All of us are quite familiar with the effect of condensation – be it a foggy windshield in a car, steamy mirror in the bath or appearing apparently out of nowhere droplets on a bottle of beer freshly taken out from the fridge. All the above effects are results of the same physical phenomenon leading to condensation of vapor on cold objects exposed to warm air.
In practice, while we may be briefly annoyed with the effects of condensation, we take it easy (to say it nicely instead of – we ignore it). It’s because in traditional housing structures (except for special cases), condensation does not have a major impact neither on the structure itself nor on our health. No wonder, we do not pay much attention to the condensation pretending rather that it does not exist.
Traditional houses are not “airtight”; walls have some level of permeability, and in general – structures are not “continuous” so there are always places from where air can penetrate interior or drift to the outside. In contrast, most shipping containers are airtight and converted to houses, they pretty much preserve this characteristic. And this completely changes the situation!
Typical effect of vapor’s condensation on container’s ceiling. (Source: Container Hire Surrey)
BTW – Recently, new concepts of energy-saving designs made inroads into the traditional residential market and they are all based on “air-tightness”, so inherently, they will face similar problems with condensation as container houses.
Condensation: what it is and why it happens?
The dry air is composed of nitrogen (78%), oxygen (21%) and small quantities of other gases – mostly argon (about 0.9%). However, in the presence of water, the air will also contain varying amounts of water vapor.
The water vapor is the gaseous form of water. Its level (amount) is determined by air’s temperature – basically, warmer air can absorb more vapor. Water changes its form from liquid to gaseous in a process of evaporation. The opposite effect (condensation) transforms water’s gaseous form back to the liquid (moisture).
Both processes (evaporation and condensation) occur simultaneously, with intensities depending on air’s temperature and a level of vapor in the air. The presence of vapor in the air we experience as “humidity” (mostly relative, compared to maximum possible at a given temperature).
In an initially dry air environment, evaporation is dominant, however growing levels of humidity increase the intensity of condensation. Finally, once the air reaches the so-called “dewpoint”, the amount of evaporating water will be balanced by condensation. At such a moment the air is “saturated” with water vapor, in other words, at a given temperature the amount of water vapor (humidity) cannot increase anymore). If the ambient temperature changes, the equilibrium between evaporation and condensation will be disrupted. So, when the temperature rises further, the evaporation will increase and a new dewpoint at higher level of vapor in the air will be established.
However, when the temperature drops, the condensation process will intensify creating the new dewpoint at lower temperature corresponding to lower level of vapor. In many cases, we see condensation as a local effect (windshield, bathroom’s mirror or window etc…) but in most cases it is just an optical effect. In the bathroom, vapor will also condensate on the walls, in the car – on all exposed metal parts, but droplets will not be easily visible. Note that the condensation zone may also be very “local” (for example an evaporator’s coil in an A/C unit or dehumidifier).
Condensation in order to happen requires an extra element, typically a colder surface where it will appear in a form of small droplets, eventually merging into bigger drops and trickles of water. When it’s happening in the atmosphere (clouds), vapor needs micro-particles (pollution) to condensate around them.
Sources of Vapor
Among the sources of home-generated vapor (and moisture), the most important are:
a) Breathing (Respiration)
While the amount of exhaled water vapor is relatively small, it’s the continuous 24/7 process affecting all living and sleeping areas in the house. Obviously, a larger family generates a proportionally larger amount of vapor through the process of breathing.
Remaining sources of vapor are usually related to specific activities and confined to particular locations (rooms).
Cooking requires higher temperatures which increases the intensity of evaporation of water contained in the cooked/fried/baked food. In particular, the evaporation reaches an excessive level when the water is boiling (it’s not anymore a surface evaporation, but a bulk one). The stove hood with an exhaust fan evacuating the vapor outside of the house can significantly reduce the level of vapor in the kitchen.
Shower is a major source of vapor, because it scatters the water into numerous tiny streams exposing this way significantly larger “surface” to evaporation. Next in line in terms of contribution to the vapor will be a bathtub (large surface of water exposed to evaporation) and then water running from the regular faucet.
