Protection against moisture from wall condensation

In the first part of this discussion on condensation in residential containers, you learned how condensation moisture forms on walls and why it is a relevant concern for container homes.

If you haven’t had the opportunity to read it, we recommend starting by reading the previous article by clicking here. Here, in Part 2, we will discuss the problems that condensation moisture can cause on the walls of a container home and how you can prevent them.

What problems can condensation moisture cause on walls?

We have established that condensation leads to a small amount of moisture inside the building. But you might be thinking, “So what?”

Well, that small amount of moisture can cause more problems than you imagine:

  • Metal damage: Oxidation can weaken the structure and be visually unappealing.
  • Masonry damage: Bricks, stones, and concrete exposed to cycles of condensation, freezing, and thawing can lead to cracks.
  • Wood damage: Condensation and moisture in contact with wood can cause wet rot (caused by certain strains of fungi), mold, swelling, and deformation.
  • Coating and adhesive damage: Paints, varnishes, and floor and ceiling adhesives can be damaged.
  • Equipment damage: Condensation can cause chemical reactions that lead to corrosion in materials such as fasteners, wiring, and air conditioning coils. Additionally, moisture can increase the conductivity of permeable insulators in electronic devices, causing short circuits and other malfunctions.
  • Stains on materials: Moisture stains and similar visible damage can stain building materials.
  • Insulation performance: The presence of moisture in permeable or open-cell insulation will reduce its R-value due to the high thermal conductivity of water.
  • Slip hazards: Larger amounts of condensation that form or migrate to floors can cause slip hazards.
  • Health problems: Moisture and condensation can cause unpleasant odors (often due to mold growth), allergy and asthma symptoms, general discomfort and lack of productivity, and may even contribute to sick building syndrome.

Dealing with moisture leading to condensation.

You can control (1) the amount of moisture entering the structure and (2) the amount of moisture exiting:

  1. Controlling moisture sources:
    • Showers: ensure proper ventilation with forced ventilation.
    • Kitchen: use covers while cooking or use a range hood over the stove.
    • Laundry drying: make sure the dryer exhaust is directed outside the building.
    • Construction materials: avoid enclosing wet construction materials during construction.
    • Exterior sealing: prevent rain, snow, melted ice, groundwater, surface runoff, and humid air from entering the building through roof and wall penetrations.
    • Plumbing leaks: make sure there are no leaks in pipes or fittings in any of your plumbing ducts that can accumulate and evaporate.
  2. Removing internal moisture:
    • Dehumidification: use a portable electric dehumidifier to remove moisture from the air, but only if it’s in a cool environment (dehumidifiers will increase the air temperature).
    • Air conditioner’s “dry mode”: use the dry mode setting found in many windowless air conditioner units to reduce the fan speed and remove moisture from the air without significantly cooling it, as long as it’s at an appropriate temperature with high relative humidity.
    • Ventilation: use windows, doors, and ventilation grilles to replace indoor air with outdoor air when the absolute humidity of the outdoor air is lower (and therefore the air is drier).


Energy conservation, temperature control, condensation prevention, and indoor air quality often come into conflict, but ventilation affects all of them.

For example, bringing fresh air inside can be great for indoor air quality but can substantially change indoor temperature and humidity.

Factors related to ventilation include:

  • Air quality: due to the typical “airtightness” of container homes, ventilation is important, even if not needed for moisture control, as it prevents stale air (filled with odors, contaminants, and lower levels of oxygen).
  • Air conditioner mixing: despite a common misconception, most air conditioning systems do not bring in outdoor air as part of their operation. Instead, they filter, cool, and dehumidify the indoor air before recirculating it within the structure. Ventilation must be provided through intentional openings (open doors, windows, and vents) and unintentional openings (building envelope leaks).
  • Ventilation frequency: ventilation can be represented by the number of air changes per hour (ACH) or the cubic feet per minute (CFM) of fresh air introduced into the space. Recommendations vary based on building/room use and are governed by different codes in different geographic areas, such as ASHRAE 62.1 and 62.2, IECC R403.6, IRC R303.4 and M1507, IMC 403.1 and 403.3, etc.
  • Relative humidity: insufficient ventilation supply can cause a cumulative increase in relative humidity over time in a sealed building, without other techniques described in the previous section. With ventilation, if dry air is introduced into the building from the outside, it will dehumidify the indoor air. If humid air is introduced, it can significantly increase the moisture load that the air conditioning system must remove.
  • Pressurization: both inside and outside the building, air is constantly moving from areas of high pressure to areas of low pressure. A negatively or positively pressurized room (observable through smoke testing) may have air and water vapor either inside or outside it. With closed doors, windows, and ventilation grilles, air will try to flow through wall penetrations and may end up inside the wall envelope.

Vapor barriers and condensation control

Vapor barriers are materials that retard the diffusion and infiltration of moisture leading to condensation in walls through their system. Vapor barriers are just one type of vapor retarder, as shown below. Vapor retarders are classified based on their permeability measured in “perms.”

