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How to control indoor humidity to avoid indoor air quality complaints, mold and dust mites.

This article 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.

We also discuss where and how to measure indoor humidity, what indoor humidity targets to set, and we explain relative humidity, dew point, and moisture condensation in and on building materials.

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Problems Caused by Excessive Indoor Humidity include Mold Growth & Dust Mites

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Article Contents

What are the more 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.

DUST MITES & BACTERIA - Excessive Humidity Encourages Dust Mites, Dust Mite Allergen Production, Bacterial and Insect Hazards Indoors

High indoor humidity can encourage more problems than indoor mold. The same moisture conditions that support growth of problematic indoor molds also encourage the development of bacterial hazards, dust mite populations, a mite fecal allergen problem, and possibly other insect problems in buildings.

The same measures to control humidity to prevent mold growth are what's needed to discourage the dust mite population that exists in all living areas. Measures discussed in this article including choosing and maintaining the proper humidity level to avoid indoor mold will also work to minimize the level of dust mites and dust mite allergens.

What humidity level is needed? - How low should we keep the interior moisture level to avoid a mold problem?

Suppose a building does not currently have a mold problem, or a mold cleanup project has been completed. How can we avoid a future mold problem in the building?

1. be sure there are no ongoing building leaks, water entry, or venting problems.

2. keep the indoor humidity level in the mid-comfort range. A maximum indoor relative humidity of 55% RH may be acceptable, 50% RH better, 45% RH for an attic knee wall provided there are no ongoing leaks and the attic space is not one which is being vented to outside (in that case you're not in control of the humidity. If you run humidity too low or too high the building occupants will be uncomfortable.

The text below offers more technical background on indoor relative humidity (RH) control. This is getting slightly more technical about measuring the relative humidity - knowing a little more about how indoor air moves, how moisture levels vary in air and in building materials, and how to set the best humidity targets will improve the management of indoor moisture levels.

HUMIDITY IN BASEMENTS - How do We Control Basement Humidity to Reduce Mold and Dust Mite Allergen Risk

If 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. 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?

Wet basement inspection points (C) Carson Dunlop Associates InspectApedia

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.

Generally your best approach to reducing the high humidity in a basement combines

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

HUMIDITY IN ATTICS - How do We 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.

TARGET HUMIDITY - How to Choose the Target Humidity Level or Relative Indoor Humidity Level to avoid Mold and Dust Mite Problems

One client said he could keep the basement at 55% Relative Humidity (RH) but he didn't want to push it below that. Is this enough safety margin?

At 60% indoor RH we're entering the indoor problem mold-formation risk zone of high interior moisture in building wall or ceiling cavities or on wall and floor surfaces, possibly conducive to mold growth.

If you set the RH target at 55%, you're operating with **not much safety margin** of dryness. A small change in outdoor conditions (spilling water by the foundation) or indoor conditions (a nearby roof, wall, window, plumbing leak) can increase the moisture and RH into the problem zone. If for reasons of dehumidification cost you have to operate close to the edge, extra attention to leaks, moisture proofing, roof and surface drainage are even more important.

REACHING THE TARGET - When Have You Reached the Indoor Humidity Target to Avoid Mold and Dust Mites?

When have you reached your mold avoiding relative humidity RH target? If a building has been damp for some time, moisture has been absorbed into various materials such as wood framing and masonry surfaces. It may take weeks or even longer to drop the humidity in such an area, as the moist materials also have to dry out, not just the air. Using a fan to increase air movement in the area being dehumidified can speed this process.

Warning: if you cannot get the indoor RH down to a low level in a below-grade area such as a basement or crawl space, I'd suspect that too much moisture is continuing to enter through the slab or masonry walls. Attention to outside drainage may not be enough. In such cases, coating the walls with a masonry sealer (Thoro-Seal™ or Dry-Lok™ are example products) might help.

If you want to get past this practical discussion of indoor humidity and mold, check out "Understanding Ventilation," by John Bower. The Healthy House Institute, 1995.

More than a normal person can stand to read about what to do about mold in buildings is at our website. You might start at the "Mold Information Center - What to Do About Mold in buildings"

MEASURING HUMIDITY - How do you measure indoor humidity and relative indoor humidity?

