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Mobile ViewINTERIORS of buildings ACOUSTICAL SEALANT CHOICES AIR BYPASS LEAKS AIR LEAK DETECTION TOOLS AIR LEAK MINIMIZATION AIR LEAK SEALING PROCEDURE AIR SEALING STRATEGIES ANIMAL ALLERGENS APPLIANCE EFFICIENCY RATINGS ASBESTOS FLOORING HAZARD REDUCTION ASBESTOS-FREE INSULATION MATERIALS ASBESTOS IDENTIFICATION IN buildings ATTIC LEAKS, CONDENSATION & ATTIC MOLD ATTIC VENTILATION BASEMENT CEILING VAPOR BARRIER BASEMENT HEAT LOSS BASEMENT LEAKS, INSPECT FOR BASEMENT WATERPROOFING BATH & KITCHEN DESIGN GUIDE BATHROOM VENTILATION BEST CONSTRUCTION PRACTICES GUIDE BLOWER DOORS & AIR INFILTRATION BLOWER FAN CONTINUOUS OPERATION BLOWER FAN OPERATION & TESTING BLOWN-IN INSULATION BRICK LINED WALLS BRICK VENEER WALL INSULATION BRICK VENEER WALL Loose, Bulged BRICK WALL DRAINAGE WEEP HOLES BUCKLED FOUNDATIONS due to INSULATION? BUILDING NOISE DIAGNOSIS & CURE CATHEDRAL CEILING INSULATION CATHEDRAL CEILING VENTILATION CEILING FINISHES INTERIOR CEILINGS, DROP or SUSPENDED PANEL CEILINGS, PLASTER TYPES CHIMNEY INSPECTION DIAGNOSIS & REPAIR COOLING LOAD REDUCTION by ROOF VENTS COMBUSTION AIR for TIGHT buildings CONDENSING BOILERS/FURNACES DAMAGE CONDENSATION or SWEATING PIPES, TANKS COOLING LOAD REDUCTION by ROOF VENTS 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 DEHUMIDIFICATION PROBLEMS DEW POINT CALCULATION for WALLS DEW POINT TABLE - CONDENSATION POINT GUIDE EARTHQUAKE DAMAGED FOUNDATIONS EFFLORESCENCE, Salts & White / Brown Deposits ELDERLY & VETERANS HOME SAFETYREMOTE ELECTRIC POWER, PHOTOVOLTAIC ELECTRIC HEAT ELECTRIC POWER, PHOTOVOLTAIC, REMOTE SITE ENERGY SAVINGS in buildings ENERGY STAR PROGRAM EVAPORATIVE COOLING SYSTEMS FIBERGLASS INSULATION FIBERGLASS INSULATION MOLD FLASHING MEMBRANES PEEL & STICK FLAT ROOF MOISTURE & CONDENSATION FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP FLOOD DAMAGED FOUNDATIONS FLOOD VENTS & FLOOD PORTS FLOODS IN buildings-mold FLOOR, CERAMIC TILE FLOOR, CONCRETE SLAB CHOICES FLOOR, CONCRETE SLAB POURED FINISH FLOOR DAMAGE DIAGNOSIS FLOOR, ENGINEERED WOOD & LAMINATES FLOOR FRAMING & SUBFLOOR for TILE FLOOR, KITCHEN & BATH OPTIONS FLOOR, LAMINATE PLASTIC FLOOR RADIANT HEAT Mistakes to Avoid FLOOR, RESILIENT VINYL or CORK FLOOR, STONE, GRANITE, MARBLE, AGGLOMER FLOOR & SUBFLOOR MOLD, HIDDEN FLOOR TYPES & DEFECTS FLOOR TILE HISTORY & INGREDIENTS FLOOR, ENGINEERED WOOD & LAMINATES FLOOR FRAMING & SUBFLOOR for TILE FLOOR TYPES & DEFECTS FLOOR TILE ASBESTOS IDENTIFICATION FLOOR TILE HISTORY & INGREDIENTS FOOTING & FOUNDATION DRAINS FOUNDATION BULGE or LEAN MEASUREMENTS FOOTING & FOUNDATION DRAINS FOUNDATION CRACKS & DAMAGE GUIDE FOUNDATION WATERPROOFING FRENCH DRAINS FRAMING DAMAGE, INSPECTION, REPAIR FRAMING DETAILS for BETTER INSULATION FRAMING DETAILS for DOUBLE WALL HOUSES FRAMING METAL STUD PERFORMANCE FREEZE-PROOF A BUILDING FROST HEAVES, FOUNDATION, SLAB GREEN BUILDING CONSTRUCTION CODES GUIDES GREENHOUSE DESIGN for SOLAR HEATING HEAT LOSS in buildings HEAT LOSS RATE CALCULATIONS HEAT LOSS DETECTION TOOLS HEAT LOSS INDICATORS HEAT LOSS PREVENTION PRIORITIES HEAT LOSS R U & K VALUE CALCULATION HEAT TAPES & CABLES on Roofs for Ice Dams HEATING COST FUEL & BTU Cost Table HEATING COST SAVINGS METHODS HEATING LOSS DIAGNOSIS-BOILERS HEATING LOSS DIAGNOSIS-FURNACES HOT ROOF DESIGNS: Un-Vented Roof Solutions HOUSEWRAP AIR & VAPOR BARRIERS HOUSE DOCTOR, how-to be HUMIDITY LEVEL TARGET ICE DAM PREVENTION INSULATION CHOICES Insulation Air & Heat Leaks INSULATION FACT SHEET- DOE INSULATION for GREENHOUSE or SOLARIUM INSULATION IDENTIFICATION GUIDE INSULATION INSPECTION & IMPROVEMENT INSULATION LOCATION - WHERE TO PUT IT INSULATION LOCATION & QUANTITY for ATTICS INSULATION LOCATION for BASEMENT FLOORS INSULATION LOCATION for BASEMENT WALLS INSULATION LOCATION for BRICK or BLOCK WALL CAVITY INSULATION