The lower the perm value, the less vapor can pass through the material. Therefore, a lower perm value means a better vapor barrier.

Vapor retarders are classified into three classes according to the International Building Code (IBC), with examples of materials in each class detailed below (source, source, source, source):

  • Class I (0.1 perm or less): vapor impermeable
    • Note: Class I vapor retarders are also known as “vapor barriers”
    • Examples: polyethylene plastic film, non-perforated aluminum foil, sheet metal, glass
  • Class II (0.1 – 1.0 perms): semi-vapor impermeable
    • Examples: kraft facing (as in fiberglass insulation batts), 1/4″ exterior plywood, 2″ closed-cell polyurethane foam, vinyl wall coverings
  • Class III (1.0 – 10 perms): semi-vapor permeable
    • Examples: common latex paint or enamel, 2″ open-cell polyurethane foam
  • Not rated (10 perms or more): vapor permeable
    • Examples: 1/2″ gypsum board (drywall), 3.5″ unfaced fiberglass insulation panels, 3.5″ mineral wool (rock) insulation.

Vapor retarders are intended to prevent wall assemblies from becoming damp. However, as an unintended side effect, they can also hinder wall assemblies from drying effectively by trapping moisture. That’s why proper application is so important.

The impact of climate on vapor retarders

Initially, they were primarily used in cold climates, but now there is a greater (often misguided) use in warmer environments.

If used incorrectly, vapor retarders can cause an increase in moisture-related problems, exactly the opposite of what is intended.

In a cold environment, vapor retarders are typically used on the interior (warm side) of a wall assembly (typically between the gypsum board and insulation) to prevent insulation and other wall materials from being exposed to the warmer, more humid indoor air. Otherwise, moisture can condense inside the wall. This works well for those cold climates.

Moisture from the warm indoor air condenses on the gypsum board but cannot diffuse beyond the vapor barrier towards the exterior

However, if used in the same way in a hot and humid environment, moisture will migrate through the wall system from outside to inside, encounter the cool vapor barrier (due to proximity to the cool indoor air), and condense inside the wall.

Moisture from the humid outdoor air condenses on the vapor barrier while infiltrating/spreading into adjacent insulation and cavities if they are permeable

Therefore, in hot and humid climates, it is sometimes better to have the vapor barrier on the outside of the wall system or not have any vapor barrier at all.

In fact, Section 1404.3.1 of the 2018 IBC prohibits the use of a Class I vapor barrier (and in some cases even Class II) on the interior side of a wall system for areas in the southern United States.

This may seem somewhat contradictory if you live in a place that is hot and humid during some parts of the year and cold at other times.

The fact is that you are asking a material to perform different functions in different seasons, and that is not very realistic. However, stay with us, and we will provide recommendations on what to do later in the article.

Now that you understand the vapor barrier dilemma for traditional construction, let’s delve a little deeper and look at vapor barriers from the perspective of container homes.

Vapor retarders in container homes

As mentioned in Part 1, the most common situation in a container home where condensation will occur is with a heated interior and a cold external environment, so we will focus on that.

The moisture sources mentioned earlier can turn this heated interior into a heated and humid interior.

The previous recommendation for vapor retarders in traditional construction in a cold environment does not take into account the fact that with container construction, the container itself is also a very effective vapor retarder.

However, the vapor retarder formed by the container is located on the exterior of the wall system, the opposite of the recommendation!

Therefore, placing a vapor retarder on the warm side of the interior wall, as recommended, actually encapsulates the insulation between two vapor retarders.

When moist air penetrates the wall system (and eventually it will, as it is nearly impossible to construct a perfect vapor barrier), it can condense on the cold metal walls of the container and then infiltrate the insulation if it is permeable.

Surrounded by vapor barriers on both sides, it will be very difficult for condensation to evaporate and for the insulation to dry.

Issues in the wall system are likely to occur as discussed earlier. If this seems like bad news, don’t worry!

There are several ways to deal with moisture from condensation in walls, considering the constraints we face with containers.

The warm, humid air from the heated interior space condenses on the wall and eventually migrates through it, despite the vapor retarder, getting trapped in the wall cavity

Recommended methods for dealing with condensation in containers:

