Energy Efficient Building Envelope Toolkit

The envelope of a building is defined as the physical separator between the exterior and the interior of the building. This includes all exterior walls, windows, doors, and roofing. Enhancing the insulation of the building envelope will help to prevent thermal exchanges between the inside and outside to save energy on heating and cooling. Below are steps you can take to increase your energy savings through the building envelope.

Walls

Lining the inside of exterior wall with a high R-Value insulation is the least expensive way to create an energy efficient building envelope. The R-Value represents how resistant a material is to heat flow.  A higher R-Value means the insulation is more efficient. Insulation comes in several different forms. For new construction, standard fiberglass insulation is typically used. Older construction sites may have exterior walls that are hard to reach. A foam spray insulation may be a better option here. The 2012 IECC for commercial buildings has become stricter when constructing new building envelopes. Typically, your architect will know the rules for designing new construction.

Ensure that insulation is installed properly. This means eliminating convection paths (gaps), particularly around windows, door jams, and wiring. Use the appropriate type of insulation for the job and ensure quality workmanship.

Roofs

Roofs are a major source of heat loss. As we know, heat rises. If your roof is not properly insulated, the rising heat will easily escape out of your roof. If your building has unused and non-insulated attic space, it will be easy to insulate and, according to the DOE, can save up to 40% of your energy costs!

The U.S. Department of Energy also has good resources for energy conservation and insulation available here.

Windows

Windows are the main source of heat loss in the building envelope. Choosing the correct window for the building is determined by climate, building orientation, retrofitting (if old construction), and cost. Most older buildings have single pane windows. These are the least efficient type of windows as they have only one thermal barrier and a high U-Factor. The U-Factor is the standard measurement for a windows resistance to heat flow. It is same concept as the R-Value only inversed. The lower the U-Factor, the greater the resistance to heat flow.  Doubled-Paned, or even Triple-paned windows will have lower U-Factors than single paned. Special gases, such as argon, are used between the panes to slow the thermal flow through the window and lower their U-Factor. High-efficiency windows can be costly, so it is helpful to have an expert find the proper balance between cost and energy savings. ENERGY STAR certifies energy-efficient windows. The ENERGY STAR label will be clear on the product if it is certified. The National Fenestration Rating Council has created a label for windows and doors that displays the U-Factor for that particular model. Look for the sticker at right for the energy performance rating.

More information regarding window efficiency can be found on the following websites:

• ENERGY STAR Windows, Doors, and Skylights
• National Fenestration Rating Council
• Efficient Windows Collaborative

Doors

When it comes to entryways and exit doors, the most important energy saving criteria is the seal and the latch. Over time, the weather stripping on exterior doors tends to deteriorate causing leaks and drafts. Latches can start to wear down as well, causing the doors to incompletely seal when they are closed. These are quick and inexpensive ways to save on heating and cooling costs. For new construction, there are many new exterior doors available with a low U-Factor. Look for the NFRC label when choosing the correct door for your climate.

Dealing with Air Leakage

Air leakage within a building can occur in a variety of areas, all of which lead to higher energy bills as heating and cooling systems must work harder to maintain comfortable temperatures. The three main types of air leakage in a building come from:

• Unbalanced fan systems  – when fans in one part of the building exhaust air at different rates than fans bringing air in
• Wind –  when wind speeds create a pressure differential on the inside and outside of the building, resulting in air flow
• The Chimney Effect –  when warmer temperatures inside the building create lower pressure conditions, resulting in cold outside air flowing in at the base of the building

Each air leakage source causes a difference in air pressure on the inside and outside of the building, resulting in air flow. While air leakage can’t be completely eliminated, ways to reduce air leakage include:

• Sealing doors and windows
• Maintaining seals over time
• Ensuring insulation is properly installed with little to no gaps

Helpful Equations

Heat transmission (loss and gain) through windows, walls, ceilings, floors, etc.:

Q=U x (surface area) x ∆T

• Q: rate of heat flow
• U: U-value (thermal conductivity)
• ∆T: temperature differential

R =1/U

• R: R-value (thermal resistance)