GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

How tight must a home be to no longer require mechanical humidification?

John_Hansen | Posted in Mechanicals on

In an online forum discussion of home inspectors in the USA, the topic of humidifiers and infiltration is receiving a lot of comments. The basic character of many of the comments mirror the position presented by Dr Allison Bailes in his article: “A Humidifier Is A Bandaid – The Problem is Infiltration”.

While we can all agree that in cold climates, cold air that enters the home and is then warmed and mixed with conditioned interior air lowers the indoor relative humidity. And a tight house may be able to maintain a desirable RH without the need for a humidifier. I note that Dr Bailes did not say that all homes in all climates for all customers with all lifestyles can be made tight enough nor insulated well enough that none will need a humidifier.

When professional weatherization contractors tighten up an existing home and add insulation, does that effort always achieve a “tight and well insulated home” that can maintain indoor RH at a desirable level without additional humidification? The question is: “How often (what percentage) is the weatherization efforts successful enough that home owners can be comfortable in cold or dry climates without a humidifier?”

Is there a sweet spot of Air Changes per Hour that can reliably keep the humidity levels of a home high enough that no additional humidification is needed and what is that ACH level is it economically achievable in existing homes?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. _Stephen_ | | #1

    I don't think anyone here is going to be able to give you a satisfactory answer. It's going to depend on how good your crew is, how leaky your home is, and what your budget is.

    I'm very confident that a $100k deep energy retrofit in a 90's home can achieve comfortable living without a humidifier, if you have a good crew.

    I'm very confident that a $100 worth of spray foaming an 1800's stone house will not.

    Your reality is going to be somewhere in the middle, and we just don't know enough to be able to predict success.

  2. Jon_R | | #2

    There are lots of examples of commonly achievable ACH@50 results. But much more accurate on new construction.

    The need for humidification is going to depend greatly on your moisture sources, outdoor dew point, ERV vs HRV, ventilation CFM, wind, etc. There are moisture source tables available online and if you do the math (evidently few people do), you can come up with some uncomfortable, very dry homes even if the air sealing were perfect - because of ventilation. "tight homes don't need humidifiers to be comfortable/healthy" is often false.

    This says down to 40%, but I'd accept the personal/building risk trade-off at 30% in cold weather - less if poor windows have condensation. I wouldn't go over 40% when cold.

    Of course measurement is more accurate than estimates. Air seal as well as you reasonable can and address low or high humidity if it later turns out to be a problem.

  3. walta100 | | #3

    There is just way too much gray for any real answer to your question. What is comfortable varies from person to person, place to place and house to house. Some people complain at 48% yet people that live in the desert are comfortable at 25%. Some people will have 50 house plants and other will have none. Some will keep hundreds of gallons of aquariums. How many people are taking showers how often and how long. A house with triple pain windows will better handle higher indoor humidity than one with single pain windows.

    So there is no clear cut answer to the question, does this house have to have a humidifier. It is very likely family A does not feel the need for a humidifier but when family B moves into the same house they will be uncomfortable without one.

    ACH50 has proven to be a poor predictor of how much air will move thru a house under its own power but it is the only one we have so we use it.

    I see a humidifier a lot like a gun in that it is a useful tool and a dangerous object all in one package it depends on the user. If you give someone a humidifier and they decide to set it to 55% and 72° and they put a 20° setback into their thermostat every night. Without fail some corner of some of the rooms will fall below the 55° dew point some nights and when it does the drywall will soak up water from the air and may grow mold or rot given enough time. If your energy bills do not matter to you a humidifier maybe a useful tool in making a drafty house comfortable.


  4. GBA Editor
    Martin Holladay | | #4

    In my opinion, a humidifier should only be used in homes where one of the residents has an unusual medical diagnosis that requires high indoor humidity. Most of us are perfectly healthy without indoor humidificiation -- and houses without humidifiers are at a much lower risk of mold or rot than houses with humidifiers.

    If your house is unusually dry during the winter, you should (a) invest in air sealing work, or (b) consider a reduction in your rate of mechanical ventilation. Here is a link to a relevant article: "Blower-Door-Directed Air Sealing."

  5. Expert Member
    Dana Dorsett | | #5

    +1 on avoiding the use of humidifiers when not medically necessary. Running a humidifier to keep the place north of 40%RH @ 70F in winter will raise the indoor mold spore counts in spring. More than just a band-aid, it's a "solution-problem".

    >"Is there a sweet spot of Air Changes per Hour that can reliably keep the humidity levels of a home high enough that no additional humidification is needed and what is that ACH level is it economically achievable in existing homes?"

    The sweet spot varies dramatically with climate & weather. Some of the "A" in "ACH" is a LOT more humid than others, varying by location, season, and recent weather patterns.

    Most houses sheathed with 2x8 goods like OSB or CDX can be economically sealed to the current 3ACH/50, but it varies. Like the local air, "existing homes" has a huge variability.

    Even older plank sheathed wood framed homes can often be retrofitted to under 3ACH/50 by dense-packing cellulose or fiberglass in walls after fixing all of the big and obvious air leaks are dealt with, easier still if using infra-red imaging during blower door testing. Whether that's economic or not depends on your energy costs, and how leaky the place was prior to the air sealing/insulation.

    Below 3ACH/50 some amount of mechanical ventilation would usually be required, but it doesn't necessarily have to be the full on ASHRAE 62.2 levels ( occupancy x 7.5cfm + square feet of conditioned space x 0.03cfm). Even leaky homes need ventilation- the "natural" ventilation isn't always entering or leaving the most needed locations (or by a sufficiently clean path). Once it's tight enough that it's staying above 30%RH @70F all winter long it's time to consider installing balanced ventilation, not just exhaust-only from bath & cooking areas.

  6. tommay | | #6

    Get some plants.....

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |