The more effective the building envelope--the walls, roof, windows and foundation of the house--the more energy efficient and comfortable the house will be. Tight construction and high performance windows reduce heat loss during the winter months, block heat gain in summer, and result in significant energy savings.

Other benefits include improved indoor air quality (by keeping out pests, dust, radon and outdoor air pollutants), reduction of outside noise, and fewer condensation problems, which can lead to mold and mildew. Careful attention to the building envelope during construction will also help eliminate the need for difficult and costly improvements in the future and can increase the resale value of the home.

CONTENTS Insulation
Appropriate levels of insulation, properly installed, are key to preventing the flow of heat and air into or out of the home. The U.S. Department of Energy (DOE) provides detailed guidance on proper insulation and air sealing techniques as well as moisture control and ventilation strategies to improve the effectiveness of the air sealing and insulation techniques used.

Minimum insulation levels for each component of the building envelope are specified in local building codes and vary from region to region depending on climate conditions. The DOE has developed recommendations for cost-effective levels of insulation for new and existing homes, based on local heating and cooling costs and climate conditions in different parts of the country.

In this house, the walls were constructed with 2"x6" framing (rather than 2"x4" framing) to allow room for thicker wall cavity insulation. Batt-type insulation material was used--R-19 batt insulation in the exterior walls, R-38 batt insulation in the roof, and R-19 batt insulation in the basement walls. With hindsight, the basement slab should have been insulated as well. Moreover, DOE has raised its recommended insulation levels since the house was built.

It is particularly important to make sure, during construction, that no gaps or voids are left in the insulation. This can happen when different trades work after the insulation has been installed.

Sealing air leaks
Taking steps to minimize air leakage is one of the most important features of tight construction. Cracks at joints and seams in the exterior walls, gaps around windows and doors, penetrations for plumbing, wiring, and ducts--all allow air to move through the building envelope. Methods used to reduce air infiltration in this house included:

  • taping all joints and seams;
  • careful attention to sealing ducts and other air leakage paths; and
  • installing special electrical outlet boxes on exterior walls.
Value-engineered framing
Conventional construction practices produce many redundant framing members, resulting in a much higher percentage of wood in the wall cavity than is needed. Value-engineered framing, on the other hand, promotes improved thermal performance, while maintaining structural integrity, as well as more efficient use of increasingly scarce wood resources.

Attention to the optimal use of wood both during the design phase (by laying out roof trusses over wall studs, for example) and during construction (by avoiding extraneous framing) reduces the amount of lumber used and maximizes the wall cavity space available for insulation. Since wood gains and loses heat more quickly than insulation, replacing unnecessary wood with insulation increases the overall R-value and integrity of the building envelope.

Value-engineered framing techniques used in this house included:

  • increasing the spacing between framing members by using 2"x6" (rather than 2"x4") framing;
  • aligning wall framing with joists;
  • aligning door and window openings with stud spacing, where possible; and
  • eliminating unnecessary framing at corners and intersections--and the pockets of uninsulated space that result (a technique known as "California corner" framing).

An important concern in tight construction is proper ventilation. When a house is tightly sealed, an active ventilation system is needed to bring in fresh air, exhaust stale air, and allow sufficient air flow to operate the range hood and bathroom vents.

In this house, a Carrier energy recovery ventilator (ERV) fulfills this function. It is a form of air-to-air heat exchanger: in winter, outgoing heated stale air preheats the incoming fresh air; in summer, the incoming fresh air is pre-conditioned by the outgoing cool air. The ERV is also able to remove excess humidity or add humidity to the ventilating air brought into the house. It works automatically to provide a preset number of air changes per day--eight air changes/day, in the case of this house--or it can be manually controlled.

High performance windows
Windows play a big role in a home's energy efficiency. They can be a major source of heat loss in the winter and heat gain in the summer, accounting for the loss of as much as 30 percent of the typical home's heat and air conditioning energy. Recent technological advances, however, have significantly improved the energy performance of windows.

There are a number of window energy properties to consider in choosing new windows:

  • U-Factor. Windows lose--and gain--heat by conduction (the direct transfer of heat through the glass and frame), air leakage through and around the window, and radiation (the movement of heat as infrared energy through the glass). The rate of heat transfer is expressed in terms of U-values and U-factors. The smaller the U-factor, the better the thermal performance of the window.

    (U-values are the mathematical inverse of R-values. R-value is a measure of conductivity and indicates a product's resistance to heat loss. The higher the R-value, the more energy efficient the product. U-factor is a measure of the rate of heat transfer through a product and takes into account air flow and radiant-heat loss/gain as well as conductivity.)

  • Low-Emissivity (Low-E). When the sun's energy strikes a window, visible light, heat and ultra-violet (UV) radiation are either reflected, absorbed or transmitted into the building. Low-E glass coatings are applied inside the air space of a double-paned (or triple-paned) window and reduce the heat flow between the panes of glass. They admit visible light while blocking heat from entering the home during summer and preventing heat from escaping through the glass in winter.

