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Building Science

The Difficulty of Updating Georgia’s Energy Code

Trying to get airtightness below 7 ach50 has been a struggle

Blower door testing has been required in Georgia since 2011.
Image Credit: Energy Vanguard

Seven years ago, Georgia led the nation. Yep. We were the first state to adopt an energy code that made blower door testing mandatory. All new homes built in the state had to show through performance testing that they had an air leakage rate of less than 7 air changes per hour at 50 Pascals of pressure difference (ach50).

When I wrote about it in 2010, I called our new code “groundbreaking.” Since then, we’ve been passed many times by other states. Now we’re finally updating the energy code here, and it’s been a struggle to move that threshold from 7 ach50 to something lower. And the reason has nothing to do with the difficulty of air sealing.

In 2010, Georgia adopted the 2009 International Energy Conservation Code (IECC) with amendments and supplements. The 2009 IECC didn’t actually have blower door testing as a mandatory requirement. That was a Georgia thing. At that time, the state also started requiring duct leakage testing and R-5 insulation for attic hatches. And we banned powered attic ventilators (except solar) and electric resistance heat used as a primary heat source.

In 2014, Georgia adopted the 2012 International Residential Code (IRC). One requirement in the mechanical section is that mechanical ventilation be installed if a house comes in more airtight than 5 ach50.

Now, in 2017, the Georgia Department of Community Affairs has been holding meetings to update to the 2015 IECC. We usually do every other code cycle, so we skipped over the 2012 version. The air leakage requirements in the 2015 IECC are in section R402.4.1.2 and stated thus:

The building or dwelling unit shall be tested and verified as having an air leakage rate not exceeding five air changes per hour in Climate Zones 1 and 2, and three air changes per hour in Climate Zones 3 through 8.

In the 2015 version, testing is now…

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  1. Charlie Sullivan | | #1

    north vs. south
    I would think that the value of air sealing would not be all that different throughout the state, particularly if regions with milder winters have hotter, more humid summers. Is there something about the climate I'm missing? Now a moot point since the standard is moving toward being uniform, but I'm curious.

  2. User avater
    Dana Dorsett | | #2

    The energy use at any ACH/50 varies with climate . @ Charlie
    The energy use per cooling degree-day is generally lower than the energy use per heating degree-day. The simple explanation for this is the higher COP efficiency of heat pumps at lower peak indoor to outdoor temperature differences in the cooling season than is seen in the heating season. eg:

    The 1% outside design temp in Savannah (Zone 2, on the edge of zone 3) is 93F, which 15F above a 78F indoor design temp.

    The 99% outside design temp is 29F, a 41F difference from a 70F indoor design temp.

    All else being equal, pumping heat across a 15F difference takes less energy input per unit of heat moved than pumping it across a 41F difference. Put another way, efficiency rises as the temperature difference shrinks.

    So even though there are more total CDD than HDD in that location, the average operating efficiency of the equipment is higher during the cooling season than in the heating season, and there is more energy used during the heating season. The average COP of a typical minimum-legal efficiency heat pump during the cooling season will be ~3.5 (=SEER 14), whereas the average COP during the heating season would need to hit an as-used HSPF of 12. That is impossible to get out of minimum-legal efficiency equipment (= HSPF 8.2), and requires at least some sizing optimization to get there even with modulating ductless mini-splits.

    Stack effect pressures driving 24/7 infiltration are also higher at the larger heating season delta-Ts than the lower cooling season delta-Ts. That results in both higher air volume infiltration, but also higher BTU content per unit volume.

    The higher infiltration, higher BTU content, and the lower heat pump efficiency at the larger seasonal delta-Ts make it rational to specify a tighter ACH/50 leakage in zone 4 than in zone 2.

  3. Charlie Sullivan | | #3

    Thanks Dana
    Thanks Dana. That makes sense. In particular, I had missed the point that stack effect is reduced with the lower delta T, making infiltration weaker.

    The flip side is that infiltration drives latent load more than other heat gains do. So the HVAC system input energy needed at a given delta T, given COP and given CFM of air leakage would be higher in the summer. But with a lower delta T, lower infiltration, and higher COP, the summer energy is lower despite the latent load.

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