Building design has evolved substantially in order to meet the demands, comfort, function, and energy needs of the public. This evolution has caused massive innovation in the structure of buildings and the design space as well. One of the most notable developments is the double-skin facade, which has raised many questions.
What are the disadvantages of a double skin facade? Beyond the coatings themselves being sensitive to humidity and the weather, double-skin façades potentially decrease the amount of natural light entering the building, while increasing temperature within the air gap, and creating condensation on the external skin.
Though these disadvantages are severe, these all can be mitigated with careful planning and consideration of the materials and method of installing. Before we go into detail on all these disadvantages, it is worth clarifying a bit what a double-skin facade is.
What Is A Double Skin Facade?
A double skin facade system consists of two layers – usually glass – that allows airflow through the in-between space. This space – which usually ranges between 20 cm to 2 meters – serves as a method of insulation against extreme temperatures, winds, and sound. All the while, it improves the building’s thermal efficiency regardless of temperature.
This airflow through that cavity (also called an air gap) can occur in two different ways: either the airflow occurs naturally or it can be mechanically driven. Depending on which method is used will determine whether the double-skin facade is using an active or passive system for ventilation purposes.
The Ventilation Types Of Double Skin Facade
Since both of these systems are dramatically different from one another, it’s worth exploring more about each of these systems.
An active system is a system that is mechanically driven. The outer skin of this system is an airtight insulated glass. Air exchange is then artificially induced and takes place inside of the building through heat exchangers. During wintertime, this can be useful in lowering your utility bill since a lot of the heat can be saved through heat recovery. You’ll find this system to be the most common of the two.
Passive, on the other hand, can be found more frequently in European countries, particularly with commercial buildings. Through this system, the air exchange takes place between the air gap and the environment. You’ll also find openings in this glass both at the top and bottom of the outer glass.
How Do They Work?
Regardless of what system is used, the double skin facade is an adaptable system in both cool and warm weather. With only a few minor modifications – like opening or closing inlet or outlet fins or activating air circulators – the behavior of the facade is changed.
In colder climates, the air buffer serves as a barrier to heat loss. Sun-heated air contained in the air gap will heat spaces outside of the glass, which reduces the demand for indoor heating systems.
In hotter climates, the air gap can be vented outside the building to mitigate solar gain and reduce cooling load. Excess heat then gets pushed up and out of the system through a process called the chimney effect. The chimney effect is that air density creates a circular motion – cold air is pushed down while hot air rises – and so this facade functions in that manner.
This system depends on several external conditions such as solar radiation and external temperature. These all directly influence the internal comfort and users’ quality of life. Because of this, it’s vital to be careful in the design compared to other designs since design team will need to consider solar orientation, context, local radiation, temperature conditions, how many people are in the building, and more.
Now that we have a bit more clear what a double-skin facade is, let’s look at the disadvantages it has.
What Are The Disadvantages Of Double Skin Facades?
Even though there are a good number of benefits to having double skin facades, there are some costs and challenges to it. They go as follows:
- Higher construction cost.
- Fire protection.
- Reduction of rentable space.
- Extra maintenance and operational costs.
- Overheating issues.
- Higher air flow velocity.
- More weight to the structure.
- Daylight hindrance.
- Acoustic insulation.
We’ll explore each one but do note that some of the disadvantages from these stem from poor installation rather than innate problems with the design itself.
Higher Construction Costs
Compared to other conventional facades, the price will be higher, which is to be expected. Instead of a single layer being installed, there are two. This means more material and more labor. Further work will be needed in the event that an active system is being installed since airflow will be mechanically driven by that point. All of this will impact the bottom line.
Not much information is available about the performance of double-skin facades in the event of fires and best practices are not clearly laid. However, the fact that the air gap connects all the different floors is a very critical point and, if not dealt with correctly, can help to speed up dramatically the spread of smoke and fire throughout the whole building.
