CEILINGS AND ATTICS

Install Insulation and Provide Ventilation

BENEFITS OF CEILING INSULATION

Insulating ceilings is one of the most

cost-effective energy efficiency measures. In

addition to reducing heat loss in the winter and

heat gains in the summer, ceiling insulation

improves comfort by bringing ceiling temperatures closer to room temperatures and providing an even temperature distribution throughout the house.

When planning and managing ceiling insulation projects, make sure

• Ceiling is properly sealed

• Correct insulation levels are selected

• Insulation is properly installed

• Insulation coverage is continuous and

complete

• Attic ventilation is maintained

Attic floors over flat ceilings are often the easiest

part of an exterior building envelope to insulate.

They are usually accessible and have ample

room for insulation. However, many homes use

cathedral ceilings or have attic knee walls that

present unique insulation requirements.

ATTIC VENTILATION

Most building codes require roof vents to expel

moisture that could cause insulation or other

building materials to deteriorate during winter.

In summer, ventilation may reduce roof temperatures, thus lengthening a roof’s life.

However, researchers are investigating whether

attic ventilation is beneficial for all climates. For

years, researchers have believed the cooling

benefits of ventilating a well-insulated attic are

negligible. Some experts also question whether

ventilation effectively removes moisture. Until

the research results are available and accepted,

builders should follow local code requirements,

which usually dictate attic ventilation.

A combination of continuous ridge vent along

the peak of the roof and continuous soffit

vents at the eaves provides the most effective

ventilation. A rule of thumb is to use 1 sq. ft. of

net vent opening for every 150 sq. ft. of insulated ceiling or 1:300 if the insulation has a

vapor barrier. Vent area should be divided

equally between the ridge and soffits.

Cap vents and gable vents can supplement a

roof design that has insufficient ridge vent

area. Turbine vents can also be used, although

they require annual maintenance. Electrically

powered roof ventilators are not recommended

because they consume more energy than they

save. Powered vents can also remove conditioned air from a home through ceiling leaks

and bypasses, pull pollutants from the

crawlspace into a home, and cause exhaust

gases from fireplaces and combustion appliances to enter a home.

OFFICE OF BUILDING TECHNOLOGY, STATE AND COMMUNITY PROGRAMS

ENERGY EFFICIENCY AND RENEWABLE ENERGY • U.S. DEPARTMENT OF ENERGY

Buildings for

the 21st Century

Buildings that are more

energy-efficient, comfortable,

and affordable...that’s the

goal of DOE’s Office of Building

Technology, State and

Community Programs (BTS).

To accelerate the development

and wide application of energy

efficiency measures, BTS:

• Conducts R&D on technologies and concepts for energy

efficiency, working closely

with the building industry

and with manufacturers of

materials, equipment, and

appliances

• Promotes energy/money

saving opportunities to both

builders and buyers of homes

and commercial buildings

• Works with State and local

regulatory groups to improve

building codes, appliance

standards, and guidelines for

efficient energy use

• Provides support and grants

to States and communities

for deployment of energyefficient technologies and

practices

T e c h n o l o g y F a c t S h e e t

Soffit

vent

Ridge vent

ATTIC VENTILATION

Continuous ridge and soffit

vents form an effective attic

ventilation system.
Rafter baffle

Soffit vent

Rafter baffle

Soffit vent

Soffit dam 

(batt or rigid foam board)

Additional or raised top plate

Sheathing

Band joist

Batt

insulation

Loose-fill

insulation

A raised top plate increases the height for insulation and ventilation at

the eaves.

This oversized truss shows loose-fill insulation that is blocked or dammed

at the eave with a soffit dam (a piece of fiberglass batt or rigid insulation).

A rafter baffle creates a channel for air flow.

ATTIC EAVES

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HDD=HEATING DEGREE DAYS

(Consult your local weather bureau for your city’s actual annual

heating degree days.)

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2

3

4

5

CEILING INSULATION R-VALUES

The 1995 Model Energy Code (MEC) and DOE Insulation

Fact Sheet provide recommended R-values for geographical

locations in the continental United States. The following

table provides some general guidance.

HDD Zone Ceiling R-value

1 (0-500) R-19

2 (501-3,000) R-30

3 (3,001-5,000) R-38

4 (5,001-6,000) R-38

5 (6,001-10,000) R-49

INCREASING THE ROOF HEIGHT AT THE EAVE

One problem area in many roof designs occurs at the eave,

where there is often insufficient space for full insulation without blocking air flow from the soffit vents. Often the insulation is compressed to fit the space, diminishing its R-value.

For a truss roof, consider raised heel or oversized (cantilevered) trusses that form elevated overhangs in combination

with rafter baffles and soffit dams. These should provide clearance for both ventilation and full-height insulation. Use of 2- to

21⁄2-foot overhangs also provides more room for insulation at

the wall junction and additional window shading.

In stick-built roofs, where rafters and ceiling joists are cut and

installed on the construction site, laying an additional top

plate across the top of the ceiling joists at the eave will raise

the roof height, prevent compression of the attic insulation,

and permit ventilation. When installing a raised top plate,

place a band joist at the open joist cavities of the roof framing. The band joist helps prevent windwashing of the attic

insulation—where air entering the soffit vents flows through

the attic insulation—which can reduce attic insulation

R-values on extremely cold days or add moisture to the

insulation. The band joist also serves as a soffit dam for the

insulation.
ATTIC ACCESS

When the attic access is

located in the conditioned

space of the house, build an

insulated attic access cover to

provide continuous ceiling

insulation coverage and use

blocking to prevent fullheight, loose-fill insulation

from falling through the

access.

CEILINGS AND ATTICS

ATTIC INSULATION TECHNIQUES

Loose-fill or batt insulation is typically installed in an attic.

Although installation costs may vary, blowing loose-fill attic

insulation—fiberglass, rock wool, or cellulose—is usually less

expensive than installing batts and provides better coverage.

STEPS FOR INSTALLING LOOSE-FILL AND BATT INSULATION

1. Seal all attic-to-home air leaks, especially chases, dropped

ceilings, wiring and plumbing penetrations, light fixtures,

and bathroom fans. Most insulation does not stop air flow.

2. Install blocking (metal flashing) to maintain clearance requirements (usually 3 inches) for heat-producing equipment found

in an attic, such as flues, chimneys, and exhaust fans.

3. Use only IC-rated recessed lights because they are airtight and

can be covered with insulation.

4. Select insulation levels in accordance with the 1995 MEC or

the DOE Insulation Fact Sheet. The Insulation Fact Sheet

(DOE/CE-0180) can be ordered from the Energy Efficiency

and Renewable Energy Clearinghouse or accessed from the

Internet at www.ornl.gov/roofs+walls.

5. Locate the attic access in an unconditioned part of the

house if possible. Otherwise, weatherstrip the attic access

and insulate it by attaching insulation to the cover or by installing an insulated cover box.

6. If mechanical equipment or storage areas are located in the

attic, elevate the attic decking to allow full-height insulation

to be installed.

ADDITIONAL STEPS FOR INSTALLING LOOSE-FILL INSULATION

1. Prior to hanging ceiling drywall, install rafter baffles to preserve ventilation from soffit vents and use insulation dams

at the soffit, porch, garage, and attic access to prevent the

insulation from spilling.

2. Follow the manufacturer’s specifications (number of bags

per square feet) to obtain complete coverage of the blown

insulation at consistent depths and to avoid fluffing the insulation.

3. As required by the 1995 MEC, make sure the installer:

• Provides attic rulers to show proper blown depth (facing

the attic entrance, one ruler for every 300 sq. ft.)

• Provides an accurate attic “report card” showing that

sufficient density was installed

ADDITIONAL STEPS FOR INSTALLING BATT INSULATION

1. Cover the top of the ceiling joists or the bottom cord of the

truss with insulation.

2. Obtain complete coverage of full-thickness, non-compressed insulation. Make certain batts completely fill the

joist cavities. Shake batts to ensure proper loft. If joist spacing is uneven, patch gaps in the insulation with scrap

pieces. Do not compress the insulation with wiring, plumbing or ductwork (cut slits in the insulation if necessary).

E N E R G Y -E F F I C I E N T C A T H E D R A L C E I L I N G S

Cathedral ceilings must provide space between the roof deck and

ceiling for adequate insulation and ventilation. For most areas of the

U.S., the 1995 MEC recommends R-25 to R-38 insulation in a

cathedral ceiling. This can be achieved through the use of truss

joists, scissor truss framing, or sufficiently large rafters. For example,

cathedral ceilings built with 2x12 rafters have space for standard

10-inch, R-30 batts and ventilation.

