Photograph of client using tissue to look for air conditioning air flowIndoor Humidity Control
Advice for Attic, Upper Floors, Basement, Crawlspace RH Control

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How to control indoor humidity:

Advice on controlling indoor relative humidity in attics, upper floors, basements, crawl spaces, or on sweating walls & pipes.

This article series answers the question "What indoor humidity level should I maintain to avoid mold and indoor air quality issues?" We explain the need for maintaining an anti-mold low humidity level indoors to avoid mold and other indoor pathogen growth in buildings.

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Indoor Humidity Control by Area

Photograph of chopped fiberglass insulationArticle Contents

Also see these articles:

Excessive Humidity Encourages Indoor Air Quality & Health Hazards

[Click to enlarge any image]

Once indoor relative humidity is in the 60% range we are inviting indoor mold growth.

High indoor humidity can encourage more problems than indoor mold.

The same moisture conditions that support growth of problematic indoor molds also cause or contribute to these hazards

Our citations refer to research articles found at the end of this article and also in this article's separate REFERENCES section.

Common problematic indoor molds

We recommend use of dehumidifiers and humidity instruments or humidity transmitters to monitor your building. But no dehumidification system will be up to the task of preventing mold if a building has serious leaks, flooding, or water entry.

No dehumidifier, no "air cleaner," no "ozone generator," nor other magic machine, spray, or air treatment will correct a mold problem in a building if there is a significant problem reservoir.

For that case, what's needed is to find the mold problem, remove it, and correct its cause. And as a last warning, there are about 1.5 million mold species - some of them may be able to grow in very dry or very wet or other inhospitable conditions.

Mold spores are everywhere all the time, entering from outdoor air as well as on pets and clothing.

A mold spore landing on an indoor surface is likely to be insignificant and amount to little more than a common component of indoor dust, until such a mold spore lands on an organic surface (such as drywall) and the indoor humidity level and thus the humidity or moisture level of the surface on which the mold spore rests, is sufficiently high.

Since a mold spore requires moisture to propagate and grow, the indoor humidity level is a key gating factor in the control of indoor mold (and dust mites) in buildings.

Certain common mold genera and species, such as some members of the Aspergillus sp. and others grow readily on common building materials if they also have enough moisture. While there are fungal species that are able to grow under a remarkably wide range of environmental conditions, keeping indoor humidity at the appropriate level will reduce the chances of growth of the most common indoor problem molds.

We refer to common problem indoor toxic or allergenic molds such as Aspergillus sp., Penicillium sp., Stachybotrys sp. /S. chartarum /Memnoniella echinata, Trichoderma sp. /T. viride, Ulocladium sp. /U. chartarum, and at a less significant level of concern, Cladosporium sp. and its common indoor species such as C. sphaerospermum and C. cladosporioides.

A number of Basidiomycetes and Ascomycetes also appear indoors as wood rotters and on other wet or damp building materials, though they may as a group be less often toxic or pathogenic to humans and more often an indicator of wet or damp mold-conducive indoor conditions.

Our table of the most commonly found indoor molds growing in buildings has been moved to a separate online document.

See TABLE OF MOST COMMON INDOOR MOLDS FOUND IN BUILDINGS. Use the back button on your browser to return to this page.

Control Attic Humidity to Reduce Mold Risk

In the case of an attic crawl space, perhaps a knee-wall area abutting an upper floor bedroom, the risk of excessive inside humidity at a wall is much less than in a basement. In the attic we don't face a cool concrete-block wall surface in the attic.

But what about an un-vented attic in a cold weather climate? Heat loss into such a space and warm moist air leaking into such a space can indeed create high levels of problem moisture - enough to wet surfaces or even form frost and later drip onto the attic floor.

On the other hand, if the attic is vented to outside (ridge vents and soffit vents as I recommend) you'll never control the attic RH. You'll be trying to control the whole outdoors.

On the third hand (if that's possible), if an attic is not vented to outside, the RH there is most-likely a function of and approach the levels of the humidity levels in the air in the rooms abutting and below the attic area.

Articles on Attic Ventilation

Upper Floor Humidity Control

Reader Question: how can I decrease the indoor humidity level upstairs

I have 3 dehumidifiers going plus the central air and the basement and first floor are at 50 - 55% humidity but I can not get the top floor where the bedrooms and bathroom is below 60% in the spring/summer/fall. Some days it even goes to 65%.

We had a mold problem 7 years ago and had professional remediation and have not seen any evidence of mold since but I have developed chronic sinus and bronchial problems that I wonder if it is being caused by mold spores.

I thought maybe humidity coming from the attic but then the floor downstairs would be affected also as it is a multilevel house and the kitchen/dining/living room area is directly under the attic also and it is 50% down there so I do not know how to fix the humidity problem.

