Insulating Attics for Performance

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Increasing attic insulation, typically one of the most cost-effective energy-effiency measures you can take, can reduce heating and cooling bills and improve homeowner comfort.

January 01, 2004

Just as a hat keeps you warm in winter, attic insulation, a home's hat, improves the building envelope's thermal performance and boosts the home's energy efficiency. Increasing attic insulation, typically one of the most cost-effective energy-efficiency measures you can take, can reduce heating and cooling bills and improve homeowner comfort.

Most attics start with some insulation deficiency. Or a service worker will move insulation to service another system and forget to replace it. Even if a remodeling project's plans do not include attic insulation, get up there and inspect it.

This article details a strategy that considers the appropriate level of insulation, air sealing and ventilation to help ensure that the attic does its part in bundling up the home.

How much do you need?

Choose the insulation type based on the attic's design and framing complexity, the climate zone and the type of heating and cooling system. Contractors often use loose-fill and batt insulation for uniformity and to insulate hard-to-reach attic spaces.

Local building codes often specify the absolute minimum level of insulation, which might not optimize energy efficiency and occupant comfort. To find the level recommended for your area, check the Department of Energy's R-value map. You also can determine the optimal insulation for your needs by using the DOE and Oak Ridge National Laboratory's interactive, ZIP code-specific insulation tool.

 

The depth of this insulation cannot be determined.
Gauge tapes provide a quick visual reference for insulation depth.

Most U.S. attics should be insulated from R-22 to R-49, depending on location and type of heating fuel. To determine the current R-value, measure the insulation's depth (photo 1) or, with faced batts, look for a label. If the value is less than R-22 (7 inches of blown fiberglass or rock wool or 6 inches of blown cellulose), you probably should add insulation.

When you blow in loose-fill insulation, ensure that the depth is continuous at its specified level for the entire space. Depth gauges help because once you cover the ceiling joists or the bottom chords of roof trusses, you cannot determine insulation depth. Use a gauge tape for quick visual depth reference (photo 2). If not provided by the insulation installer or manufacturer, create gauge tapes from scrap plywood or oriented strand board. Space them equally along the length of a truss and repeat every fourth or fifth truss.

Keep insulation continuous, seal air leaks

No matter which type of insulation you use, its thermal performance will be only as good as the installation. To control conductive heat transfer, an attic should be insulated continuously without gaps. Air infiltration from the interior can sabotage the thermal benefits of an otherwise good installation, but controlling air leakage can be tricky. Structural complexity and the many mechanical vents, ducting, lighting and electrical elements penetrating the ceiling plane introduce leaks. Take the following key steps to ensure you do it right.

 

 

 

Low levels of insulation and an insulation void at the back, where there are non-IC-rated canned lights, make this installation a no-no.
This shows uniform insulation, continuous to the edge, with ventilation baffles.

 

 

 

  • Assess and correct air-leakage paths from the conditioned space into the attic. Before insulating, do a visual inspection to detect open chaseways and other unsealed penetrations. This requires shifting the existing insulation to see the attic floor and locate all interior walls, soffits, dropped ceilings and potential chaseways. Wear appropriate protection. (For details on working with synthetic mineral and synthetic vitreous fibers.

    A blower door can help you identify air-leakage paths and ensure that you don't air-seal too much. ASHRAE Standard 62.2 offers minimum ventilation guidelines for residential structures. If you're unfamiliar with them, hire an experienced air-sealing crew who can test with a blower door. Seal all penetrations, such as top plates, pipe and wire chases, soffit spaces and chases.

     

  • Address heat-producing sources in the attic. When air-sealing and insulating near heat-producing equipment such as exhaust fans, combustion venting, chimneys, knob-and-tube wiring, and canned lights not rated for insulation contact, use appropriate materials and clearance. For example, noncombustible blocking such as metal flashing maintains the required clearance between the insulation and the equipment (usually 3 inches, but check local requirements). When air-sealing around these devices, use appropriately rated materials such as high-temperature caulk. (Check out http://www.eere.energy.gov/buildings/documents/pdfs/26450.pdf.)
  • Frame around the attic access with plywood to maintain full thickness up to the hatch or ladder edge, and build an insulated closure for the attic hatch.
  • Replace lighting fixtures that are not rated for insulation contact with ICrated airtight fixtures (photo 3). Follow the manufacturer's instructions.
  • In tray ceilings, where the center ceiling is flat but raised from the room's perimeter, frame up the sides with plywood and insulate the sides to maintain insulation depth for the full tray width.
  • In a storage attic, build a raised storage platform to allow for at least 75% of the insulation's full thickness. Build it as an open deck, with open sides and a slatted platform to allow the natural movement of moisture from the rest of the house to the attic. A raised deck supports stored items without crushing insulation.

Ventilation

Most building codes require roof vents to expel moisture that could damage building materials. Researchers are investigating whether attic ventilation helps in all climates, but you probably work on vented attics, so consider these guidelines:

 

 

  • Don't vent interior exhaust fans into the attic or soffit, as moisture would be dumped into the attic and could condense on cold places and cause problems. All ventilation fans should be ducted, with the shortest equivalent duct length, to the outside rather than into attic spaces.
  • To insulate to the edge of the attic without compromising ventilation, install ventilation baffles to the roof deck first. Ventilation baffles let you install the recommended level of insulation to the attic's edge without blocking the soffit vents (photo 4).

Jennifer Kent is communications manager for IBACOS Inc., a research and consulting firm specializing in building science. Ananda Hartzell is the firm's existing buildings research project manager.

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