In contrast to the kitchen, usually the bathroom’s door stays closed so the vapor is contained and can be quickly evacuated outdoors by an efficient ventilation system.
Simple method to reduce the amount of vapor from the shower, is to reduce its temperature 😊! Source: Mirashowers (UK)
d. Laundry room
Washing machines (if we use cold or lukewarm water for rinsing) does not contribute much into the overall amount of vapor generated in the house. In contrast, the drying process may! In houses connected to the grid, a large amount of vapor is evacuated outside of the house directly from the dryer, but in an off-grid environment the dryer wouldn’t fit into the available power budget. While in warm seasons the drying can be (actually, it should be) done outdoors in traditional grandma-way, winter (freezing temperatures) make it quite difficult. As a result, the moisture still contained in the centrifuged clothing will evaporate indoors into the air.
Condensation in Houses
Important note: The information and examples described below mostly correspond to houses located in colder climate zones where the length of the heating season is longer than then one requiring air-conditioning (if at all). However, understanding problems and learning how to solve them in colder climates should help to address condensation-related issues in hot and humid climate zones.
In cold days when the outdoor temperatures are low, warm interior air will naturally migrate to the outside through ceiling and walls across building materials like drywall, insulation, press-wood (process is known as diffusion) and/or through discontinuities and holes (process known as infiltration or air leaks). Along the way, migrating air will face continuously dropping temperature and correspondingly lower dewpoints.
Note that the process of vapor diffusion does not depend on the movement of air. The physics behind the vapor diffusion is the difference of vapor pressure on both sides of the “barrier”. In other words, we can say that the diffusion is driven by the difference between the levels of humidity) – a phenomenon known as “Vapor Drive”. An airtight material will act as a barrier for the air, but it may still allow the vapor to migrate across (a good example is a drywall). This physical effect is described as permeability and construction materials are rated according to their ability to transfer water vapor and absorb liquid water is expressed in units called “perms”.
Vapor Barriers & Retarders
Classification according to International Building Code (IBC),
- Class I (less than 0.1 perm) represents vapor impermeable materials
These are so-called Vapor Barriers and their most popular examples are metals and glass. Now it should be obvious, why container-based houses face serious problems with interior-generated water vapor.
- Class II (01. To 1.0 perm) represents almost- vapor impermeable materials.
Typical construction materials belonging to Class II are closed-cell polyurethane spray foam, vinyl wallpaper, polyethylene film, few coats of oil-based paint….. Given the fact that “nothing is perfect”, the construction industry considers them as Vapor Barriers and it is a well-justified approximation, because in practice, the diffusion process across these materials is so low that it has negligible impact.
- Class III (1.0 to 10 perms) represents semi-permeable materials.
They are known as Vapor Retarders (in other words these materials will slow-down diffusion of vapor). Popular construction materials belonging to this class are open-cell polyurethane spray foams and latex and enamel paints. BTW- gloss paints are less permeable than their flat versions and for obvious reasons two layers of paint will do better than one. To give some reference numbers: Epoxy-polyamide (gloss version) have 0.14 perm while latex (semi-gloss version) correspondingly 5 perms), oil-based paints between 1.6 to 3 perms (single coat).
- Vapor Permeable materials (more than 10 perms)
This group encompasses popular construction materials like drywall, blanket fiberglass and rockwool insulations, wood and press-wood boards, etc…
In contrast, leaks of vapor fully depend on discontinuities in the wall (holes, gaps, unsealed joints etc…). It’s important to underline the fact, that in typical houses, the overwhelming majority (up to 80%) of vapor migrates via air leaks.
At the first opportunity when the combination of temperature and vapor matches the corresponding dewpoint, the water vapor will start to condensate. The process will intensify when vapor migrates further across the wall, because lower the temperature, less vapor the air can hold. Usually, the condensation will happen in outer layers of insulation. If this process lasts for a longer period of time, the outcome will be a large amount of collected moisture and coming with it mold, mildew, fungus and decay of building materials.
When the building materials are “breathing” (in other words are partially permeable), with time and warmer seasons, accumulated water will eventually evaporate to the outside, walls will dry-out and the destructive process will be stopped.