LOCATION for BRICK VENEER WALLS INSULATION LOCATION for CAPES, CRAWLSPACES INSULATION LOCATION for CATHEDRAL CEILINGS INSULATION LOCATION for GREENHOUSE or SOLARIUM INSULATION LOCATION for PASSIVE SOLAR FLOOR SLAB INSULATION LOCATION & EXTENT for SLABS INSULATION LOCATION for SOUND CONTROL INSULATION LOCATION for SUSPENDED PANELS INSULATION LOCATION for SWIMMING, INDOOR INSULATION MOLD INSULATION R-Values & Properties KIT HOMES, Aladdin, Sears, Wards, Others KITCHEN & BATH DESIGN GUIDE LEED GREEN BUILDING CERTIFICATION LOG HOME ENERGY EFFICIENCY LOG HOME GUIDE MOBILE HOME INSPECTIONS MOISTURE CONTROL in buildings ATTIC CONDENSATION CAUSE & CURE Attic Moisture or Mold Sources BASEMENT CEILING VAPOR BARRIER BASEMENT HEAT LOSS BASEMENT LEAKS, INSPECT FOR BASEMENT WATERPROOFING CATCH BASINS Chimney Leaks CONDENSATION or SWEATING PIPES, TANKS CRAWL SPACE DRYOUT PROCEDURES DEW POINT CALCULATION for WALLS DEW POINT TABLE - CONDENSATION POINT GUIDE DRYER VENTING DRYWELL DESIGN & USES EFFLORESCENCE, Salts & White / Brown Deposits FLOOD Damage Assessment & Repairs FLOOD DAMAGED FOUNDATIONS FLOOD VENTS & FLOOD PORTS FLOODS IN buildings-priorities FOUNDATION WATERPROOFING GRADING & SITE WORK, EXTERIOR 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 MOISTURE CALCULATIONS MOISTURE METER STUDY MOISTURE PROBLEMS: CAUSE & CURE MOLD in buildings Mold in Fiberglass Insulation Mold on Books, Book Conservation Mold on Fiberboard Insulating Sheathing MOLD PREVENTION AFTER FLOODING MOLD RESISTANT CONSTRUCTION ROOF VENTILATION SPECIFICATIONS Soffit Intake Vents & Attic Condensation VAPOR BARRIERS & CONDENSATION in buildings VENTILATION DESIGN PROBLEMS & SOLUTIONS VENTILATION, WHOLE HOUSE STRATEGIES WATER ENTRY in buildings WINDOW LEAKS INTO BASEMENT MOLD in FOAM INSULATION, RESISTANCE MOLD INFORMATION CENTER 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 PAINT FALURE, DIAGNOSIS, CURE, PREVENTION PASSIVE SOLAR DESIGN METHOD PASSIVE SOLAR HEAT PERFORMANCE PASSIVE SOLAR HOME, LOW COST PHOTOVOLTAIC POWER SYSTEMS PLASTER & BEAVERBOARD & DRYWALL PASCAL CALCULATIONS RADIANT BARRIERS RADIANT HEAT RADIANT HEAT Floor Mistakes to Avoid RADIANT HEAT TEMPERATURES RADIANT SLAB FLOORING CHOICES RADIANT SLAB TUBING & FLUID CHOICES REFLECTIVE INSULATION RIGID FOAM USE INDOORS ROOF VENTING ENERGY SAVING DETAILS ROOF VENTING NEEDED? ROOF VENTILATION SPECIFICATIONS ROT, FUNGUS, TERMITES ROT, TIMBER FRAME SEARS KIT HOUSES SINKHOLES, WARNING SIGNS SINKING BUILDINGS SLAB CRACK EVALUATION SOLAR ENERGY SYSTEMS SOUND CONTROL in buildings Splits in Structural Wood Beams STAIN & BIODETERIORATION AGENT CATALOG STAINS on buildings - QUICK GUIDE STAIN DIAGNOSIS on BUILDING EXTERIORS STAIN DIAGNOSIS on BUILDING INTERIORS STAINS on INDOOR SURFACES: PHOTO GUIDE STAIRS, RAILINGS, LANDINGS, RAMPS STONE CLEANING METHODS STUCCO WAll FAILURES DUE TO WEATHER STUCCO WALL METHODS & INSTALLATION STUCCO OVER FOAM INSULATION STUCCO PAINT FAILURES SUMP PUMPS GUIDE SWEATING (CONDENSATION) on PIPES, TANKS THERMAL EXPANSION of MATERIALS THERMAL MASS in buildings THERMAL TRACKING Indicates 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 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 Leaks into vinyl-sided building Select & Use House Wrap Code Requirements for Building Wrap Sheathing Wrap Performance Measures Water Resistance of Housewraps Air Infiltration of Housewraps Performance Table for Housewraps Can the Vapor Barrier be Omitted? VAPOR CONDENSATION & BUILDING SHEATHING WOOD SIDING FLASHING DETAILS WATER BARRIERS, EXTERIOR 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 BLOWER DOORS & AIR INFILTRATION BLOWN-IN INSULATION BRICK or BLOCK WALL CAVITY INSULATION BRICK VENEER WALL INSULATION CATHEDRAL CEILING INSULATION CATHEDRAL CEILING VENTILATION CLOTHES DRYER VENTING COOLING LOAD REDUCTION by ROOF VENTS CRAWL SPACE VENTING & Dryout Procedures 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 Inspect Attics for Moisture or Mold Inspect Attics for Blocked Soffit Intake Vents Inspect Basements for Moisture or Mold Inspect Building Exterior - Roof Venting Inspect the Ridge Vent System from the Attic Inspect the Soffit Vent System from the Attic Insects & Foam Insulation HOUSE DOCTOR, how-to be Insulation Air & Heat Leaks MOISTURE CONTROL in buildings MOISTURE CALCULATIONS 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, BALANCED HEAT COST SAVINGS VENTILATION DESIGN PROBLEMS & SOLUTIONS VENTILATION, WHOLE HOUSE STRATEGIES WALL SIDING TRIM & FINISHES WALL FINISHES INTERIOR WATER BARRIERS, EXTERIOR BUILDING WATER ENTRY in buildings WIND ENERGY SYSTEMS WIND TURBINES & LIGHTNING WIND WASHING INSULATION At EAVES WINDOWS & DOORS WINTERIZE A BUILDING Wood Burning Heaters Fireplaces Stoves Woodstove Safety More Information |
This article discusses vapor barriers and indoor condensation: explaining when and why condensation occurs inside buildings, explains the problems caused by excessive indoor condensation, explains how moisture enters building wall and ceiling cavities, and summarizes the best approaches to prevention of indoor moisture and condensation problems. InspectAPedia offers impartial, unbiased advice without conflicts of interest. We will block advertisements which we discover or readers inform us are associated with bad business practices, false-advertising, or junk science. Our contact info is at InspectAPedia.com/Contact.htm.Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. This discussion of vapor barriers and condensation in buildings in this article series begins at part I, VAPOR BARRIERS & CONDENSATION in buildings, (when and why condensation occurs inside buildings, explains the problems caused by excessive indoor condensation, explains how moisture enters building wall and ceiling cavities, and summarizes the best approaches to prevention of indoor moisture and condensation problems), continues with part II at VAPOR CONDENSATION & BUILDING SHEATHING (detailed questions and answers about various building wall sheathing and insulating materials and their impact on building condensation problems) followed by VAPOR BARRIERS & AIR SEALING at BAND JOISTS. Readers should also see VAPOR BARRIERS & HOUSEWRAP and FELT 15# ROOFING, as HOUSEWRAP/VAPOR BARRIER. Because Grade D paper tends to deteriorate under prolonged wetting, the trend in three-coat stucco is to use two layers of 30-minute paper. Because the paper tends to wrinkle, the two layers tend to form a small air space, creating a rain-screen effect. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © 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. Article One on Vapor Barriers & Building Condensation"Vapor Barriers, Part I - science and common sense point the way to effective strategies" - links to the original article in PDF form immediately below are followed by an expanded/updated online version of this article.
This timeless building condensation and vapor barrier placement article explains the fundamentals of condensation in buildings: what causes building condensation, how to control building condensation, and the problems that condensation causes in structures.
I think a lot about vapor barriers and dew points. It's an occupational hazard. For guidance in these matters I pore over arcane volumes of DOE conference proceedings and muddle through the ASHRAE Handbook of Fundamentals. When puzzled, I talk to the experts in person. And to find out what the real world is doing, I talk to builders around the country. At times, it's rather confusing. Over the phone and at recent conferences (in the 1980's), I've heard builders and designers ask many of the same questions I've wrestled with. Few of these questions have definitive answers. Often