Part 1 of our two-part discussion of condensation, you learned how cond">
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How to protect your family and your shipping container from condensation

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In Part 1 of our two-part discussion of condensation, you learned how condensation forms and why it is a relevant concern for shipping container homes.  Here in Part 2, we’ll discuss the problems that unaddressed condensation can cause and how you can prevent them.

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What problems can condensation cause?

We’ve established that condensation leads to a bit of water in the interior of the building.  But you may be thinking, “So what?”  Well, that little bit of water can cause more issues than you may think:

  • Metal damage: Rust can cause structural weakening as well as being visually unappealing.
  • Masonry damage:  Brick, rock, and concrete exposed to condensation and freeze-thaw cycles can lead to cracking.
  • Wood damage: Condensation and moisture in the presence of wood can cause wet rot (caused by particular strains of fungi), mold, swelling, and warping.
  • Coating and adhesive damage: Damage to paints, varnishes, and flooring/roofing adhesives are possible.
  • Equipment damage: Condensation can lead to chemical reactions that cause corrosion in materials like fasteners, wiring, and conditioning coils.  Additionally, moisture may increase the conductivity of permeable insulators in electronic devices and lead to short circuiting and other malfunctions.
  • Material staining: Water spots and similar visible damage can stain building materials.
  • Insulation performance: The presence of condensed water in permeable insulation (like fiberglass) will decrease its R-value because of the high thermal conductivity of water.
  • Slip hazards: Larger quantities of condensation that form or migrate onto floors can cause slipping hazards.
  • Health concerns: Moisture and condensation can cause unpleasant smells (usually from mold growth), allergy and asthma symptoms, an overall lack of comfort and productivity, and may even be a contributing factor to Sick Building Syndrome.

Dealing with the moisture that leads to condensation

You can control (1) the amount of moisture coming into the structure and (2) the amount of moisture leaving:

  1. Controlling sources of moisture:
    • Showers: Ensure proper ventilation with forced ventilation.
    • Cooking: Use lids when cooking or use an exhaust hood over your stove.
    • Drying clothes: Ensure your dryer vent exhausts outside the building.
    • Building materials: Avoid enclosure of wet building materials during construction.
    • Exterior sealing: Keep rain, melting snow and ice, groundwater, surface runoff, and humid air from leaking into your building via roof and wall penetrations.
    • Plumbing leaks: Ensure there aren’t pinholes in pipes or leaking fittings and connections on any of your plumbing runs that can pool and evaporate.
  2. Removing interior moisture:
    • Dehumidification: Use a portable electric dehumidifier to remove moisture from the air, but only if you’re in a cold environment (the dehumidifiers will raise the temperature of the air).
    • Air conditioner “dry mode”: Use the dry mode setting that many window and ductless AC units have to slow the fan and remove moisture from the air without cooling it significantly if you’re at a suitable temperature with too much relative humidity.
    • Ventilation: Use windows, doors, and vents to replace inside air with outside air when the outside air’s absolute humidity is lower (and hence the air is drier).

A Note on Ventilation

Energy conservation, temperature control, condensation prevention, and indoor air quality are oftentimes are in conflict, but ventilation affects them all.  For instance, bringing fresh air inside can be great for indoor air quality, but may substantially alter indoor temperature and humidity.  Factors related to ventilation include:

  • Air quality: Due to the typical ‘tightness’ of container homes, ventilation is important even if it’s not necessary for moisture control because it prevents air from getting stale (full of odors, contaminants, and containing lower oxygen levels).
  • Air conditioning confusion: Despite a common misconception, the majority of air conditioners do not provide outside air as part of their operation.  Instead, they filter, cool, and remove moisture from the indoor air before recirculating it back into the structure.  Ventilation must instead be provided by intentional (open doors, windows, and vents) and unintentional (building envelope leaks) means.
  • Ventilation rate: Ventilation can be represented by the number of air changes per hour (ACH) or the cubic feet per minute (CFM) of makeup air introduced into the space.  The recommendations vary by building/room use and are governed by different codes in different geographic areas, such as ASHRAE 62.1 & 62.2, IECC R403.6, IRC R303.4 & M1507, IMC 403.1 & 403.3, etc.
  • Relative humidity: Failure to provide proper ventilation can cause a cumulative increase in relative humidity over time in a sealed building, absent other techniques discussed in the section above.  With ventilation, if dry air is pulled into the building from outdoors, it will dehumidify the indoor air. If humid air is pulled in, it can greatly add to the humidity load that must be removed by the air conditioner.
  • Pressurization: Air, whether inside or outside the building, is constantly moving from areas of high pressure to areas of low pressure.  A negatively or positively pressurized room (observable via smoke test) can have air pushed into it or out of it, along with the water vapor contained in the air.  With doors, windows, and vents closed, air will try to flow through wall penetrations and may end up inside the wall envelope.

What role do vapor retarders and barriers play in condensation control?

Vapor retarders are materials that slow the diffusion and infiltration of moisture through a wall system.  Vapor barriers are just one type of vapor retarder, shown below.

