How to Dehumidify a Shipping Container House

Dehumidify Shipping Container

Why it is Important to Dehumidify a Shipping Container House

In the previous writing (Condensation in Container Houses) we discussed various ways of installing layers of thermal insulations in order to prevent condensation. As could be expected, there is no easy answer to the problem of condensation to which all container-based houses are exposed much more than traditional wooden-frame houses. We will provide here information on how to dehumidify a shipping container house and what supply to use.

Now, it’s time to see the situation from the opposite point of view asking the question: If we cannot win the “condensation battle” with the indoor-generated vapor, can we reduce its amount reducing also this way the temperature of indoor dewpoint to the level manageable by the existing insulation?

Well, the answer is: Yes, it is possible and in fact, very beneficial. Good engineering practice tells us that the best way to solve complex problems is to use a combination of different methods because only combined effort can provide practical and economically viable solutions! And so here, comes the idea of dehumidification.

Dehumidification literally means a reduction of the level of humidity in both of its forms: gaseous (vapor) and liquid (moisture). To give you a better idea about how lowering the level of vapor helps to minimize the danger of condensation, let’s examine the drawing below:

AC-condensation container

By lowering the amount of vapor in the air we also lower the temperature of the dew point at which the condensation will start moving this way the point of condensation further from the inner wall (DP2 on the drawing). As a result, we can install a narrower layer of interior insulation (purely determined by comfort and cost). Note that technically, it may be even possible to reduce the amount of indoor vapor and lower the (DP3) dewpoint’s temperature to a level comparable with the outdoor one fully eliminating the condensation. Source: FineHomebuilding (slightly modified original drawing)

The most popular methods of dehumidification are:

1. Ventilation

It’s probably the most common method for all of us (who did not open windows to let fresh air inside?). In terms of physical phenomenon, by opening the window we open the “door” for indoor-accumulated vapor to escape to areas of lower vapor pressure (lower humidity). It’s the same force that pushes vapor to migrate toward cold container walls. This time, however, it happens in a “controlled” way via open window(s) or it is forced by air-evacuating fans.

To keep it short, proper ventilation, especially in almost airtight container-based houses is of utmost importance and so it deserves to be discussed in separate writing (see: TBD)

2. Forced Condensation and/or Absorption

The word “Forced” in this case expresses two facts: one – that the dehumidification is accelerated by special means (dehumidifier in this case) and the second one – that it does not come for free. The necessary condition is the availability of electrical energy which may be a problem in off-grid locations.

3. Passive Absorption

As the name may suggest, passive absorption is a natural process of dehumidification taking advantage of materials (known as “Desiccates”) that have the intrinsic ability to absorb vapor (or moisture) from the air. It does not need exterior energy, and we like it or not – it progresses at its own pace.

Forced Dehumidification to Dehumidify a Shipping Container House

Refrigerant Dehumidifiers

Condensation-based dehumidifiers use the same concept as A/C units. Practically, the only difference is that A/C units evacuate outdoors the product of condensation (water) and hot air, while dehumidifiers recirculate the hot air indoors. It’s because their only task is to remove the moisture, without changing the interior temperature. They are automatic On/Off cycles that are not activated by the thermostat (like A/Cs) but by the humidistat detecting the level of humidity.

MeacoDry ABC 10L dehumidifier is ideal for the kitchen, bath, and laundry room as well as for any smaller room. Source: Meaco

a) Power consumption

The model is rated at 160W although its actual power requirements vary from about 125W to 195W depending on ambient temperature and the level of humidity in the air. The good news is that it is feasible to feed it from a reasonable-size solar power source (Inverter to AC is required).

b) Water extraction rate

This parameter is a bit tricky and can be misleading. The Meacodry ABC 10L is advertised to extract up to 10 liters (2.6 gals) per day. The “up” means that the real extraction rate depends on the ambient temperature and the level of humidity. The table below shows some details:

Notes:

– The water extraction per day highly depends on the amount of vapor in the air (which is understandable because the less vapor in the air, less you can extract). Note that at the same Relative humidity (RH), there will be more vapor in the air at higher temperatures, and so the dehumidifier will extract more water. The bottom line is that a model advertised as extracting up to X liters/Y gallons per day will mostly not live up to such promises. The “promise” expresses theoretical limits!

– There is also another factor: Condensation-based (refrigerant) dehumidifiers have low efficiency (low water extraction rate) at temperatures below 20 deg C (68 deg F).
– Required power depends on the amount of condensation (water). If the dehumidifier is fed by the solar system you must provide a power budget for the worst-case scenario.

