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What indoor humidity should we maintain in order to avoid a mold and dust mite problem? InspectAPedia®  -     How to use humidity control & dehumidifiers to avoid or help eliminate a problem with dust mites and dust mite allergens

How to control indoor humidity to avoid mold and dust mites. This article explains the need for maintaining an anti-mold low humidity level indoors to avoid mold and other indoor pathogen growth in buildings. It answers the question "What indoor humidity level should I maintain to avoid mold and indoor air quality issues?" 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.

Also see DEW POINT TABLE - CONDENSATION POINT GUIDE for an explanation of dew points and indoor humidity in buildings, and see MOISTURE PROBLEMS: CAUSE & CURE and HOUSEWRAP AIR & VAPOR BARRIERS and VENTILATION in BUILDINGS.

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.

HUMIDITY ENCOURAGES MOLD - Too Much Humidity Encourages Mold Growth and Dust Mites - What are the more common problematic indoor molds?

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 - 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.

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.

Instruments to Measure Relative Humidity Indoors

When you buy an instrument to measure indoor humidity, you must either buy one which reads the result in relative humidity or you'll have to do some calculations to factor in temperature. The old, amusing, but very accurate sling psychrometer made an "end run" around the calculation problem by swinging a wet bulb thermometer through the air, literally slinging it around in a circle at the end of a string. This motion caused the water on the bulb to evaporate. The instrument then measured the temperature drop from evaporation and enabled reading of the relative humidity.

Modern electronic instruments which read relative humidity are available from many suppliers and read out immediately in relative humidity, or "RH". I've found that the less costly instruments, perhaps in the $40. to $100 range, are not precise "lab-grade" instruments, but they'll generally be within 1-3 percentage points of one another - accurate enough for our purposes, particularly if the instrument is consistent in its behavior. Since these instruments have to know the temperature as well as the humidity in order to calculate the relative humidity of the air, they'll usually give the temperature reading as a bonus along with the RH reading.

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:

• RH 48% - Outdoors
• RH 45% - Indoors, main floor kitchen, center of room, 5 ft. from floor
• RH 49% - Indoors, basement, center of single large area, 5 ft. from floor
• RH 57% - Indoors, basement, 1" from rear wall, 3/4" from floor
• RH 49% - Indoors, basement, at intake side of the dehumidifier, with the machine operating
• RH 35% - Indoors, basement, at the outlet side of the dehumidifier, with the machine operating

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, 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 I've 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 "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.

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