However, any impermeable vapor barriers (like sheets of polyethylene in traditional houses and metal walls in container-based houses) will inhibit the transfer of moisture (water) and vapor out of the wall to the outside. As a result – the water will stay trapped inside of the wall!
Typical structure of the traditional wall includes drywall, permeable thermal insulation, press-wood, vapor barrier, siding. The warm, humid indoors air migrating through the wall, reaches the press-wood (with vapor barrier behind). The vapor condensates, at the outer side of the insulation layer, because its temperature is close to the outdoor one. The vapor barrier inhibits further transfer of the moisture to outdoors at favorable temperatures.
However, in hot and humid climate zones, the migrating air changes direction and so the exterior vapor barrier does an excellent job. Humid exterior air cannot migrate inside of the house and water does not condensate on the vapor barrier because the latter is at the temperature close to the exterior one.
Source: Don Mannes (FineHomebuilding: How it works: Vapor Drive)
In traditional houses constructors use one of the following solutions:
- Installation of vapor-retarders characterized by some level of permeance (breathability) allowing for limited transfer of vapor across the wall or elimination of vapor barrier at all. This is the most common solution as it allows indoor-generated vapor to “naturally escape from the house. Note that in practice, the major part of vapor escapes through discontinuities in the “envelope of the house structure” (air gaps around doors, windows etc…)
- Exterior layer of thermal insulation. This changes the gradient (distribution) of temperature across the wall, there is no need for any interior Vapor Barrier. The wall will be exposed to the indoor temperature (well above the potential dewpoint) and as the result, there will be no condensation. In practice, coats of paint on the walls will act as some sort of vapor barrier or retarder, but it wouldn’t change mush when it comes to condensation.
This solution is widely used especially in modern buildings (concrete or steel structures) having poly foam sheathing attached on exterior walls. It is also more and more used in traditional residential housing.
Partially, the problem with indoor moisture can be solved by using a dehumidifier. Unfortunately, while a dehumidifier will decrease the gravity of the problem, it will not fully eliminate the condensation of water in external walls. Also, for obvious reasons, dehumidifiers are not practical solutions in an off-grid environment.
Note that the similar process takes place in the opposite direction, especially in hot, humid climate zones. However due to much lower difference between interior and exterior temperatures (typically, 22-to-24 oC (72-75 oF) indoor versus 30-to 35 oC (86-95 oF) outdoor), the difference between dewpoints is much smaller compared to those experienced during winters in cold climate zones. As a result, the probability (and eventually intensity) of condensation is much lower.
Additionally, inside of the house, the AC’s evaporator (for obvious reasons also called “condenser”) will create the local lowest dewpoint. When recirculated interior air passes over the condenser’s coil, the water vapor condensates on it, merged droplets drip into a water collector from where they are evacuated to the outside.
A/C reduces the amount of vapor in the house by converting it to the water (condensation process), then evacuate the liquid water outdoors. Source: AirExpertsNJ
Condensation in container houses
In contrast to traditional housing structures, container-based ones have “built-in” impermeable vapor barrier – steel. Container houses have many benefits but understandably, also some disadvantages and susceptibility to condensation is just one of them. The fact that metal is a very good heat conductor makes it even worse.
Exposed outer walls will very closely follow outdoor temperatures, and if this is already not bad enough, also large sections of internal walls will be acting as “thermal bridges” distributing the “cold environment” throughout the whole metal structure. As a result, also interior walls (in multi-unit structures) will be prone to condensation. Note that in traditional, wooden-frame housing structures, interior walls practically do not need any thermal insulation and do not suffer from condensation.
Below we will discuss the most popular solutions for elimination or reduction of condensation or for at least lessening their impact. Note however, that the answer “Right for You” largely depends on the climate zone where your house is located!
On the positive side – container houses with their “built-in” exterior vapor barriers (metal walls) will not have big problems with interior condensation in locations with hot and humid climates. It’s because:
- Vapor Barrier is on the right (warm) side of the wall
- Conditions for condensation can only exist in air-conditioned houses, but then the AC’s evaporator will “mostly” take care of it).