Several studies, such as this one or this one talk in-depth about the issue, being the chimney effect the most dangerous characteristic. Having said that, other studies, such as this one, propose the use of double-skin facades to control the smoke spread throughout the building.
What is clear is that, if not taken into account, a bad-designed double-skin facade can become a big fire hazard. Certain design decisions, such as the installation of sprinklers or the use of non-combustible materials in air-cavity can help reduce the problem.
Less Rentable Space
As mentioned before, the width of the air gap can vary from 20 centimeters to a few or more meters. While that space can provide greater airflow, there is a cost to it in the form of smaller space to work with. This creates a disadvantage, especially in plots where the buildability is maximized.
For business owners, this means there is less space to work with, so that could require a business to change the floor plan for their offices. Employees will have smaller rooms to work in and that could make them uncomfortable. Maybe not in the case of an air gap being 20 centimeters, but the difference will be noticeable if the gap is over a meter.
This also doesn’t favor those who wish to rent this space. Similar to apartments, part of the determining price is based on the amount of space that’s offered. Even though a double skin facade could increase the price for various reasons, there is still less space that a landlord is charging for, so they could suffer more losses long-term. This can also apply to business owners who are looking to rent out space as well.
Extra Maintenance & Operational Costs
Double skin facades will have higher initial costs, but they’ll also demand more in maintenance and operational costs too. Because there are two layers to it, this translates to double the cleaning, inspection, servicing, maintaining, and operating of the design. This will cost a business more and can also hamper employees as well since they may have to deal with the sounds of cleaners or other repairs on occasion.
Another thing to note is that poor designs will also lead to a shorter operating life of electric motors, heat exchange systems, and other equipment used for airflow and temperature purposes. The reason for that is poor designs lead to higher temperatures in the air gap that these systems won’t be able to keep up with.
While these expenses can be written off, this still impacts the financial survivability of a business since more money will be put into repairing old systems or replacing old air systems with new ones. This cost can, of course, be mitigated by proper planning, but a business will still have to take the initial cost of these systems if it wants a double skin facade for the building.
In an ideal scenario, this shouldn’t be a problem. However, if the design team poorly constructs this design, businesses can run into various issues. One such problem is overheating. This problem is mitigated by the width of the air gap as many professionals iterated that designs should be a minimum of 20 centimeters between layers.
In the event that the width is too small, this can lead to overheating in interior spaces since the heat won’t properly flow upwards and out. Also considering the nature of these systems, that hot air will seep into the building itself making it uncomfortable for people to work in those areas.
This issue can also present issues of fires as well since overheating from external factors can lead to various items catching fire. This is particularly concerning with cables and wires. Even though they have a protective coat, that doesn’t mean wires are immune to higher heat and temperatures.
And in the event it does lead to a fire, there is the other problem mentioned above with fire safety. The fire might spread much quicker through the air cavity and affect several floors almost at the same time.
Higher Air Flow Velocity
This disadvantage stems from taller buildings with this design since there is a shift in air pressure between each office depending on the floor. Airflow velocity is the factor that contributes to the comfort of individuals in a building. The idea is that the air will work as a coolant to prevent the air from getting too warm.
When dealing with taller buildings, there will be more airflow velocity which can lead to people developing a compromised immune system and ease the onsets of colds and hypothermia.
More Weight to The Structure
With double the materials, there is going to be more weight added to the building and force placed on the foundation of the building. This also is a contributing factor to the cost of construction if a business is building this structure from the ground up.
Applying a second skin to a pre-existing building will also put a strain on the foundation of the building. Since it is extremely expensive to change the foundation of an existing building, there has to be further planning on considering whether the building will be structurally sound once the second skin is installed.
In extreme cases, failure to do so can cause the structural integrity of the building to be put into question and can lead to the building collapsing from the additional weight.
