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 Mobile ViewENVIRONMENTAL HAZARDS - INSPECT, TEST, REMEDY ACCEPTABLE MOLD LEVEL ACCURACY OF VARIOUS MOLD TEST METHODS ACTION GUIDE - WHAT TO DO ABOUT INDOOR MOLD ACTIVITY of MOLD in BUILDINGS AGE of MOLD - Old is the Mold? AIR CLEANER PURIFIER TYPES AIR FILTERS for HVAC SYSTEMS AIR POLLUTANTS, COMMON INDOOR AIR TEST FOR MOLD: ACCURACY AIR TEST SAMPLING CASSETTE STUDY AIRBORNE MOLD SPORE COUNT ACCURACY AIRBORNE PARTICLE ANALYSIS METHODS ALLERGEN TESTS for BUILDINGS ALLERGENS in BUILDINGS, RECOGNIZING ALLERGY & MOLD IAQ PRODUCTS ALLERGY TESTS for PEOPLE ALLERGY TEST ACCURACY ATTORNEYS and EXPERT WITNESSES ATTIC MOLD BASEMENT MOLD BASEMENT MOLD WATER IMPACT BATHROOM MOLD BROWN HAIRY BATHROOM MOLD BIBLIOGAPHY for ENVIRONMENTAL HEALTH, MOLD, IAQ BIOLOGICAL POLLUTANTS BLACK MOLD, HARMLESS COSMETIC BLACK MOLD, TOXIC & ALLERGENIC BLEACHING MOLD, Advice about BLUE vs YELLOW COMBUSTION FLAMES BOOK MOLD, Moldy Book Cleaning BOOKSTORE - ENVIRONMENTAL CAR MOLD CONTAMINATION CARBON DIOXIDE - CO2 CARBON MONOXIDE - CO CARPETING & INDOOR AIR QUALITY CAT DANDER in buildings Cell phone Radiation Hazards CHIMNEY INSPECTION DIAGNOSIS REPAIR CHIMNEYS & Flues - Asbestos Transite Pipe CHINESE DRYWALL HAZARDS CHEMICAL CONTAMINANTS in WATER CHLORINE IN DRINKING WATER CHLORINE IN SEPTIC WASTEWATER COMBUSTION GASES & PARTICLE HAZARDS CONDENSATION or SWEATING PIPES, TANKS CPSC Indoor Air Pollution Book Online Copy DIRECTORY of MOLD / ENVIRONMENTAL EXPERTS DIRT FLOOR MOLD CONTAMINATION Disinfectants Disinfecting Buildings with Bleach DRYWALL MOLD DRAFT HOODS - gas fired DRAFT MEASUREMENT, CHIMNEYS & FLUES DRAFT REGULATORS, DAMPERS, BOOSTERS DRINKING WATER Diethylstilbestrol - DES DUCT SYSTEM & DUCT DEFECTS DUST ANALYSIS for FIBERGLASS DUST CONTAMINATION FROM HVAC? DUST SAMPLING PROCEDURE EMERGENCY RESPONSE, IAQ, GAS, MOLD EMF MEASUREMENT PROCEDURES ENERGY SAVINGS in BUILDINGS ENVIRO-SCARE - PUBLIC FEAR CYCLES EXTERIORS of BUILDINGS FEAR of MOLD - MYCOPHOBIA Fiberboard Insulation Sheathing Mold Fiberglass Enviro-Scare FIBERGLASS HAZARDS FIBERGLASS INSULATION FIBERGLASS INSULATION MOLD FIBERGLASS PARTICLE CONTAMINATION TEST FIBERGLASS INSULATION MOLD FIBERGLASS PARTICLE CONTAMINATION TEST FIRE DAMAGE vs MOLD DAMAGE Fireplace Inserts Fireplaces & Woodstove Contaminants FLAME COLOR, BLUE vs YELLOW COMBUSTION FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP FLOODS IN BUILDINGS-mold FLOOR DAMAGE DIAGNOSIS FLOOR & SUBFLOOR MOLD, HIDDEN FLOOR TILE ASBESTOS IDENTIFICATION FLOOR TILE HISTORY & INGREDIENTS FORMALDEHYDE HAZARDS FORMALDEHYDE HAZARDS FUNGICIDAL SPRAY & SEALANT USE GUIDE MOLD INFORMATION CENTER ACTION GUIDE - WHAT TO DO ABOUT INDOOR MOLD CLEARANCE INSPECTIONS - MOLD CLEANUP CONDENSATION or SWEATING PIPES, TANKS COOLING LOAD REDUCTION by ROOF VENTS DEHUMIDIFICATION PROBLEMS DEW POINT CALCULATION for WALLS DEW POINT TABLE - CONDENSATION POINT GUIDE DUST ANALYSIS for FIBERGLASS DUST SAMPLING PROCEDURE EFFLORESCENCE, Salts & White / Brown Deposits EMERGENCY RESPONSE, IAQ, GAS, MOLD FEAR of MOLD - MYCOPHOBIA Fiberboard Insulation Sheathing Mold FIBERGLASS INSULATION MOLD FIND MOLD, ESSENTIAL STEPS FIND MOLD in buildings, HOW TO HUMIDITY LEVEL TARGET HUMIDITY CONTROL TO PREVENT MOLD Humidity Encourages Mold Dust Mites & Bacteria What humidity level? Humidity in Basements Humidity in Attics Target Humidity Reaching the Target Measuring Humidity Tools for Measuring Humidity INDOOR AIR QUALITY & HOUSE TIGHTNESS INDOOR AIR QUALITY IMPROVEMENT GUIDE INDOOR AIR QUALITY METHODS COMPARED LEAD POISONING HAZARDS GUIDE LEED GREEN BUILDING CERTIFICATION LEED Building Designation & IAQ MEDIA BLASTING for MOLD REMOVAL METHANE GAS SOURCES MICROSCOPE DIGITAL PHOTOGRAPHY MILDEW in BUILDINGS ? MILDEW ERRORS - MOLD PHOTOS MILDEW REMOVAL & PREVENTION MOISTURE CONTROL in BUILDINGS MOLD: A COMPLETE GUIDE TO MOLD MOLD ACTION GUIDE - WHAT TO DO ABOUT MOLD MOLD ACTIVITY in buildings MOLD AGE - Old is the Mold? MOLD APPEARANCE - WHAT MOLD LOOKS LIKE MOLD APPEARANCE - STUFF THAT IS NOT MOLD MOLD ATLAS & PARTICLES INDEX MOLD CLEANERS - WHAT TO USE MOLD CLEANUP COMPANIES MOLD CLEANUP, DO IT YOURSELF MOLD CLEANUP GUIDE- HOW TO GET RID OF MOLD MOLD CLEANUP by MEDIA BLASTING MOLD CLEARANCE INSPECTIONS MOLD CLINICAL REFERENCE TEXTS MOLD CONSULTANTS/INSPECTORS MOLD CONTAMINATION LEVELS MOLD CULTURE PHOTOS MOLD CULTURE TEST KIT VALIDITY MOLD DETECTION & INSPECTION GUIDE MOLD or INDOOR AIR EMERGENCY RESPONSE MOLD EXPERT, WHEN TO HIRE MOLD GROWTH on SURFACES MOLD INSPECTORS & MOLD TESTERS MOLD PREVENTION GUIDE MOLD RELATED ILLNESS GUIDE MOLD RELATED ILLNESS SYMPTOMS MOLD TEST KITS Nanomaterials Hazards NOISE / SOUND DIAGNOSIS & CURE ODORS, Smells, Gases in buildings-Diagnosis & Cure More Information |
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. InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers nor with topics or services discussed at this website.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. 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. 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. © Copyright 2012 InspectAPedia.com, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use page top links to major topics or use links at the left of each page to navigate within topics and documents at this website. Green links show where you are in a document series or at this website. 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 IndoorsHigh 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 RiskIf 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 RiskIn 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 ProblemsOne 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 buildingsA 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 TypeThe 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 PointTo 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 MaterialsWater 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
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