Cracking the Code: Understanding Air Tightness in Idaho Homes


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Cracking the Code: Understanding Air Tightness in Idaho Homes

Air Tightness
While it is no secret that most Idahoans benefit from some of the lowest cost energy in the country, it might surprise you to know that not all Idaho residents benefit equally. We explored the effects of air tightness in homes, and how the current Idaho Energy Code (IEC) could be financially impacting households.

Air tightness relates to the air leakage between the exterior and interior and directly affects the heating and cooling energy consumption, indoor comfort, air quality, and even durability of materials. It is often measured in ACH (air changes per hour), or CFM (cubic feet per minute). Idaho adopted the 2018 International Energy Conservation Code (IECC), which for climate zone 5 sets a maximum of 3ACH50 for residential. However, in hopes of minimizing the burden on new construction the State of Idaho allowed for some changes creating the Idaho Energy Code (IEC). One of these changes raises the maximum to 5ACH50. Using NEEA’s Residential Building Stock Assessment and energy modeling we were able to estimate the effects on energy cost on homes built at different ACH as allowed by the IEC and create a clear picture of the actual cost to households of living in leakier homes.
alt_text(Bartlett, Halverson and Xie 2019)
Idaho Homes
We looked at homes built during the 1970s, 2000s, and 2018. The first code adopted included a max. ACH50 was the 2009 IECC adopted by Idaho in 2011 with a max of 7. The homes built under those requirements and surveyed in recent years averaged 5.17, some were higher and some lower. In 2015 the 2012 IECC which calls for 3ACH50 with a blower door test requirement, is adopted but the max, is again changed to 7 with only a visual inspection. Homes built with those requirements average now at 4.03. Currently and since 2021 the IEC allows 5ACH50 with testing of 1 in 5 homes. These builds now average 4.42. Surprisingly we can see how even though a leakier build was allowed, results were still mostly good. For example, we can estimate that 70% of homes built in 2018 were around 4ACH50. That is much better than the maximum of 7. So, let’s consider that in construction it is not possible to just build to achieve a specific air tightness number, and that if the desired result is not realized after completion when the blower door test is done, changes to the building could be costly and time consuming. Rather a builder must apply techniques and methods to exceed the desired result to avoid not hitting it. It is currently not feasible for a builder to adapt these methods and techniques to each specific home, so they instead must adapt them to all buildings. So, maybe if we ask for 7, the majority will have to be at 5, if we ask for 3 the majority will be at 1? Anyways, enough speculation.
alt_textIECC changes to ACH requirements overtime.
Our Research
Using these different numbers, we built identical energy models that differ only on the Air Tightness. We took one home built at 3ACH50 and compared it first to one built at 7, the tighter home paid around $115 more a year for energy bills. This mounted to $3,450 for in 30 years and that is not considering likely raises to the cost of energy in the coming years. In 2015, 7,784 homes were built, this could mean that all those homes combined could be paying up to $895,160 more each year than if they had been built at 3ACH50.

For 2021 we compared 5ACH50 (IEC) to 3ACH50 (IECC2018) the results were not as big but still important. Unrealized savings add up to $56 per household, or $1,680 over 30 years. But 16,154 homes were built in the state that year meaning the combined potential savings would be $904,624 a year.

Why It Matters
There are different ways to interpret these results and I am sure each reader will decide for themselves if change is needed or necessary. But there are a couple more things to consider. Starting with the fact that in this cost estimate we only used the rates of Idaho Power and Intermountain Natural Gas Company. While most Idahoans are in their service network not all are and many must rely on significantly more expensive energy sources, additionally the effects of air tightness grow with the increase in outdoor-indoor temperature difference and to simplify the results we only considered the weather of Boise, Idaho.

According to the 2020 census there are just under 200,000 renter occupied homes in Idaho. Rented homes tend to be leakier than owner occupied homes, and on average have a lower efficiency heating system.

We can then estimate that on average a renter occupied home in let’s say Salmon Idaho, a CZ6 area where delivery of propane gas is the norm for heating fuel could pay significantly more to keep the home comfortable. When I considered other research currently done at the Integrated Design Lab that involves different regions in Idaho, I worry the potential financial savings could be up to 3 times what they are in Boise.

Air tightness in homes is not like insulation where we can just up the R-value and have a very good idea of both the effects and the cost of the upgrade. Air tightness requires much from the builder, and for many builders it is very hard to effectively estimate the cost of the upgrade, even know how to achieve it. Many new methods and materials are available to help, and some certification entities such as PHIUS, help push the industry in the right direction by creating very stringent standards, but more awareness is needed to help Idahoans make more informed decisions and save money and energy.

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University of Idaho - Integrated Design Lab

322 East Front Street Boise, ID 83702