Relative Humidity vs. Absolute Humidity in buildings

A variety of instruments can measure the amount of moisture in air, which we call "humidity." For example an inexpensive indoor "weather station" often includes a "humidity" gauge along with a barometer and thermometer. But just knowing the level of moisture in air (absolute humidity) is not enough. Usually, the humidity targets we use in these articles, and in academic or scientific texts are numbers expressed as relative humidity which takes into account not only the absolute water level in the air, but also the air temperature.

Relative humidity, by taking into account both the absolute humidity in the air and the temperature of the air, is telling you the humidity level as a function of the maximum amount of water that the air is capable of containing at a given temperature. If we're trying to control mold and other indoor pathogens for which water is a gating factor, it's relative humidity that is important.

Why? Because water condenses out of air onto a building surface (and thus supports mold or other indoor pathogens) only when the air at that surface contains more water than it can hold at that temperature. When warm, moist air contacts a cool surface, your basement drywall near the floor, for example, the air touching that surface may cool and give up some of its moisture to condense on the surface.

See TOOLS for MEASURING HUMIDITY for accuracy and options for indoor humidity measurement equipment.

Variations in Indoor Relative Humidity by Building Area and Surface Type

The relative humidity, or "RH" will vary significantly in a building at a given moment, depending on where you make your observations.

Here are some example RH measurements from a recent investigation at a 1970's wood frame two story home in generally good condition, after an extensive mold remediation and dryout project, where the owner had been running two dehumidifiers in the basement, and where there were no building leaks:

Notice, with no surprise, that the RH is higher close to the (cool) masonry surface? This explains our reasoning in suggesting a fairly low basement RH target for buildings if we're going to measure the RH in the center of the room.

Some dehumidifiers have an RH meter built right into the machine, so it will tell you what RH level it's seeing in its incoming air. But for operating efficiency you'll often run the machine in the center of the room.

The target humidity for a building, if measured at room center, needs to be low enough to avoid condensation out on cool surfaces at the room perimeter or floor.

Humidity, Condensation, and the Dew Point

To avoid moisture condensation on cool basement or other building surfaces, we need to keep the RH down below the dew point at those surfaces. The "dew point" is the temperature at which moisture will condense out of the air. The dew point is determined by the combination of the current temperature of the surface, the air temperature, and the humidity level.

If we were being scientifically precise we'd monitor all of the pertinent data - surface temperature, air temperature, relative humidity, and indoor air movement across surfaces. For our purposes, setting a reasonably low room-center target RH will usually be enough. But remember, even if you don't see water condensing on and running down your basement walls, it doesn't mean that the walls won't be at a notably higher moisture level than the air in the center of the room.

See DEW POINT TABLE - CONDENSATION POINT GUIDE for details about the dew point and how to measure or calculate it for a building area or surface.

Moisture Movement in Building Materials

Water molecules are very smart. They will naturally move from a moist area or surface to a more dry one, tending to seek equilibrium moisture across all surfaces and materials in a building, always considering the factors We have discussed above: temperature, relative humidity, and dew point. So if humidity increases in a basement from warm moist air entering that space, moisture will begin to enter the more dry drywall and insulation materials.

Conversely, as you run a dehumidifier in the basement, moisture will be removed first from the basement air, and then as that dry air contacts more-moist basement surfaces (drywall and insulation, for example), moisture will move from those materials back into the air. Moisture moves in either direction, into the air from materials, or into materials from air, always moving from the more-moist to the less moist substance, seeking equilibrium. This is why there will be a lot of water output from a basement dehumidifier when it is first run in an area, and then later water output will slow.

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.

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 MOISTURE CONTROL in BUILDINGS and also with more focus on sources of indoor moisture
or water beginning
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.

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.

Supply-only Vent System Operation: PIV System Humidity Level Variations

Nuaire Drimaster Positive Input Ventilation System PIV from Nuaire in the U.K. -

A U.K. reader wrote that he had recently installed a Nuaire "positive input ventilation" PIV system sold in the U.K. The installer monitored the temperature and humidity in the most suspect corner a building room. The PIV system generally maintained 55% RH but sometimes the humidity increased to 60-62%.

The operating premise of a Positive Input Ventilation (PIV) system is the continued introduction of filtered outdoor air into the building at a continuous rate, presumably putting the building at positive pressure with respect to the outdoors and thus causing indoor air to move outside through other building leaks or vents.

Shown at left, the Nuaire Drimaster Positive Input Ventilation System (PIV). In the U.K. & Ireland Nuaire is at +44 (0) 29 2085 8200, www. Email: or

This product is sold in the U.K. specifically intended to address indoor dampness & condensation problems.