good research is lacking or the theory, research, and anecdotes fail to confirm one another. In many of the case studies reported, the full story is not known. What was the relative humidity in the house with the rotting rafters? But all the research and analysis has not been in vain. For the major issues, consistent findings have emerged. Better news is that there is little cause for alarm. The energy-efficient housing stock does not seem to be rotting beneath our feet. [For an exception, see leak, rot, and mold concerns involving residential installations at SIDING EIFS & STUCCO.] Here are some frequent questions on the fundamentals of vapor barriers and condensation, with attempts at brief answers. In a following discussion, VAPOR CONDENSATION & BUILDING SHEATHING, we look at more specific materials and applications. Question: When does condensation occur in a building or a building cavity?Answer: Condensation occurs on or in building surfaces and materials when warm moist air hits a cold surface, or when moisture vapor flow through a wall or ceiling gets dammed-up and sufficiently cooled. To find the dew point (the point at which condensation will occur on a surface) for a given air temperature and relative humidity (RH), you need a chart or graph. See DEW POINT CALCULATION for WALLS and see Question: Where does condensation occur on or in buildings?Answer: Building condensation occurs on cold interior window surfaces and within building walls and roof cavities. In walls, condensation occurs generally on the inside surface of the building's exterior wall sheathing, or on the back side of the exterior siding itself. In summer, with air conditioning, the situation may be reversed in very humid climates such as in Florida, and even further north (New York) when outdoor temperatures and relative humidity are high,m causing condensation to occur on concrete or other masonry floors, walls, and even under wallpaper. It is possible for condensation to drip and collect on wall plates or under windows. Question: Is condensation in buildings a serious problem?Answer: Mold and wood-staining fungi grow well on a wetted organic (wood, paint, paper) surface at 60 deg F. and 60 percent RH. For wood destroying fungi to grow, though, wood fibers must be saturated (about 30-percent moisture content) and warm. Most building experts consider wood above 18 to 20 percent moisture-content to be at risk of rot or mold. These decay causing fungi grow fastest at 50 to 60 degF, but can grow at lower temperatures, as low as 32 degF. depending on the mold genera and species. Indoor mold is both a rot or building damage hazard and a potential indoor air contaminant that can be a serious problem especially for people who are sensitive such as people who are immune-impaired, allergic, asthmatic, or have other medical or respiratory vulnerability. Both building rot repair and building mold remediation jobs can be very costly where large areas are involved. See ROT, FUNGUS, TERMITES and for an extensive reference on building mold detection, testing, cleanup, and prevention, see MOLD INFORMATION CENTER. Also see MOLD ACTION GUIDE - WHAT TO DO ABOUT MOLD, and MOLD PREVENTION GUIDE. If you are already concerned about a mold problem in a building, MOLD EXPERT, WHEN TO HIRE offers help in deciding when it is appropriate to hire a professional. Question: How does the moisture get into a building wall or ceiling cavity?Answer: Water vapor is generated in the building from normal human activities (bathing, cooking), or moisture can enter a building from other sources such as plumbing leaks, roof leaks, surface runoff or even roof spillage leaks into the building (see WATER ENTRY in buildings), and on more dangerous occasions from gas-burning appliances. In winter, water vapor moves outside by passing through permeable materials (a process called moisture diffusion), and as research has shown, the most significant moisture movement in buildings occurs as moisture is carried by air leaking around windows, doors, or other gaps in the building shell. Moisture diffusion is the movement of water vapor (that is, water molecules, not water droplets) from areas of higher moisture level into areas of lower moisture concentration. Diffusion occurs independent of air movement. But the most significant moisture movement into building walls occurs by air leaks at wall openings or penetrations. Two key articles you'll want to read are MOISTURE CONTROL in buildings and MOISTURE PROBLEMS: CAUSE & CURE. Also see VAPOR CONDENSATION & BUILDING SHEATHING, also HOUSEWRAP AIR & VAPOR BARRIERS and Air Bypass Leaks, Thermal Tracking. Question: How can I predict how and where moisture is going to be a problem in a building?Answer: It's tricky to predict where moisture problems will occur in a building, although experienced home inspectors and contractors who have seen or perhaps even disassembled and repaired buildings with moisture damage often have an eye for just were problems are most likely to occur. Taking a more technical approach to building moisture, even if you can do the math, many of the moisture variables such as perm ratings and building air leakage rates will not be precisely known. (See BLOWER DOORS & AIR INFILTRATION for determining a building's air leakage rate.) Still, a simple moisture behavior model is useful for building design purposes. One approach is to plot the temperatures through the wall on a graph and to overlay a plot of the dew point temperatures. Wherever the actual temperature falls below the dew point temperature, condensation may occur. This method is detailed in the ASHRAE Handbook of Fundamentals and in the National Bureau of Standards (NBS/NIST) Report BMS 63 or this more detailed building moisture model article from NIST. Question: How common is moisture condensation in insulated 2x4 and 2x6 building walls?Answer: Researches think that small amounts of liquid or frozen condensation (frost) occurs normally in these cavities. Question: So why hasn't wall moisture caused more buildings to rot away?Answer: Wood and other porous building materials safely store a lot of the moisture at well below saturation levels until it re-evaporates from daily or seasonal warming. Question: If building materials can safely store moisture until it re-evaporates, why is moisture a building worry?Answer: As houses get smaller and tighter, indoor humidity levels are rising, which increases the risk of problem-causing condensation. Also, the more insulation in the wall cavity, the colder the exterior building sheathing - another factor in condensation. Finally, the use of low permeance sheathings has raised many questions. They go against the conventional wisdom of keeping the outside of a wall five to 10 times more permeable than the inside. An example of a moisture-related indoor problem that was not widely recognized until around 2001 was the development of large reservoirs of potentially airborne toxic or allergenic mold hidden in building insulation (see Mold in Fiberglass Insulation) or on the wall cavity side of drywall in buildings with leaks or moisture problems. (See FIND MOLD in buildings, HOW TO). Sometimes (not always) mold from these reservoirs becomes an air quality and health problem for building occupants and sometimes a costly cleanup is needed. In other structures, such as homes sided with low permeance EIFS synthetic stucco, trapped moisture from building leaks or from moisture leaks into walls has led to severe rot damage, also leading to costly building repairs. (See SIDING EIFS & STUCCO.) Question: What is the best approach to avoiding building problems from moisture condensation?Answer: The safest