Even though there is more shade being provided in certain designs, daylight still plays an important role in the overall comfort and wellbeing of people. Even when not using metallic perforations, there is a reduction in the quality of light that enters a room due to that external skin. There can be an even greater difference based on the glazing too.
Even though this is listed as an advantage below as well, being able to hear other sounds can be important in certain circumstances. Because the second skin is blocking more sound, there is a stronger barrier of sound between rooms and floors.
While this is good news most of the time, it has also a negative side. In the event of an emergency – like a fire, or an unauthorized individual entering the building, or someone getting hurt – being able to react quickly is important. Acting quickly typically won’t happen if the rooms are now semi-sound proof.
It’s also worth noting that if the double skin facade is installed poorly, there won’t be any sound reduction at all. Installing it could even worsen the effect.
Another large issue is condensation. This is prominent in passive designs, however, another contributing factor is the position of the building and climate conditions. Condensation can occur on the inner side of the outer glass pane which will disrupt people’s view. This problem becomes more problematic during morning hours in the spring and autumn times.
All that said, this too can be mitigated based on careful planning and consideration. In the case of condensation, there are anti-fog coatings for glass that have been designed to deal with this issue. This can be applied to the outer glass pane which will reduce the chances of condensation greatly.
The other thing to note with this coating is that it’s impossible to apply this coating once the installation is complete. It’s key to determine this option during the project planning stage.
What Are The Advantages Of Using This System?
The fact that we have listed so many disadvantages should not make you think that double-skin façades are a bad idea. On the contrary, they can be great choices. When done right, this system can provide a number of benefits. Below are its main advantages.
As mentioned already, the air gap serves as an insulator and thermostat, able to capture heat and coolness and cycle through it. Since this system can provide heat during cool months, there is less need for heating systems, thus reducing the energy consumption.
Buildings will also enjoy a lower utility bill during the warmer months as well when using this method. The reason for that is that the two facade layers essentially monitor their own temperature. Similar to the colder months, these facades monitor the heat within that space. Whenever there is a lot of heat, the system is built in a way to drain out the heat by using the stack effect.
The stack effect is the difference between air density, creating air buoyancy. In other words, the higher the temperature of the air in that air gap is, the more that’s pushed out of that air gap.
This results in the temperature of the inner skin to be lower and there is less transfer of heat from the building’s exterior into its interior. Since this process will make any building or room cooler during the warmer months, there is less demand for air conditioning regardless of the system being active or passive.
As we mentioned in the disadvantages above, one remarkable thing about double-faced facades is that, even when the windows are open, there is still acoustic insulation similar to a single-skin facade. The effectiveness of this sound insulation can also be improved in this manner by adding perforated metal panels or grilles in the proper way.
There are two key methods for going about this.
First is sound transparency which incorporates soundproofing elements and has perforated sheets as a protective cover. The idea with this method is the sheets work in allowing sound to pass through them while the extra sound controlling material soaks up the rest of the frequencies.
Second method is tuned resonance which needs specialized perforation sizes. The reason for that is that these perforations are matched to the frequencies of sounds that are being blocked. In this method, the sound waves will go through the holes as opposed to passing through them. This results in reducing noise further.
These systems are designed to enhance comfort and improve working conditions. When designed properly, these can pay off in various ways. Social benefits can include more employee satisfaction, higher productivity, and healthier workers.
Some other notable benefits are:
- Flexibility in perforation size and profile. Custom images are also an option with many businesses who create these.
- A wide array of colors and finishes to choose from.
- A long life span due to the coatings.
- Many options offer environmentally conscious materials.
What Materials Are Ideal For Double Skin Facades?
Since design plays a crucial role in this, it’s key to know what materials are being used. The wrong kind of materials can lead to poor design and building problems.
To best understand the materials, it’s also important to know the Double Skin Facade (DSF) system. This system can be installed in many buildings including renovation and reconstruction projects. It’s possible to have this installed in existing buildings with minimal adjustments as well, provided the planning is done right.
