Chart describes moisture profiles of building walls (C) Daniel FriedmanVapor Barriers & Building Condensation - Part I
     


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Vapor barriers & building condensation: this article discusses vapor barriers and indoor condensation: explaining the function of building wall and ceiling vapor barriers and the effects of indoor condensation: answering a series of questions about when and why condensation occurs inside buildings, what are the problems caused by excessive indoor condensation, how moisture enters building wall and ceiling cavities, and what are the best approaches to prevention of indoor moisture and condensation problems.

We also explain where the vapor barrier should be placed in building walls - it may vary by climate, and he answers a much debated question about whether or not vapor barriers are really needed in building ceilings.

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Article One on Vapor Barriers & Building Condensation

Water trapped in poly vapor barrier (C) D Friedman"Vapor Barriers, Part I - science and common sense point the way to effective strategies". Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

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.

Here Steve Bliss discusses the function of building wall and ceiling vapor barriers and the effects of indoor condensation: answering a series of questions about when and why condensation occurs inside buildings, what are the problems caused by excessive indoor condensation, how moisture enters building wall and ceiling cavities, and what are the best approaches to prevention of indoor moisture and condensation problems.

Mr. Bliss also explains where the vapor barrier should be placed in building walls - it may vary by climate, and he answers a much debated question about whether or not vapor barriers are really needed in building ceilings.

Basement ceilign insulation and vapor barrier placement (C) D FriedmanBecause 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.

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.

Moisture meter check for waer in walls (C) D Friedman

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
DEW POINT TABLE - CONDENSATION POINT GUIDE.

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 / ENVIRONMENTAL EXPERT, 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

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:

AIR BYPASS LEAKS
AIR LEAK DETECTION TOOLS
AIR LEAK MINIMIZATION
AIR SEALING STRATEGIES

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.

Watch out: But watch out for multiple vapor barriers that are spaced apart and exposed to potential leakage. DJ Friedman reports finding severe rot to floor joists in a 10-year old home built over a wet crawl space that had a concrete floor. Kraft-faced insulation had been installed with the vapor barrier "up" towards the warm side or building floor underside (correctly) but an owner, hoping to avoid a problem from the wet crawl area, had loosely stapled poly over the underside of the floor joists.

Condensation and ultimately so much water had accumulated on the upper side of this poly that it was visible as pools or stains on the plastic. The lower few inches of floor joists over this area were so badly rotted, after just ten years, that the inspector (Friedman) was able to tear off the bottom portions of rotted floor joists with his bare hand.

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.

This article series about building vapor barriers and condensation in buildings 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).

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) is followed by VAPOR BARRIERS & AIR SEALING at BAND JOISTS. Readers should also see VAPOR BARRIERS & HOUSEWRAP.

Original article

Links to the original article in PDF form immediately below are preceded by an expanded/updated online version of this article.

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.

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