approach to avoiding building problems from condensation is to install a lapped and sealed 6-mil poly vapor barrier - Saskatchewan style - on the winter warm side of the wall, combined with paying very very meticulous attention to sealing at every wall penetration so that air leaks do not send moisture into the wall cavities. So little moisture will then diffuse or leak into wall cavities that it won't matter what insulation or sheathing material was used. For more details see these articles: Question: How important is it not to puncture the vapor barrier?Answer: ASHRAE lists a typical foil vapor barrier at 0.02 perm if un perforated, and 0.08 to 0.16 perm if it has "a few holes larger than pinholes per square foot. After monitoring a number of test walls for two years, researcher Gerald Sherwood of the Forest Products Laboratory (FPL) in Madison WI concluded: "Puncturing the vapor retarder, as with an electrical outlet, can completely change the moisture patterns in the wall," and that once punctured, 6-mil poly performed no better than paper. Question: Is it really necessary to make the vapor barrier continuous around band joists (rim joists) at floor structures?Answer: This seems like the preferable way when feasible. Many builders, though, prefer to caulk pieces of foil-faced rigid-foam insulation board between the band joists and caulk or tape these to the wall vapor barrier. See FRAMING DETAILS for BETTER INSULATION and also FRAMING DETAILS for DOUBLE WALL HOUSES. This seems adequate. Non-hardening acoustical sealant remains the best bet for polyethylene patchwork. See ACOUSTICAL SEALANT CHOICES. Question: How about putting the vapor barrier a third of the way into the wall cavity?Answer: This approach was developed by the National Research Council of Canada for use in 10,000 degree-day climates. So it should be all right in milder climates. If it is 70 degF. indoors and 0 degF. outdoors, the vapor barrier temperature will be 70 - (1/3x 70) = 46.7 degF. If the indoor RH is above 45 percent at 70 deg. F. this could be a problem. And since many homeowners feel more comfortable with the indoor RH at 50 to 55 percent, that condition is likely in many homes. Consequently I would not recommend this approach if you are anticipating similar conditions for prolonged periods. Question: Are there problems with using multiple vapor barriers?Answer: There is no theoretical reason why this should be a problem as long as neither of the vapor barriers falls below the dew point. For example, using foil-faced insulation plus poly on the wall inside surface should pose no problem.
Question: Do you need a vapor barrier in the building ceiling?Answer: In mild climates, some researchers claim you can safely omit the ceiling vapor barrier if you have good attic (under-roof) ventilation. Exactly how mild and how much ventilation is not clear. I would not leave the vapor barrier out of a cathedral-type ceiling where there is little space for moisture vapor to disperse. OPINION-DJF: and it seems to me a foolish "economy" to save the small cost of installing a poly ceiling vapor barrier as well as careful sealing against air leaks around ceiling penetrations for lights or plumbing, in view of the frequency with which home inspectors find severe attic condensation problems. For details about use of acoustical sealants or tapes to seal polyethylene vapor barriers, see ACOUSTICAL SEALANT CHOICES. This discussion of vapor barriers and condensation in buildings continues at VAPOR CONDENSATION & BUILDING SHEATHING. Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. Questions & Answers regarding this articleQuestions & answers about vapor barriers and moisture condensation in buildings Ask a Question or Search InspectAPediaHTML Comment Box is loading comments...
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