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What is the Thermal Envelope & How Does It Affect Energy Efficiency?

If you are in the process of designing or building an energy efficient home, you may have heard the term “thermal envelope” used again and again. But what exactly does this phrase mean, and how does it affect the energy efficiency of your house? 

Simply put, a “thermal envelope” is anything and everything about the house that serves to shield the interior from the outside environment. This may include wall and roof assemblies, insulation, energy-efficient windows, doors, finishes, weather-stripping and air/vapor retarders.

Maximizing a home’s energy efficiency means creating a tightly sealed thermal envelope. If you have ever been inside a building that feels drafty, or a building that leaks when it rains or snows, then you have experienced the effects of a poor thermal envelope.

Below, we will explain the various components of creating a tightly sealed thermal envelope.

1. Wall and Roof Assemblies

There are several alternatives to the conventional “stick” (wood stud) framed wall and roof construction that can help to strengthen the thermal envelope. These include:

  • Optimum Value Engineering (OVE): A method that uses the minimum required amount of wood, thus reducing construction costs and saving space for insulation. However, there is very little room for errors, requiring workmanship to be excellent.
  • Structural Insulated Panels (SIP): Plywood or oriented strand board (OSB) sheets are laminated to a core of foamboard, which may be 4 – 8 inches thick. Since the SIP acts as both the framing and the insulation, construction is much faster than OVE or “stick-framing.” The quality of construction is often superior, since there are fewer places for workers to make mistakes.
  • Insulating Concrete Forms (ICF): These often consist of two layers of extruded foamboard (one inside the house and one outside the house) that act as the form for a steel reinforced concrete center. This is the fastest and least likely technique to have construction mistakes. Such buildings are also very strong and easily exceed code requirements for tornado or hurricane prone areas.

2. Insulation

One thing that all energy-efficient homes have in common is insulation with a much higher R-value (the measurement of how well a material resists heat flow from one side to the other) than is required by most local building codes.

For example, a typical house in New York might have haphazardly installed R-11 fiberglass insulation in the exterior walls and R-19 in the ceiling, while the floors and foundation walls may not be insulated at all.

A similar, better-designed and constructed home’s insulation levels would be in the range of R-20 to R-30 in the walls (including the foundation) and R-50 and R-70 in the ceilings.

3. Air/Vapor Retarders

In cold climates, pressure differences can drive warm, moist indoor air into exterior walls and attics, which condenses as it cools. In very warm climates, the reverse is true: as humid outdoor air enters the walls to find cooler wall cavities, it condenses into liquid water.

In any climate, water vapor condensation is a major threat to the structure of a house. It is important to minimize water vapor migration by using a carefully designed thermal envelope and sound construction practices.

Any water vapor that does manage to get into the walls or attics must be allowed to get out again. Some construction methods and climates lend themselves to allowing the vapor to flow towards the outdoors. Others are better suited to letting it flow towards the interior so that the house ventilation system can process it.

4. Foundations and Slabs

Foundation walls and slabs should be at least as well insulated as the living space walls. Uninsulated foundations have a negative impact on home energy use and comfort, especially if the family uses the lower parts of the house as a living space.

Additionally, because many appliances that supply heat as a by-product (such as hot water heaters, washers and dryers) are often located in basements, these appliances can assist in the heating of the house if the foundation walls and floors are properly insulated.

5. Windows

Did you know the typical home loses over 25% of its heat through its windows? Determining window placement and design will depend largely on the local climate:

  • Warm climates: Homes should have a minimal number of windows on the north, east and west exposures. A rule-of-thumb is that window area should not exceed 8-9% of the floor area, unless your designer is experienced in passive solar techniques. If this is the case, then increasing window area on the southern side of the house to about 12% of the floor area is recommended.
  • Cool climates: Select east, west and south facing windows with low solar heat gain coefficients (these block solar heat gain). A properly designed roof overhang for south-facing windows is important to avoid overheating in the summer. In general, the best sealing windows are awning and casement styles since these often close tighter than sliding types. Metal window frames should be avoided.

Review Energy Star regional climatic guidelines for windows for more information.

6. Air-Sealing

A well-constructed thermal envelope requires that insulating and sealing be extremely precise. Sealing air leaks throughout the thermal envelope reduces energy loss significantly.

Good air-sealing alone may reduce utility costs by as much as 50% when compared to other houses of the same type and age. Homes built in this way are so energy-efficient that specifying the correct sizing heating/ cooling system can be tricky. Rules-of-thumb system sizing is often inaccurate, resulting in oversizing and wasteful operation.

Understanding the thermal envelope and how it affects your home’s energy efficiency is an important step to making the right design decisions. Unique Home Solutions has been designing energy efficient houses since 1983 – contact us today for more information and a free estimate for your home!

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