What is Infiltration?
Infiltration describes air that comes into the building in an uncontrolled way from the outside.
The most readily accessible example of infiltration is draught / draft which comes into a building through cracks under doors and in windows. In winter this infiltration can represent lots of cold air coming into the building, increasing the amount of energy the heating needs to add to keep the space warm. It can also increase the overall size of the heating system.
Infiltration still happens in modern buildings, but codes and standards have become increasingly tighter and many modern buildings are pressurised, reducing the amount of infiltration that occurs (this still of course means that conditioned air can be lost to the outside through cracks!)
Why do I care about Infiltration?
When you're designing a building, you might be looking to weigh up the relative merits of different levels of infiltration control in the design. Some building facade systems have better infiltration control. Operable windows often have higher infiltration rates than fixed windows.
It's often worth testing to see the impact of the added expense of infiltration control on the building's energy and operating costs. Sometimes, especially in cold climates, the effect will be substantial.
You can see how infiltration affects your space loads by looking at the Peak Loads tab. Often Infiltration is one of the top 3 contributors to heating load.
How is infiltration measured?
There are many ways to measure infiltration. Most standards set targets that can be tested with infiltration testing, such as blower-door testing. These tests pressurise a building up to a certain pressure differential, then test the amount of air that leaks out. This leakage rate is converted then into a rate per area of facade (eg m3/m2.h) or a rate in terms of air changes (the number of times per hour the whole volume of the space is replaced).
Infiltration is also less commonly measured in terms of crack flow - especially for naturally ventilated buildings.
How does Sefaira use EnergyPlus infiltration?
Sefaira use's EnergyPlus to determine infiltration rates for the plugin and the web-applications. In the web-application there are 4 infiltration options available:
- Air Changes (Per Hour, at approximately 50Pa)
- Facade Area (Airflow over time as a proportion of the facade area, at 50Pa)
- Facade Area (Airflow over time as a proportion of the facade area, at 75Pa)
- Crack Infiltration (airflow at approximately 50Pa, per perimeter length of glazing)
Only Perimeter Zones (zones that are denoted with a P eg P02) see the effects of infiltration in any instance. In the case of Air Changes, the size of the perimeter zone has a big impact on the amount of infiltration (because flow is based on the zone volume).
Many other software tools assume constant infiltration flow for all hours of the year. EnergyPlus determines the pressure drop at the facade based on wind and temperature differentials and calculates infiltration hourly based on this differential. This means that hour on hour the actual infiltration can vary substantially, depending on these conditions.
For Facade Area infiltration values, EnergyPlus defaults to an outside pressure of 4Pa. We have used conversion factors to adjust this to 50Pa and 75Pa which are thresholds commonly used for blower-door tests.
Where does the default come from?
The default for new uploads is Facade Area, with a default value of 7.2 m3 of air per m2 of facade per hour (m3/m2h) (0.394 cfm/ft2 in IP units). Many codes and standards reference this metric to set goals and this value represents the equivalent of code compliance in the UK.
Which methodology is used for calculating infiltration?
We now use the DoE-2 methodology for establishing input constants for infiltration. This puts a high emphasis on the impact of wind on infiltration and far less of an emphasis on the temperature difference between in side and outside. This methodology was changed on 18 February 2019.
Which metric should I use?
We've provided 4 metrics to support different customers from different regions with different use cases.
If you're not sure what metric to use, the Facade Area one is fine. If you're in a region where infiltration is less heavily regulated, you might want to increase the value.
There are some issues you should be aware with for each metric.
- If your model is single zone per floor it is recommended NOT to use Air Changes. This is because your whole floor plate is considered a perimeter zone, so you'll probably end up with more infiltration that you might expect
- Natural Ventilation ONLY works with Crack Flow infiltration. If you want to compare options that don't have Natural Ventilation with options that do have natural ventilation, it's recommended to use Crack Infiltration for all options.
- Crack Infiltration is susceptible to models that have lots of small windows. Some models get translated with triangulation of glazing, which can substantially increase the length of the glazing perimeter. Similarly if you've just drawn the entire wall as being glazing (or you've used the glazing override) then you'll possibly be under-estimating infiltration with this default value.
- Remember, unlike some other software tools the infiltration rate is not constant, it can vary greatly hour by hour according to wind pressure.
Contact firstname.lastname@example.org if you believe you are seeing very high infiltration rates with any infiltration option used.