The DSF has three components to it: the exterior wall, the air gap and the interior wall.
The exterior wall will be the one providing protection against the weather. With that in mind, the glass is going to be either heat-strengthened or laminated safety glass. That said, there is some flexibility to this glass based on specifications. There are also glazings that can be applied to the surface of the glass too. Another thing to note is that high-reflective flint glass can also be used for higher transparency.
The interior wall is used for thermal-insulating single or double pane glass. It’s on this interior wall where coatings can be applied. The coatings will serve as a way to reduce radiative heat gain and are low-emittance coatings. That said, there are other coatings that serve other purposes such as preventing condensation and such.
The last aspect is the air gap itself. While that is where the air flows, there is plenty of materials that can be applied there. Many businesses opt to have metal floors or grilles on each level that allow access for maintenance, cleaning, and even fire escape.
Also depending on how a business is promoting airflow in that cavity will require more materials. For example, for fan-assisted ventilation, a fan will be needed. Businesses will also need to consider computer systems or other devices should they desire an automated computer system or mechanical methods to provide airflow respectively.
Beyond that, the space also offers solar protection, though buildings can use shading devices in these instances. There are metallic perforations, but blinds, roller shades, louvre systems, or motorized openings can be placed in the cavity too.
With these shading devices, there is plenty of materials that can be provided, including wood and stone. The louvres systems are typically made from glass. If perforated sheets are used, various metals can be used like aluminum or even fabric.
Keeping all of these design choices in mind can ensure the installation of these systems will go smoothly. However, there is one other big determining factor, and that is the type of DSF system used.
The Main Types Of Double Skin Facade Systems
There are two types of ventilation systems in place: active and passive. However, there are four main types that will impact the overall design choice. Below is detailed information of each type as well as requirements for these systems to work.
Buffer Double Skin Facade System
This system was in before insulating glass was invented as a means of maintaining daylight into buildings while increasing sound and insulating properties of the wall system. Like with all double-skin facades, there are two layers of (in this case single) glazing that’s spaced out between 25 and 90 centimeters apart.
The spacing is then sealed and allows fresh air into the building through a controlled method like an HVAC system. Box type windows also work which cut through the overall double skin. Shading devices can also be included in the cavity.
Extract Air Double Skin Facade System
The extract air system has a second layer of glazing, though it’s applied to the interior of the wall. This allows the air within the air gap to become part of the HVAC system.
Through this system, the heated used air is extracted through the gap with fans. The fans also work as ways of cooling the inner layer of the glazing while the outer layer of insulating glass minimizes the heat. Shading devices can also be mounted in the air gap too.
Most of these systems are located in places where natural ventilation isn’t possible. That or there is a lot of noise, wind, or fumes.
Twin Face Double Skin Facade System
The twin-face system offers a unique design compared to the other two systems. This system consists of a conventional curtain or thermal mass wall system with a single glazed building skin. The outer glass can either be safety or laminated glass or insulating glass. Shading devices can be used and the range of space between the walls must be at least 50 to 60 centimeters apart for cleansing purposes.
The unique thing about this system is there is an opening that allows natural ventilation. This allows the outer skin to be used for protecting air-gap components – such as shading devices – while the internal skin works as insulation for minimizing heat loss.
Hybrid Double Skin Facade System
The final system is the hybrid system which is a combination of two of the three systems mentioned above. These kinds of systems work in a way where a single one of those systems can’t accommodate the building systems needs. For example, a building could use a layer of screens or non-glazed materials on the inside or outside of the exterior wall which would make it a hybrid system.
Whether to implement this design choice or not, there are several considerations that have to be made. There will be budget demands since it’ll be key to know the exact cost of implementing this system. Afterwards there are the designing and planning considerations which can take weeks or months to plan out depending on the scale of the project.
It’s not an easy decision, but by understanding the advantages, disadvantages and the choices of systems, the process should be easier to go through.