Vapor retarders are rated according to their measured permeance in ‘perms’.  The higher the number of perms, the more vapor that can pass through the material.  Therefore, lower perms means a better block against vapor.

Vapor retarders are categorized into three classes by the International Building Code (IBC), with examples of materials in each class given 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: plastic polyethylene sheet, unperforated aluminum foil, sheet metal, glass
  • Class II (0.1 – 1.0 perms): Vapor semi-impermeable
    • Examples: kraft-facing (as on fiberglass insulation bats), exterior 1/4″ plywood, 2″ closed-cell polyurethane spray foam, vinyl wall coverings
  • Class III (1.0 – 10 perms): Vapor semi-permeable
    • Examples: normal latex or enamel paint, 2″ open-cell polyurethane spray foam
  • Unclassified (10 perms or more): Vapor permeable
    • Examples: 1/2″ gypsum board (drywall), 3.5″ unfaced fiberglass insulation bats,  3.5″ mineral (rock) wool insulation

Vapor retarders are supposed to prevent wall assemblies from getting wet. However, as an undesirable side effect, they can also prevent wall assemblies from drying effectively by trapping moisture.  This why their proper application is so important.

The Impact of Climate on Vapor Retarders

Initially, they were mostly used in cold climates, but now see increased usage (often, erroneously) in warmer environments.  Used incorrectly, vapor retarders can actually lead to an increase in moisture-related problems, exactly the opposite of what’s intended.

In a cold environment, vapor retarders are typically used on the inside (warm side) of a wall assembly (normally in between the drywall and the insulation) to keep the insulation and other wall materials from being exposed to the warmer, more humid inside air which could otherwise condense inside the wall.  This works fairly well for these cold climates.

Condensation from heated indoor air

Moisture from the heated indoor air condenses on drywall but is unable to diffuse past the vapor barrier toward the outside

However, if used in this same manner but in a warm and humid environment, moisture would migrate through the wall system from the outside in, then encounter the cool vapor retarder (because it is close to the cold interior air) and condense inside the wall.

Condensation from humid outdoor air

Condensation from the humid outdoor air condenses on the vapor barrier while soaking/diffusing into the adjacent insulation and studs if they are permeable

Therefore, in warm, humid climates, it’s sometimes better to either have the vapor retarder on the outside portion of the wall system, or to not have any vapor retarder at all.  In fact, Section 1404.3.1 of the 2018 IBC prohibits using a class I vapor retarder (and in some cases, even a class II) on the interior side of a wall system for areas in the southern United States (Climate Zones 1-4, excluding Marine 4).

US Climate Zone Map

US Climate Zone Map (source)

This may sound a little conflicting 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, you’re asking a material to do different things at different times of the year, and that’s not very realistic.  Stick with us though, and we’ll give recommendations on what to do later in the article!

Now that you understand the vapor retarder dilemma for traditional construction, let’s dive a little deeper and look at vapor retarders through the lens of shipping container homes.

Vapor Retarders in Shipping Container Homes

As we discussed in Part 1, the most common container home situation in which condensation will form is with a heated interior and a cold outdoor environment, so we’ll focus there.  Previously mentioned sources of moisture can make that warm interior into a warm and moist interior.

The above recommendation for vapor retarders in traditional construction in a cold environment fails to account for 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 outside 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 finds its way into the wall system (and it eventually will, as a perfect vapor barrier is almost impossible to construct), it can condense on the cold metal walls of the container, then diffuse into the insulation if it is permeable.  Surrounded by vapor barriers on both sides, it will be very difficult for the condensation to evaporate and the insulation to dry.  More than likely, problems in the wall system will result as discussed previously.

Warm, moist air from the heated indoor space condenses on the wall and eventually migrates through it despite the vapor retarder, becoming trapped in the wall space

If this seems like a lot of bad news, fear not!  There are several ways we can deal with condensation given the constraints we face from shipping containers.

Recommended approaches to shipping container condensation

  • Concealed condensation: When condensation does occur, try to keep it from being concealed condensation.  We don’t want humid air entering the wall space, whether humid air from outside, or from inside, depending on where you live and season.  still, you want to keep the wall/ceiling cavity airtight so any warm humid air that enters the envelope does so into the interior space and causes only visible condensation.
    • Preventing diffusion into wall and ceiling cavities
    • Preventing infiltration into walls by being careful when installing wiring, plumbing, windows, doors etc. and sealing around wall penetrations well
    • Using insulation that is resistant to moisture movement and impregnation
  • Visible condensation: If visible condensation persists, you can wipe it off with a towel, but if it returns, you really need to figure out why and how you can fix it.
  • Dewpoint temperature: Ultimately, condensation of any type can only form if you have surfaces in the building envelope that are below the dewpoint temperature.   he ac should quickly lower the RH and the visible condensation will evaporate.  it’s a little confusing because insulation is also needed for temperature control.  Nighttime sky radiative cooling
  • Windows: Use premium insulated windows to help keep glass temperatures above the dewpoint temperature (in warm, moist environments, the condensation can actually appear on the outside of the window strangely enough)
  • Thermal bridging: Keep anything (especially metal) in the interior of your structure from touching the exterior or metal frame of your container.   Use thermal ‘breaks’ where possible, which is an insulating material placed in between two pieces of metal that will slow the conduction of heat.  Ensure there is insulation completely surrounding the thermal bridging item and keeping it from contacting the inside air.

