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AIR CONDITIONING & HEAT PUMP SYSTEMS
A/C - HEAT PUMP CONTROLS & SWITCHES
AIR BYPASS LEAKS
AIR LEAK DETECTION TOOLS
AIR LEAK MINIMIZATION
AIR LEAK SEALING PROCEDURE
AIR SEALING STRATEGIES
ASBESTOS FLOORING HAZARD REDUCTION
ASBESTOS-FREE INSULATION MATERIALS
ASBESTOS IDENTIFICATION IN buildings
ATTIC LEAKS, CONDENSATION & MOLD

BASEMENT WATERPROOFING
BATHROOM VENTILATION
BIOGAS PRODUCTION & USE
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING

CRAWL SPACES
  CRAWL SPACE DRYOUT PROCEDURES
  CRAWL SPACE GROUND COVERS
  CRAWL SPACE INSULATION RETROFIT
  CRAWL SPACE SAFETY ADVICE
  CRAWL SPACE VAPOR BARRIER
  CRAWLSPACE MOLD ADVICE
  MOLD CLEANUP by MEDIA BLASTING
  MOLD ON DIRT FLOORS
  SUMP PUMPS
CONDENSATION or SWEATING PIPES, TANKS
COOLING LOAD REDUCTION by ROOF VENTS

DEHUMIDIFICATION PROBLEMS
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
ENERGY SAVINGS in buildings

ENVIRONMENTAL HAZARDS - INSPECT, TEST, REMEDY
EXTERIOR WALL SIDING TRIM & FINISHES

FIBERGLASS INSULATION
FIBERGLASS HAZARDS
FIBERGLASS PARTICLE CONTAMINATION TEST
FIBERGLASS INSULATION MOLD

FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP
FOOTING & FOUNDATION DRAINS
FOUNDATION CRACKS & DAMAGE GUIDE
FOUNDATION WATERPROOFING
FRENCH DRAINS
FRAMING DETAILS for BETTER INSULATION
FRAMING DETAILS for DOUBLE WALL HOUSES
FRAMING METAL STUD PERFORMANCE
FREEZE-PROOF A BUILDING
FOUNDATION WATERPROOFING
FRENCH DRAINS
HEAT LOSS in BUILDINGS
HEAT LOSS DETECTION TOOLS
HEAT LOSS INDICATORS
HEAT LOSS PREVENTION PRIORITIES
HEAT LOSS R U & K VALUE CALCULATION
HOT ROOF DESIGNS: Un-Vented Roof Solutions
HOUSEWRAP AIR & VAPOR BARRIERS
HOUSE DOCTOR, how-to be
HUMIDITY LEVEL TARGET

ICE DAM PREVENTION

INDOOR AIR QUALITY & HOUSE TIGHTNESS

ICE DAM PREVENTION
INSECT INFESTATION / DAMAGE
INSULATION IDENTIFICATION GUIDE
INSULATION INSPECTION & IMPROVEMENT

INSULATION CHOICES
Insulation Air & Heat Leaks
INSULATION FACT SHEET- DOE
INSULATION for GREENHOUSE or SOLARIUM
INSULATION IDENTIFICATION GUIDE
INSULATION INSPECTION & IMPROVEMENT

LEAD POISONING HAZARDS GUIDE
LEED GREEN BUILDING CERTIFICATION
LIGHT, GUIDE to FORENSIC USE
LOG HOME ENERGY EFFICIENCY
LOG HOME GUIDE

