The US Department of Energy websites report that that as much as 45 percent of heat loss from an un-insulated house is lost through the ceiling as warm air rises. Inadequate ceiling insulation to keep heat inside and air infiltration that causes hot or cold drafts are the leading causes of wasted energy in the average home. Eliminating air infiltration and insulating the ceiling to recommended levels will keep your home more comfortable by maintaining a more uniform temperature throughout the house. Energy savings from a properly insulated ceiling will also be substantial.

Current building codes call for ceiling insulation if the attic is unheated or if there is no attic. In cases where the attic is unheated, the flat ceiling is commonly insulated from above by placing a thick layer of insulation on the attic floor and adding a radiant barrier under the roof deck or rafters. The ceiling can also be insulated from the bottom of the joists.

Cathedral ceilings have no attic above them to create a buffer between inside and outside air. As such, the insulation system in a cathedral ceiling is particularly important. A properly insulated cathedral ceiling will reduce both winter heat loss and summer heat gain. The US DOE recommendations for the correct amount of insulation in a cathedral ceiling range from R-38 for most of the country and R-60 in the far north if the building uses electric heat. The DOE also recommends using reflective insulation as a radiant barrier in all warmer areas.

click here for ceiling insulations

Ventilate to allow water vapor out

Most building codes still call for ventilation in a cathedral ceiling of at least 1" of air space between the roof decking and the insulation to allow vapor water to flow out and to cool the roof shingles. These codes are developed primarily to accommodate the use of fiberglass batt insulation. Fiberglass batts are the worst alternative for cathedral ceiling insulation. Fiberglass has the lowest R-value per inch of all of the insulation options to start with making it unsuited to a ceiling cavity of limited depth. In addition, any moisture in the insulation negates the effectiveness of fiberglass. Some manufacturers make special high density batts for cathedral ceilings to fit in the limited space available in the ceiling cavity but it has the same problems. Fiberglass does not impede convective air flow. Condensation within the ceiling cavity forms drops on the insulation leading to mold and mildew. Fiberglass needs ventilation for evaporation and leaves too much possibility for air leakage.

Ceiling Insulation Vapor Barrier

Pack the ceiling cavity to prevent water vapor penetration

In this scenario, installers pack the cavity with either spray-in foam or blown-n cellulose and install a vapor barrier on the warm side of the ceiling under the drywall. The installer will blow in cellulose that is so densely packed that moisture in convective air flows cannot penetrate. If used with a vapor barrier, this eliminates the need for ventilation. It may also eliminate the need for the vapor barrier in dryer climates. Spray foam applied directly to the bottom of the roof deck will seal all air leaks and also needs no ventilation. Studies by the Florida Solar Energy Research Center show that the temperature and longevity of roof shingles are unaffected by the presence or absence of a ventilation air space.

Either scenario will still necessitate the use of a radiant barrier. While other types of insulation are made to resist or impede the flow of warm air, a radiant barrier reflects back infra-red energy from the sun so it does not penetrate the building. It can also reflect back radiant heat inside the house so it does not escape. A radiant barrier is a thin layer of aluminum sheeting placed in a building airspace to block radiant heat transfer between a surface that will radiate heat (such as a hot roof) and a surface that will absorb heat (such as conventional attic floor insulation).

Heat moves through wall cavities, between roofs and ceilings or between floors and basements by a combination of radiation, conduction, and convection with radiation being the dominant method of heat transfer. Research shows that control of radiant heat transfer is the core of heating/cooling climate control.

A radiant barrier will cut air-conditioning costs by blocking a major portion of the downward heat gain into the building in summer. In the milder weather of spring and fall, radiant barriers can save additional cooling dollars. While outdoor air temperatures are comfortable much of the time, solar energy still heats up your roof, insulation, and ceiling to temperatures that can make you uncomfortably warm. An attic radiant barrier will stop almost all of this downward heat transfer so that you can stay comfortable without air conditioning during mild weather.

Radiant barriers are made of one or two layers of aluminum foil sheeting. Multi-layered foil products use a central layer of foam, plastic bubbles or fiberglass for additional insulating value. This reflective insulation is very cost effective as it gives you protection from both radiant and convective heat in one.

Reflective Insulation

 Use reflective insulation in a cathedral ceiling – it has the highest R-value per inch of any insulation on the market and will fit in the narrowest spaces. Use alone or in conjunction with rigid foam board or spray foam insulation to achieve the r-values for the climate and type of heat used.

Installing reflective insulation in a flat ceiling

• Fasten reflective insulation to the bottom of the joists using 1/2" staples every 4" - 6". The foil side should face towards the floor.
• Securely tape all seams with metalized tape to create a vapor barrier.
• Ensure that a 3/4" air space is maintained between the product and the outer surface in order to optimize system R values. 

Installing reflective insulation under the ceiling has many advantages over using traditional fiberglass batts.

•  Moisture-proof - will not allow moisture to pass through in either direction
• Eliminates condensation within the ceiling and walls when properly installed as a vapor barrier
• Unaffected by humidity with lower moisture transfer and absorption rates than mass insulation - no mildew, mold or fungus growth
• No significant mass to absorb and retain heat
• Very low emittance values "E-values" (typically 0.03 compared to 0.90 for most insulation) which significantly reduces heat transfer by radiation
• No change thermal performance over time due to compaction, disintegration or moisture absorption - common concerns with mass insulation
• Easier to install than fiberglass – no strapping is necessary to keep the insulation up until the ceiling drywall is installed - can be stapled, nailed, or glued
• Safer for workers to install than fiberglass - there are no fibers to breath or cause skin irritation, or eye irritation
• Nontoxic and non-carcinogenic - does not irritate the skin, eyes, or throat and contains no substances which will out-gas
• Carries a Class A / Class 1 Fire Rating
• Not a nesting material for rodents, birds or insects

 Installing reflective insulation in a cathedral ceiling

 The easiest time to retrofit a cathedral ceiling and add insulation is when re-roofing. Add a layer of reflective insulation under the roof decking. Alternately, blown cellulose can be added through holes in the drywall side.

 Combination of rigid and reflective insulation in a cathedral ceiling

From top layer to bottom: Roof Rafters (Ventilated Air Space) Rigid Insulation Wood Strapping Reflective Insulation Wood Strapping Drywall

• Secure rigid foam board insulation between the rafters or against the rafters to prevent thermal bridging through the wood.
• Install wood strapping at every 16” c/c.
• Staple reflective insulation to the strapping and seal with aluminum tape.
• Install the second wood strapping perpendicular to the first one, and then screw on the drywall.

US Department of Energy,
“Ceilings and Attics, Install Insulation and Provide Ventilation”,
February 2000.

Bonneville Power Administration,
“How to Insulate a Ceiling”,
http://www.bpa.gov/energy/n/energy_Tips/weatherization/InsulateCeiling.cfm.

US Department of Energy,
“Cathedral Ceiling Insulation”,
http://www.eere.energy.gov, Sept 12, 2005.