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Community and Q&A

Installing Controlled Outside Air Introduction

HhsHwZFSCs | Posted in Energy Efficiency and Durability on

I have a two story house in Atlanta GA that was built in 1984. I recently had a blower door test and received a rating of .47 ACH Natural. The top floor is 1700 sq ft and the lower floor is 1400sq ft. I was told by individuals I should be concerned the house is too tight and I should installed fresh air system on the upper floor. I have some additional insulating and sealing projects above the garage in the bonus room and knee walls which will likely tighten the house up a bit more.

How do I evaluate whether I need this fresh air system? Are there symptoms of too tight a house?

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  1. jklingel | | #1

    Scott: Keep reading here. "Too tight" is a bit controversial, but now that you are pretty closed in, keep going. Obvious signs of "too tight" are wet windows, stuffy smelling, etc. Less obvious are the long-term effects of breathing non-fresh air. Ventilate. ASHRAE recommends 7.5 cfm/person, plus 1 cfm/100 sf of living area; some folks say that is a bit overkill, but it is a good starting point. In Atlanta, I can not fathom you really noticing any heat loss costs; cooling is likely the same, but I have zero knowledge of cooling costs. My calcs show that I'll be spending something like $250/yr for heat lost to air exchanges, and Fairbanks is a tad colder than Atlanta. The cost of health is easy to justify, IMO.

  2. JuTtXteKqJ | | #2

    I agree, ventilation is healthy, but over ventilation wastes energy, and can affect comfort in some cases. Keep sealing and insulating, and try out some online energy and ventilation calcs. RetScreen International is a good one, it's produced by Natural Resources Canada, . IMO the health implications are very important, tight houses keep air out and airborne pollutants in, including VOCs and other toxins we live with and take for granted. Controlling ACH lets the good air in and the bad air out, this benefit negates reasonable energy losses.

  3. GBA Editor
    Martin Holladay | | #3

    I agree with John: the first sign of an underventilated house is condensation on the interior of the window panes in winter. Lingering smells may or may not be a sign of an underventilated house -- it depends on the origin of the smells.

    There is no such thing as a house that is too tight -- only a house that is underventilated.

  4. user-945061 | | #4

    One of the more common standards for retrofits is Building Performance Institute's, which cites ASHRAE 62-89 for determining if ventilation should be installed. BPI calls for considering mechanical ventilation at <.35 ACHn, and requires it around .25 ACHn. If your numbers are correct, then you're well above the standard. There's certainly much to be said for planned ventilation, but for me it wouldn't be a starting point in a large and leaky house. Unless your house was occupied by 50 cigar smokers or some other ridiculous thing. Of course you'll want to do more testing as your retrofit work progresses.

  5. user-723121 | | #5

    .47 NAC is far from tight, with a climate correction factor of 20+, you are at 10ach50.

  6. jklingel | | #6

    Doug: Is there a simple way to convert NAC to ACH50? I was under the impression that NAC is pretty "loose", as in not easy to determine accurately. Apparently not?? Thanks.

  7. GBA Editor
    Martin Holladay | | #7

    “Natural infiltration” (ACHnat) varies greatly by season and by climate. It is not the same as normalized leakage, although its value may be similar. Rules of thumb for calculating ACHnat vary by climate. In Minnesota, ACHnat equals approximately ACH50 divided by 17, while in Florida, ACHnat equals approximately ACH50 divided by 30.

  8. user-723121 | | #8

    John ,

    This Home Energy article has been linked here before, describes the regional calculation for converting ACH50 to ACHnat.

  9. jklingel | | #9

    thanks, martin and doug. will read. j

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