Any help would be appreciated as I am tired of being sick. Thank you. (this is the first summer I have been sick like this and can not get over it) - Anonymous 9/12/11

Reply: a strategy for correcting high indoor moisture levels and possible mold or dust mite reservoirs

Although sometimes we find surprising down-currents of air and moisture from a building attic, that's not the most common indoor moisture problem source.

Since we haven't inspected your building and know next to nothing about it, we have to outline a more general strategy for reducing high indoor moisture:

  1. Find and fix sources of high indoor moisture; If your building uses central air conditioning also
  2. Then run dehumidifiers seasonally as needed, if necessary using fans to improve indoor air circulation - a step that will in turn significantly increase the effectiveness of your indoor dehumidifiers.
  3. Make a more expert and thorough inspection of your building for a remaining mold reservoir; the fact that you previously had enough of an indoor mold problem to justify paying a professional mold remediator suggests that the problem was significant. Too often a "remediator" focuses on spraying, or better, removing moldy materials and cleaning the building.

    But still, if s/he forgot to find and fix the sources of high indoor moisture, a problem with mold, dust mites, or other indoor allergens or air quality problems is likely to return.

At CONDENSATION on WINDOWS, SKYLIGHTS we discuss the causes & cures for excessive condensation on windows

At DEW POINT CALCULATION for WALLS we discuss how and why moisture can accumulate within building walls

At INDOOR AIR QUALITY & HOUSE TIGHTNESS we discuss indoor air quality problems in homes with very little ventilation or fresh-air makeup

At MOISTURE CONTROL in BUILDINGS and also with more focus on sources of indoor moisture

At MOISTURE METER STUDY we include examples of the difficulty of measuring moisture in building walls and ceilings and we show points of hidden leaks that may affect indoor humidity levels.

At WATER ENTRY in BUILDINGS we discuss the importance of finding the source of excessive building moisture and doing what we can to correct that problem first.

Control Basement Humidity to Reduce Mold and Dust Mite Allergen Risk

Wet basement inspection points (C) Carson Dunlop Associates InspectApediaIf the RH in the center of a basement is 55% it is likely that at the walls or corners, where there is less air circulation, the RH may be different, often higher.

The local temperature difference close to a cool masonry wall surface means that both temperature and measured relative humidity close to the wall will be different than in the center of a room.

But it's at the cooler wall surface that condensation may be expected to occur. If you measure the RH at the worst-case location such as the most-suspect-of-dampness corner of a basement and you're 55% close to the wall you're likely to be ok.

Reader Question: what is the best way to reduce humidity level in a basement? Vent the drop ceiling?

I have a 1,400 square-foot basement with a drop ceiling below the main level subfloor. The house is on a slope, so one wall is not underground. In some areas, the RH is 62 percent.

I was thinking of installing an attic-style vent fan horizontally through that wall in the area between the drop ceiling and the subfloor to create air movement and to vent the air to the outside. Will that help reduce the RH? Thanks. - D.M. 6/4/2013

Our illustration at left shows where to look for moisture in and leak problems in basements, courtesy of Carson Dunlop Associates. Click to enlarge.

Reply: here are the basic steps in reducing & controlling basement humidity level

Interesting question; I think we need more information to make an answer beyond mere arm-waving;

While years ago we thought that we should vent crawlspaces and perhaps basements with outdoor air - still reflected in crawl space or foundation vent opening size ratios found in some building codes.

Current best construction practices, now better informed, have shifted to making the indoors a closed, conditioned space. That's because the moisture level and temperature of outdoor air vary so widely in many areas that under many conditions mere venting makes indoor moisture levels worse not better.

Your best approach to reducing the high humidity in a basement or crawl space combines these methods

Dehumidifying a crawl space (C) Daniel Friedman

Also take a look at the collection of suggestions under CRAWL SPACE DRYOUT - home

Do Water Pipes or Stone or Brick Surfaces "Sweat" in buildings?

No. Why does water condense on your cold water pipes overhead in the basement before it condenses on the steel Lally columns supporting your main girder?

Perhaps because cold water (at 40 deg. F.) is running through the water pipe, cooling its surface to a lower temperature (40 deg. f.) than that of the Lally column (perhaps 55 deg. f.). Water pipes do not "sweat" as people say - water is not exuding out of pores in the pipe. Water is condensing from moist air onto the surface of the cold water pipe. Insulate your cold water pipes to avoid condensation and drips onto the floor. It looks like sweat, but it's not.

For a different reason, that of energy efficiency, you might want to insulate your hot water and heating pipes in a basement as well, though in some conditions we are so desperate to warm and dry a problem area that we deliberately leave the hot water and heating pipe insulation off so that we can steal some of their heat to warm and dry an area.

Similarly, moisture will condense out of moisture-containing air on cool building surfaces like stone, brick, metal, concrete floors or walls or ceilings, and on tile floors or walls set over cool or cold surfaces.


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