What indoor humidity should we maintain in order to avoid a condensation, mold, or dust mite problem?
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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. This article 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. If you are having trouble getting the humidity down to an acceptable level in your building, also see DEHUMIDIFICATION PROBLEMS.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Problems Caused by Excessive Indoor Humidity include Mold Growth & Dust Mites
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.
Related articles: CONDENSATION or SWEATING PIPES, TANKS. 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. Related article: CONDENSATION or SWEATING PIPES, TANKS.
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.
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:
- 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 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.
Bottom Line on Excessive Indoor Condensation: What to Do About It
- Identify and correct sources of un-wanted indoor moisture
- If necessary use a dehumidifier in damp areas like crawl spaces or basements
- Review building ventilation details in general, such as attic venting (avoiding attic condensation)
- Investigate the details of construction of building exterior walls and top floor ceilings to see what vapor barrier is present or absent.
- Review indoor temperatures, relative humidity, and air movement.
Frequently Asked Questions (FAQs) about how to measure, diagnose, & correct high indoor humidity level problems
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 brochial 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:
- Find and fix sources of high indoor moisture; If your bulding uses central air conditioning also see WET CORRODED DUCT WORK
- 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.
- 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.
Question/Comment: excessive indoor humidity traced to high velocity air conditioning system coil condensation blow-off
For years I've been trying to figure out the excessive humidity problem in my home too. I finally found it after working with foundation people, plumbers, a/c techs -- nobody could figure it out, but I finally did. When the cooling kicks on, the moisture level skyrockets. It has affected the inside of my home tremendously. We thought it was the a/c drain. They re-routed it, made sure it was draining well and clear. It is. Leak near or under the foundation. We checked everything-that wasn't it.
What's happening is the fan is actually sucking the moisture out of the evaporator coils before the condensation off the coils can drain away. The design of this Lennox horizontal system in the attic is such that the small space right above and right below the squirrel type fan, creates a venturi effect, increasing the velocity of the air being sucked into the fan.
The velocity of the air is so strong that the coils (about 12 in. away) have the moisture sucked right off of them and into the fan, which, of course is then blown into the ducting. I'm not sure if this is an engineering design problem, if the a/c co. wired the fan to a speed that's too high, or if perhaps a part is missing that is supposed to prevent this.
Regardless, the inside of the unit is now so covered with mildew and mold and the electrical connections on the heating elements as well as all the electrical connections on the inside of the unit, are so corroded and rusted, it's a wonder that love thing works at all. (This also probably explains why sometimes the heat works and sometimes it doesn't. The a/c repair guys have never been able to figure out why. They always seem to think it's the t'stat. It's not. It's new and has recently been completely re-wired when I moved it from an outside wall to an inside wall.)
Anyway, that's where MY moisture problem is coming from. Good luck! - Mike / DFW 12/13/2012
Reply:
Mike, gold star to you for good detective work. We will keep your note in this article, as it will surely help other readers.
More about dehumidifcation problems traced to central air conditioning systems can be read at DEHUMIDIFICATION PROBLEMS.
Do you think that the root problem, then, is an improper duct or plenum size or design?
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- "Damp Indoor Spaces and Health", Institute of Medicine, National Academy of Sciences, 24 May 2004. - Web Search 6/23/2010 - original source: http://www.iom.edu/Reports/2004/Damp-Indoor-Spaces-and-Health.aspx
Damp Indoor Spaces and Health - executive summary- Quoting:
Almost all homes, apartments, and commercial buildings will experience leaks, flooding, or other forms of excessive indoor dampness at some point. Not only is excessive dampness a health problem by itself, it also contributes to several other potentially problematic types of situations. Molds and other microbial agents favor damp indoor environments, and excess moisture may initiate the release of chemical emissions from damaged building materials and furnishings. This new book from the Institute of Medicine examines the health impact of exposures resulting from damp indoor environments and offers recommendations for public health interventions. "Damp Indoor Spaces and Health" covers a broad range of topics. The book not only examines the relationship between damp or moldy indoor environments and adverse health outcomes but also discusses how and where buildings get wet, how dampness influences microbial growth and chemical emissions, ways to prevent and remediate dampness, and elements of a public health response to the issues. A comprehensive literature review finds sufficient evidence of an association between damp indoor environments and some upper respiratory tract symptoms, coughing, wheezing, and asthma symptoms in sensitized persons. This important book will be of interest to a wide-ranging audience of science, health, engineering, and building professionals, government officials, and members of the public.
