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Moisture Problems in Buildings: Causes & Cures InspectAPedia®  -     How to identify causes of high indoor moisture How to eliminate sources of un-wanted moisture & condensation indoors Role of vapor barriers, infiltration barriers, building condensation Sources of building mold, rot, paint failures Major vehicle of moisture movement out of homes What to do about high indoor moisture levels

Here we discuss the concepts behind the cause and cure of indoor moisture problems. T This website discusses how to inspect, diagnose problems in, and install or repair building insulation & ventilation systems including heat loss, moisture, & interior stains. Our page top photo shows extreme condensation at the header of a basement window in a home exposed to interior leaks. High indoor moisture levels can lead to costly mold contamination problems as well as insect attack and rot on buildings.

Readers should also see ROT, TIMBER FRAME which demonstrates severe damage due to moisture, insulation, and vapor barrier defects, and see INDOOR AIR QUALITY IMPROVEMENT GUIDE which includes details about whole house ventilation systems. Also see DEW POINT TABLE - CONDENSATION POINT GUIDEfor an explanation of dew points and indoor humidity in buildings, HOUSEWRAP AIR & VAPOR BARRIERS and VENTILATION in BUILDINGS. Also see WATER ENTRY in BUILDINGS where we discuss the cause and prevention of water leakage into buildings.

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Moisture-Caused Building Problems

This article is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Excessive indoor moisture problems on buildings are often difficult to diagnose and cure, largely because although the physics of moisture vapor transmission, air flow, and dew points is well known (but complicated), the movement of moisture in buildings is often complicated and not obvious unless invasive measures (cutting holes to look) are used.

Controlled experiments and field investigations of condensation in building cavities have turned up fewer problems than anticipated. Condensation on windows (page top photo), frost in building attics, and in some cases biological pollutants & mold contamination or peeling exterior paint are common effects of high indoor moisture levels, but rotted building sheathing and rotted or insect-damaged framing are more often due to actual leaks into the structure.

Dry rot - a misnomer for a wood eating fungus such as Meruliporia incrassata - will in fact attack a home where there is no light, saturated wood, and temperatures above 50 degF. In a typical uninsulated wall, these conditions rarely occur together, which is why so many older wood-frame homes are still going strong. With the warmer walls and higher moisture levels of today's tighter homes, care should be taken to avoid this type of decay. As for the effect of moisture on thermal efficiency (heat loss and heating costs) of fiberglass insulation, reports vary widely.

Sources & Movement of Water Vapor in Buildings

The National Bureau of Standards says that a family of four typically produces two to three gallons of water vapor a day. (More detail on building moisture contributed by building occupants is at MOISTURE CALCULATIONS.) Additional moisture migrates up through the building from basements and crawl spaces, particularly where no vapor barrier was placed on or below those floors or surfaces. Once in the house, water vapor enters wall and ceiling cavities by two primary means: diffusion and convection.

Water Vapor Diffusion in Buildings

Water vapor diffusion refers to the migration of water vapor from areas of greater vapor pressure (more humid or wetter building areas) into areas of lesser vapor pressure (drier areas). This movement is roughly from warm moist areas into cool dry areas, such as from a humid heated building occupied space into drier, cooler wall or ceiling cavities. This moisture movement occurs in buildings at a molecular level (molecules of water, H₂O, and it is independent of actual air currents. The rate at which water vapor passes through building materials varies according to the vapor pressure differentials and the permeability of the materials.

Water Vapor Movement in Buildings by Convection (Movement on Air Currents)

Air convection is the movement of air across air pressure differentials - up into ceilings and attics via the "stack effect" or out through wind-swept building walls. Warm air rises in buildings. Tall buildings or any home with open or leaky upstairs windows (or any other air infiltration or exfiltration leaks high in the building) become a natural air "chimney" drawing air from the lowest levels of the building upwards, increasing moisture movement up from damp areas (crawl spaces and basements) as well as increasing heating costs when heated or conditioned air ultimately escapes to the outdoors.

Studies performed in the 1990's demonstrated that the movement of water vapor through drywall (gypsum board) into wall cavities was much lower than people previously believed, and the same research demonstrated that the majority of moisture movement in (or out) of building wall or ceiling cavities occurs at leaks and penetrations such as around electrical receptacles, light switches, lights, plumbing piping, etc.

It is now well established that convection, not diffusion, is the major vehicle of moisture transport out of homes. Computer simulations of a typical small home with an average vapor barrier and one air change per hour predict that diffusion will account for less than two percent of the total moisture expelled.

