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Building Envelope

The Building Envelope - Walls, Attic, Basement, Doors and Windows


This is a consumer page on the building envelope. For more resources, visit the building envelope topic page.


Before buying a new heating or cooling system, it makes sense to tighten up your house first. The following steps will reduce your heating and cooling load, improve your comfort, and maybe even allow you to purchase a smaller—and less expensive—furnace or air conditioner when the time comes to replace your existing system.

Step 1: Consider an Energy Audit
Step 2: Find and Seal Air Leaks
Step 3: Insulate
Step 4: Upgrade Inefficient Windows & Doors
Step 5: Replace Old Windows

*If you are replacing your roof, find out how to reduce energy with "cool" roofing materials

 

Consider an Energy Audit

 

Find out from a pro where heat is being lost through your home's shell and what you should do about it. Energy auditors use sophisticated equipment, like a blower door and infrared camera, to help pinpoint air leaks and areas with inadequate insulation. Depending on the service, you may be able to have your heating or cooling system cleaned, tuned up, and tested at the same time. They will often perform air sealing work as they go, and can usually connect you with qualified contractors to complete other major work. Some utility companies provide basic energy audits free of charge.

Finding a contractor who can perform high-quality energy auditing services can be difficult, and they may be listed under a variety of names, such as home performance contractor, energy rater, or energy doctor. Some key resources to help you get started are listed below. Also contact your state energy office or cooperative extension service for information on qualified auditors and contractors in your area. To learn more about contractor services, visit Home Energy Tune-Up.

  • Building Performance Institute. Some of the most reliable contractors are certified by the Building Performance Institute. Find certified contractors and learn about their services on their site.
  • RESNET. One of the largest lists of certified energy auditors is maintained by RESNET, which offers home energy ratings. A home energy rating entails the same kind of on-site diagnostic tests that an energy auditor would do; but with a rating, your house will be given a point score between 1 and 100 that compares your house to others. For existing houses, this score acts as a tool to evaluate and pinpoint the most cost-effective improvements. Find a rater that is certified by RESNET.
  • Weatherization. Depending on your income, you may qualify for a free energy audit and energy efficiency improvements to your home through the Weatherization Assistance Program. To see if you qualify, contact your local weatherization agency at this link and click on “How Do I Apply for Weatherization.”

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Find and Seal Air Leaks

 

Hidden air leaks cause some of the largest heat losses in older homes. Common air leakage sites include:

  • Plumbing penetrations through insulated floors and ceilings
  • Chimney penetrations through insulated ceilings and exterior walls
  • Fireplace dampers
  • Attic access hatches
  • Recessed lights and fans in insulated ceilings
  • Wiring penetrations through insulated floors, ceilings, and walls
  • Missing plaster
  • Electrical outlets and switches, especially on exterior walls
  • Window, door, and baseboard moldings
  • Dropped ceilings above bathtubs and cabinets

The best material for sealing these hidden air leaks depends on the size of the gaps and where they are located. Caulk is best for cracks and gaps less than about 1/4" wide.

Expanding foam sealant is an excellent material to use for sealing larger cracks and holes that are protected from sunlight and moisture. Today's products are safe for atmospheric ozone. Backer rod or crack filler is a flexible foam material, usually round in cross-section (1/4" to 1" in diameter), and sold in long coils. Use it for sealing large cracks and to provide a backing in very deep cracks that are to be sealed with caulk.

Use rigid foam insulation for sealing very large openings such as plumbing chases and attic hatch covers. Fiberglass insulation can also be used for sealing large holes, but it will work better if wrapped in plastic or stuffed in plastic bags, because air can leak through exposed fiberglass. Don't use plastic in places where high temperatures may be reached, and always wear gloves and a dust mask when working with fiberglass. Specialized materials such as metal flashing and high-temperature silicone sealants may be required for sealing around chimneys and flue pipes. Check with your building inspector or fire marshal if unsure about fire-safe details in these locations.