MOISTURE CONTROL in BUILDINGS
  ATTIC CONDENSATION CAUSE & CURE
  Attic Moisture or Mold Sources
  BASEMENT CEILING VAPOR BARRIER
  BASEMENT De-Watering Systems
  BASEMENT HEAT LOSS
  BASEMENT LEAKS, INSPECT FOR
  Chimney Leaks
  CONDENSATION or SWEATING PIPES, TANKS
  CONDENSATION on WINDOWS & SKYLIGHTS
  CORROSION in ELECTRICAL PANELS
  CRAWL SPACE DRYOUT PROCEDURES
  DEW POINT CALCULATION for WALLS
  DEW POINT TABLE - CONDENSATION POINT GUIDE
  DRYER VENTING
  DRYWELLS, FRENCH DRAINS for FLAT SITES
  COMMERCIAL SEPTIC DESIGN
  DRYWELL DESIGN & USES
  EFFLORESCENCE, Salts & White / Brown Deposits
  ELECTRIC PANEL MOISTURE
  EXTERIOR WATER SOURCE ELIMINATION
  FLOOD Damage Assessment & Repairs
  FLOOD DAMAGED FOUNDATIONS
  FLOOD VENTS & FLOOD PORTS
  FLOODS IN buildings-priorities
  FOUNDATION DRAINS, INTERIOR
  FOUNDATION WATERPROOFING
  GEOTEXTILES & DRAINAGE MATS
  GRADING, DRAINAGE & SITE WORK
  GUTTERS & DOWNSPOUTS
  HUMIDITY CONTROL & TARGETS INDOORS
  HOUSEWRAP AIR & VAPOR BARRIERS
  MOISTURE in BUILDING WALLS, EFFECTS
  MOISTURE in CELLULOSE INSULATION
  Mold in Fiberglass Insulation
  Mold on Books, Book Conservation
  Mold on Fiberboard Insulating Sheathing
  MOLD PREVENTION AFTER FLOODING
  MOLD RESISTANT CONSTRUCTION
  MOISTURE CALCULATIONS
  MOISTURE CONTROL in BUILDINGS
  MOISTURE METER STUDY
  MOISTURE PROBLEMS: CAUSE & CURE
  MOLD in buildings
  ROOF VENTILATION SPECIFICATIONS
  Soffit Intake Vents & Attic Condensation
  WATER ENTRY in buildings
  VENTILATION, WHOLE HOUSE STRATEGIES
MOLD INFORMATION CENTER
MSDS Material Safety Data Sheets
MVOCs & MOLDY MUSTY ODORS
MYCOPHOBIA, STAINS MISTAKEN for MOLD
MYCOTOXIN EFFECTS of MOLD EXPOSURE

Nanomaterials Hazards
NOISE / SOUND DIAGNOSIS & CURE
NOISE CONTROL for HEATING SYSTEMS
NOISE CONTROL for FLOORS
NOISE CONTROL for PLUMBING
NOISE CONTROL for ROOFS