    Combining low-E coatings with low-conductance gas fillings--such as argon or krypton--boosts a window's energy efficiency still further. Originally, the space between the panes of glass was filled with air prior to sealing. Filling that space with a less conductive, slower-moving gas minimizes the convection currents within the space and reduces the transfer of heat between the panes of glass.

  • Solar Heat Gain Coefficient (SHGC). The SHGC is a measure of the solar energy that passes through a window. It is expressed as a number between 0 and 1. The lower a window's SHGC, the better it is at blocking unwanted heat gain.

    Regions vary, however, in the amount of solar heat gain that may be desirable, and a variety of low-E coatings have been designed to accommodate these differences. In regions with cold winters and cool summers, for example, a high-solar-gain, low-E coating will reduce heat loss but admit solar gain, helping to keep the house warm during the heating months. In regions with hot summers and mild winters, a low-solar-gain, low-E coating will reduce heat loss in both winter and summer, helping to keep the house comfortable during the cooling months.

The National Fenestration Rating Council (NFRC) label provides information about the energy performance of windows, doors, and skylights. In addition to reporting a product's U-Factor and Solar Heat Gain Coefficient, it rates the product on the amount of light admitted (Visible Transmittance), the amount of air that passes through (Air Leakage), and the ability of the product to resist the formation of condensation (Condensation Resistance).

All ENERGY STAR-qualified windows, doors and skylights are independently tested and certified according to NFRC procedures. The ENERGY STAR label helps consumers select products best suited for their climate zone.


For this house, superior energy performance was a top priority since windows comprise so much of the exterior wall area to take advantage of the lake views. Triple glazed, low-E, argon-filled Loewen windows and glass doors with wood frames were selected. Thermal bridges are minimal, and weatherstripping is high quality.

The center-of-glass R-value of the windows is 7.9, and the solar heat gain coefficient is 0.34. The rated performance, combined with the conscientious use of interior shades, results in a highly efficient window system.

Energy-efficiency evaluation
When the house was completed (2003), its energy efficiency was evaluated by Informed Energy Decisions, a certified "home energy rater," to measure its performance against the Home Energy Rating System (HERS) standard. The house met the requirements for ENERGY STAR certification: a HERS score of at least 86 on the pre-2006 scale then in effect, verifying that the house was at least 30 percent more energy-efficient than a standard built-to-code home. EPA has since strengthened the ENERGY STAR requirements for new homes to ensure that the label continues to identify homes that are significantly more efficient than non-labeled standard construction homes.

The HERS evaluation for this house involved an analysis of the construction plans and an on-site inspection that included a blower door test and a duct test to determine how tight the house was and where any infiltration might be occuring. For the blower door test, a fan was temporarily installed in a door opening using an adjustable frame, and the house was depressurized, replicating the effect of a roughly 20-mph wind blowing on all sides of the house. Any leaks (air infiltration) were measured as the air moved through the fan and out the door.

Using the volume of the home, the number of air changes per hour was calculated. This number, along with the other thermal characteristics of the house, was used to determine the energy rating of the home compared to a computer-simulated reference house of identical size and shape meeting minumum requirements of the 1993 Model Energy Code. The reference house was assigned a score of 80, and each point above (or below) that score represented a 5 percent reduction (or increase) in energy usage compared to the reference house. In 2003, an ENERGY STAR-qualified new home needed to achieve a HERS score of at least 86, signaling that it was 30 percent more energy-efficient than a house built to the 1993 Model Energy Code.

Based on the audit performed by Informed Energy Decisions, this house achieved a HERS score of 92.7 and earned a "5 Stars Plus" ENERGY STAR label. Points were lost because the boilers are larger than needed and the basement slab is not insulated.

[When the audit was conducted, the higher a home's HERS Score, the more efficient the home. In 2006, the HERS Index replaced the HERS Score. With this rating system, the lower the value the better. A HERS Index of 100 represents the energy use of a standard new home, while a net zero energy home has a HERS Index of 0.]

Links and Resources

  • Home Energy Saver is a web-based do-it-yourself energy audit tool designed to help consumers identify the best ways to save energy in their homes and find the resources to make the savings happen. The project is sponsored by the U.S. Department of Energy (DOE) and the U.S. Environmental Protection Agency (EPA), as part of the national ENERGY STAR program for improving energy efficiency in homes.

  • Detailed information about the technologies and building practices that contribute to improved energy efficiency can be found on the ENERGY STAR website. Topics covered include tight construction, tight ducts, improved insulation, high performance windows, and energy-efficient heating and cooling equipment. Information about current federal tax credits for energy efficiency is also provided.

  • The Efficient Windows Collaborative is a useful source of information about the benefits of energy-efficient windows, descriptions of how they work, and recommendations for their selection and use.

  • The Residential Energy Services Network works to ensure the success of the building energy performance certification industry. Consumers can find a certified rater in their state through the RESNET Certified Rater Directory.

Website prepared by Eleanor Revelle.
Last revised: February 2014