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INSULATION & VENTILATION INSPECTION & IMPROVEMENT AIR BYPASS LEAKS AIR LEAK DETECTION TOOLS AIR LEAK MINIMIZATION AIR SEALING STRATEGIES ASBESTOS IDENTIFICATION IN BUILDINGS ATTIC LEAKS, CONDENSATION & ATTIC MOLD BASEMENT WATERPROOFING BATHROOM VENTILATION BLOWER DOORS & AIR INFILTRATION CRAWL SPACES Crawl Space Dryout Procedures Crawl Space Safety Advice Crawlspace Mold Advice Media Blasting for Mold Removal Mold on Dirt Floors DEW POINT CALCULATION for WALLS DEW POINT TABLE - CONDENSATION POINT GUIDE ENERGY SAVINGS in BUILDINGS ENERGY SAVINGS PRIORITIES ENERGY SAVINGS RETROFIT CASE STUDY ENERGY SAVINGS RETROFIT LEAK SEALING GUIDE ENERGY SAVINGS RETROFIT OPTIONS ENVIRONMENTAL HAZARDS FIBERGLASS INSULATION FIBERGLASS HAZARDS FIBERGLASS MOLD FRAMING DETAILS for BETTER INSULATION FRAMING DETAILS for DOUBLE WALL HOUSES FREEZE-PROOF A BUILDING FOUNDATION WATERPROOFING HEAT LOSS in BUILDINGS HEAT LOSS DETECTION TOOLS HEAT LOSS INDICATORS HEAT LOSS R U & K VALUE CALCULATION HOUSEWRAP AIR & VAPOR BARRIERS HOUSEWRAP - TYVEK INSTALLATION DETAILS HUMIDITY LEVEL TARGET ICE DAM PREVENTION INDOOR AIR QUALITY & HOUSE TIGHTNESS INSULATION & VENTILATION INSPECTION & IMPROVEMENT INSULATION CHOICES INSULATION FACT SHEET- DOE INSULATION IDENTIFICATION GUIDE INSULATION LOCATION for BASEMENTS INSULATION LOCATION for CAPES, CRAWLSPACES INSULATION R-Values & Properties Insulation Values of Log Home Walls INTERIORS of BUILDINGS 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 MOLD INFORMATION CENTER MVOCs & MOLDY MUSTY ODORS ODORS & SMELLS DIAGNOSIS & CURE ROT, FUNGUS, TERMITES TERMITE SHIELDS vs TERMITICIDE ROT, TIMBER FRAME SOLAR ENERGY SYSTEMS SOUND CONTROL in BUILDINGS STAIN DIAGNOSIS THERMAL MASS in BUILDINGS THERMAL MASS in UPSTAIRS THERMAL TRACKING & HEAT LOSS Ceiling Thermal Tracking Marks Wall Thermal Tracking Stains Floor Carpet Thermal Tracking Stains Air Bypass Leaks Marks on Insulation Thermal Tracking to Diagnose IAQ Stains HVAC Supply Registers Pet Stains on Floors Pet Stains on Walls Human Occupant Stains on Walls Stains from Candles, Woodstoves, Fireplaces Other Stains on Indoor Walls & Ceilings What to Do About Thermal Tracking VAPOR BARRIERS & AIR SEALING at BAND JOISTS VAPOR BARRIERS & CONDENSATION in BUILDINGS VAPOR BARRIERS & HOUSEWRAP VAPOR CONDENSATION & BUILDING SHEATHING VENTILATION in BUILDINGS Air Bypass Leaks, Thermal Tracking ATTIC CONDENSATION CAUSE & CURE BATHROOM VENTILATION Blocked Soffit Intake Vents BRICK VENEER WALL INSULATION CATHEDRAL CEILING INSULATION CRAWL SPACE VENTING & Dryout Procedures BLOWER DOORS & AIR INFILTRATION HEAT LOSS: How to Calculate Heat Loss in a Building HOUSEWRAP AIR & VAPOR BARRIERS HUMIDITY LEVEL TARGET ICE DAM PREVENTION MOISTURE CONTROL in BUILDINGS Crawl Space Ventilation MOISTURE CALCULATIONS MOISTURE PROBLEMS: CAUSE & CURE ROOF VENTILATION SPECIFICATIONS ROOF VENTING ENERGY SAVING DETAILS ROOF VENTING NEEDED? Soffit Ventilation VENTILATION in BUILDINGS WATER ENTRY in BUILDINGS ATTIC LEAKS Moisture or Mold BASEMENT De-Watering Systems BASEMENT HEAT LOSS INSULATION for GREENHOUSE or SOLARIUM INSULATION CHOICES BASEMENT LEAKS Moisture or Mold BASEMENT WATERPROOFING 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 LOG HOME Leak Diagnosis & Cure LOG HOME Condensation &Moisture MOISTURE CALCULATIONS MOISTURE PROBLEMS: CAUSE & CURE MOLD in BUILDINGS SEWAGE CONTAMINATION SUMP PUMPS GUIDE WATER ENTRY in BUILDINGS WIND TURBINES WINDOWS & DOORS SKYLIGHT LEAK DIAGNOSIS & REPAIR SITE BUILT DOUBLE GLAZED WINDOWS SLOPED GLAZING DETAILS VERTICAL GLAZING DETAILS WINDOW / DOOR ENERGY EFFICIENT, DOE WINDOW LEAKS INTO BASEMENT WINTERIZE A BUILDING More Information InspectAPedia Blog - News Updates Air Conditioning & Heat Pumps Bookstore Electrical Environment Exteriors Heating Home Inspection Insulate Ventilate Interiors Mold Inspect/Test Plumbing Water Septic Roofing Structure Accuracy & Privacy Policies Contact Us |
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 ProblemsThis 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 BuildingsThe 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 BuildingsWater 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 MoistureAn 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.
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 ProblemsCondensation 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.
-- Adapted with permission, from original material appearing in Solar Age Magazine and written by Steven Bliss. ... Technical Reviewers & References
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11/04/2009 - 08/05/2009 - InspectAPedia.com/Energy/Moisture_Problems.htm - © 2009 - 1988 Copyright Daniel Friedman All Rights Reserved - InspectAPedia® is a Registered U.S. Trademark