ODORS & SMELLS DIAGNOSIS & CURE

PASCAL CALCULATIONS
ROT RESISTANT LUMBER
ROT, TIMBER FRAME
ROT, TIMBER ASSESSMENT

SOUND CONTROL in buildings
STAIN DIAGNOSIS on BUILDING INTERIORS

THERMAL MASS in buildings
THERMAL TRACKING & HEAT LOSS

VAPOR BARRIERS & CONDENSATION in buildings
  ATTIC CONDENSATION CAUSE & CURE
  BASEMENT CEILING VAPOR BARRIER
  CONDENSATION or SWEATING PIPES, TANKS
  CRAWL SPACE VAPOR BARRIER
  DEW POINT CALCULATION for WALLS
  DEW POINT TABLE - CONDENSATION POINT GUIDE
  FELT 15# ROOFING, as HOUSEWRAP/VAPOR BARRIER
  HOUSEWRAP AIR & VAPOR BARRIERS
  HOUSEWRAP INSTALLATION DETAILS
  HOUSEWRAP PRODUCT CHOICES
  HOUSEWRAP at SILLS, SOLES, TOP PLATES
  HUMIDITY LEVEL TARGET
  MOISTURE CONTROL in BUILDINGS
  RAIN SPLASH-UP SIDING DAMAGE
  VAPOR BARRIERS & AIR SEALING at BAND JOISTS
  VAPOR BARRIERS & CONDENSATION in buildings
  VAPOR BARRIERS & HOUSEWRAP
  VAPOR BARRIERS, VINYL SIDING
  VAPOR CONDENSATION & BUILDING SHEATHING
  WATER BARRIERS, EXTERIOR
  WOOD SIDING FLASHING DETAILS
VENTILATION in BUILDINGS
  Air Bypass Leaks, Thermal Tracking
  AIR CHANGE RATE ACH HEAT SAVINGS
  ATTIC CONDENSATION CAUSE & CURE
  BALANCED VENTILATION, HEAT COST SAVINGS
  BATH & KITCHEN DESIGN GUIDE
  BATHROOM VENTILATION
  Blocked Soffit Intake Vents
  BRICK or BLOCK WALL CAVITY INSULATION
  BRICK VENEER WALL AIR LEAKS
  CATHEDRAL CEILING INSULATION
  CATHEDRAL CEILING VENTILATION
  HOT ROOF DESIGNS: Un-Vented Roof Solutions
  CRAWL SPACE VENTING & Dryout Procedures
  BLOWER DOORS & AIR INFILTRATION
  BLOWER FAN CONTINUOUS OPERATION
  BLOWER FAN OPERATION & TESTING
  HEAT LOSS: How to Calculate Heat Loss in a Building
  HOT ROOF DESIGNS: Un-Vented Roof Solutions
  HOUSEWRAP AIR & VAPOR BARRIERS
  HUMIDITY LEVEL TARGET
  ICE DAM PREVENTION
  MOISTURE CONTROL in BUILDINGS
  Crawl Space Ventilation
  MOISTURE CALCULATIONS
  MOISTURE CONTROL in BUILDINGS
  MOISTURE METER STUDY
  MOISTURE PROBLEMS: CAUSE & CURE
  ROOF VENTILATION SPECIFICATIONS
  Roof Venting: Eaves Intake if no Overhang
  Roof Venting: Soffit Intake Vent-Continuous
  Roof Venting: Un-Vented Hot Roof Solutions
  ROOF VENTING ENERGY SAVING DETAILS
  ROOF VENTING NEEDED?
  SKYLIGHT VENTILATION DETAILS
  SOFFIT VENTILATION
VENTILATION in BUILDINGS