Evidence that Moisture Condensation is an Indoor Problem?

At LOG HOME GUIDE we include a case study that distinguished between moisture coming from the dry-out of new green logs and moisture that came from building leaks. Here we discuss moisture problems on conventional wood-framed or masonry homes. At ROT, TIMBER FRAME we include a case study that diagnosed severe structural rot on a timber framed building where moisture combined with vapor barrier & insulation problems to cause significant damage.

How much moisture passes through or is trapped in building wall and ceiling cavities, and how much moisture moves right to the outdoors? One study at the National Bureau of Standards (cited in Solar Age, March 1983, p. 37), found 28% of the air in a pressurized room leaked right through the walls (through hairline cracks and penetrations) in typical drywall construction. Combined with leaks at floor and ceiling joints and around door and window frames (usually the points of greatest air leakage in buildings), this adds up to a tidy sum of air and moisture flowing into (or through) wall cavities.

Moisture becomes a problem only if it condenses in sufficient quantities and remains in liquid form long enough to saturate building and insulation materials. Condensation occurs when moisture laden air is cooled to its dew point. At the dew point, surplus water vapor condenses and wets the nearest surface. Our article on THERMAL TRACKING relies on this fact to explain why moisture on cooler areas of a wall or ceiling cause higher deposition of house dust, leading to dark streaks sometimes mistaken for mold contamination. See DEW POINT TABLE - CONDENSATION POINT GUIDE for a guide to finding the dew point in building cavities and on building surfaces.

If more water vapor is supplied or if temperatures drop further, more water condenses out of the moisture laden air.

Moisture entering & leaving wood materials: Fortunately, wood building sheathing and framing can store and later release large quantities of this moisture before reaching fiber saturation levels. (Moisture levels below 18% in wood are generally safe from rot and mold growth.) As wall temperatures rise again, or when humidity levels drop, the water re evaporates and is expelled from the wall (framing and sheathing, insulation, or other moisture-absorbed materials) by diffusion or convection.

Fortunately, building materials do not normally get wet enough in these daily and seasonal moisture uptake and moisture release cycles to be damaged, although the R-value of insulation may be degraded. Even when the dew point is reached within the insulation, the bulk of condensation seems to occur on the inner surface of the exterior wall sheathing or on the wall cavity side of the drywall (depending on just where the dew point is reached in the wall cavity), not within the insulation. [This point remains under debate.]

What to Do About Indoor Moisture

An insulated home should have two barriers: an air barrier and a vapor barrier. A single material in a once location, such as polyethylene, can perform both functions. Or the builder can use two different materials at two locations in the structure.

Air barriers control heat loss through infiltration and exfiltration - air movement through building walls or ceilings - which together account for up to 50% of the annual heat loss in a well-insulated home. An air barrier must be carefully planned and well-executed to be effective. This means lapping joints over solid backing, caulking seams with flexible sealants, and tightly sealing around electrical and plumbing penetrations, doors, and windows.

The air barrier material should run continuously between building floors and over plates. If the air barrier is installed on the building exterior, for example on a heavily windswept wall, then it should consist of a material that allows water vapor to diffuse out, such as Tyvek™ or Typar™. If a separate vapor barrier is installed in conjunction with a proper air barrier, then it probably needn't be so meticulously sealed. Care should be taken, however, to seal interior spaces from wall and ceiling cavities.

Generally the most economical (and therefore the most common) solution in new construction is to combine both air and vapor control in one barrier - usually comprised of 4- to 6-mil polyethylene or thin foils. This material is carefully installed on the warm side of the insulation.

Generally in climates where the higher humidity (vapor pressure) is outdoors and air conditioning is run for much of the year, the vapor barrier is installed near the outside surface of the exterior wall; in climates such as the Northeast where vapor pressures are generally higher indoors than outdoors during cold weather, the vapor barrier is installed near the interior surface of exterior walls.

Harold Orr at the Building Research Division of the National Research Council of Canada, has developed a rule of thumb that places the air-vapor barrier within the inner one-third of insulation value. In thick, superinsulated walls, this protects the barrier from plumbing and electrical penetrations and interior finish work.

With 70 degF. indoor temperatures, outdoor temperatures would have to drop below -20 degF. to reduce the temperature at the one-third point to 40 degF., the temperature at which condensation in building walls is likely to occur.

Condensation on double-insulated glass is Harold Orr's indicator that inside relative humidity is too high for outdoor temperatures and that ventilation is necessary. Water is no friend of interior millwork, either, as our photograph of a moldy window frame shows here.

