Opaque envelope performance

The opaque envelope is the barrier that helps maintain comfortable indoor conditions regardless of prevailing outdoor conditions. This page describes how envelope design impacts its thermal performance, and the performance of the building.

Envelope performance metrics

U-factor

The U-factor of a wall, also known as the thermal transmittance, is a numerical value that indicates how well the wall conducts heat. In simpler terms, it represents the rate of heat transfer through the wall assembly from one side to the other.

A lower U-factor signifies a better insulated wall. This means the wall resists the flow of heat more effectively.

The units for U-factor are:

IP units: Btu / (hr * ft2 * °F)         

SI units: W / (m2 * °C)

R-value

The R-value of a wall is also called its thermal resistance. It is a numerical value that indicates a wall's resistance to conducting heat. It is the inverse value of the U-factor. A higher R-value represents a better insulated wall. The R-value is often printed on insulating products.

The units for R-value are:

IP units: (hr * ft2 * °F) / Btu        

SI units: (m2 * °C) / W

Note that insulation behaves differently at different temperatures. As the delta T on either side increases, thermal resistance decreases; as delta T decreases, thermal resistance increases. This phenomenon is captured by simulation engines.

Specific heat

This property represents the amount of heat energy required to raise the temperature of a unit mass of a material by one degree. Materials with higher specific heat require more heat to experience a temperature change. Materials with lower specific heat require less heat for the same temperature increase.

Materials with high specific can act as thermal mass, absorbing and releasing heat slowly, helping to regulate indoor temperature fluctuations. This is beneficial in spaces where maintaining a stable temperature is important. (e.g., concrete walls in buildings)

Calculating overall U-factor and R-value for a construction assembly

Calculation of overall r-value for a wall assembly with uniform layers

Most walls, roofs, and floors consist of multiple layers of materials—these multi-layered surfaces are referred to as construction assemblies. For simple assemblies with uniform layers, an overall R-value can be calculated by summing the individual R-values of each layer. However, for more complex assemblies where thermal bridging may occur, additional calculations must be performed.

Sources for material thermal property data

Sources for material thermal properties include:

• ASHRAE Handbook of Fundamentals, Chapter 26 "Heat, Air, and Moisture Control in Building Assemblies—Material Properties"
• R-values of Insulation and Other Building Materials, www.archtoolbox.com

Note that published R-values are often in terms of R-value per unit of material thickness (e.g. R-value per inch). The actual r-value for a layer is determined by multiplying its thickness by this published value. Sometimes, insulation r-values are published for a specific product and may be the r-value for the total thickness, so always be sure to read the published values carefully.

Simple, uniform-layered constructions

For simple assemblies with uniform layers, an overall R-value can be calculated by summing the individual R-values of each layer. Be sure to include air-film resistance as discussed below.

Constructions with insulation between framing

Many wall and roof constructions include structural framing members with insulation between the frames. This framing layer is often called a "composite layer" because it is represented as a single layer but consists of multiple materials. It is NOT appropriate to represent this layer with just the insulation R-value because the structural members act as a thermal bridge allowing some heat to flow "around" the insulation, thus lowering the R-value of the composite layer compared to the insulation. Wood framing may reduce the R-value (compared to the insulation filled between frames) by about 15% whereas metal framing can reduce it by over 60%!

In order to model a composite layer in a simulation tool, the overall u-factor or r-value for the composite layer must be calculated before entering it into the simulation too. Some simulation tools may have built-in calculators to specify framing size, spacing, and insulation levels.

A commonly-used calculation approach is called the "parallel path" method. However, this does not account for 3-dimensional heat transfer mechanisms so it is a simplification, and not advisable to use especially for steel frame constructions. A better approach is to use a heat transfer calculation tool to model 2-D or 3-D heat transfer effects to calculate a more accurate composite layer R-value. Some software tools include a library of composite material layers with thermal properties calculated using advanced 3-D heat transfer approaches.

Accounting for air film resistance

In addition to the layers of construction materials, some additional resistance to heat transfer is due to air movement at the surface of the construction. As heat is transferred between the outer layers of the construction and the air, the air will move due to convection, and create a thin film of stagnant air that results in a small amount of resistance. This will impact the overall U-factor. Note that in most energy simulation software tools, the air film resistance is calculated automatically, so the air film layers do not need to be manually input by the energy modeler.

Additional thermal bridging considerations

Differences in heat transfer through framing members and the insulation between frames is one of the most important thermal bridging concepts to represent in the model, however there are other thermal bridges that must also be accounted for. Read more at: Thermal bridging - modeling approach

Impact on building performance

The opaque envelope has a large impact on the performance of many building types. Better envelope design can reduce heating and cooling loads and in turn, heating and cooling energy consumption. Additionally, better envelope design leas to improved occupant comfort.

Buildings with relatively low internal gains, or with a large surface area to volume ratio are sometimes called "envelope-load dominant designs" in that a large portion of the building's loads are due to heat transfer through the envelope. Buildings that have very large internal gains (e.g. a data center, or equipment-intense laboratory) are sometimes called "internal-gain dominant designs" — these types of buildings may not benefit as much by improvements to the envelope, however it is always valuable to perform analysis to determine the optimal envelope design.

Energy code requirements

Most energy codes require minimum levels of opaque envelope performance. This is often specified as a maximum allowable U-factor, or a minimum allowable R-value.