H1/VM1 - Infiltration and Ventilation
For those familiar with the H1 Energy Efficiency Verification Method H1/VM1, it is an option for establishing compliance with the performance criteria in Building Code clauses H.1.3.1(a), H1.3.2E, and H1.3.3. Like all modelling methods, H1/VM1 has areas that can be improved. One key area is the application of Infiltration and Fresh Air Ventilation within the model. Let's explore this in more detail.
Infiltration
Infiltration, or unintentional outdoor airflow into a building through openings such as cracks and exterior doors, is addressed in D.1.13. The assumptions for both the proposed and reference buildings must be the same and reasonable for the building's construction, location, and use.
D.1.13 Infiltration
D.1.13.1: Infiltration assumptions for the proposed and reference buildings shall be the same and reasonable for the building construction, location, and use.
Factors influencing infiltration include the building envelope's air tightness, wind, stack effect, and building pressurization. Modellers determine what is reasonable, as no specific value or unit is provided for infiltration.
There are four ways to specify infiltration in calculations, provided an Airflow Network is not used:
ac/h: Infiltration is defined in air changes per hour (ac/h), i.e., zone volume per hour under normal operating pressures.
m³/h-m² at 50 Pa: Infiltration is defined as flow rate per unit exposed surface area, with a pressure difference of 50 Pa.
m³/h-m² at 4 Pa: Infiltration is defined as flow rate per unit exposed surface area, with a pressure difference of 4 Pa.
n50 (ac/h at 50 Pa): Infiltration is defined in air changes per hour (ac/h), i.e., zone volume per hour, with a pressure difference of 50 Pa.
Speckel defaults to an ac/h rate of 0.35 for H1/VM1. This means that 35% of the home's volume is exchanged constantly at normal operating pressures, which is a significant assumption.
Fresh Air Ventilation
In addition to Infiltration, intentional fresh air ventilation is necessary for occupant health, comfort, and amenity. According to D.3.2, the fresh air ventilation rate and schedule must be the same for both the proposed and reference buildings. Unlike infiltration, fresh air ventilation has specified values.
D.3.2 Fresh Air Ventilation
D.3.2.1: The fresh air ventilation rate and schedule must be the same for both the proposed and reference buildings. The minimum fresh air ventilation rate is:
0.5 air changes per hour for housing
As specified in NZS 4303 for other buildings
Speckel defaults to an ac/h rate of 0.35 for outdoor air, referenced in NZS 4303 for H1/VM1. This equates to another 35% of the home's volume being exchanged constantly at normal operating pressures, which is another substantial assumption. The 0.5 air changes per hour can be adopted manually, but may result in modelling inaccuracies.
Key Takeaway
For housing, both infiltration and fresh air ventilation rates may be set using air changes per hour. When combined, the total volume of air exchange at normal operating pressures (e.g., 4 Pa) can be very high. This requires heating large amounts of cold air to maintain control temperatures, with a minimum temperature of 18°C as specified in D3.1. This can result in a model being insensitive to fabric improvements as the heat demand to heat the cold air becomes the main heat loss component of the design.
For modellers looking for more reasonable values to adopt, setting an allowance for fresh air ventilation per person as per G4 Ventilation requirements, where outdoor air supply can be designed and equipment installed to comply with NZS 4303, or AS 1668.2 is recommended. This results in per person outdoor air rates of between 7.5 l/s and 12 l/s.
Download NZS 4303:1990 Ventilation for acceptable indoor air quality here
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