Copies of Damp Indoor Spaces and Health [buy at Amazon] are also available from the National Academies Press,
500 Fifth Street, N.W., Lockbox 285, Washington, DC 20055; (800) 624-6242 or (202) 334-3313
(in the Washington metropolitan area); Internet, http://www.nap.edu. The full text of this
report is available at http://www.nap.edu.
- Professional Equipment is a retailer of inspection tools and test equipment - selling temperature, humidity, and RH measurement tools from under $20. to over $1,300. U.S.
- Tecpel Co., Ltd., 4F-1, No 225, HePing East Road, Sec 3, Taipei 11056 Taiwan ROC 888-2-2737-5866
- WHO Guidelines for Indoor Air Quality: Dampness and Mould (World Health Organization Europe), WHO Regional Office for Europe, ISBN-10: 9289041684, ISBN-13: 978-9289041683
When sufficient moisture is available, hundreds of species of bacteria and fungi -- particularly mold -- pollute indoor air. The most important effects of exposure to these pollutants are the increased prevalence of respiratory symptoms, allergies and asthma as well as disturbance of the immune system. Preventing (or minimizing) persistent dampness and microbial growth on interior surfaces and building structures is the most important means of avoiding harmful effects on health.
This book provides a comprehensive overview of the scientific evidence on the health problems associated with this ubiquitous pollution and provides WHO guidelines to protect public health. It also describes the conditions that determine the presence of mould and provides measures to control its growth indoors.
- Olalekan F. Osanyintola, Carey J. Simonson, Moisture buffering capacity of hygroscopic building materials: Experimental facilities and energy impact, article within Energy and Buildings,
Abstract
Research into dynamic moisture storage in hygroscopic building materials has renewed interest in the moisture buffering capacity of building materials and shown the potential for these materials to improve indoor humidity, thermal comfort and indoor air quality in buildings. This paper complements previous research by estimating the effect of hygroscopic materials on energy consumptions in buildings. The results show that it may be possible to reduce heating and cooling energy consumption by up to 5% and 30%, respectively, when applying hygroscopic materials with well-controlled HVAC systems. The paper also describes two different experimental facilities that can be used to measure accurately the moisture buffering capacity of hygroscopic building materials. These facilities provide different convective transfer coefficients between the hygroscopic material and ambient air, ranging from natural convection in small, sealed jars to fully developed laminar and turbulent forced convection. The paper presents a numerical model and property data for spruce plywood which will be used in a companion paper [O.F. Osanyintola, P. Talukdar, C.J. Simonson, Effect of initial conditions, boundary conditions and thickness on the moisture buffering capacity of spruce plywood, Energy and Buildings (2006), doi:10.1016/j.enbuild.2006.03.024.] to provide additional insight into the design of an experiment to measure the moisture buffering capacity of hygroscopic materials.
Keywords
Moisture buffering capacity;
Energy savings;
Experimental facility;
Uncertainty;
Indoor air quality;
Convective transfer coefficients;
Spruce plywood
- Olalekan F. Osanyintola, Prabal Talukdar, Carey J. Simonson
Effect of initial conditions, boundary conditions and thickness on the moisture buffering capacity of spruce plywood
Energy and Buildings, Volume 38, Issue 10, October 2006, Pages 1283-1292
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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- Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
- Environmental Health & Investigation Bibliography - our technical library on indoor air quality inspection, testing, laboratory procedures, forensic microscopy, etc.
- Adkins and Adkins Dictionary of Roman Religion discusses Robigus, the Roman god of crop protection and the legendary progenitor of wheat rust fungus.
- Kansas State University, department of plant pathology, extension plant pathology web page on wheat rust fungus: see http://www.oznet.ksu.edu/path-ext/factSheets/Wheat/Wheat%20Leaf%20Rust.asp
- "A Brief Guide to Mold, Moisture, and Your Home",
U.S. Environmental Protection Agency US EPA - includes basic advice for building owners, occupants, and mold cleanup operations. See http://www.epa.gov/mold/moldguide.htm
- US EPA - Mold Remediation in Schools and Commercial Building [Copy on file at /sickhouse/EPA_Mold_Remediation_in_Schools.pdf ] - US EPA
- US EPA - Una Breva Guia a Moho - Hongo [Copy on file as /sickhouse/EPA_Moho_Guia_sp.pdf - en Espanol
