By Jan Brandjes – a seasoned construction and building specialist with years of experience in sustainable and eco-living.
For most people the term `building envelope’ is as foreign as black hole science, and yet it is something they look at and experience every day of their lives.
Simply put, the building envelope is made up by the external walls, the floor and the ceiling of every building, including the house.
In many countries around the world the building envelope has been during the last 4 decades a major discussion point for building scientists and construction professionals. And it stands to reason that it was, is and will be a major topic for many years to come.
You see, the building envelope is the only thing standing between you and discomfort, indoor health, and high energy bills to name a few. It is the barrier that prevents rain, wind and cold/hot weather entering your home. It forms the largest element of the house and could be compared to the skin of a person. It controls moisture and temperature flow and allows us to be comfortable under almost all conditions.
Yet, just like we tend to forget about our skin being as important as all our main organs, so do we pay very little attention to the building envelope. It’s true that a beautiful kitchen and bathroom count for more when we purchase or build a home. If your skin doesn’t perform as intended, your organs have to work a lot harder. The same applies to a house. A poorly constructed building envelope will make a heating/cooling system work a lot harder, with you footing the bill.
In terms of sustainability, the building envelope provides us with three barriers: A vapour barrier, a thermal barrier, and a rain screen. I briefly will discuss each barrier to give you the reader a better understanding of the importance:
The rain screen as a barrier does just that; it keeps the rain from entering the house. This is an important function, as wet external walls, floors and ceiling quickly will deteriorate a building and is also the major cause for mould, a known asthma trigger. A rain screen sits on the exterior of the house, and can be made from materials such as bricks or weatherboards, to name a few. The part of this barrier you don’t see is the house wrap, which sits behind these building facades. House wraps should allow moisture to leave the inside wall cavity, while stopping moisture from entering. Like everything else, the most important part is the attention to detail when it gets installed. A house wrap, which has been well sealed, with all joints taped up and no holes, will perform a lot better and keep your building envelope a lot dryer.
To be effective, a thermal barrier needs to be continuous (no gaps and holes) and have lots of still air trapped inside. It is this still air inside the bulk insulation that provides the thermal barrier. The same applies to double-glazing; it’s the air between the two panes of glass that gives it its thermal efficiency. Also here applies the rule that the devil is in the detail. No point in putting insulation in the ceiling or walls and leave plenty of gaps: Miss 5% of the total insulated area and lose 50% of the thermal efficiency.
The last barrier and also the most important one of the three is the vapour barrier (or air barrier). Its function is to prevent air from leaking through the building envelope. Air leakage happens when you have holes/gaps/cracks in the envelope and you have a pressure difference between inside and outside. As the wind blows around the house, as a light breeze, or with gale force, it pushes air on one side of the house and pulls air on the other side. Combined with a leaky building envelope, it will suck all the heated or cooled air out of the house, and replace it with air from outside. The more wind force, the more leakage!
The reason I like to emphasize the importance of the vapour barrier, is that if you spend all your money on good insulation and double-glazed windows, it will all be for nothing with a leaky building envelope. This (largely unrecognized) problem needs the urgent attention of the building regulators and construction industry and can only be solved through proper training in building envelope technology.