Understanding Building Envelope Performance: Key Metrics for Energy Efficiency and Comfort

The building envelope (walls, windows and roof) doesn't use energy, but it has a significant impact on efficiency and comfort. However, it can be difficult to measure its performance. By understanding metrics and ratings associated with building envelope components, you can evaluate envelope efficiency and take steps to improve energy performance.

Wall and roof insulation

Insulation is critical to optimizing heating and cooling system efficiency and occupant comfort. R-value is the measure of thermal performance; the higher the R-value, the greater the insulating effectiveness.

The R-value depends on the type of insulation and includes its material, thickness and density. Standards such as ASHRAE 90.1 Energy Standard for Buildings Except Low-Rise Residential Buildings and the International Energy Conservation Code (IECC) provide insulation recommendations based on building type and climate.

Windows and doors

Performance measures include thermal loss, heat gain, visibility and moisture protection:

• U-factor indicates heat loss. The lower the number, the better the window is at keeping heat in. U-factor includes not only the glass but also the entire window assembly.
• Solar heat gain coefficient (SHGC) measures how well a window blocks solar radiation. SHGC is a number between 0 and 1; the lower the value, the more efficient the window is at blocking heat.
• Visible transmittance (VT) measures how much light comes through a product. The higher the VT (between 0 and 1), the greater the potential for daylighting.
• Air leakage (AL) quantifies how much outside air comes in through a door or window. AL rates typically fall between 0.1 and 0.3. The lower the number, the better a product is at keeping air out.

The National Fenestration Ratings Council oversees performance ratings and certifications. ASHRAE 90.1 and the IECC provide minimum requirements for U-factor and SHGC.

Roofing materials

Roofing materials have a major impact on heating and cooling costs. Important metrics include:

• Reflectance measures the ability of a roof's surface to reflect sunlight on a scale of 0 to 1. The higher the value, the more efficient the product is at reflecting sunlight and heat away, reducing roof temperature.
• Emittance indicates how well materials release absorbed heat, expressed as a number between 0 and 1. On warm, sunny days, highly emissive roofs reduce cooling load by freeing heat absorbed from the sun.

A cool roof has a high solar reflectance and thermal emittance. Cool roofs are generally lighter in color and deliver substantial savings in lower cooling costs—especially in warmer climates. ASHRAE 90.1 defines a cool roof as having a minimum solar reflectance of 0.70 and a thermal emittance of at least 0.75. The Cool Roof Ratings Council certifies cool roof products.

Looking inside out

Armed with all this new knowledge, you'll be better prepared to measure and evaluate building envelope performance, opening up all kinds of opportunities for saving money and improving comfort. These metrics are important in meeting building energy codes and achieving green building ratings, such as LEED and ENERGY STAR.

We are here to help you navigate energy efficiency upgrades, and our Greenback Rebate program could help offset the cost of an upcoming or existing project. Connect with your TXU Energy sales executive or email us at business@txu.com to learn more.