Measuring infiltration accurately matters for your PH design Coefficient E explained

14 July 2021 by Sara Wareing

We not infrequently get questions about the values in the ventilation tab in PHPP, or need to review these entries with Passive House designers. This month, we get very technical about the Coefficient E for wind protection class value. Bear with it: there is no way to make this topic sexy but, as always, our goal is to make your (Passive House) life clearer and easier.

An example from the Ventilation tab in PHPP.

Firstly, “Coefficient E for wind protection class” is a technical way to record how windy a building site is. This value sets the assumed infiltration rate. That value influences both the heating demand and the heating load numbers so it’s worth paying attention to. On a project close to the margin, an underestimated infiltration rate could, once corrected, tip your design over the heating load/demand maximums and be a barrier to certification. Wellington projects in particular are often being submitted with inaccurately low coefficient e for wind protection class.

The Passive House use of airtight building membranes significantly reduces infiltration. This is good, because high infiltration rates mean draughts, heat loss and probably lower indoor air quality (because some incoming air is bypassing the filtration on the MVHR). Yet, a house standing on a Wellington ridgeline with views to the south will have a much greater rate of infiltration compared to the exact same house nestled in Reefton*. The co-efficient is our best estimate at accounting for this difference. 

NZS 3604 already requires an assessment of the expected wind speeds for design purposes.

I’ve compiled a table translating wind zone classifications to PHPP coefficient E inputs. Note the two columns with different values depending on how many sides of a building are exposed to the wind. Don’t be quick to use the lower set of values: almost all buildings will have more than one side exposed to the wind. The exceptions would be projects like retrofitting a unit within an apartment block or constructing an earth sheltered living design. Also note that unless you are building your Hobbiton dream home, the value for Coefficient F will always be 15. 

Earth-sheltered design at Hobbiton, image by othree CC BY 2.0

Please note this is not a literal translation of wind speeds between German and NZ standards. It’s simply Sustainable Engineering’s guidance on the easiest way to determine the value to enter for Coefficient E for your PHPP model. You’re welcome.

3604 Wind zone classification Maximum ULS wind speed (m/s) PHPP Coefficient E (several sides exposed) PHPP Coefficient E (one side exposed)
Low Below 32 0.04 0.01
Medium 37 0.07 0.02
High 44 0.07 0.02
Very High 50 0.07 0.02
Extra High 55 0.10 0.03
Specific Design Over 55 0.10 0.03

* Fun fact: Reefton is one of the least windy places in New Zealand.