What is the Building Envelope?
“What is the Building Envelope?” It’s a very common question that we thought we might answer in this article, and while we are at it, show you where it is generally located.
There are several different perspectives of what the building envelope is, for example:
- Physical protection
- Rain resistance
- Noise protection
- Light control
- And finally thermal and air separation
However, in this article, we focus on the building physics side of the building envelope.
The building envelope, also known as the air barrier, refers to section of the building fabric that separates the interior air volume from the external environment. It includes the floor of the bottom floor, the walls/window facades and the ceiling of the top floor. Any part of the building that connects directly to the ground is considered outside the air barrier.
It’s critical to identify exactly where your air-tight envelope is located. False ceilings and raised floors are not part of the air barrier. The air barrier must also align with the insulation layer. For example, in a commercial building with a metal deck roof and suspended ceiling tiles, remediation work may be required above the ceiling tiles to improve airtightness. Air leakage testing should be conducted with the tiles removed to allow pressure to distribute behind the false ceiling.
The benefits of reducing the permeability rate of a building envelope are:
- Lower volume of air requiring conditioning – reduces heating and cooling costs
- Smaller HVAC systems required
- Increased thermal comfort for occupants
- Better control over internal humidity
- Improved indoor air quality
The building envelope consists of an:
- Air Barrier
- Vapour Barrier and a
- Thermal insulation barrier.
The air barrier and thermal barrier need to be thoroughly and consistently in contact with each other.
The same issue arises in roof designs. Some assume the false ceiling acts as the airtight layer, leading contractors to attempt to seal ceiling tiles with caulking to improve airtightness. This is not a solution. It is essential to consult experienced contractors who specialise in airtightness and building envelope integrity.
Air leakage testing with blower door testing.
Air leakage testing, otherwise known as blower door testing or fan pressurisation testing, allows us to test a building for air tightness scientifically. This process produces a number which can be used to compare buildings and their build quality directly.
The building envelope permeability rate is a measure of the amount of air that “permeates” through gaps, cracks and penetrations in the building envelope. This is driven by the application of an external force such as wind, a poorly balanced air conditioning system or the thermal stack effect (Chimney effect). It is expressed in cubic metres of air per hour that passes through each square metre of the building envelope (m3/h/m2).
50Pa of fan pressure from a blower door or multiple blower doors is equivalent to approximately 32km/h winds onto all facades of a building.
Contact Efficiency Matrix online or give us a call should you have any queries about improving the performance of your building envelope.