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NGBS flexibility = builder choice of air infiltration calculations

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Chinedu Moneke
February 12, 2021

2 Critical Calculations for Consumer Comfort: ACH and ELR

“There’s no place like home!” But surely even Dorothy Gale wouldn’t have wanted to go back if home was hot and humid or cold and dry. Comfort is the appeal that draws residents home every day. Whether they think about it or not, the climate and air quality in homes can make the difference in how comfortable the residents and their guests feel. Most home owners or renters turn on the air conditioner in the summer and the heater in the winter with limited knowledge about how these systems work. Builders don’t have that option – you must know all the important factors that go into calculating building envelope tightness, which ultimately lead to resident comfort.

For both energy and budget efficiency, the less time a home’s heater or air conditioner is actively running, the better. The amount of time they run depends on how much infiltrated air is leaking into the home from the outside. The inside air is called conditioned air, while the outside air is called unconditioned air. To quantify how much infiltrated unconditioned air is leaking into the home, a Blower Door test is used. The Blower Door depressurizes the home to 50 Pascals, and the CFM (cubic feet per minute) is displayed. The CFM is then used to calculate the Air Changes per Hour (ACH) and Envelope Leakage Ratio (ELR).

The ACH is the calculation builders are probably most familiar with – it indicates how many times the volume of conditioned air in the home would be replaced by the outside unconditioned air in an hour. So, if we have a 7 ACH, this means that in an hour the entire conditioned air in a home would be replaced by unconditioned air from the outside seven times. This also means that the HVAC system would have to heat or cool the air seven times in an hour. The lower the ACH, the better for the home and its occupants. To get a lower ACH, all visible leaks and flaws around the building envelope need to be air sealed. For the 2015 and 2020 NGBS, a maximum value of 5 ACH is allowed for Climates Zones 1 and 2, and a maximum value of 3 ACH is allowed for Climate Zones 3 through 8. The formula for ACH is (CFM50 x 60 ÷ conditioned volume). The ACH should always be used for detached homes because all the infiltration air is coming from unconditioned space.

However, attached homes – townhomes, duplexes, and multifamily units – should be calculated differently because not all infiltrated air is coming from unconditioned space. Air infiltration through the building envelope can come from adjacent conditioned homes or units. An equivalent ACH is necessary to account for only unconditioned air infiltration when dealing with these homes or units. To obtain the equivalent ACH, we first need to calculate the ELR (CFM50 ÷ building envelope area), then convert it to the equivalent ACH, which is [(20 x ELR) – 1.6]. This equivalent ACH is usually smaller than the actual ACH since some of the infiltration air would come from conditioned spaces and would not technically contribute to heat loss. This equivalent ACH should also be used in the energy modelling software for attached homes or dwelling units.  

How It Works for NGBS

The NGBS Green scoring tool provides both ACH and ELR entries for practice 701.4.3.2. Depending on the calculated value, points can be earned for practice 703.2.4 in the Energy Prescriptive path. For ACH values less than 5, or ELR values less than 0.33, ventilation in 902.2.1 is mandatory.

Bottom line, it is best to use the ELR calculation method for small- to medium-sized attached homes or dwelling units. The actual ACH should always be used for detached dwelling units because all air infiltration through the building envelope is from unconditioned space.

NGBS Green is the building certification of choice for builders because it provides flexibility for compliance without compromising the level of building performance expected from certified buildings. Builders have the flexibility to determine which air tightness calculation best suits their building type —ACH or ELR — when calculating building envelope tightness. And buyers and renters gain a more comfortable home. Interested in seeking NGBS Green certification for your next building project? Contact us.

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