Air flow, Temperature and the Pressure Plane
By B. Adams
In our second article on building science, we discuss air flow, temperature, and the pressure plane as it relates to the stack effect. The temperature difference between inside and outside as well as the height of the building determines how much stack effect a building experiences and where the interior pressure plane should exist. The presence of holes in the building envelope is another important factor determining stack effect, the location of the pressure plane as well as occupant comfort and energy costs. Air leakage is a major factor affecting the amount of energy waste related to heating and cooling the air in buildings, as well as in allowing higher volumes of water vapor to enter the building.
A building envelope should be designed with a continuous air and thermal insulation barrier system that separates the conditioned space from the outside. All systems consist of individual parts. Air and thermal barrier systems are no different.
An air barrier system includes a principal air-stopping material, accessories that seal or fasten materials together and components like windows and doors. Air barrier material can be self-adhered, liquid-applied, sprayed or mechanically fastened building products tested by the American Society for Testing and Materials (ASTM). Air “permeance” is not the same as air leakage. Air permeance is the amount of air that migrates through a material, while air leakage occurs as a result of holes or gaps in the air barrier system (hence the need for air barrier system continuity). Air-barrier system accessories include products such as sealants, tapes, membranes and fasteners. An example of air barrier material and accessories applied in the field includes exterior walls built with green laminated ZIP wall sheathing sealed along its seam with an advanced acrylic adhesive tape.
If the goal is to achieve a continuously sealed air barrier, cutting large holes on every side of a building presents challenges. Windows, doors and other manufactured elements that a building must have to be considered habitable are components in an air barrier system. Each component must be installed according to manufacturer’s recommendations, if the product is to function as intended or qualify for under-warranty replacement. Air barrier accessories like the advanced acrylic adhesive tape work very well in sealing these components to the air barrier material, while mechanical fasteners secure the components to the building structure. Air barrier materials, accessories and components must be installed perfectly – no matter the weather conditions of the day or the mood of the installer. A properly designed and installed, continuously sealed air barrier system is the primary defense against infiltration, exfiltration, outdoor pollutants and water vapor. To keep the hot side hot and the cold side cold, a building also needs a thermal insulation barrier fully aligned with the air barrier system.
A thermal insulation barrier is not nearly as complicated as an air barrier system. The thermal insulation barrier is typically made of some type of insulating material designed to reduce the transfer of heat energy through materials of drastically different temperatures or from one area to another. The thermal insulation barrier reduces the effects of convection, conduction and radiation. Not all products are created equal when it comes to insulating against the transfer of heat energy. To determine which product carries the highest level of insulating value, refer to the product’s R-value. R-value is the standard unit of measure that determines the heat-resistive properties of insulation. There exists a vast array of insulating products with varying insulating properties. If I could impart words of advice regarding insulation it would be: install it perfectly and in alignment with the air barrier system — otherwise you’re wasting time, money and energy!
Wearing a wind breaker over a wool sweater on a cold day is an apt analogy that best describes the air barrier system and thermal insulation barrier. The openings for head and hands allow cold air into your personal space, thus increasing your personal discomfort and ability to stay warm. The same principles apply to buildings with holes and gaps in the air barrier system and thermal insulation barrier. If buildings were designed as windowless boxes the air and thermal barriers would be exceptionally easy to construct and align in perfect harmony. In the real world, buildings are edifices of human ingenuity, pushing the limits of form, function and height. No matter the style or size, continuously sealed air barrier systems with perfectly installed and fully aligned thermal barrier systems are as necessary as windows and doors.
Look for the next building science article discussing moisture management and how to prevent your building from growing old with mold.
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.