For at least two reasons, it is the most typical solution used in container houses:
- Uniqueness of architectural forms
It preserves the unique architectural form of container-based structures. In fact, corrugated steel walls are often considered as one of the most important assets of container houses, offering new, distinctive visual effects clearly differentiating them from traditional residential architecture.
CargoTecture has its specific aesthetics. Source: Alino.info
Metal walls (if correctly protected against corrosion) represent robust and weather-resistant barriers. Due to their inherent strength, they are also resistant to mechanical impacts, abrasions etc. Eventual exterior deteriorations (corrosion, scratches….) are usually easily visible so they can be quickly patched-up. Note that any external layer of insulation will need protective siding, making it economically less attractive.
Unfortunately, interior insulation has also some disadvantages:
- Thickness (and so effectiveness)
It is limited by the container’s width and height (just to remind you – only 8 ft wide and 8.5 ft high unless the cube version is used). Every extra inch of added walls shrinks the precious interior space by 2 inches, making the indoor area smaller.
- Thermal bridges
Most interior insulations do not provide continuousness (exception is only spray insulation). This will leave active many thermal bridges lowering structure’s overall R-factor. Note that in traditional, wooden-frame houses, thermal bridges do not have such a strong impact on overall R-factor like in container houses. It’s because metals are very good heat conductors compared to wood!
Traditional interior insulation.
Most owners of container houses will use well-known designs taken from traditional housing. It means – wooden frame along all walls and ceiling supporting blankets of fiberglass or rockwool insulation and the final finish – drywall panels. Unfortunately, all these components (especially insulation) are highly vapor-permeable so in cold climate zones, the water vapor will be migrating towards the cold, impermeable steel walls.
Usually, condensation will start within the outer layers of insulation and culminate at the walls. If such a process lasts for a prolonged amount of time, mold, fungus and corrosion will start to develop. For quite long hidden from our eyes this decaying process will have costly consequences.
Popular, blanket-type rock-wool insulation. Source: Greenspec (UK)
Note, that even a thicker layer of insulation will not change the situation. The vapor-permeable insulation cannot inhibit the diffusion of vapor and to make it worse, the most popular ones also easily absorb the water. The impermeable steel walls will trap the water inside of the wall! That’s why the traditional residential solution will not work in metal structures!
Open-cell foam insulations like EPS (Expanded Polystyrene) or PU (Polyurethane) have lower vapor permeability than blankets of fiberglass or rockwool, however they do not act as vapor barriers, but rather as vapor retardants. What makes it worse is that such insulation is mainly used in the form of blocks attached side-by-side to each other, so it cannot provide continuous vapor-retardant barrier. The bottom line – it will take more time, but at the end, the vapor will find its way to the container walls and condensate.
The Indoor dewpoint is established by the amount of vapor and temperature indoors. At outdoor temperature T2 (higher than existing interior dewpoint) there will be no condensation at all. However, at lower outdoor temperature T3, the temperature across the insulated wall correspondingly drops and once it reaches the interior dewpoint, the condensation will start, because colder air cannot hold all the vapor contained in the migrating air.
2. Traditional interior insulation w/extra vapor barrier
At first, it may look like placing the vapor barrier right behind the drywall is a good solution. One may think that migration of air and vapor will be inhibited by the vapor barrier and since the latter is almost exactly at room temperature there will be no condensation on it. Well, nothing is as it appears to be. The sad truth is that there is no perfect vapor barrier. If you remember Murphy’s laws – “All sealed joints will leak”.
First of all, a typical class II vapor barrier like the sheet of polyethylene has only limited, but still non-zero permeability. Also, it’s impossible to guarantee the continuity of the barrier. It will be perforated by drywall’s nails, screws, there will be openings for electrical outlets, plumbing, intersections of sheets, light fixtures etc. Bottom line – the migration of air and vapor may take significantly more time, but it will happen.
Once the vapor migrates towards the container’s metal wall, it will condensate and the water will be trapped between two barriers with little chance to evaporate in warmer seasons. In the similar way will act vapor retarders (as their name points out, they only slow the process of migration of vapor through the wall, but they do not stop it).
