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What is a good ventilation and dehumidification strategy for an existing home?

user-2310254 | Posted in Mechanicals on

At some point soon I will complete the remodel on my new (old) townhouse.

This project involves a lot of updates, including improving air sealing and insulation, eliminating combustion devices, and installing two new high efficiency split systems (based on Manual J). Overall, the townhouse will be much tighter and more efficient when all the work is completed.

But I am wondering how to address ventilation and dehumidification.

I may be able to maintain a safe humidity level just with the new HVAC systems. And I may be able to ensure adequate ventilation with a supply-only approach.

My inclination is to install a few humidity monitors (in the attic and around the living space) and see what happens with moisture levels over the next few months. For ventilation, I am considering introducing 20 to 40 CFM of outdoor air and then checking for “stuffiness.” (My rough guess is the townhouse (2,800 square feet with two occupants) needs about 43 CFM.)

There seems to be a lot of debate about what “adequate ventilation” actually means. I don’t want to over- or under-ventilate. If necessary, I am comfortable with installing a dehumidifier/ventilator and/or a dehumidifier and an ERV.

If this was your project, what would you do?

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  1. Expert Member
    Dana Dorsett | | #1

    In general terms, the higher the tested SEER, the higher the sensible heat ratio usually is, and less dehumidification you get out of the cooling system. But right-sizing it to the loads delivers longer cycles, which compensates quite a bit for equipment that has a higher sensible heat ratio.

    In a high-performance house with high performance heat rejecting windows delivering lower than typical sensible loads you may still need active dehumidification beyond what the cooling system(s) provide.

    Air sealing lowers both the latent & sensible loads, but insulation & heat rejecting windows lower only the sensible load. If the predominant envelope improvement has been air sealing, odds are in your favor of having lowered the latent load more than the sensible load, which would be good.

    Unless you're smoking or doing a lot of frying or using a lot of aerosol products indoors you'll probably do fine with 40cfm.

    1. user-2310254 | | #2

      Thanks for your input, Dana.

      The top-floor unit is a ducted Daikin with a SEER of 15.5. and an HSPF of 9.2 The main-plus-ground-floor unit is an Amana heat pump with a SEER of 18 and an HSPF of 10. I guess performance-wise they qualify as pretty good but not spectacular.

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