Thanks to technological progress, these days it will be hard to say that such dehumidifiers are power-hungry and noisy. By all accounts, the MeacoDry ABC 10L is low power and ultra-quiet unit. We have to accept the fact that all compressor-based dehumidifiers by the laws of physics will need some power and will generate noise.
There are also other, significantly lower power and lower noise, condensation-based dehumidification technologies using the thermoelectric Peltier effect for cooling (you may be familiar with it because it’s used in portable, 12Vdc Ice Boxes for cars). Elimination of power-demanding and noisy compressors is a big benefit. Another one is the use of 12Vdc (24Vdc) power supply which makes them ideal for off-grid applications.

Dehumidify a Shipping Container House

Concept of condensation-based dehumidifier using the Thermo-electric (Peltier) effect. Note that the only component with moving parts is the fan, so such units are much less noisy. Source: HumidityExpert.org

A good example can be the Ivation IVADM35 dehumidifier. It’s a Mid-Size Thermo-Electric unit able to capture up to 0.6 liters (0.16 gals) of water per day. With a power rating of just 72W max, the water content of 2 liters (0.53 gals), and lightweight and almost silent operation, it is designed for spaces of up to 62 cubic meters (2,200 cu. ft) Assuming a typical 2.4m (8ft) container’s headroom, means that it can be efficiently used in any room with the surface of up to 26 sq. meters (275 sq. ft) like baths, kitchens, and even small bedrooms (the main benefit – it can work during the night because it’s quiet …).

Dehumidify a Shipping Container House

Ivation thermo-electric dehumidifier – model IVADM35. Source: Ivation

Desiccant Dehumidifiers

For the completeness of the subject, we will also mention here the process of dehumidification based on forced absorption and evaporation. Frankly, it is a more power-hungry process, but very efficient at low temperatures (up to +20 0C /68 0F). For this reason, they are mainly used for dehumidification of low-temperature areas like storages, garages, basements, etc… They are durable and quiet (no compressor).

Left: Concept of an Absorptive (Desiccant-based) dehumidifier. Source: Aerial Control Technologies
Right: Honeycombs structure of the vapor absorbing wheel. Source: Ecor Pro

How it works: The concept is based on the properties of water-absorbing materials known as desiccants. They can absorb vapor from the cold air and then release it back when heated. The practical implementation consists of a large wheel with a honeycomb (some may call it a “corrugated cardboard”) structure lined with Zeolite.

Humid air is pushed through the section of the wheel, where the vapor is captured by the absorber. The dry air coming out is split into two streams. The majority (typically 75%) is released back to the room. The remaining 25% is heated up to about 100-to-120 OC (212-to- 248 OF) and is forced back through the rotating wheel. Hot air triggers the evaporation of the captured water reactivating the absorber to its initial dry form.

The hot, humid air is then evacuated outdoors. This is a continuous process with subsequent sections of the wheel exposed to humid cold air (absorption) and then dry hot air (evaporation and evacuation). The whole idea works well when the indoor humidity is higher than the outdoor one because the evacuated air (25%) must be replaced by a fresh one from the outside.

Aerial-desiccant-dehumidiifer.

Aerial Absorptive Dehumidifier: Visible is the inlet for air to be processed (bottom hole) and the wet-air outlet (upper left corner). Main characteristics: dehumidification capacity of 25.7 kg of water per day at 20°C / 60% Relative humidity. Maximum power 1,040 W.

Notes:

  • In their basic implementations, absorption-based dehumidifiers do not generate liquid water (the moisture is converted back to vapor and then evacuated). Note that the evacuated air is much warmer than the processed one, so it can contain much more vapor. As an example – the cubic meter of air at 20 0C (68 0F) at 100% RH can contain 17.3 g (1.07 x 10-3 lb/cu.ft) of water, while at 60 0C (140 0F), correspondingly 130 grams (8.1 x 10-3 lb/cu.ft). The weak point of the process is that 25% of the air must be replaced and heat energy is lost!
  • More complex versions of absorption-based dehumidifiers use an extra stage with the Heat Exchanger where the hot, humid air is cooled down by the input air (before its dehumidification). As a result, the vapor condensates, and water is collected and evacuated, while the “reactivation” air is recirculated instead of being evacuated outdoors together with the accumulated heat energy. The weak point is that the efficiency of the water extraction is compromised (the inlet air is heated in Heat Exchanger while the vapor in the “reactivation” air may not fully condense). That’s also the reason, that absorptive dehumidifiers do not perform well at ambient temperatures higher than 20 0C (68 0F).

Dehumidify a Shipping Container House

Concept of an absorptive (desiccant-based) dehumidifier with the extra stage of condensation. Source: How does a dehumidifier work? By Christina Woodger; Which? (UK)

  • Desiccant materials have very long lifespans (in other words their ability to extract vapor does not change with time), so such dehumidifiers are much more reliable than compressor-based ones (the wheel may need to be periodically cleaned).
  • The technology can efficiently work even at temperatures below the freezing point (conditions where refrigerant dehumidifiers cannot work at all).