Summarizing – the extra interior vapor barrier is a recipe for disaster.
Contribution of air leaks to the migration of vapor through the wall (Canada is a perfect example of the impact of condensation, due to its long heating season). Source: EcoHome: (The Difference Between Air Barriers and Vapor Barriers)
The above pictures emphasize the fact that an overwhelming amount of vapor migrates via air leaks. Given the fact that walls are never airtight, even the best layer of vapor barrier behind the drywall cannot stop this process. And then, in container houses, the vapor faces the impermeable and airtight metal structure preventing its escape to the outdoors! In contrast – familiar Tyvek sheets covering outer sides of exterior walls in traditional housing are airtight (if we ignore perforations by staples and nails), however, they are permeable for vapor allowing it to escape from inside of the wall! (to make it clear – Tyvek sheets also offer protection from bulk water(rain)).
3. Impermeable Interior Insulation
Closed-cell foam insulations like PF (Phenolic Foam) or CC-SPF (Closed-Cell Spray Polyurethane Foam) are almost impermeable. In other words, they create quite efficient vapor and air barriers. Additionally, when applied as a spray, they can also provide a continuous vapor barrier, strongly adhering to protected surfaces. They will fill all cavities, “seal” areas around pipes, wires creating a very effective layer of protection from water-vapor for all vulnerable components behind.
Unfortunately, closed-cell foams are significantly more expensive than traditional insulations. What makes it worse – they are not eco-friendly. The main culprit is the “blowing agent” used to create the foam. So far, the dominant industry standard is a hydrofluorocarbon (HFC) known to be a very potent factor of Global Warming (its effects are about thousand times more disastrous than those caused by a more familiar villain – carbon dioxide). Eco-conscious customers may be encouraged by the fact that Solstice, (hydrofluoro-olefin (HFO) based liquid blowing agent recently introduced by Honeywell) is much more eco-friendly and nonflammable. But as usually, every new product on the market comes with many unanswered questions.
The thing is that on top of the impact of the blowing agent, you should also weigh potential health-hazards related to the chemical composition of the polyurethane itself. All PU-based insulations must include fire-retardants (in their natural form, they will not meet relevant standards). As you may expect – more chemicals lead to more health-hazards due to the off-gassing process. That’s why, many would not use SPF for indoor applications!
Another practical aspect – once cured, the CC-SPF is difficult to trim (if necessary). Also, with time, closed cells will eventually “open” (deteriorate) allowing the vapor to migrate across. The good news is that such degradation is really a very slow process, so it will take long years for the vapor barrier to lose its properties.
Note that the SPF is also available in an open-cell form. It is less expensive and less harmful (instead of HFC, it uses carbon dioxide as a blowing agent), however the open-cell insulations will act only as vapor-retardants (in other words, they are semi-permeable). And what was written above about required fire-retardant chemicals still applies.
Summarizing: Closed-Cell foam (especially in the form of spray) acts as good thermal insulation (high R-factor) and an almost impermeable vapor barrier. It provides a robust and continuous (no perforations, holes, cuts, rips, air gaps) layer of protection from condensation. It efficiently prevents migration of moisture towards low temperature areas (walls), preventing this way the condensation. Given the fact, that in general CC-SPF insulations are more expensive than blanket-type ones, if higher R-value is required, the extra inner layer of inexpensive insulation is added.
If the interior insulation is properly designed for a given climate zone, closed-cell insulation can fully prevent condensation. Properly designed means – the thickness of the CC-SPF must be adequate for a given climate zone, because the whole concept works only as long as the dewpoint is “buried” within the impermeable zone. In other words, if in moderate climate zones 1 inch of CC-SPF may be enough, in cold ones, you may need few more inches!
Note that due to potential health-hazards and known impact on the environment, the use of CC-spray foam insulations should be carefully evaluated on the individual basis by future users.
