CIBSE is updating the Domestic Heating Design Guide, and one of the biggest changes affects ventilation heat loss. Here’s what you need to know:

1. New air change defaults

The guide will now align with BS 12831-1:2017, which sets simple defaults:

• 0.5 ACH for habitable rooms
• 0.0 ACH for non-habitable rooms

These apply to all homes, regardless of age or construction.

2. Airtightness becomes a bigger factor

Air change rates (ACH) isn’t the only thing driving ventilation losses. The building’s air permeability (AP) — how airtight it actually is — makes a big difference.

• Without a blower-door or pulse test, you can’t reliably know the AP.
• UK studies show airtightness varies widely and does not correlate well with age, type, or construction.

The national average AP is around 8.6, with 75% of homes below 10.2 (but with very long tail of values all the way up to 46!)

3. The impact on MCS and heat loss tools

At the moment, the MCS Heat Load Calculator estimates airtightness indirectly by using the minimum ventilation rates from the CIBSE Domestic Heating Design Guide.

This can lead to wide variations between properties because the calculation is sensitive to how a home’s internal volume relates to its external envelope area.

To improve consistency and align with BS 12831-1:2017, MCS are shifting to using the default airtightness values from the standard instead. This reduces the sensitivity caused by differing room volumes and building shapes.

Installer Toolkit already includes BS 12831-based defaults as an option, but we will shortly make this the standard approach, rather than the CIBSE ventilation rates when no measured airtightness value is available.

4. Default AP will be 12

To stay consistent with the BS 12831 standard, MCS and tools like Toolkit will move to a default AP of 12 unless there is evidence to use something else.

• If the home has a tested value — use it.
• If not — use 12.
• For post-2006 homes (where airtightness testing became mandatory), you can check the EPC for the airtightness test result.

5. Why this matters

Once the CIBSE guide is updated, ventilation heat loss will depend much more on the AP number you use. Because airtightness is hard to estimate, the safest approach is to:

• Use the measured value if you have one
• Otherwise use the BS 12831 backstop of 12 

6. When will these changes to the ACH and AP default values happen?
   We plan to put these changes into Toolkit early in the New Year (assuming the new CIBSE guide does come out in December). We wanted to give you a heads up and an explanation before we made the changes. We don’t know exactly when MCS will make their changes but we’ll let you know when we do.