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Why does IAQ go up when ACH goes down? I thought outside air was healthier.

cold_feet | Posted in General Questions on

I’ve been trying to educate myself on the topic of building science.  This site, the Building Science podcast, the EnergyVanguard blog, there’s a lot there for a layman.  I’m a bit confused about something I’ve been hearing though and I feel like it’s being oversimplified and maybe even distorted.  Can someone help me sort this out?  Let me be clear before I dive into my question, that this is not about efficiency, it is about IAQ.

Here’s what I’ve been hearing and need help working through.  Air quality has an impact on our health (no argument or confusion from me there).  Outside air is healthier than air inside a home because of all the contaminants, off-gassing from manufactured products, off gassing from cooking/heating/fireplaces, and mold found in many homes (okay, that sounds very plausible).  Why then, would just decreasing the number of air changes per hour make the IAQ better if less “fresh” air is being brought in?  Isn’t that an oversimplification?  If the walls of a home are already being kept dry, wouldn’t it take more than just lowering the ACH to improve the IAQ?  Would it be more accurate to say that it’s not really about lowering the ACH, but that it is instead about choosing to use the best MERV rated air filter you can, avoiding or buying fewer manufactured products that are going to off-gas and hence increase the contaminants in your air, by controlling the humidity in your home, and whatever else I’m not thinking of?

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  1. jberks | | #1

    I think you're confusing the results from a blower door test (measured in ACH).

    Blower door tests measure how much a building leaks air through gaps and cracks in framing. You want to lower this number to avoid moisture issues and increase HVAC efficiency.

    Once the envelope of the home is handled, (ie controlling the air infiltration by verifiby a lower the ACH low) you now have better control of your IAQ. This is where you introduce mechanical ventilation systems, to purposefully, timelingly and controllingly bring in the amount of filtered outside air that you need.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Cold Feet.

    First of all, I agree with Jamie on this. IQA doesn't go up because a house is tighter. IAQ can be improved in a tighter house because we have now have control of ventilation rates and know exactly where that fresh air is coming from. And, we can filter it.

    However, the most important things to control for better IAQ are the pollutants themselves, in other words, source control. Though it is specifically about VOCs, I suggest you read Indoor Air Quality and VOCs. Towards the end of the article there is a long list of things you can do to improve IAQ including source control, ventilation, and filtration.

  3. tommay | | #3

    IAQ is in my opinion can be improved by introducing fresh air from the outside, especially night time air in the summer. Learning how to control the flow of air by strategically opening windows that allow for flow across and up and out of your house is the key.

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