As an approximation of the absorption-based dehumidifiers, one can say that they act as a big rotating “sponge” that absorbs the water in one location and then squeezed, and releases it at another one.

Passive Absorption

The main differences between passive and forced dehumidification processes are:

  • vapor extraction rate (in other words how long it will take to capture a given amount of vapor). Frankly, it is difficult to provide some numbers, because it is a natural process. It will certainly be slow in areas with stagnant air where the vapor can only move by diffusion due to the so-called Vapor-Drive (difference of vapor pressures). In contrast, in areas where the air is circulating (natural convection or forced by fans), the process will be faster.
  • Level of relative humidity (in this case how much it can be reduced). The limits of Passive absorbers (desiccants) are determined by the fact that they work at ambient temperatures. The limits of refrigerant dehumidifiers are determined by the temperature of the condensing coil (well below the ambient).

Before going further, we have to clarify the physical process of passive dehumidification. While for convenience we may still call it Absorption (from the user’s point of view what’s important is to get rid of moisture). The truth is, however, that most desiccants do not absorb water vapor, but rather absorb moisture.

Absorption is a volume (bulk) process.  The whole body of the absorber takes part in it and in the end, the concentration of the absorbed substance (gas or liquid) will be uniform throughout the body of the absorber.

Adsorption is a surface process driven by another physical phenomenon known as adhesion (that’s the force that drives the water against the forces of gravity up through plant cells). Thanks to the adhesion, ions, and molecules of gas or liquid (in this case vapor and moisture) are trapped on the surface of an Adsorber. In contrast to absorption, at the end of the adsorption process, the concentration of the adsorbed substance on the surface will be higher than in the surroundings.

Most desiccants are Adsorbers! They are characterized by a very porous structure, high adhesion force, and low surface vapor pressure. The first characteristic results in a very large ”effective” surface exposed to the surrounding air (note that a little, few millimeters wide bead of desiccant material with zillions of nano-meter size pores and capillaries can have an effective surface of few hundred square meters exposed to the air). The remaining properties help to attract and capture the water and vapor molecules present in the air (as long as vapor pressure in the air is higher). Interestingly, the higher the level of humidity in the air, the more water can be adsorbed by desiccants! Desiccants can be “reactivated” by exposing them to hot dry air.

Popular desiccants:

  1. Silica gel

Silica gel is an artificially produced amorphous, non-crystalline form of silicon dioxide SiO2 (the latter is known as quartz and is a major component of sand). Due to its porous structure (literally zillions of voids and pores) it attracts and absorbs molecules of water (it can capture water in the amount of up to 40% of its own weight). Usually, it is available in the form of granules or beads. What also makes it attractive is its good efficiency over the wide temperature range as well as the fact that it is non-toxic and non-corrosive. The small, permeable packets of silica gel are widely used to protect moisture-sensitive merchandise (food, drugs, leather, etc…) during transport and/or on the shelf.

Left: Familiar silica-gel packets (Source: Lifehacker.com), Right: Silica gel beads (Source: Author – Wiebew, Wikipedia)

  1. Calcium oxide (CaO)

Calcium oxide is produced by the calcination of limestone at high temperatures. Thanks to the abundance of limestone in nature, it is an inexpensive compound widely used in numerous applications. It has hygroscopic properties, easily absorbing water at low levels of humidity (which is not the case with many other desiccants), and is able to retain moisture at higher temperatures.

  1. Calcium Chloride

Calcium chloride CaCl2 is a crystalline salt, widely available in seawater. In contrast to other desiccants, calcium chloride is an Absorber (it acts like a sponge). Solid at room temperature and highly soluble in water, it has excellent hygroscopic properties being able to absorb more water than its own weight. Its ability to absorb moisture increases at higher levels of humidity.

  1. Zeolite (Molecular Sieves)

It’s a synthetic material based on aluminum silicates with a uniform structure of crystalline pores and empty adsorption cavities. It has the capability to absorb (surface retention) and to absorb (sponge-effect) vapor and moisture. These properties together with its structural uniformity allow it to capture more water than any other desiccant. It performs well at low levels of relative humidity and holds adsorbed water over a wider range of temperatures than other desiccants. Unfortunately, it is more expensive than naturally occurring desiccants, so it is mostly used in more demanding, industrial applications.

Notes:

  • Inherently, desiccants (especially these natural ones) are non-toxic and environmentally friendly. (BTW – some are used in the food and pharmaceutical industry). Occasionally, when exposed they can cause irritation of the skin, eyes, and/or respiratory tracts. When inhaled in a form of powder (dust) they may cause respiratory problems and even serious illnesses, while when swallowed, they may cause interior burns and serious health problems!
  • Calcium chloride (if not properly handled) may also contribute to corrosion (after all it’s a sort of sea salt)
  • Silica gel is also available in versions doped with a “moisture level indicator”. It gradually changes the color of silica beads undergoing a transition from the dry (anhydrous) to the wet (hydrated) state. Cobalt chloride makes silica beads look blue when dry and pinkish when wet, while methyl violet is correspondingly orange when dry and green when wet. Color coding greatly helps the control and managing process, however as both dopants are toxic, such color-coded desiccants should not be used in any habitable spaces.

