Condensation at a basement window (C) Daniel FriedmanMoisture Problems in Buildings
Causes & Cures

  • MOISTURE PROBLEMS: CAUSE & CURE - CONTENTS: 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
  • POST a QUESTION or READ FAQs about building moisture & humidity problems: causes, effects, cures

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How to cure high indoor humidity, moisture or "sweating" surfaces & pipes:

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.

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Moisture-Caused Building Problems

Condensation caused mold on window (C) Daniel FriedmanExcessive 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 or PEELING PAINT are common effects of high indoor moisture levels.

In contrast rotted building sheathing and ROTTED / 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.

This article is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

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.

[Click to enlarge any image]

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.

-- Adapted with permission, from original material appearing in Solar Age Magazine and written by Steven Bliss.

How to Track Down the Cause of & Cure Bathroom Moisture Problems

Reader Question 2/19/2014 teri  said:

almost 2 years ago i had the drain line below my bathroom lined with CIPP. ever since that time i have had a moisture problem in my bathroom. after showering in the morning, i hang my towel on the rack, come home after working 8 hours and the towel is still damp.

my toilet tissue is always 'damp' feeling and lots and lots of mildew, which i never had prior. there is no exhaust fan, but there never has been, there is a window and this house was built in 1950. could it be possible, when the plumber performed the CIPP that he blocked off the vent pipe that leads from the sewer line thru the roof?

my paint is peeling off my walls in the bathroom on the wall that the shower toilet and sink are on. help please !

Reply: Teri, the CIPP that I know about is described by ISTT as: CIPP Lining

Cured in Place pipe (CIPP) can be used to rehabilitate sanitary sewers, storm drains and pressure pipelines for water, gas and process effluents. Circular pipe from 100-2,700mm and a variety of noncircular pipe such as egg shapes, ovoids, and box culverts can be lined.

Lining with CIPP removes the pipe from service for the duration of the installation and reinstatement process so overpumping or provision of an alternate source of supply may be necessary.

From your note I understand that you had some sort of drain treatment like this. We're talking about fixing a plumbing drain, presumably to stop it from leaking. By no means would stopping a drain leak explain an excessive bathroom moisture problem.

On the contrary that ought to reduce bath moisture if in the process we stopped leaks from leaving water under the building.

The only relationship between a CIPP drain line repair and increased bath moisture that occurs to me is listed as item 4 in my notes below:

Here are some things to check:

1. How is the bathroom ventilated? As there is no working exhaust fan, only ventilation by opening a window can reduce moisture and the effectiveness of that approach varies by season and window use and the ability of air to enter and exit the bathroom.

2. Is the bathroom window (which permits omission of an exhaust fan in some jurisdictions) actually operable, and if so is it opened to ventilate the bathroom? When, for how long=?

3. Had the use of the bathroom changed in some way: long hot showers?

4. Did the CIPP installation (or some other event) damage or block or disconnect the vent piping system? A clogged vent or a vent that leaks could cause poor drain performance (you'd hear gurgling for example) and a disconnected vent could vent moisture into the building floor or walls depending on where the opening occurs.



Continue reading at DEW POINT TABLE - CONDENSATION POINT GUIDE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

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