WALL FINISHES INTERIOR
WALL CONSTRUCTION BARRIER vs CAVITY
WATER BARRIERS, EXTERIOR BUILDING
WATER ENTRY in buildings
  ATTIC LEAKS Moisture or Mold
  BASEMENT De-Watering Systems
  BASEMENT CEILING VAPOR BARRIER
  BASEMENT De-Watering Systems
  BASEMENT HEAT LOSS
  INSULATION for GREENHOUSE or SOLARIUM
  INSULATION CHOICES
  BASEMENT LEAKS, INSPECT FOR
  BASEMENT WATERPROOFING
  BASEMENT WATER ENTRY PREVENTION
  CATCH BASINS
  Chimney Leaks
  CONDENSATION or SWEATING PIPES, TANKS
  CONDENSATION on WINDOWS & SKYLIGHTS
  CORROSION in ELECTRICAL PANELS
  CRAWL SPACE DRYOUT PROCEDURES
  DEW POINT CALCULATION for WALLS
  DEW POINT TABLE - CONDENSATION POINT GUIDE
  DRYER VENTING
  DRYWELLS, FRENCH DRAINS for FLAT SITES
  COMMERCIAL SEPTIC DESIGN
  DRYWELL DESIGN & USES
  EFFLORESCENCE, Salts & White / Brown Deposits
  ELECTRIC PANEL MOISTURE
  EXTERIOR WATER SOURCE ELIMINATION
  FLOOD Damage Assessment & Repairs
  FLOOD DAMAGED FOUNDATIONS
  FLOOD VENTS & FLOOD PORTS
  FLOODS IN buildings-priorities
  FOUNDATION DRAINS, INTERIOR
  FOUNDATION WATERPROOFING
  GEOTEXTILES & DRAINAGE MATS
  GRADING, DRAINAGE & SITE WORK
  GUTTERS & DOWNSPOUTS
  HUMIDITY CONTROL & TARGETS INDOORS
  HOUSEWRAP AIR & VAPOR BARRIERS
  LOG HOME Leak Diagnosis & Cure
  LOG HOME Condensation &Moisture
  MOISTURE in BUILDING WALLS, EFFECTS
  MOISTURE in CELLULOSE INSULATION
  Mold in Fiberglass Insulation
  Mold on Books, Book Conservation
  Mold on Fiberboard Insulating Sheathing
  MOLD PREVENTION AFTER FLOODING
  MOLD RESISTANT CONSTRUCTION
  MOISTURE CALCULATIONS
  MOISTURE CONTROL in BUILDINGS
  MOISTURE METER STUDY
  MOISTURE PROBLEMS: CAUSE & CURE
  MOLD in buildings
  SEWAGE CONTAMINATION
  SUMP PUMPS GUIDE
  SWEATING (CONDENSATION) on PIPES, TANKS
  WATER ENTRY in buildings
WIND ENERGY SYSTEMS
WIND TURBINES & LIGHTNING
WIND WASHING INSULATION At EAVES
WINDOWS & DOORS
  ALUMINUM WINDOWS
  Best Practices Guide for Windows & Doors:
  CLIMATE, WINDOW CHOICES FOR
  CONDENSATION on WINDOWS & SKYLIGHTS
  Doors, Exterior, Energy Efficiency Guide
  Doors, Exterior Flashing Details
  Doors, Exterior, Frames
  DOORS, EXTERIOR, Selecting & Installing
  DOORS, INTERIOR
  FIBERGLASS WINDOWS
  GLASS vs HEAT MIRROR SOLAR GAIN/Loss
  HURRICANE, WIND, & STORM-Resistant WINDOWS
  LOW-E WINDOW GLAZING
  LOW-E VS QUAD-GLAZING
  LOW-E RETROFIT ADD-ON FILMS
  POLYCARBONATE GLAZING
  SITE BUILT DOUBLE GLAZED WINDOWS
  SKYLIGHTS, Guide to Choosing & Installing
  Skylight Condensation Problems
  Skylight Design Issues
  Skylight Energy Efficiency
  Skylight Installation Procedures
  SKYLIGHT LEAK DIAGNOSIS & REPAIR
  SKYLIGHT VENTILATION DETAILS
  SLIDING GLIDING WINDOW DEFECTS
  SLOPED GLAZING DETAILS
  SOLAR SHADES & SUNSCREENS
  STORM WINDOW INTERIOR
  STORM WINDOW PLASTIC CHOICES
  STORM WINDOW WEEP HOLES
  SUNGAIN, FILMS, LOW-E GLASS
  SUNSPACE GLAZING for SUNTANNING
  VERTICAL GLAZING DETAILS
  VINYL WINDOWS
  VINYL / PVC WINDOW WARPING
  WINDOW / DOOR ENERGY EFFICIENT, DOE
  WINDOW / DOOR AIR LEAK SEALING HOW TO
  Window Certification
  WINDOW EFFICIENCY Features & Ratings
  WINDOW FLASHING & SEALING Guide
  WINDOW GLAZING Based on Climate
  WINDOW HARDWARE PHOTOS
  Window Installation, Flange-Type
  Window Installation, w/ Integral Brickmold
  WINDOW LEAKS INTO BASEMENT
  Window Materials & Construction
  Window Types, Guide
  WINDOW TYPES - Photo Guide
  Window & Door Sources

WINTERIZE A BUILDING

More Information

Condensation at a basement window (C) Daniel Friedman Moisture Problems in buildings: Causes & Cures
     

  • 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
  • Questions & answers about building moisture & humidity problems: causes, effects, cures
  • References