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Insulate

 

Insulation is your primary defense against heat loss through the house envelope. However, putting insulation into a house after it is built can be pretty difficult. If there isn't any insulation, the best option is to bring in an insulation contractor to blow cellulose or fiberglass into the walls.

  • Attic. Adding insulation to an unheated attic is easier than insulating existing walls and is likely to have a greater impact on comfort and energy use. If there is no floor in the attic, simply add more insulation between the ceiling beams, either loose fill or unfaced fiberglass batts. In most of the country, a full foot of fiberglass or cellulose insulation is cost-effective in the attic floor. However, it is critical to install fiberglass batts properly in order for them to do the job. If you do it yourself, make sure you read up on correct installation practices.
  • Basement. Materials that could be damaged by moisture, such as fiberglass batts and cellulose, should almost never be used to insulate a basement. Interior vapor barriers can also be very damaging because they prevent basements from drying to the inside. Interior basement insulation should start with rigid foam installed against the basement walls. If you are considering finishing your basement and using it as a living space, seek the advice of an experienced professional.

    If you have a crawl space, it should be sealed, not ventilated. To do this, use 6-mm thick polyethylene sheeting as a moisture barrier to cover the ground and seal tightly to walls and columns. Then use rigid foam to insulate the foundation walls. In the South, it is important to keep an uninsulated band for inspection of possible termite tunnels.

For more information about insulation, or to locate a contractor, dealer, or manufacturer please visit the insulation trade association links below:

Association

Acronym

Website

Air Barrier Association of America

ABAA

www.airbarrier.org

Blow-In-Blanket Contractors Association

BIBCA

www.bibca.org

Cellulose Insulation Manufacturers Association

CIMA

www.cellulose.org

Extruded Polystyrene Foam Association

XPSA

www.xpsa.com

Insulation Contractors Association of America

ICAA

www.insulate.org

National Insulation Association

NIA

www.insulation.org

North American Insulation Manufacturers Association

NAIMA

www.naima.org

Spray Polyurethane Foam Alliance

SPFA

www.sprayfoam.org

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Upgrade Inefficient Windows and Doors

 

About one-quarter of the home's total heat loss usually occurs through windows and doors. If your existing windows have rotted or damaged wood, cracked glass, missing putty, poorly fitting sashes, or locks that don't work, you may be better off replacing them.

If your windows are generally in good shape, it will probably be more cost-effective to boost their efficiency with inexpensive products purchased from your local building supply or hardware store.

1. The quickest and cheapest option is to seal all window edges and cracks with rope caulk.

2. Another solution is to weatherstrip windows and doors with a special lining that is inserted between the window and the frame. For doors, weatherstrip around the whole perimeter to ensure a tight seal when closed. Install quality door sweeps on the bottom of the doors if they aren't already in place.

3. Install storm windows. Removable or operable storm windows with glass or rigid acrylic panes generally make more sense if you plan to stay in the house for more than a few years. Otherwise, there are storm window kits that consist of a plastic film taped inside the window frame; these kits cost just $3-$8 per window.

4. Finally, prevent radiative heat gain and loss in the summer and winter by installing insulating curtains or drapes on the interior.

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Replace Old Windows

 

Whether replacing windows in an older house or choosing windows for a new house, your decisions on what type of windows to buy will be among the most important decisions you will make in terms of energy use. Because of the impact windows have on both heat loss and heat gain, proper selection of products can be confusing. To get the most up-to-date information on what to look for in energy-efficient windows for your home, visit the Efficient Windows Collaborative.

