Container Condensation Science

Design & Build

Container Condensation Science Cover

If you’ve done any research into shipping container homes, you know that condensation is a common concern for a lot of people.  However, like many of these more technical topics, there is a lot of confusion and misinformation out there.  Our purpose with this two-part series is to pull back the curtain of mystery surrounding condensation and how to deal with it.  Here in Part 1, we’ll discuss the origin of condensation and why it’s important to understand in the context of shipping containers.

What is condensation?

If you’re ever seen early morning dew on your grass, noticed water droplets form on the outside of a cold drinking glass, or found your bathroom mirror covered in ‘fog’ after a hot shower, you’ve experienced condensation.

Condensation refers to water in its gaseous form (known as water vapor) that changes phase into a liquid (in the form of water droplets).  This phase change is caused by a temperature decrease, usually in the presence of a solid material onto which the droplets form (the grass, drinking glass, and bathroom mirror in our examples).

How and when does condensation form?

You may have noticed that in our examples of condensation above, the conditions have to be just right for condensation to form.  You don’t always have dew on your grass in the morning, for example.  But what are these conditions?  Hold onto your hat, it’s about to get technical!  We’re going to dive into psychrometrics, which is the study of the properties of gas-vapor mixtures.

To understand condensation, we first need to understand humidity.  Normally, when people talk about humidity, they’re referring to ‘relative humidity’, which is a percentage of the amount of water vapor in a volume of air compared to the maximum amount of water vapor that could be in that same volume of air at a given temperature.  A relative humidity of 30% means that the air contains 30% of the moisture it could possibly hold at that temperature.

Another measure of humidity is absolute humidity, which is the mass of the water in a given volume of air at a certain temperature, often expressed as grams per cubic meter (g/m3).

As air temperature increases, the amount of water vapor that air can hold increases as well (In other words, a relative humidity of 100% will correspond to a higher absolute humidity at higher temperatures.  However, if the air temperature rises but the moisture content remains the same, the relative humidity decreases).  The opposite is also true: as the air temperature decreases, the amount of water vapor it can hold decreases.

It’s easier to understand with an example.  Let’s say we’re at sea level and the air is saturated with water vapor (meaning it has 100% relative humidity and is unable to hold any more moisture).   If the temperature is 86°F (30°C), that air will contain around 28 grams of water per cubic meter. But at a temperature of 46°F (8°C) that air will only have 8 grams of water per cubic meter (if we assume we’re still at 100% relative humidity).

Instead, let’s say that we’re again at sea level with air at 86°F (30°C), but now the relative humidity is only 50% (and therefore the absolute humidity is around 15 g/m3).  As you reduce the temperature of the air, the relative humidity starts to increase above 50%, while the absolute humidity holds steady at 15 g/m3.

At some point, you’ll have dropped the temperature enough to where the relative humidity gets to 100% (while the absolute humidity will STILL be at 15g/m3).  This temperature is called the dewpoint temperature and is the temperature at which the air has the maximum amount of water vapor that it can hold (a state we previously defined as being saturated).  In our example, the dew point temperature is about 65°F (18°C).

What does this mean?  Well, if we try to drop the air temperature below 65°F (18°C), we’ll have a problem.  The lower temperature air has a reduced capacity for water vapor, but the water vapor that is already in the air needs to go somewhere.  If you guessed that this excess water vapor turns into condensation, you’re right!

Staying with our example, let’s say that we want to further reduce the temperature down to 50°F (10°C).  Fully saturated (100% relative humidity) air at this temperature and at sea level has an absolute humidity of around 9.5g/m3.  But remember, when we reached the dewpoint of 65°F (18°C), our cubic meter of air had 15g/m3 of water vapor.  This means that to get to 50°F (10°C), 5.5 grams of water vapor will have to condense out of each cubic meter of air!

Where does the condensation physically occur?  On any surface or object that is below the dewpoint temperature.