  • Hidden condensation: when moisture occurs, try to prevent it from turning into condensation. We don’t want humid air to enter the wall cavity, whether it’s external or internal air, depending on where you live and the season. Keep the wall/ceiling cavity airtight so that any humid, warm air that enters does so in the interior space and causes only visible condensation.
    • Prevent diffusion in wall and ceiling cavities.
    • Prevent infiltration into walls, be careful when installing wiring, pipes, windows, doors, etc., and seal around wall penetrations.
    • Use moisture-resistant and moisture-impermeable insulation.
  • Visible condensation: if moisture from condensation on walls is visible and persistent, you can wipe it with a towel, but if it returns, it is essential to figure out why it is there and how to fix it.
  • Dew point: ultimately, moisture from condensation on walls of any type can only form if there are surfaces in the building envelope that are below the dew point temperature. Air conditioning should quickly reduce the relative humidity (RH), and visible condensation will evaporate. This is a bit tricky because insulation is also needed to control temperature.
  • Windows: Use high-quality insulated windows to help keep the glass temperatures above the dew point temperature (in hot and humid environments, condensation may appear on the exterior of the window, strange as it may seem).
  • Thermal bridging: Avoid any elements (especially metal) inside your structure from coming into contact with the exterior or the metal structure of your container. Use “thermal bridges” whenever possible, which are insulating materials placed between two metal pieces to slow down heat conduction. Ensure that there is insulation that fully encloses the thermal bridge element and prevents contact with the interior air.

Condensation moisture in container walls in cold and mixed climates

  • Closed-cell insulation: Closed-cell spray polyurethane foam (ccSPF) is what we recommend for nearly all situations, and it is especially good for colder environments. Once open-cell foam or other porous insulation materials are exposed to moisture, they are difficult to dry and become a breeding ground for mold, etc. Closed-cell foam serves as both insulation and a vapor retarder, keeping moisture out of the wall cavity. Unlike a plastic film vapor retarder, ccSPF is not easily damaged, punctured, or cut and maintains the integrity of its protection. Additionally, the spray application fills all gaps in the corrugation, around outlets, etc., to form a good seal. While it is a more expensive option, we believe it is a worthwhile investment.
  • Exterior insulation: Placing wall insulation on the exterior of the container is a less common option as many people want their building to retain the aesthetics of the container. However, exterior insulation has some benefits, such as increased interior space and the elimination of the possibility of condensation within the interior wall cavity. It is also not as critical to use expensive ccSPF since there is no limited space, and permeable insulation has the ability to dry from the outside in. If you insulate the exterior, you will need to cover the insulation with some type of cladding to protect it from the elements and provide a more visually appealing look. Some variation of wood or cement is a common choice.

Short cycling of air conditioning:

  • Earlier, we discussed how air conditioners have the ability not only to cool the air (sensible heat removal) but also to remove humidity (latent heat removal) and reduce moisture. However, these processes can be greatly affected by the size of your air conditioning unit.
  • Air conditioners remove moisture from the air by allowing the cooling coil inside the building, called the condenser, to cool below the dew point temperature. When a fan blows the humid indoor air over the condenser, water vapor condenses on the coil and slowly drips down a condensate drain line as it exits the building envelope.
  • Every time the air conditioner turns on, it spends several minutes operating in a dry coil condition before the condenser is cold enough for water vapor to condense. However, note that air cooling may take place before this temperature is reached if the coil is below room temperature but above the dew point.
  • An undersized system will run continuously and never reach your desired space temperature. That’s obviously bad. An oversized system will have short runtimes throughout the day and spend more of its daily operating time in the dry coil phase before the condenser is cold enough to remove water vapor from the air. This causes three problems. First, your air will have more moisture than you want. Second, your equipment will wear out faster as the most demanding operating time is during startup and shutdown. Third, you will have paid more for the oversized system.
  • Curious if your current air conditioner was sized correctly? On a hot afternoon, with the thermostat set to your normal temperature, measure how long your system runs. If it’s less than 10 minutes (or comes on more than three times per hour), but the indoor temperature is good despite the high indoor relative humidity, you likely have an oversized system.

How can I measure condensation moisture on walls inside my container?

  • You need to know the temperature, relative humidity, and dew point of the indoor and outdoor air to make conclusive judgments about condensation. The indoor temperature is a personal preference, but the indoor relative humidity should generally be between 30-60%.
    • You can get a good estimate of external conditions by finding a weather station near your location on Weather Underground, but the further the data is collected from your specific site, the less accurate it will be.
    • It’s best to find out the actual conditions at your location with your own weather monitor that can measure temperature and relative humidity, and then use a calculator or table to find the dew point.
  • A digital thermometer/hygrometer like these can measure humidity and temperature indoors and outdoors, with the base unit plus a wireless remote sensor for outdoor measurements.
  • Another option is a portable unit that can measure temperature and humidity wherever you take it.
  • If you know the indoor dew point and are concerned that some of your building surfaces may be colder and prone to condensation, an infrared laser thermometer like this can be very useful.
  • If you’re also concerned about CO2 levels in your building due to perceived lack of ventilation, a desktop thermometer/hygrometer that also includes CO2 monitoring is a good investment.

If you’ve followed along with parts 1 and 2 of our series on condensation in shipping containers, congratulations!

We spent a lot of time working on these two articles to help improve your understanding, and we hope the series on condensation moisture on walls has been helpful to you.

Let us know what you think about condensation moisture on walls in the comments below!

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