draft lobby.

The image below shows a porch with over 2m² opening onto a corridor. The space in the porch creates a genuine airlock so that a person, even with a pushchair can close one door before opening another. 

If the main door opened into another room, rather than a hallway or corridor (except a cloakroom or WC) then it would not have been a draft lobby.

The image below shows a porch that is more than 10% of the total floor area so should be included as an extension. 

However, in this example, the area of the porch is at least 2m² and creates a genuine airlock so should also been included as a draft lobby.

For example:

• An open porch with only one door is not a draught lobby.
• A small vestibule that’s too tight to close one door before opening the other doesn’t count either.
• Flats that are accessed via an enclosed heated or unheated corridor or stairwell should be treated as having a draught lobby, because this enclosed space serves the same purpose — it prevents direct draughts into the flat.

Getting this right is important because a draught lobby affects the air infiltration rate in the calculation, which can impact the property’s overall EPC rating. Overlooking it, or wrongly including a porch that doesn’t meet the conditions, could lead to an inaccurate assessment.

Always check:

• Is there a true airlock arrangement with two doors?
• Is the space large enough (2 m² or more)?
• Does it lead into a circulation area?

If in doubt, refer back to Convention 2.09b to ensure your assessment is correct and consistent.