In our bathroom example, almost all surfaces of the bathroom (walls, floor, sink, mirror, etc.) take on the same temperature as the interior air of your house after hours (or days) of exposure.  As the moist air from your hot shower (with near 100% relative humidity) contacts these cooler surfaces (below the dewpoint temperature), water condenses on all of them.  However, unless you take an extremely long, hot shower, you’ll probably only notice condensate on the mirror, as the water bends the light of your reflection and makes it very easy to see compared to other surfaces.  Heat up your mirror and you won’t get condensation there anymore.  Or, take a cold shower and you won’t have any condensation in the entire bathroom!

To further explore the relationship between air temperature, dew point temperature, and humidity, we’ve created the quick reference tables shown below.  Note that the temperatures may be slightly off for altitudes above sea level.

Alternatively, you can try using several online calculators that provide even more interesting data (There may be slight differences in the results between different tables and calculators as a result of using different estimating equations).

English and Metric Psychrometric Tables

Tables linking temperature and humidity for English and Metric units

What are the sources of moisture in a building?

It’s important to understand that condensation, usually by way of moisture-filled air, must be passed into a structure.  There are a number of ways in which this can occur, some of which you may not have considered:

  • Respiration (Breathing): Every time you exhale, water vapor is included in your breath. On cold days, you’ll notice the vapor condensing into fog when it encounters the cold air!
  • Perspiration (Sweating): The human body’s primary cooling mechanism is the evaporation of sweat. When that sweat evaporates, it is turning into water vapor in the interior air.
  • Showers: We’ve already discussed how hot showers introduce air saturated with water vapor into a bathroom. Without proper ventilation, that water vapor stays inside the building.
  • Cooking: Many forms of cooking, but especially cooking that involves boiling water, are introducing water vapor into your kitchen. Similar to a bathroom, without the use of proper ventilation, this water vapor stays inside the building.
  • Washing dishes: If you’ve ever opened a dishwasher right after the cleaning cycle is complete and been greeted by a face full of steam, you’ve witnessed the humidity that a dishwasher can introduce to an interior space.
  • Drying clothes: While washing clothes at a very high temperature (or with a steam cycle) could introduce moisture into the interior space after opening the washer door, the more common culprit is an improperly vented clothes dryer that fails to send the hot, moist air outside the structure.  If you dry your clothes naturally on a rack, the same issue applies unless you do it outside the building.
  • Ironing clothes: It should be no surprise that when using the steam setting, a clothes-iron vents water vapor into the air of your space.
  • Non-electric space heaters: Gas, oil-fired and propane space heaters (and even wood stoves using inadequately seasoned wood) give off moisture through the process of combustion, which can enter your space if not properly vented out through the flue.
  • Damp building materials: During construction, ‘green’ wood or other materials that have been exposed to rain or other water can evaporate their moisture into the air. If you have your material closed in during part of your construction, you could end up having these damp materials trapped behind your walls where they can cause problems.
  • Improper exterior sealing against liquid water: Rain, melting snow and ice, groundwater, and surface runoff all have the capacity to bring liquid water into your building if roof and wall seams and penetrations are not adequately sealed.
  • Plumbing leaks: Pinholes in pipes or leaking fittings and connections can let water into your house as well, often occurring in places that are difficult to access or see.
  • Pressure Differentials: When parts of your house are under negative pressure compared to the outside environment, outside air can be pulled into the structure via open doors and windows, or air leakage through smaller openings.  If the outside air is warm and humid, this can introduce moisture into the structure.

Moisture has many possible paths into and around your building, but most of the above sources exist regardless of whether you use shipping containers or not.

Why is condensation especially important to consider with shipping container homes?

Condensation is not a phenomenon that applies exclusively to shipping containers, but all metal buildings do have some properties that make condensation more of a concern than with typical construction:

  • Air Leakage: Traditional construction can be prone to air leaks due to the number of individual pieces used, gaps between pieces that occur when craftsmanship is less than ideal, improper sealing around penetrations, etc.  In contrast, many metal buildings (and especially shipping containers) tend to be more tightly sealed with less unintentional ventilation.  While this does have some benefits like weather and pest proofing, it can also mean that if you get moist air inside, it’s less likely to leak out.  Without being proactive, the moisture can be trapped indoors.
  • Permeability: Permeability is the measure of the ability of a porous material to have fluids (liquids and gases) pass through it. Traditionally constructed buildings typically incorporate more permeable materials (such as wooden studs, sheathing, etc.) that can safely absorb (and later release, like a rechargeable battery) moisture before condensation occurs. In comparison, metal buildings are built almost exclusively out of non-permeable materials like steel (except for insulation), and thus visible condensation forms and pools more easily.
  • Specific Heat Capacity: Specific heat capacity is the amount of heat energy required to increase a mass of material by one degree (or the amount of heat energy that must be lost to decrease it by one degree). As an example, for a pound of material, wood has a specific heat capacity that is about 4x greater than steel (Source).  This means when exposed to a set amount of heat energy from the environment, the pound of steel will get much hotter than the pound of wood.  In a metal building such as a shipping container, this means that the external temperature can have a rapid and drastic effect on the temperature of the building’s metal skin and structure.  In the summer, this means heating it up quickly, but in the winter it can just as easily lose temperature and move below the dew point temperature in certain conditions.
  • Thermal Conductivity: Thermal conductivity measures the speed with which heat energy moves through a material. The thermal conductivity of steel is about 300x greater than wood (Source).  This means that heat can move very quickly through the steel skin and structure of a shipping container, including across any thermal bridges that exist.  In the summer, these thermal bridges could cause inefficient hot spots.  In the winter, the thermal bridges could cause cold spots within a container home that provide a location for condensation to form.

The two types of condensation you need to understand

  1. Visible condensation: Moisture that condenses on surfaces you can easily see just by walking around and without tearing into walls, such as on windows, walls surfaces, exposed pipes, etc.
  2. Concealed condensation: Moisture that migrates into and condenses inside the structure of a building in locations like insulation, wall studs, and wall and ceiling cavities.  This type of condensation is more damaging and difficult to deal with as it is hidden behind wall coverings.  Often, you don’t even know it exists until tremendous damage has already been done.  There are two types of migrating moisture that cause concealed condensation:
    • Diffusion (Vapor Drive): A process by which water vapor migrates through solid but permeable materials.  For instance, if one side of gypsum board (drywall) is moist and the other is dry, the moisture can make its way through the material despite it not having any visible holes.
    • Infiltration (Air Leakage): A process by which air (including the water vapor contained in it) migrates through visible holes in materials and wall assemblies and gets inside the wall assembly.  Examples problem areas include electric switch plates, light fixtures, plumbing penetrations, and the perimeter of windows and doors.

The important thing to realize is that if you have an ongoing visible condensation problem, you most likely also have concealed condensation lurking inside your walls.  If you catch concealed condensation early and give your building time to ‘dry out’, you may be ok.  But ongoing concealed condensation can cause problems that are difficult and expensive to fix.

The two primary conditions in which condensation can form in insulated shipping container homes

  1. A cold environment with interior heating: The container’s metal skin assumes the cold temperature of the exterior environment.  The heated interior air can pick up moisture from sources discussed previously.  The moisture can sometimes work its way through the wall system via infiltration and diffusion.  When it encounters the cold outer metal skin, it can form concealed condensation.  A vapor retarder can potentially help in this situation, but that can sometimes cause more problems than it solves as we’ll discuss in Part 2!
  2. A warm, humid environment with interior air conditioning: The container’s metal skin assumes the warm temperature of the exterior environment.  When you open doors/windows, or have an improperly sealed penetration, some warm, humid air enters the container and mixes with the interior air.  If the interior of the container is kept very cool, you could have some limited visible condensation on interior surfaces that were at the cooled temperature but exposed to the warm, humid air.  However, there would be little risk of concealed condensation in this case as the metal skin behind the interior wall surfaces is usually hot enough to be above the dewpoint temperature.  Furthermore, the air conditioner, on top of cooling the newly introduced warm air, also reduces its humidity by allowing it to condense on the evaporator coil and exit the structure. Therefore, most of the moisture would be removed from the container automatically, and this scenario isn’t as likely.

In Part 2 of our two-part series on condensation, we’ll uncover the effects of condensation and discuss how you can address and mitigate them.

Questions about the origin of condensation?  Still not sure why condensation is such an important subject?  Let us know in the comments below!

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