Combined closed-cell spray foam and traditional fiberglass batts insulation. In this particular case (interior and exterior temperatures and the temperature gradient across the wall), vapor migrating across the layer of fiberglass will reach the impermeable CC-SPF barrier at temperature higher than the corresponding dewpoint. The condensation will not happen!
However, if the exterior temperature drops lower, the temperature gradient across the wall will get steeper, moving the dewpoint back into the layer of permeable fiberglass insulation. In such a case, the condensation will happen! (Source: FineHomebuilding)
As shown above, the interior insulation is far from being an ideal solution for containers’ houses, especially when potentially harmful off-gassing prevents you from using closed-cell spray foam as the insulation and vapor barrier.
It’s then the right moment to change our approach to interior-generated vapor and subsequent condensation. Instead of protecting walls from vapor condensation we may try the following methods:
- Allowing the condensation (once it already happens) to escape from the wall
- Lowering the level of indoor vapor by ventilation and dehumidification (these will be discussed in the separate article).
4. Interior insulation with Controlled Condensation
Let me quote here another of famous Murphy’s laws: “If something can happen, it certainly will”. That’s why it’s essential to discuss solutions that help to minimize damaging effect of condensation on container’s structure (corrosion), walls (rot and mold) and health of inhabitants.
One of suggested solutions proposed by people knowledgeable in the matter is to leave an airgap between the container’s metal walls and the layer of thermal insulation. In this case, we accept the fact that the vapor this or other way will migrate towards container’s walls and when outdoor temperatures and the level of indoor vapor create conditions for condensation, it will take place.
The ventilated airgap in the condensation zone allows for evacuation of moisture and vapor to outdoors, significantly limiting the potentially disastrous impact of trapped in the wall water on the structure and health.
However, if the airgap is properly ventilated, it will offer a sort of “open-doors” for continuous evacuation of the moisture outdoors. For sure, the adequate ventilation is the key parameter of this protection system. It must be designed for that purpose and most likely “assisted”. In other words, the natural (passive) ventilation based on convection may not be efficient enough. Depending on location (climate zones) you may need either roof-mounted wind-propelled turbines helping to move air, moisture and vapor across mentioned airgaps or even strategically located, small electrical ventilators.
It seems that the airgap of about 1-to-1.5” (2.5-to-3.5cm) thick may be enough to guarantee efficient airflow. While this extra 1+ inch will have an impact on the overall thickness of the wall (and free indoor living space), it may be still a low-cost solution to the condensation problem.
Lomanco BIB-12 Black Whirlybird Turbine Ventilator
Few technical details:
a. Corrosion protection
Shipping containers are built from the special corrosion resistant Corten steel. In practice however, it does not mean that they are corrosion free, but rather that the corrosion process is much slower thanks to the “self-healing” ability of Corten steel (which BTW does not work well in a salty atmosphere at shore locations). That’s why, in order to minimize the impact of condensation on the metal structures, it is strongly suggested to cover metal with a thin layer of an impermeable coating.
There is a large choice of impermeable coatings starting from marine-grade paints (low cost DIY solution), ceramic coating (lasting almost forever but requiring professional help) and so on… One of the recently introduced materials of interest seems to be a GrafoTherm. Being still largely unknown, it deserves more detailed description (see below)
b. Low-permeability insulation
It is crucial to choose low permeability insulation (in other words not only slowing down migration of vapor, but also characterized by low absorption of water). What we want in this case is to quickly evacuate the moisture outdoors. Note that any water-absorbing insulation material will slow the process of releasing it to the airgap and so the evacuation to outdoors. To keep it short, block-foam insulations like Phenolic Foam, Polyurethane Foam (PU), Expanded (EPS) and Extruded (XPS) Polystyrene are good candidates, while wood-fiber boards, and popular blankets of mineral glass and rock wools are bad ones.
As mentioned earlier, up to 80% of vapor migrates across the wall by “discontinuities” in layers of insulation and fixing all potential air-leaks is almost impossible (it’s sort of a Sisyphean Task).
c. Insulation thickness
As discussed above, to prevent condensation, the required thickness of insulation is not only determined by the energy-efficiency of the structure and personal comfort, but also by the need to keep the dewpoint in the layer of impermeable CC-SPF. In cold climate zones meeting such requirements may be costly (not only in terms of $$$ but also due to shrinking living space).
