Energy efficiency measures for buildings often focus on equipment upgrades and automatic controls. However, the building envelope design also has a significant impact on energy performance. Even if heating and cooling systems use the most efficient equipment in the market, deficient insulation and air leaks will increase energy expenses.

An efficient HVAC design minimises the cost per BTU of heating or cooling, but the loads served are determined by building conditions. At the same time, automatic controls ensure that the available HVAC capacity is used efficiently; when the full heating or cooling capacity is not needed, operating at maximum output is a waste of energy.

Specifying a High-Performance Building Envelope

To enhance energy efficiency, the building envelope must minimise heat transfer in both directions. Consider that summer heat gain is added to the air conditioning load, while winter heat loss is added to the space heating load. Thus, a high-performance building envelope achieves HVAC savings all year long.

Heat transfer across the building envelope can occur by conduction through walls, fenestration and other building elements. Air leaks also transfer heat, due to the temperature difference between indoor and outdoor air. The following are some effective strategies to improve the energy efficiency of a building envelope:

• Increasing the R-value of the building envelope with insulation.
• Using energy efficient windows with triple-pane glass and low-emissivity coating.
• Ensuring airtightness, to minimise heat loss and heat gain from air leaks.

Deficient insulation and air leaks are invisible, but they can be detected with thermal imaging, and insulation performance can be modeled based on the properties of construction materials. Air leaks in particular can be detected with pressurization tests, which use a blower door and smoke infiltration to pinpoint their locations.

In new building projects, insulation can be improved drastically with innovative products such as insulated concrete forms (ICF), which serve as formwork during construction while increasing the R-value of finished concrete elements.

Window orientation can have a significant impact on air conditioning loads. While windows allow natural lighting, they also cause glare and solar heating if their locations are not planned carefully. In the northern hemisphere, the southern face of a building gets the most sunshine throughout the year, while the northern face gets the least. North-facing windows do not cause significant glare and solar heating, but windows with other orientations may require shading or outdoor vegetation.

Envelope Enhancements in New Buildings and Renovations

An efficient building envelope design is highly recommended in new constructions and major renovations. Deploying a new envelope for an existing building is expensive and disruptive, since walls and facade elements must be modified. However, a renovation is an excellent change for an envelope upgrade, since the building will be altered anyway. In the case of new constructions, a high-performance envelope can be specified from the design phase.

An efficient envelope design can also lower the upfront cost of HVAC equipment in new constructions and renovations. Since heating and cooling loads are reduced, the corresponding equipment can be specified with a smaller capacity. This reduces all ownership costs associated with HVAC installations: the initial purchase, operation and maintenance.

High-performance windows can be deployed easily in a new project, since there are no existing windows to replace. On the other hand, a window upgrade in an existing building can be challenging: window frames are not normally designed for triple-pane glass, and must be replaced completely. Scheduling a window upgrade along with a major renovation is strongly recommended, since two disruptive projects are combined in one.

Fixing Issues in Existing Building Envelopes

As mentioned above, an envelope upgrade is more challenging in an existing building due to the disruptive nature of the project. However, there are ways to improve the thermal performance without taking apart building elements.

Storm windows are a lower cost alternative to triple-pane windows, if a property owner prefers not to disrupt the existing envelope. Storm windows are installed on top of the existing ones, creating an air gap that reduces heat transfer. Although triple-pane glass offers better insulation, this project does not require the removal of existing building elements. As a complementary project, air leaks can be fixed with special foams that are sprayed in place.

Property owners who are planning an HVAC upgrade should consider a building envelope assessment. If insulation and airtightness issues are fixed first, the new HVAC equipment can have both a higher efficiency and a reduced workload.

Michael Tobias, PE, LEED AP, CEM.

Michael Tobias is the founder and principal of New York 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 New York, New
Jersey, Pennsylvania, Connecticut, Florida, Maryland and California, as well as
Singapore and Malaysia.