The importance of building envelopes

Q. William, your company, Green Building Solutions, focuses on inspecting and testing the quality of the building envelope. What exactly is the building envelope?

This is often misunderstood because it is not simply the glass façade, the insulation, sealant or even the complete wall assembly or roofing materials. It is rather the combination of all those things plus mechanical and electrical service penetrations, connections between dissimilar wall construction types, the required flexible expansion joints, service shaft openings and even minor final details such as the installation of a handrail or signage on the building exterior.

The building envelope is actually a system, and a very complex one, with many, many points of possible failure. As you can also see, unlike other building systems such as the HVAC system, where most questions and concerns can usually be answered by two members of the project team, the HVAC engineer and the installer, the ultimate quality of the building envelope is the result of many, many participants of the project team, and isn’t finalised until the last gasket and wall fixture is installed.

Q. What is the most significant factor in constructing a proper quality building envelope?

The air / water vapour barrier, the vital membrane that keeps the outside out and the inside in, is far more critical in our hot, humid area of the world than has been recognised.

Let’s talk about the significance of those two factors, air and water, one at a time:

1. The recently developed urban areas of the GCC region are in essence 24-hour, seven-days-a-week air conditioned indoor environments. Unlike many moderate climates, our outdoor air gathers dust, salt and biological spores as it travels the trade currents from distant subtropical land masses across vast deserts and high salinity bodies of water. The un-planned outdoor / indoor air exchange caused by poor building envelope system failure allows the desert inside, where it eventually accumulates in the A/C system.

The respirable particulates and micro-organisms can become the constantly re-circulated conditioned indoor air we breathe and negatively impact human health. For example, according to Dubai Statistical Centere data in the year 2012, the population of Dubai was 2,105,875 people and in that very same year the largest single category (by over two times any other category) was respiratory illness, with 520,726 medical clinic visits. Obviously, that number contains multiple visits by the same individuals; however, I believe it is worth noting since in fact nearly all the air we breathe here is mechanically produced air-conditioned air.

2. The water vapour in our hot, humid climate is a more elusive and effective delivery system than rain for creating the corrosive effects of un-planned water inside the building envelope. Unwanted water inside our buildings causes material degradation and fosters mould growth.

The higher the air temperature, the more water vapour is in any given body of air. Research ‘building details’ on the internet and you find many examples of how to keep rain – liquid water – from entering a building and causing internal damage – however, very few about creating an effective air barrier.

Meanwhile, here in the UAE we have 341 rainless days a year. From April to October the average temperature is 36 degrees and annual average relative humidity is 58.8%. The basics physics of the challenge we face comes down to the fact that the liquid water molecule is larger than the water vapour molecule and therefore much harder to protect against.

Q. Having observed air leakage in 90% of the buildings you have inspected, what design and construction practices contribute to this?

In terms of building mechanical system design, there tends to be a standard practice of slightly over-pressurising the building by 5 to 10% in the belief that this will effectively guard against outside air entering the building – infiltration. What is seldom recognised is that in real-time conditions any breech in the integrity of the building envelope is either infiltrating or exfiltrating at any given time, depending on the localised atmospheric conditions.

To illustrate what I mean, let’s take a typical multi-storey office building where a large tenant has taken several floors. The A/C system capacity is calculated for the load by well-proven international codes and practices, and as a measure of safety the capacity is boosted to achieve a 7-10 Pascal positive pressurisation. That may counteract infiltration on any one given day, but what about at night when the system is reduced or shut off, or when there is an internal pressure imbalance because the surrounding floors are vacant with no active A/C to counteract the positive pressure? Or even more to the point, AC capacity calculations often do take into effect the prevailing wind direction and the fact that one side of the building may be at low pressure (inducing exfiltration) or at a high pressure (inducing infiltration); however, the real-time conditions don’t always follow the rules.

Here in the UAE, the yearly average wind speed is 19.4km/h, which converts to an average exerted pressure of 21.1 Pascal, and the extra positive pressurisation may be ineffective most of the time. Also important to note is that we have sand storms 20-50 days of the year. Wind speeds during a common sand storm or Shamal are 40km/h and often go much higher. 40km/h equals 200 Pascal of pressure that will significantly increase infiltration or exfiltration if you have poor building envelope integrity.

The building‘s architectural design may also negatively enhance infiltration; an office building sitting on top of an open three-four floor parking garage often creates the perfect wind tower, where cooler air near the ground naturally flows upward through the building by the rising warmer air in what is known as the stack effect. As mentioned, architectural detailing and construction practices here have yet to take on the mindset of air tightness, and individual construction activities impacting the building envelope are not seen holistically as part a single system to insure a robust air and water barrier.