As mentioned above, popular insulation materials known like glass or rock wools (known for being harmless for health) do not meet permeability requirements for this application. On the opposite side, all synthetic materials like polyurethane, polyester and to lesser extent phenolic blocks are not neutral (if not known for potentially creating health-hazards by themselves, then for sure by mandatory fire-retardants).
The forced-ventilation airgap significantly limits the impact of interior off-gassing, as most of it will be automatically evacuated to outdoors.
Off-gassing is never so visible as on this picture, but unfortunately, it is there 24/7. Source: ArchitecturalDigest
Important: Note that in climate zones with significantly varying amplitudes of daily temperatures (hot during a day due to the exposure to the sun and cold nights), wetting (condensation) and drying (evaporation) cycles will be occurring on a 24/7 basis. We may be unaware of this danger, because most of us are assuming that for condensation to happen, we need freezing temperatures.
The ventilated airgap (if properly designed) will efficiently take care of this effect.
5. Exterior insulation
Exterior insulation is the textbook example of doing it right. The truth is that the vapor barrier should be always on the hot side of the structure. It means that in cold climate zones where heating is the must, the vapor barrier should be installed on the inner layer of the wall (in practice, behind the drywall). In warm climate zones (where cooling is the must), the vapor barrier should be installed on the outer side of the wall.
And in moderate weather zones (where both – heating and cooling will be used) the choice should be made based on “lesser evil”. Practically, it will be the same scenario as in the cold zone, because winter temperatures will be usually much lower than our zone of comfort compared to the difference between the later and typical summer peak temperatures. Also, usually wintertime will be longer than heat waves in summer (well, at least so far, because the Global Warming is rapidly changing weather patterns, so what we learned from the past may not be valid in the future).
Note that while in traditional wooden-frame houses the potential condensation problems can be solved by installing the vapor barrier on the “right side of the wall”, while in container’s houses the vapor barrier is already there, so the problem can be only solved by installing the wall on the right side of the barrier.
Due to the Vapor Barrier (Corten steel) the warm indoor vapor cannot migrate outdoors. The temperature across the exterior insulation will drop to corresponding outdoor temperatures (T2 or the lower one T3). However, in both cases the outdoor dewpoints are established by exterior weather conditions and by definition they are never higher than those corresponding to air temperatures (T2 or T3). Note that the cold air moving across the exterior insulation towards the Corten wall will warm up and actually will be able to hold even more vapor then it is carried with outdoor air.
Benefits of exterior insulation
a. Elimination of condensation
The first and most important benefit is elimination of indoor condensation. This is because when the thermal insulation is installed on the exterior side, the container’s walls will closely follow the house’s interior temperature. Indoor air may be humid (due to all vapor-generating activities), but the walls’ temperature will be higher than the indoor-related dewpoint which will prevent the condensation.
In the summertime, the opposite effect takes place when the interior is air-conditioned. However, in sharp contrast to the case of interior insulation (if the inner sides of the walls are not finished) the condensation (if any) will be visible and can be quickly removed. Needless to repeat, that:
- If you won’t use the A/C (most likely scenario in an off-grid environment) the indoor temperature will be rather similar to the outdoor one (windows opened) or higher (windows closed), so there will be no condensation, or it will be negligible.
- If you will air-condition your house, the A/C unit, by itself will act as the dehumidifier evacuating moisture to outdoors.