List of Indoor Moisture Control Measures to Avoid Indoor Problems

Condensation in walls may not pose the problems some suspect in conventional homes. However in smaller, tighter homes - some with added moisture of a greenhouse or earth coupling - caution should be exercised. IN all but extreme situations, the following guidelines should steer us free of trouble.

• Keep the building's outside skin five to ten times as permeable as the inside skin so moisture is not trapped in the wall, but keep it tight to water and wind.
• Make the air barrier, wherever it is, as airtight as possible. We prefer it on the winter-warm side of the wall.
• Seal well all cracks and joints connecting wall cavities to inside and outside air
• The vapor barrier, if separate from the air barrier, may be of conventional type except in high moisture areas
• Provide at least twice as much insulation outside the vapor barrier as in. In high moisture areas, keep the vapor barrier on the warm side of the insulation.
• If air exchanges are kept to 0.3 air changes per hour or less in a building, or if window condensation becomes a problem, then ventilate with air-to-air heat exchangers or use a quiet bath on a timer and good backdraft dampers to provide basic whole-house ventilation. (See Whole House Ventilation Strategies).
• Avoid thermal short-circuits and gaps in the insulation. These will cause cold spots and condensation.
• Provide adequate under-roof airflow through attic and cathedral ceiling spaces with venting from eaves to ridge. See ICE DAM PREVENTION.
• See HOUSEWRAP AIR & VAPOR BARRIERS for more information about house wrap and air barrier installation requirements.
• See DEW POINT TABLE - CONDENSATION POINT GUIDE - how to find the dew point in building cavities & on surfaces

-- Adapted with permission, from original material appearing in Solar Age Magazine and written by Steven Bliss.

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• Daniel Friedman - principal author/editor of the InspectAPedia^® Website
• Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education. His recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) is available from the publisher, J. Wiley & Sons, and also at Amazon.com. See our book review of that publication.
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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

AIR SEALING STRATEGIES
DEW POINT TABLE - CONDENSATION POINT GUIDE
MOISTURE CONTROL in BUILDINGS
  Attic Moisture or Mold Sources
  BASEMENT HEAT LOSS
  BASEMENT LEAKS Moisture or Mold
  Chimney Leaks
  CRAWL SPACE Dryout Procedures
  DEW POINT CALCULATION for WALLS
  DEW POINT TABLE - CONDENSATION POINT GUIDE
  EFFLORESCENCE, Salts & White / Brown Deposits
  FLOOD Damage Assessment & Repairs
  FLOOD DAMAGED FOUNDATIONS
  FLOOD VENTS
  FLOODS IN BUILDINGS-priorities
  FOUNDATION WATERPROOFING
  HUMIDITY CONTROL & TARGETS INDOORS
  HOUSEWRAP AIR & VAPOR BARRIERS
  MOISTURE CALCULATIONS
  MOISTURE PROBLEMS: CAUSE & CURE
  MOLD in BUILDINGS
  Soffit Intake Vents & Attic Condensation
  WATER ENTRY in BUILDINGS
  Whole House Ventilation Strategies
MOISTURE PROBLEMS: CAUSE & CURE
PAINT FALURE, DIAGNOSIS, CURE, PREVENTION