Click to Show or Hide Related Topics

  • ENERGY SAVINGS in buildings - home
  • AFUE DEFINITION, RATINGS
  • AIR BYPASS LEAKS
  • AIR CHANGE RATE ACH HEAT SAVINGS
  • AIR CONDITIONING HEAT PUMP SAVINGS
  • AIR LEAK DETECTION TOOLS - home
  • AIR SEALING STRATEGIES
  • BASEMENT HEAT LOSS
  • BTU USAGE MONITORS
  • CATHEDRAL CEILING INSULATION
  • COOLING LOAD REDUCTION by VENTS
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  • ENERGY SAVINGS RETROFIT CASE STUDY
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  • ENERGY SAVINGS RETROFIT OPTIONS
  • ENERGY STAR PROGRAM
  • ENERGY USE MONITORING, SOLAR
  • EVAPORATIVE COOLING SYSTEMS
  • FRAMING DETAILS for BETTER INSULATION
  • GLASS vs HEAT MIRROR SOLAR GAIN/Loss
  • HEAT LOSS in BUILDINGS - home
  • HEAT LOSS DETECTION TOOLS
  • HEATING COST APPORTIONMENT, BTU MONITORS
  • HEATING COST SAVINGS METHODS
  • HIGH MASS TRADEOFFS, HEATING vs COOLING
  • HOUSE DOCTOR, how-to be
  • INDOOR AIR QUALITY & HOUSE TIGHTNESS
  • INSULATION INSPECTION & IMPROVEMENT
  • INSULATION LOCATION - WHERE TO PUT IT
  • INSULATION R-Values & Properties
  • LOG HOME ENERGY EFFICIENCY
  • RADIANT BARRIERS
  • RADIANT HEAT
  • REFLECTIVE INSULATION
  • RIGID FOAM USE INDOORS
  • ROOF COLOR RECOMMENDATIONS
  • SKYLIGHT ENERGY EFFICIENCY
  • SOLAR ENERGY SYSTEMS
  • THERMAL IMAGING, THERMOGRAPHY
  • THERMAL MASS in buildings
  • THERMAL TRACKING & HEAT LOSS
  • TIMERS for ELECTRIC WATER HEATERS
  • VENTILATION, BALANCED SAVINGS
  • WALL CONSTRUCTION BARRIER vs CAVITY
  • WIND ENERGY SYSTEMS
  • WINDOW EFFICIENCY Features & Ratings
  • WOOD, COAL STOVES & FIREPLACES
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

This high indoor humidity article explains the causes and cures of high indoor moisture or indoor condensation problems.

This article series 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 see DEHUMIDIFICATION PROBLEMS and MOISTURE PROBLEMS: CAUSE & CURE and also DEW POINT TABLE - CONDENSATION POINT GUIDE for an explanation of dew points and indoor humidity in buildings. HUMIDITY LEVEL TARGET explains the proper indoor humidity levels we should be trying to achieve. If you need to measure indoor humidity levels, see Tools for Measuring Humidity.

Also see WATER ENTRY in buildings where we describe the cause and prevention of water leakage into buildings. ROT, TIMBER FRAME describes severe damage due to indoor moisture, insulation, and vapor barrier defects. INDOOR AIR QUALITY IMPROVEMENT GUIDE includes details about whole house ventilation systems.

Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.

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. This article explains 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 - Air Barriers & Vapor Barriers

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.

Mold on Indoor Window Trim: Condensation and Moisture

Condensation caused mold on window (C) Daniel FriedmanHarold 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 Window Glass

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 VENTILATION, WHOLE HOUSE 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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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

  • Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: info@buildingadvisor.com
    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. Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com
  • "Moisture Problems: Causes and Cures. Understanding moisture problems can steer you free of trouble", Steve Bliss: Building it Right, Solar Age, March 1983 page 37 and page 38 [click here to view second page]. -- Adapted with permission, from original material to form this web page article.
  • "Moisture Calculations", Q&A article, Solar Age, April 1984

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

  • Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
  • Home Reference Book - Carson DunlopThe 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.

  • GO TO Carson Dunlop's Home Study Course Information - How to Become a Home Inspector: Carson Dunlop's nationally recognized Home Study Course, selected by ASHI the American Society of Home Inspectors and other professionals and associations. This website author is a contributor to this course.
  • GO TO Carson Dunlop's Home Study Course Information - How to Become a Home Inspector: Carson Dunlop's nationally recognized Home Study Course, selected by ASHI the American Society of Home Inspectors and other professionals and associations. This website author is a contributor to this course.
  • GO TO Carson Dunlop's Home Study Course Information - How to Become a Home Inspector: Carson Dunlop's nationally recognized Home Study Course, selected by ASHI the American Society of Home Inspectors and other professionals and associations. This website author is a contributor to this course.
    Building inspection education & report writing systems from Carson, Dunlop & Associates Ltd
  • 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.

  • "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 web page 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 [copy on file as/interiors/MOIST_Model_NIST_b95074.pdf ] - ", 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 [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
  • 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" SheetrockR 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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