Summarizing: No condensation, no consequences (rot, mold, mildew….) and so no (or less) headaches!
b. More interior space
While most owners of container homes will decide to line the interior with some nice looking and “visually warmer” materials compared to raw Corten steel, such linings will take only a fraction of interior space. Given the limited dimensions (especially width and height) of cargo containers, it will be a significant improvement of living conditions.
c. Better energy-efficiency
It’s very difficult to make a continuous layer of interior insulation in container-based structures (practically only spray foams can meet this requirement). Numerous thermal bridges created by the container’s structure will considerably lower the “Equivalent R-factor” from the value promised by the insulation material. In contrast, the exterior insulation can encapsulate the whole structure in a sort of “cocoon” much more efficiently isolating it from the outdoor weather.
d. Lower Health Hazards
Whatever we may think, the truth is that synthetic materials are not neutral for our health. More of them around us, higher probability of allergies and immunological problems. And while usually, their impact is not visible right away, it will be seen later in life. In some cases, we may know the risks, but there are many cases when such information will come to the public opinion much later (and often it is too late). Available thermal insulations (with the exception of mineral wools that unfortunately are poor choices for containers) are not harmless in indoor applications. However, using them outdoors substantially changes the situation for the better. Firstly, we are not continuously (24/7) exposed to off-gassing products like in the case of interior applications and outdoor environment (even the close one) is much safer due to the almost continuous “natural ventilation” process (wind).
The price? Usually, exterior insulation is more costly and requires periodic maintenance! Whatever you will use to “encapsulate” the container, it will be more fragile and mechanically less robust than the original Corten steel. Also, permanent exposure to weather elements (temperatures and their amplitudes between summer and winter, wind, rain, snow, UV – you name it….) creates far more harsh conditions than those to which interior walls are exposed to!
Note, that eventual moisture or vapor infiltrating exterior walls will naturally dry-out in favorable weather conditions. Exterior walls do not have any outer vapor barriers, they can breathe freely.
Insulation for outdoor use should be limited only to semi-permeable materials like mentioned earlier PF, PU, EPS, XPS block foam (Fortunately, they do not need any extra supporting structure). The traditional Tyvek moisture barrier should be used to prevent infiltration of rainwater (note that Tyvek sheets are permeable for vapor but act as a barrier for bulk water).
Block foam insulation, despite its rigidity, does not have any strength protecting it from mechanical impacts and abrasions. Its aesthetics is also far from acceptable, so the exterior cladding is necessary. There are a lot of suitable cladding materials on the market, so most likely the main problem you will face will be the aesthetics. Losing the stylish pattern of corrugated steel so characteristic for Cargotecture is already bad enough and creates a big challenge to come up with something equally attractive. The bottom line – the choice of exterior cladding is a personal decision largely affected by “surroundings” (neighborhood – if in the city, nature – if in the countryside).
Regardless of the cladding and Tyvek barrier, the container’s steel-structure will be exposed to weather elements (at least those that can migrate through the exterior wall (diffusion and leaks). In this case however, the outer side of container walls are not visible and not easily accessible for checkup and eventual fix. That’s why it is a good practice to protect container’s structure by a long-lasting, impermeable coating prior to installation of insulation and cladding.
Grafo-Therm is a desiccant-like acting coating that can absorb and retain the moisture generated by condensation. What makes it special however, is that at the presence of drying conditions it can equally easily release the captured moisture back to air by evaporation. For that to happen, adequate ventilation is needed. In fact, the Grafo-Therm is not designed to keep captured water for a long period of time. To be effective it has to work in conditions favorable for frequent absorption/evaporation cycles. That’s why it is mainly used in open industrial areas exposed to natural airflow (ventilation).
The Grafo-Therm is very effective and if not friendly, then at least not harmful. As advertised by the manufacturer (Grafo Products (UK)), the 1.5mm thick coating can absorb up to 1 liter of water per meter square. It is not toxic, not flammable and includes an effective fungicide preventing formation of mold and mildew.
It seems that Grafo-Therm may be a good candidate for preventive coating of container structure, provided the airgap between the metal walls and interior walls (insulation and drywall) can be efficiently ventilated.
Above: Grafo-Therm in the original container. Below: Applied on the ceiling and walls of the cargo container. Source: GrafoProducts (UK).
Note that by absorbing and temporarily trapping the moisture, the Grafo-Therm prevents the formation of drops and liquid water that will eventually leak down the wall infiltrating sensitive to corrosion (potentially unprotected) areas of the container’s structure. From this point of view, the Grafo-Therm has a clear advantage over any other impermeable coating.
On the negative side – application of the Grafo-Therm spray is not a DIY project, for that you will need a professional service.