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
• "Moisture Problems: Causes and Cures. Understanding moisture problems can steer you free of trouble", Steve Bliss: Building it Right, Solar Age, March 1983 p. 37, 38. -- Adapted with permission, from original material to form this webpage article.
• "Moisture Calculations", Q&A article, Solar Age, April 1984
• ASHRAE resource on dew point and wall condensation - see the ASHRAE Fundamentals Handbook, available in many libraries. The following three ASHRAE Handbooks are also available at the InspectAPedia bookstore in the third page of our Insulate-Ventilate section:
  • 2005 ASHRAE Handbook : Fundamentals : Inch-Pound Edition (2005 ASHRAE HANDBOOK : Fundamentals : I-P Edition) (Hardcover), Thomas H. Kuehn (Contributor), R. J. Couvillion (Contributor), John W. Coleman (Contributor), Narasipur Suryanarayana (Contributor), Zahid Ayub (Contributor), Robert Parsons (Author), ISBN-10: 1931862702 or ISBN-13: 978-1931862707
  • 2004 ASHRAE Handbook : Heating, Ventilating, and Air-Conditioning: Systems and Equipment : Inch-Pound Edition (2004 ASHRAE Handbook : HVAC Systems and Equipment : I-P Edition) (Hardcover)
    by American Society of Heating, ISBN-10: 1931862478 or ISBN-13: 978-1931862479
    "2004 ASHRAE Handbook - HVAC Systems and Equipment The 2004 ASHRAE HandbookHVAC Systems and Equipment discusses various common systems and the equipment (components or assemblies) that comprise them, and describes features and differences. This information helps system designers and operators in selecting and using equipment. Major sections include Air-Conditioning and Heating Systems (chapters on system analysis and selection, air distribution, in-room terminal systems, centralized and decentralized systems, heat pumps, panel heating and cooling, cogeneration and engine-driven systems, heat recovery, steam and hydronic systems, district systems, small forced-air systems, infrared radiant heating, and water heating); Air-Handling Equipment (chapters on duct construction, air distribution, fans, coils, evaporative air-coolers, humidifiers, mechanical and desiccant dehumidification, air cleaners, industrial gas cleaning and air pollution control); Heating Equipment (chapters on automatic fuel-burning equipment, boilers, furnaces, in-space heaters, chimneys and flue vent systems, unit heaters, makeup air units, radiators, and solar equipment); General Components (chapters on compressors, condensers, cooling towers, liquid coolers, liquid-chilling systems, centrifugal pumps, motors and drives, pipes and fittings, valves, heat exchangers, and energy recovery equipment); and Unitary Equipment (chapters on air conditioners and heat pumps, room air conditioners and packaged terminal equipment, and a new chapter on mechanical dehumidifiers and heat pipes)."
  • 1996 Ashrae Handbook Heating, Ventilating, and Air-Conditioning Systems and Equipment: Inch-Pound Edition (Hardcover), ISBN-10: 1883413346 or ISBN-13: 978-1883413347 ,
    "The 1996 HVAC Systems and Equipment Handbook is the result of ASHRAE's continuing effort to update, expand and reorganize the Handbook Series. Over a third of the book has been revised and augmented with new chapters on hydronic heating and cooling systems design; fans; unit ventilator; unit heaters; and makeup air units. Extensive changes have been added to chapters on panel heating and cooling; cogeneration systems and engine and turbine drives; applied heat pump and heat recovery systems; humidifiers; desiccant dehumidification and pressure drying equipment, air-heating coils; chimney, gas vent, fireplace systems; cooling towers; centrifugal pumps; and air-to-air energy recovery. Separate I-P and SI editions."
  • Principles of Heating, Ventilating, And Air Conditioning: A textbook with Design Data Based on 2005 AShrae Handbook - Fundamentals (Hardcover), Harry J., Jr. Sauer (Author), Ronald H. Howell, ISBN-10: 1931862923 or ISBN-13: 978-1931862929
  • 1993 ASHRAE Handbook Fundamentals (Hardcover), ISBN-10: 0910110964 or ISBN-13: 978-0910110969
• The National Institute of Standards and Technology, NIST (nee National Bureau of Standards NBS) is a US government agency - see www.nist.gov
  • "A Parametric Study of Wall Moisture Contents Using a Revised Variable Indoor Relative Humidity Version of the "Moist" Transient Heat and Moisture Transfer Model", George Tsongas, Doug Burch, Carolyn Roos, Malcom Cunningham; this paper describes software and the prediction of wall moisture contents. - PDF Document from NIST
• Understanding Ventilation: How to Design, Select, and Install Residential Ventilation Systems, John Bower
• "Weather-Resistive Barriers", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE

More Information on Building Diagnostic Inspections and Repairs

• Ice Dam Leaks in building attics and roof cavities, how to inspect for evidence of leaks, identify causes, and correct bad attic ventilation, improper roof venting, and these causes of attic mold or roof structure damage
• Mobile Home Inspections common defects unique to factory built housing, inspection methods
• Mold Cleanup, How to clean or remove mold in buildings
• Mold Resistant Drywall is mold-resistant or "mold proof" Sheetrock^R from US Gypsum or Georgia Pacific worth it?
• Mold Prevention: Avoiding Mold Problems in Buildings by Using Mold-resistant Construction Products & Practices
• Mold Information Center: What to Do About Mold, mold action plan, valid and invalid test methods, mold remediation
• Rot, fungus, termites, carpenter ants, powder post beetles, & other wood destroying organisms
• Stairways: A Checklist for Stair, Railing, Landing Safety Inspections
• Links to our list of additional information sources: Interior Defects in Buildings

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Use this simple, economical mold test kit by following our instructions on how to collect and mail mold samples to our lab

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