Note, that undoped beads of silica gel are whitish and they keep their color regardless of the level of captured moisture.

 

Color-coded beads of silica gel. Left: doped with cobalt-chloride (Source: Sorbead India). Right, doped with methyl violet (Source: Silicagel Desiccant Bags)

Familiar to most of us packets of silica gel are used to protect small individual objects. They are designed for “one-time use” and once the job is done, they are thrown away (saturated with water, they can be “regenerated” by exposure to high temperatures, but frankly, no one will bother to do it). However, desiccants’ adsorption mechanism can be also used in larger spaces as an alternative to power-demanding dehumidifiers. It’s just a matter of the amount of moisture-absorbing compound, its physical location, and circulation of air (in fact, large passive dehumidifiers are very often used in the transport industry (including cargo shipping containers).

Absortech Absorpoles

Moisture Pole Absorbers (Absortech (Sweden)) is specially designed for cargo shipping containers. They perfectly fit into the corrugated recess of container walls. Based on calcium chloride they can capture large quantities of water significantly exceeding their own weight. Soaked, the desiccant will dissolve into a saline solution (brine), however, the captured water will stay trapped in membrane-protected cells, preventing it from evaporation.

Concept and real model of Absorpol. Source: Absortech

Typically, each Absorpole will last for a few months (although at high levels of humidity, its lifespan may be shorter). At the end of its life, all calcium chloride will be dissolved dripping down into a built-in tank located at the bottom of the Absorpole. The collection tank represents another level of evaporation barrier, trapping the moisture inside.

While designed for cargo shipping containers, Absorpoles (if properly handled) are health-hazards-free and potentially can be used inhabitable spaces.

Absortech® Flex

Absortech® Flex is specially designed for habitable spaces (homes, baths, storage rooms, RVs, garages, etc.). They use calcium chloride as an active component and operate the same way as Absorpoles. The only difference is that they come in elegant, refillable plastic frames that can be attached to the wall. They are eco-friendly and harmless.

Absortech® Flex (here shown the frame where the desiccant is housed). Source: Absortech

A similar product under the name Dampstick® is manufactured by Protective Packaging Ltd. It’s also a calcium-based desiccant designed to control the level of humidity in homes, RVs, boat cabins, etc.

DampRid

It’s a popular calcium-chloride-based product easily available in Home Depot or similar stores. It comes in various packages and sizes that can cover areas of up to 1,000 sq. ft. They are easy to use (“moisture-lock” technology prevents re-evaporation of moisture), safe (non-toxic, harmless compound plus safety cover protecting from children and pets), and easy to diagnose the status (end of life when all original crystals are dissolved). Often, they also come with a fragrance!

Note that the effectiveness of DampRid is limited to the relatively small surrounding area. It will do the job in baths, wardrobes, and walk-in closets, however, in larger spaces, you will have to use multiple containers to achieve the goal.

Dehumidify a Shipping Container House

DampRid in special Easy-Fill container facilitating airflow. Source: DampRid

 

Dehumidify a Shipping Container House – Benefits

While the main reason for dehumidification in container-based houses is the prevention of condensation on the walls, the process has more benefits.

By lowering the level of humidity, we also:

  • reduce the probability of mold and mildew. As a result, we eliminate the potential breeding grounds for microbes, dust mites, allergens, and all kinds of invisible stuff that can make our life miserable
  • Eliminate unpleasant smells (in general, dehumidification prevents them from developing, also, some passive absorbers can include fragrances making it even better!)
  • Protect moisture-sensitive items (dry food, leather, clothing, bedsheets, etc… In a way, most of them (especially dry food) are more or less “moisture absorbers”!
  • Improve the living comfort – note that typically, a relative humidity of 50% is considered as comfortable (if higher, we sweat).

 

Preventive actions

This chapter will include usually the lowest cost and most effective dehumidification solutions, but at the same time, probably the most difficult to implement in real life, because they touch our everyday habits. The bottom line – we can spend much less effort (and $$$) to limit the impact of condensation in container-based houses when we pay attention to the amount of generated water vapor at its sources! A separate chapter will be dedicated to this subject.

This vapor probably also carries the aroma of freshly brewed coffee.  Nevertheless – such an idyllic moment of pleasure later ends up with condensation. (Source: ArielPeriz Blogspot)

We hope that this article helped you learn how to dehumidify a shipping container house.

Insulating Container Home

Leave a Comment

Your email address will not be published. Required fields are marked *