The fact of the matter is that you cannot adequately air condition a balloon with a hole in it; the envelope must be a sealed membrane.

Q. Building envelopes are thus important to producing livable, healthy urban environments. What about sustainability and climate change?

I am in fact very, very optimistic. I believe that building envelope integrity is actually the most viable, politically neutral and lowest-cost strategy for the world to regain sustainability. A recent study conducted at Princeton University outlined 15 sustainability strategies that could each save 25 billion tonnes of CO2 annually. Most of the strategies are seemingly out of our current ability level of international cooperation, strategies such as “Stop All Deforestation”, “Increase Wind Power 80-fold to Create Hydrogen for Cars”, “Increase Solar Power 700-fold to Replace Coal”.

However, one strategy to save those same 25 billion tonnes of CO2 was simply: “Cut Electricity Usage in Homes, Offices and Retail by 25%”. If your building envelope does not leak, this is very achievable. The least costly way to reduce energy use is by building buildings that don’t waste energy. No one plans to design a leaky building, no one plans to build a leaky building and no one plans to buy a leaky building, yet they get built all the time.

Air tightness should not be an additional cost. The architectural detailing and construction practices to create building envelope integrity are in fact a fundamental expectation of any habitable built structure.

Another study by a leading global economics research organisation took into account the carbon emissions savings, financial cost and the carbon emissions required to implement a given sustainability strategy and found that it is 10 times more efficient to improve the quality of the building envelope than to spend money and resources on increasing the efficiency of the HVAC system of a given residential or commercial building.

This only makes sense as HVAC is a reactionary process: it can only be efficient to the degree that the building‘s conditioned volume is efficient in reducing cooling load in the first place. Again, this is especially critical to our local condition of a virtually 100% air conditioned environment. Typical figures taken from many sources indicate that our residential, commercial and retail buildings consume over 40% of the electrical load in a developed country (in the US, for example, the latest US Environmental Protection Agency figure is 48%), and the built environment is often the single highest sector of power consumption.

Of that total power consumption in the GCC, air conditioning is by far the largest single use. A published study by King Abdullah University of Science and Technology (KAUST) shows that buildings account for 80% of the Kingdom of Saudi Arabia’s electrical usage and 70% of that power is consumed by air conditioning alone.

Q. What are your thoughts on a strategy for future improvement on these issues?

We must develop a local knowledge base that reflects the unique conditions we find here, not the moderate climates that are the training experience of most of our current professional consultants. We need to challenge our thinking, beginning with building architects and construction professionals, and go back to the basics to develop solutions for the regional extreme climate realities which are vastly different than the weather and climate conditions we studied and learned in our home countries. Eventually legislation and building codes will evolve, but it must start with active local knowledge leaders; legislation is pretty much the last step in process of creating progressive sustainable policy, and all legislation is a shell game unless there is viable realistic enforcement.

Also consider this. There are only two places in the world where significant testing of the building envelope air tightness is legally compulsory – northern Europe, where the oil crisis of the 1970s forced a movement toward conserving energy, and here in the UAE, where the vision of the founding father H.H. Sheikh Zayed Bin Sultan Al Nahyan has been ultimately translated into the 2030 plan of Estidama (Arabic for sustainability) and the specifics of the Pearl Rating System for Buildings and the now mandatory Dubai Green Building Regulations & Specifications. This critical inspection and analysis is not mandatory in the US, China or India.

These two green codes have broken new ground and are based on the realities and thermodynamic stresses of our harsh extreme climate. I am pleased that this movement is spreading; the Qatar Sustainability Assessment System has recently evolved to the Global Sustainability Assessment System and now includes directives on air tightness for buildings.

The MENA region, and specifically the UAE with a five-year lead in creating mandatory sustainable green codes, is really a vibrant laboratory for the extreme climate change the world’s leading scientists agree is virtually unstoppable at this point, because the needed change for the future is bogged down in the politics of the present. With world-class institutions such as the Masdar Institute of Science and Technology and IRENA (the International Renewable Energy Agency) devoted to meeting the challenges of the increasing reality of climate change, I believe that we in the building industry are in the right place at the right time.

William Whistler is the owner / managing director of Green Building Solutions (GBSI), an inspection and testing company for building envelope issues. Founded in 2010, GBSI has conducted over 300 airtightness tests in the region and is currently conducting building envelope verification services for the Louvre Abu Dhabi and the Four Seasons Hotel, Al Maryah Island, Abu Dhabi.


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