How an Efficient Building Envelope Saves Energy
Energy efficiency projects often focus on isolated measures, such as equipment upgrades. However, enhanced results are possible if the whole building is analyzed as a single system. Building improvements can achieve synergy when they are planned and specified carefully; a good example is improving the thermal envelope along with an HVAC upgrade.
When space heating and air conditioning costs are high in a building, an equipment upgrade is often considered as the first option. However, there are other factors that influence HVAC costs:
- The effectiveness of the building envelope in keeping heat outside during summer, and inside during winter.
- The thermostat settings and controls used for HVAC equipment.
Even if a building uses the most efficient heating and cooling equipment in the market, the operating cost may remain high if these two factors are not considered. HVAC efficiency projects should follow a whole-building approach, rather than focusing on specific equipment.
Importance of the Thermal Envelope in Building Projects
The main purpose of the building envelope is minimizing heat transfer in both directions. Heat gain increases air conditioning costs during summer, while heat loss increases heating costs during winter. A well-designed thermal envelope reduces both heat flows, achieving energy savings all year long.
An efficient building envelope can reduce all forms of heat transfer effectively. Adequate insulation is important to reduce heat conduction through walls, rooftops and other building elements. The building must also have an airtight construction, since air leaks can also carry heat through the envelope. Finally, the building must use fenestration that minimizes both heat conduction and solar heat gain.
Insulation materials are characterized by their variety, adapting to various applications and building needs. In existing properties, insulation is often applied on surfaces and then covered with a finishing layer. Spray foam insulation is also a common option, since it can be applied in hollow or hard-to-reach places. New constructions allow more freedom, since insulation can be integrated with the structure. For example, insulated concrete forms (ICF) are designed to be cast in place when concrete is poured.
Windows have been improved with many technologies and design features. The following are the most common examples:
- Using two or three glass panes.
- Removing the air between the glass panes to create a vacuum, or adding an inert gas.
- Applying a low-emissivity coating on the surface of glass.
The most efficient windows that use these technologies can reduce heat transfer by over 80%, compared with a single pane of uncoated glass. Since window upgrades are disruptive for a building in use, the best recommendation is installing them during the initial construction or a major renovation. An alternative to a window replacement is adding storm windows on top of the existing ones, which reduces heat transfer without removing the existing fenestration.
How to Improve an Existing Building Envelope
Detecting issues that affect a building envelope is challenging because they produce little or no visual signs. However, energy consultants use specialized methods to find them:
- Thermal imaging cameras are powerful tools, since they can reveal temperature variations on building surfaces. Areas with deficient insulation become evident, with sharp temperature changes with respect to their surroundings.
- Air leaks can also be detected with thermal imaging, also based on temperature differences with their surroundings. There are also smoke and pressurization tests that can reveal air leaks.
Building envelope improvements can be analyzed with an energy modeling tool, which can estimate the reduction in heating and cooling loads. If the building envelope is improved before a major HVAC upgrade, better purchase decisions can be taken. The new equipment will not only have a higher nameplate efficiency, but also a reduced workload. It may even be possible to downsize the heating and cooling equipment during the upgrade.
Michael Tobias is the founder and principal of Chicago Engineers, an Inc 5000 Fastest Growing Company in America. He leads a team of 30+ mechanical, electrical, plumbing, and fire protection engineers from the company headquarters in New York City; and has led over 1,000 projects in Chicago, New York, New Jersey, Pennsylvania, Connecticut, Florida, Maryland and California, as well as Singapore and Malaysia.