Hatch lid pushes up and 

out of the way for access

Air seal

gasket

Scuttle hole cover

Insulation dams 

prevent loose-fill 

insulation from 

falling through 

access

     

Pull-down attic staircase

Air seal

gasket

Seal stairs

frame gap

with caulk

or foam Weatherstripping

Panel

               

Rigid insulation box 

forms lid for pull-down 

attic staircase

Cover box pushes up 

and out of the way for 

access
For more information, contact:

Energy Efficiency and

Renewable Energy

Clearinghouse (EREC)

1-800-DOE-3732

www.eren.doe.gov

Or visit the BTS Web site at

www.eren.doe.gov/buildings

Or refer to the Builder’s Guide

Energy Efficient Building

Association, Inc.

651-268-7585

www.eeba.org

Written and prepared for

the U.S. Department of

Energy by:

Southface Energy Institute

404-872-3549

www.southface.org

Oak Ridge

National Laboratory

Buildings Technology Center

423-574-5178

www.ornl.gov/ORNL/BTC

The Model Energy Code

can be obtained from the

International Code Council by

calling 703-931-4533

MECcheck, a companion

compliance software

package, can be ordered from

DOE by calling

1-800-270-CODE

or downloaded directly

from the Web at

www.energycodes.org/

resid/resid.htm.

CEILINGS AND ATTICS

NOTICE: Neither the United

States government nor any

agency thereof, nor any of their

employees, makes any warranty,

express or implied, or assumes

any legal liability or responsibility

for the accuracy, completeness,

or usefulness of any information,

apparatus, product, or process

disclosed. The views and opinions of authors expressed herein

do not necessarily state or reflect

those of the United States government or any agency thereof.

Foil-faced batt insulation is often used in

cathedral ceilings because it has a 0.5 perm

rating and provides the permeability often

required for use in ceilings without attic

spaces. A vent baffle should be installed between the insulation and roof decking to ensure

that the ventilation channel is maintained.

If roof framing provides insufficient space for

required insulation, higher insulation values can

be obtained by either attaching furring strips to

the underside of the rafters (that permit additional insulation to be installed), using

high-density batts (high-density R-30 batts are

about the same thickness as R-25 batts and fit

into 2x10 framing), or adding rigid foam insulation under the rafters. Rigid foam insulation

offers a resistance to thermal bridging through

wood rafters. Rigid foam insulation must be

covered with a fire-rated material when used on

the interior of a building. Half-inch drywall

usually complies, but check with local building

codes for confirmation.

KNEE WALLS

Knee walls are vertical walls with attic space

directly behind them. They are often found in

houses with finished attics and dormer windows, such as in story-and-a-half designs.

One approach to constructing an energy-efficient knee wall is to first air seal the knee wall

using conventional techniques (i.e., seal the

bottom plate, seal penetrations through the

drywall, etc.). The open joist ends below the

knee wall should be plugged with squares of

cardboard, metal flashing, or rigid insulation;

cellulose insulation blown at a high density; or

batt insulation stuffed into plastic bags. The

plugs should be sealed to the joists using

caulk or spray foam.

The knee wall and attic floor in the attic space

behind it should be insulated to recommended

levels. The same techniques for achieving

higher insulation levels in cathedral ceilings

can be applied to knee walls. Twine is often

used to hold the batt insulation in place. The

technique of adding rigid foam insulation over

the framing is particularly effective. Rigid insulation can be notched to fit over the floor joists.

Sealing rigid insulation to floor joists effectively blocks open floor joists.

A better approach is to insulate and air seal the

rafter space along the sloping ceiling of the knee

wall attic space. The rafters should receive recommended insulation levels. They should be

covered with a sealed air barrier, such as drywall

or foil-faced hardboard. The barrier must be

caulked to the top plate of the exterior wall

below the attic space and to the top plate of the

knee wall itself. All other cracks and holes must

be sealed as well. One advantage of this technique is that any ductwork located in this space

is now inside the conditioned space.

Printed with a renewable-source ink on paper containing at

least 50% wastepaper, including 20% postconsumer waste.

February 2000 DOE/GO10099-771

ATTIC KNEE WALL DESIGN

Seal Attic

living

space

Seal

Hardboard

Seal all edges of rigid 

foam insulation 

Attic space

Desired 

ventilation

Unwanted air leakage Attic ventilation

Knee wall

Seal

outlet

box to

drywall