[Click to enlarge any image]

A further underlying premise is that the relative humidity of outdoor air is always below that of indoor air. This is usually true for buildings in which there are significant moisture sources (use of plumbing, cooking, occupants, or a wet basement or crawl area).

Really? Well often but not always: In some climates and under some weather conditions, if outdoor humidity is quite high, even though we are introducing outdoor air, it may be more humid than indoors. A widely-discussed example of this "reverse humidity" problem (insofar as the Nuaire design intends to bring in outdoor air) is the movement of humid outdoor air into cool crawl spaces below buildings during summer weather. The result in these areas can be a significant increase in the crawl area moisture level as moisture from warm, moisture laden outdoor air condenses out into the cooler crawl space.

Relative temperatures between indoors and outside are also a factor. A commissioned installation by experts would include measurements of building air flow, air changes per hour ACH, and humidity levels.

In the U.K. Nuaire offers an HRV Best Practices Guide (cited below) that may be helpful. The company warns that installing any ventilation system without first studying building conditions is (in my words) a bit uncertain. Here is what Nuaire says about controlling condensation or moisture when installing a PIV system:

Control of condensation/moisture

In ducted ventilation systems, condensation will occur when warm, humid air extracted from bathroom, shower, kitchen or utility room hits the cooler surface of your ventilation duct. If the extracted air is significantly cooled, the moisture will condense back to water and become trapped in your ducting, potentially causing damage to the fan and property.

For this reason building regulations stipulate the following installation guidance in order to prevent issues with condensation and moisture:

Horizontal ducting including ducting in walls should be arranged to slope slightly downwards away from the fan to prevent backflow of any moisture into the fan unit

Ducting rising vertically carrying extract air to outside requires the use of a condensate trap, this is to prevent any moisture forming inside the ductwork, dropping back down and into the fan unit. This should then be connected to a suitable soil vent pipe using 21.5mm pipe. - Nuaire Best Practice Installation Guide [for] Mechanical Heat Recovery Ventilation (MVHR), Nuaire Group, retrieved 1/1/2015, original source:

To understand why humidity might increase in a building where a PIV system is installed, start by looking at the periods of higher relative humidity levels, temperatures, and comparing indoors and outside air conditions. An expert, if one designed and installed your system would also have looked at building air flow rates or air change rates expressed as air changes per hour (ACH).

An improperly-sized, installed, or located unit can give unsatisfactory results including indoor moisture and moisture-related mould problems, poor indoor air quality or fan noise issues.

I would also look for both apparent and clandestine moisture sources in the building. The fact that you found most concern at a wardrobe (do you mean a closet or a piece of furniture?) may simply reflect temperature differences in that location.

In the U.S. we also refer to PIVs as Supply Only Ventilation Systems. The operation of supply ventilation systems or PIVs is discussed in more detail by our contributor and expert Steven Bliss at


Heat Recovery Ventilation System Installation Requirements

For U.K. readers, as of 1 October 2010 revisions to Approved Document F-F1 the Means of Ventilation, applicable to people living in England and Wales requires that all Mechanical Ventilation & Heat Recovery System (MVHR) installations require that such systems be commissioned using a qualified, competent expert in compliance with 2010 ADF2010 regulations.

Among the requirements for these ventilation systems are conditions that will improve the installation and performance of any mechanical ventilation or heat recovery ventilation system wherever you live: [Paraphrasing from the Nuaire Best Practices Guide cited above and again in detail at REFERENCES]

You might do some simple, low -cost tests using toilet tissue or talcum powder or smoke or even a match to see which way air is flowing at your windows or doors.

Don't forget to look for condensate leaks in the PIV system ducting too.

About boosting the airflow rate of the PIV system, the company says:

The unit air volume can be manually boosted to maximum speed by wiring a simple one way switch (part number 771532) to the PCB (located under the top cover). By switching the ‘boost’ all other functions are over-ridden. - Nuaire, retrieved 1/1/2015, original source

See these articles on attic ventilation:

Bottom Line on Excessive Indoor Condensation: What to Do About It

  1. Identify and correct sources of un-wanted indoor moisture
  2. If necessary use a dehumidifier in damp areas like crawl spaces or basements
  3. Review building ventilation details in general, such as attic venting (avoiding attic condensation)
  4. Investigate the details of construction of building exterior walls and top floor ceilings to see what vapor barrier is present or absent.
  5. Review indoor temperatures, relative humidity, and air movement.

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