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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"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.
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.
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.
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.
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.
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.
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.
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
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.
Answer: Researches think that small amounts of liquid or frozen condensation (frost) occurs normally in these cavities.
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.
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.)
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:
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.
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.
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.
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.
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.
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.
Continue reading at VAPOR CONDENSATION & BUILDING SHEATHING - part II of this article, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
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(Oct 9, 2011) jay Watson said:
Question: What is the best approach to avoiding building problems from moisture condensation?
Eliminating condensing surface by installing rigid foam outside of sheathing(warming sheathing). See Joseph Lstiburek for expert advice.
wow we've written too much about fixing building moisture - your question is a challenge. The basic answer is we need to find and fix the water sources.
(Oct 26, 2011) Alex in Seattle said:
I have a 1961 single story home with a cement floor crawl space. We have some moisture issues that I will address at a later time, but I'd like to install a vapor barrier on the underside of the 1st floor subfloor as well as insulation. There is currently no VB or insulation. I'll be installing 6 mil plastic and stapling it next to the subfloor. is there any benefit to caulking the edges of the plastic to keep the moisture and air from getting past the plastic? I'll be installing unfaced fiberglass and supporting it with string, lathe, or some chicken wire. I'll tape or glue the seams of the plastic.
You might be making a mistake putting the crawl space ceiling (floor above) vapor barrier down on the cool wet side - take a look at the articles under CRAWL SPACES - links at page left - for procedures for keeping the crawl space dry and putting the vapor barrier in the right place.
If you push the poly up between the floor joists, up against the subfloor above, that'd be messy but OK - it'd be in the right place.
Even so, I'd be sure to keep the crawl area as dry as possible as moisture entering the fiberglass can lead to a hidden mold problem there (search InspectAPedia for FIBERGLASS MOLD for details)
(Nov 1, 2011) Alex in Seattle said:
I'm dealing with the moisture issues the best I can. WE have a downspout drain system on the exterior that I believe to be leaking. It is made of drain tile and the only way to access it is by removing the concrete patio that goes all around the house. I have a few cracks and holes that I'm patching and I"m thinking of installing a sump pump in the lowest area of the crawlspace. we also get quite a lot of water that comes down off the hill that is behind our house. I really just want to know if I'm waisting money and time by caulking the 6mil plastic up between the floorjoist on the underside of the subfloor, or if I should just staple it and not worry about it being air tight. thanks for the help.
Try this first:
Cut off the downspout drain about 24" above ground level. Install an elbow and an above-ground downspout extension line (you can use un-perforated flexible black plastic 4" tubing or solid PVC piping, whatever is convenient)
Extend the drain line extension well away from the building and to a location that will continue to drain away by gravity - we don't want water to run back towards the building.
12 feet or more away from the house is good for most soils.
Now watch what happens to the level of water entry inside. If this fixed the problem then you may want to go ahead and install permanent buried downspout drain piping to a suitable destination like a storm drain.
In any case don't bother to install a sump pump without fixing the downspout drain- else you're just pumping water in a nearly fruitless cycle.
I wouldn't put plastic on the underside of the joists of floor above - the risk is that the vapor barrier is on the wrong side and that you trap moisture in the floor cavity leading to mold and rot problems.
You can put plastic down on the FLOOR of the crawl space, but I would first be sure that the floor is clear of debris and smooth and pitched to a common spot into which you can still install a sump pump if needed.
For detailed guidelines for drying out the crawl space see the articles beginning at
(Dec 9, 2011) Dave said:
We have a relatively large church building in a cold climate that could use more attic insulation (gable roof). There is old cellulose that is crumbling and is now only about 4" deep. There is no vapour barrier and the ceiling is acoustic tiles. Attic ventilation is very good. Would it make sense to put vapour barrier over the roof joists and cellulose before adding about R42 blown fiberglass insulation, or would there be a possibility of condensation forming between the ceiling and the 'raised' vapour barrier?
Dave I don't have a clear image of your situation; by roof joists you probably mean the rafters supporting the roof. Where is the present insulation - the attic floor or between the rafters?
Generally in a well vented attic with insulation in the floor, the vapor barrier is under that insulation, on the "warm side" or towards the upper surface of the ceiling drywall or plaster on the underside of the floor joists.
That leaves the roof sheathing and rafters free of any moisture trap.
In any case be sure not to create a "sandwich" of vapor barriers that traps moisture in the cavity.
(July 25, 2014) Barb said:
70s home, vaulted ceiling, Low slope roof . Replaced tar &gravel roof with torch on. 1 year later had indoor leaks on north and south sides of house in spring ( snow on roof was melting). Contractor claims vapour barrier is old and condensation caused leaks. Leaks stopped when he shovelled roof. Seems strange coincidence to have leakage only after new roof installed. Does his analysis make sense? Suggestions for a fix? He is suggesting installing ventilation on the roof peak.
Possibly your contractor is right - but then almost anything is possible - but his surmise would not be my first guess.
It would take quite a bit of interior moisture condensing in the roof cavity to show up as an apparent roof leak, though such can accumulate if moisture moves into the roof cavity and collects there as ice in freezing conditions affecting a poorly-insulated roof.
I'd look for an error such as a leak in the torch-down (presumably modified bitumen) roof seams, at a penetration, joint, or connection.
Adding roof venting at the upper ridge will do nothing good for you except possibly increase the affection with which your heating fuel company views your home (increased heating costs) unless there is also
- adequate intake venting opening at the lower roof edge
- adequate space for venting air to pass above the roof cavity insulation and exit at the high end (2 inches or more)
Ask: What has changed ? The vapour barrier was old before the new roof. Right?
(Aug 17, 2015) Darus Gigut said:
I have more of a question about condensation. I live in a house that is elevated because I live on a canal that goes out to the bay. So my bottom floor is the roof of my garage. It appears more and more like a condensation problem that is rotting my floor. I have lived here for more than 12 years and did not have this problem. I had a new A/c unit installed about a year ago and I keep my house at about 65 degf most of the time except when I go to work and the automatic temp control(my thermostat cuts back to about 80 degf until about 3pm when I will be home at 4 pm.
I now have a serious problem of rotting floor that is wide spread enough that I can now say must be condensation as it is far away from my water pipes. I did n ot have a water barrier when I tore out the insulation after Hurricane IKE and replaced all the insulation just like it was before. But obviously something is severely wrong as my floor is in bad shape in places. Any help would be appreciated.
I agree that you need to identify the moisture source: look under the floor from below. Let me know what you see.
(Oct 19, 2015) Jim Ringler said:
installing a wood tongue & groove ceiling on a cathedral roof , should I install a vapor barrier or not ? I live in south western Pa.
You will be installing the wood T&G over drywall, hopefully taped drywall, to comply with fire safety regulations.
Current building science argues that most of the moisture leaks into ceilings occur at the penetrations in the drywall - such as around light fixtures;
As long as you seal the penetrations, adding a vapor barrier atop the drywall and before the wood T&G wouldn't buy you much benefit.
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