Air Permeability Interpretation.

[Boris]

Hi.  On our SAP certificate it states the design value of the APT is 3 m3 @50Pa.  The maximum is 10 m3 @ 50 Pa.  Do we have to get 3 or is any value between 3 and 10 acceptable ?  Thanks in anticipation. 

Edited August 7, 2020 by Boris

[NSS]

I would imagine 3 is the target for the design to meet the SAP and hence EPC, but you also need to be aware that differing APT results impact on the type of ventilation required.

[A_L]

@Boris If your house is measured at less than or equal to 3.0m³/m²/hr it requires a formal ventilation system. If several similar houses have been measured you can assume 10m³/m²/hr, if single self-build 15m³/m²/hr has to be used.

 

The principle effect of assuming a value is to increase the energy used by the house which increases the Dwelling Emission Rate which has to be less than the Target Emission Rate. 

[ADLIan]

3 is very low so don't expect typical contractor to achieve this. Needs very careful design, great workmanship and meticulous attention to detail. The value is not known until tested on completion so if not achieved may need expensive remedial work. If measured the test result cannot be worse than 10. It is possible to avoid the air pressure test on a one-off house but a default air leakage of 15 must be used - probably resulting in a failure under Appr Doc L1A.

 

Also 3 or less means you have to install whole house mech vent.

[nod]

We scored 3 Traditional build 

With 30 odd sash windows that can never be airtight 

The final Sap will work with what you have 

It’s all it bit of a joke 

[Boris]

We have MVHR.  Am I right in interpreting the comments that anything below 10 is OK ?

[Mr Punter]

45 minutes ago, Boris said:

We have MVHR.  Am I right in interpreting the comments that anything below 10 is OK ?

 

If you have an air test and score more than 10 you will fail.  If you score between 6 and 10 you will have to have made significant improvements elsewhere with insulation and glazing.  Less than 3 and you will need your MVHR and your house is very well built.  Most new houses probably achieve between 4 and 6.

[MikeSharp01]

1 hour ago, Mr Punter said:

Most new houses probably achieve between 4 and 6.

Unless you are going for Passive house standards in which case you need a different metric, PHPP - n50≤0.6 ac/h, and even better air tightness. Sadly the conversion between PHPP units and SAP (Air Permeability - AP) units is not simple as one uses just the volume of the property to calculate total air changes per hour while the other uses the surface area as well, and is measured per m² , to calculate the AP. if you know both volume and envelope area you can do the maths and compare the two values. 

[joth]

10 hours ago, Boris said:

We have MVHR.  Am I right in interpreting the comments that anything below 10 is OK ?

3 is the target to get any payback value from the MVHR. The higher above 3 it is, the more leaky the  house is and the less value the MVHR is giving you. 

[DevilDamo]

With 3 being a very low figure, what additional measures would need to be taken to the other elements to achieve a more common figure of 5?