By Brian C. Adams
This article is the first in a series about the fascinating world of building science. As a building scientist, I like to think I know a thing or two about how buildings should be built. Unfortunately, I encounter too many that have been designed poorly, perform badly and deteriorate prematurely. While this is excellent job security, I would rather see better buildings that perform exceptionally well and deliver low-cost comfort for 100 years. This series will help explain how building science affects energy efficiency, durability, comfort and indoor air quality. Real world examples will lend context to the principles of building science and the effort to keep the outside out and the inside in. Communities are recognizing the need for resilient, durable buildings that provide low cost comfort no matter the season or weather. As a result, designers and contractors alike are starting to take building science seriously.
Illustration of the stack effect. Exterior buoyancy forces push against the first floor walls, create a negative pressure environment inside, displacing warm air to the second floor, creating a positive pressure environment which forces conditioned air through building openings. Credit: Energize New York
Early societies employed basic building science principles even if they were unable to explain the physics behind them. We can thank the Romans for indoor plumbing, for example – they were luxuriating in hot indoor baths some 2,000 years ago. Around the same time, the Greeks and Chinese knew to orient their buildings to take advantage of passive solar gain. Aeschylus and Socrates praised the southern aspect for winter sun and the use of verandahs and eaves for summer relief. After all, they didn’t have central heating and cooling yet. Modern day buildings are still built using basic materials like stone and wood, but they also include sophisticated materials like insulation, plastic and asphalt, and they’re filled with complex energy and plumbing systems our ancestors could scarcely dream of. It’s more important than ever that we understand and apply the basic tenets of building science to design and construction.
With knowledge gathered over millennia about buildings under our societal belt, one would think building science would be mainstream. Unfortunately, too many buildings are still designed and built with fundamental flaws such as lack of continuous solid surfaces and poorly designed mechanical equipment. The resulting air flow problems are perhaps the most critical to building durability, occupant comfort and energy costs, so let’s start this introduction to building science there.
Air flow is fairly straightforward when you’re standing outside. The air just blows past you from the prevailing winds. Drop a building down on that spot and the air flow starts to act differently. The presence of flat surfaces with multiple openings, a thermally controlled interior, and unpredictable occupant behavior creates positive and negative pressures inside the building. The building is exposed to what your high school physics teacher calls buoyance force on the above- grade building assemblies. Buoyance force is the directional force exerted against building surfaces as a result of temperature differences between the interior and exterior. The result is stack effect – or the movement of conditioned (warm) air escaping through the top of the building. Cold air enters a building near ground level, displaces interior conditioned air (warm air in this example) until the warm air escapes through openings in the roof, windows or other building assemblies.
What do air flow, density and pressure differences have to do with buildings? A lot! How the building performs in varying ambient temperatures, positive pressure against the building shell and positive or negative interior pressure from mechanical equipment defines the comfort level of the occupants. (Not to mention how much money is spent on maintaining that comfort level.) Buildings are complicated structures with even more complicated mechanical equipment. But sometimes they are down and dirty buildings built without considering buoyancy forces and the stack effect. Here’s an example.
I recently inspected a 30,000 square foot low-rise medical building built in 2005. The occupants had concerns about heating and cooling costs as well as inconsistent temperatures throughout. The building was slab-on-grade so I knew stack effect was not a significant issue. A multi-storied building is more susceptible to the stack effect due to its height and greater buoyancy force against the exposed surfaces. Back in the building, I removed an acoustic ceiling tile knowing full well what I would find – a plastic vapor barrier tacked to trusses with some kind of insulation. And indeed, I discovered a 30,000-square-foot plastic ceiling penetrated with staples, 16-inch round duct work and wood and steel assemblies. Let’s overlook the fact that the building code allows this type of poor construction. What happened to common sense and the building science principles of air density and pressure? I was in the building for less than thirty minutes and found a glaring violation of the rules of building science: make sure the building envelope has a solid surface and remains continuous.
In the next article we will cover in greater detail the concept of the stack effect, energy transport and the need for a continuous air barrier.
Brian is a commissioning specialist with Cornerstone Commissioning based in Boxford, MA. He has a keen interest in increasing public health and safety in the built environment and creating comfortable, energy efficient buildings. With 10 years of experience in energy accounting, building audits, modeling and financial analysis Brian delivers compelling evidence for designing resilient and healthy buildings. http://www.cornerstonecx.com/
Leave a Reply