Note on container condensation in cold and mixed-climate environments

  • Closed cell insulation: Closed Cell Spray Polyurethane Foam (ccSPF) is what we recommend for almost all situations, and it especially good for colder environments.  Once open cell foam or other porous insulation materials are exposed to moisture, they are hard 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 your wall cavity.  Unlike a plastic film vapor retarder, the ccSPF isn’t easily damaged, punctured, or cut and retains the integrity of its protection.  Additionally, the spray-in application fills up all the gaps in corrugation, around outlets, etc. to form a good seal.  While it’s a more expensive option, we think it’s 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 assume the shipping container aesthetic. However, exterior insulation does have some benefits, such as an increase in interior space and eliminating the chance of condensation inside the interior wall cavity. It’s also not as important to use expensive ccSPF since you aren’t space-constrained, and a permeable insulation has the ability to dry out from the outside in.  If you do insulate the outside, you’ll need to cover the insulation with some type of cladding to protect it from the elements and provide a more visually appealing appearance.  Some variation of wood or cement board is a common choice.

Note on air conditioner short cycling

  • We discussed earlier how air conditioners have the ability to not only cool the air (removal of sensible heat) but also to remove moisture (removal of latent heat) and reduce the humidity.  However, these processes can be greatly affected by the sizing of your air conditioner unit.
  • Air conditioners remove moisture from the air by allowing the cooling coil in the interior of the building, called the condenser, to cool down to below the dew point temperature.  As a fan blows the moist interior air over the condenser, water vapor condenses on the coil and slowly drips away down a plastic condensate line where it exits the building envelope.
  • Each time the air conditioner turns on, it spends several minutes of operation in a dry coil condition before the condenser is cold enough to have water vapor condense on it.  Note however that air cooling can still take place before this temperature is reached if the coil is below the room temperature but above the dewpoint.
  • An undersized system will run continuously and never get your space to the desired temperature.  That is obviously bad.  An oversized system will have short run times throughout the day, and spend more of its overall 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 humidity than you want.  Second, your equipment will wear down faster, as the most taxing time of operation is during startup and shut down.  Third, you’ll have paid more up front for the oversized system.
  • Curious if your current air conditioner was sized correctly?  On a hot afternoon, with the thermostat set at your normal temperature, time how long your system runs.  If it’s less than 10 minutes (or it is coming on more than three times per hour), yet the interior temperature is ok despite high interior relative humidity, you probably have an oversized system.

How can I test my container for potential condensation indicators?

  • You need to know the temperature, relative humidity, and dewpoint of the inside and outside air to make any conclusive judgements about condensation.  Indoor temperature is a personal preference, but indoor relative humidity should generally be between 30-60%
    • You can get a good estimate of the outside conditions by finding a weather station close to you at Weather Underground, but the further away the data is collected, the less accurate it is
    • It’s better to find out the actual conditions at your location with a weather monitor of your own that can measure temperature and relative humidity, then use a calculator or table to find the dew point
  • A digital thermometer/hygrometer like this one can measure indoor and outdoor humidity and temperature with the base unit plus a wireless measurement unit for the outdoors:

  • Another choice is a portable unit that can measure temperature and humidity anywhere that you carry it:

  • If you know the indoor dewpoint and are concerned that some of the surfaces in your building may be colder and prone to condensation, an infrared laser thermometer like this can be very helpful:

  • If you’re also concerned about the CO2 levels in your building due to a perceived lack of ventilation, a desktop thermometer/hygrometer that also includes CO2 monitoring is a sound investment

If you made it all the way through 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 hope you found the series helpful.  Let us know what you think in the comments below!

  1. Joe Spratt

    Great article guys, This is not common knowledge for most people. As a result most never consider it until it’s a problem. A stick-built house with vinyl siding doesn’t really experience these issues too much.

    • Discover Containers

      And oftentimes, you don’t know it’s a problem until you already have damage! Thanks for the kind words, we’re glad you enjoyed the article and found it helpful.

  2. Jimmy Sink

    I have 3 shipping containers I have used to build a hunting cabin out of and close it up when not there. I use 2 fans I use to keep the air inside circulating to help with the inside moisture. Seems to control the moisture inside. I use a humidity/temperature monitor to check on the humidity while I’m not there. Is this a logical way to help control the moisture?? Thanks jimmy

    • Discover Containers

      If by ‘circulating’ you mean exchanging inside and outside air with ventilation, that’s good. If you’re just moving inside air from room to room, that isn’t going to help your overall interior humidity level. The temperature/humidity monitor tells the whole story though, and if you can keep an adequate interior temperature and humidity, then keep doing what you’re doing. A note for others who are reading this…It sounds like you are using this building sporadically, which may be giving your humidity a chance to ‘bleed off’ via leakage before your next use. That’s to say if you were continuously occupying the building, you *might* find that your current method of condensation control is insufficient, though it may work for your current limited usage.