Basic Features to Look for in a New Window:

  • Reliability and good installation. Choose windows with good warranties against the loss of the air seal, and be sure to have experienced contractors install your high-tech windows. Consumers should recognize that the manufacturer’s quality control at the factory and care during shipping can have a big impact on the window’s air tightness at a site.
  • Certified by ENERGY STAR. Windows, doors, and skylights qualifying for the ENERGY STAR label must meet requirements tailored for the country’s four broad climate regions: northern, north-central, southcentral, and southern. ENERGY STAR windows must carry the NFRC label (discussed below).
  • Proper dimensions. To maximize energy performance, choose windows with larger unbroken glazing areas instead of multi-pane or true-divided-light windows. Applied grills that simulate true-divided-light windows are fine; they do not reduce energy efficiency.
  • Efficient frame material and sash construction. Wood is still the most common material in use, and it insulates reasonably well. Unless a thermal break is incorporated into the design, aluminum frames conduct heat very rapidly and are therefore inefficient. Vinyl (PVC) windows, or vinyl frames insulated with fiberglass, are the most efficient and tend to insulate better than wood. "Pultruded" fiberglass frames and wood-polymer composite frames are other options becoming more common in high-efficiency window products. They insulate well and their cost generally falls between that of wood and vinyl.
  • Air-tightness. Examine air leakage specifications carefully when selecting windows, but in general, casement and awning windows are tighter than double-hung and other sliding windows.
  • Glazing with low emissivity. Low-e coatings made of a thin, transparent layer of silver or tin oxide are used on high-performance windows to reduce the solar heat gain without reducing visibility as much as older tinted glass. The variety and placement of the low-e coating on the window varies for different climate zones and applications. ENERGY STAR offers purchasing tips to help you find which windows are appropriate for you.
  • Multiple layers of glazing. Double glazing insulates almost twice as well as single glazing. Adding a third or fourth layer of glazing results in further improvement. Some of these windows use glass only; others use thin plastic films as the inner glazing layer(s).
  • The right thickness of air space. With double-glazed windows the air space between the panes of glass has a big effect on energy performance.The wider the space, the less heat can be conducted through that space (as long as it stays under about 1").
  • Low-conductivity gas fill. By substituting a denser, lower conductivity gas such as argon for the air in a sealed insulated glass window, heat loss can be reduced significantly.
  • Proper edge spacers. The edge spacer is what holds the panes of glass apart and provides the airtight seal in an insulated glass window. Avoid traditional hollow aluminum spacers because they have extremely high conductivity. Instead, choose edge spacers that are thin-walled steel, silicone foam or butyl rubber. With new edge spacers, however, pay particular attention to warranties against seal failure.

Selecting New Windows for Your Home

Windows, doors, and skylights qualifying for the ENERGY STAR label must meet requirements tailored for the country’s four broad climate regions: northern, north-central, southcentral, and southern. ENERGY STAR windows must carry the National Fenestration Rating Council (NFRC) label, allowing comparisons of ENERGY STAR-qualified products on specific performance characteristics.

  • U-value. U-factor measures how well a product prevents heat from escaping. It is the inverse of R-value, which is familiar to many people as a measure of insulation thermal performance. The lower the U-value rating, the better the overall insulating value of the window. Typical U-values range from 0.20 to 1.20. The U-factor ratings listed on NFRC labels (and in the NFRC Certified Products Directory) take into account heat loss through the glass, window edge, and window frame.

  • Solar heat gain coefficient (SHGC). The SHGC describes how much solar energy is transmitted through a window. Solar heat gain can be beneficial—providing free passive solar heat during the winter months — or it can be a problem, resulting in overheating during the summer. Windows with high coefficients (above 0.60) are designed for colder climates, while windows with low coefficients are designed for hotter climates.
  • Visible light transmittance. While SHGC describes the relative amount of solar energy that can pass through a window, the visible light transmittance is simply the relative amount of sunlight that can pass through, measured on a scale between 0 and 1. The higher the number, the greater the amount of light that can pass through.
  • Air leakage. Air leakage is already listed by many window manufacturers, in terms of cubic feet of air per minute per foot of crack, with typical values ranging from 0.1 to 0.3. An optional air leakage value is included on NFRC labels and in the NFRC Certified Products Directory.
  • Condensation resistance. Finally, the ability of a window to resist the formation of condensation on the interior surface is very important in evaluating the relative durability of a window. The NFRC measures condensation resistance on a 0–100 scale. The higher the rating, the better that product is at resisting condensation formation. This rating is optional for new products, and it can not predict actual condensation.

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Page last updated December 2012