There are many good reasons to insulate your roof. Roofing insulation helps lower total building energy consumption, creates greater comfort levels for the building occupants and can reduce HVAC equipment requirements.

Roof insulation is one of the most cost effective ways of controlling the outside elements. Reflective foil insulation will reduce the amount of radiant heat that enters the home in summer by up to 97% and will also provide some winter thermal protection. Rigid or bulk insulation will keep convective heat inside in winter as warm inside air rises. Using the right kind and the right amount of insulation will reduce the amount of energy required to heat and cool a building. This can allow you to run your heating and cooling systems less making them last longer and can potentially reduce the HVAC equipment size required thereby saving additional money.

Properly installed reflective insulation can also reduce the potential for condensation occurring on interior surfaces causing mold, mildew, rot and dripping. Combining roofing insulation with adequate ventilation under the roof deck will allow moisture to escape preventing ice buildup in the eaves and damage to the building.

Reflective insulation installed under a metal roof

There are two basic types of insulation used for roofs: reflective insulation and bulk insulation in the form of blankets, blown-in cellulose, sprayed foam, or rigid boards. Most homes, commercial buildings and steel buildings combine reflective and bulk insulation to make a complete insulation system. In warmer climates, reflective insulation alone is sufficient to control indoor temperatures and moisture.

Reflective insulation

Reflective insulation is made of two layers of reflective aluminum foil with a central layer of foam, plastic bubbles or fiberglass. When installed facing an airspace of at least 2" (like the rafters), it reflects up to 97% of radiant heat while the central layer inhibits the convective flow of warm air. If sealed properly, reflective insulation will also create a vapor barrier. R-Values (the measure of resistance to convective heat transfer) are up to 14.5 per inch of reflective insulation.

Bulk Insulation

• Blankets or batts - Blanket insulation is made of fiberglass, cotton, sheep's wool, or mineral wool. It will protect against the expanding and rising flow of warm air, thereby trapping your heat inside. R-values are roughly 3.2 per inch.
• Rigid foam - Rigid foam will not compress or absorb moisture like blanket insulation which lowers insulating efficiency. Its higher r-value per inch makes it ideal for roofing insulation where space is limited. Polyisocyanurate board has an average R-value of 5.8 per inch.
• Sprayed polyurethane foam - Sprayed foam is used for metal walls and roofs to seal air and moisture leaks and control condensation. Sprayed foam average R-value is 5.9 per inch. Unlike the other forms of insulation, sprayed foam is not a do-it-yourself option.

The best defense against the intense heat of summer from the sun's radiant energy is a radiant barrier placed under the roof. Studies by the Florida Solar Energy Center have shown the effectiveness of using an aluminum foil radiant barrier. In retrofit applications, placing a radiant barrier under the attic rafters can lower air conditioning costs in a typical Florida home by up to 12%. A radiant barrier is most effective where there are long, hot summers. They are recommended in all new constructions.

How a radiant barrier works

Radiant barriers primarily block the downward flow of radiant heat from the roof deck down into the house. An aluminum foil radiant barrier under the roof will reflect 97% of solar radiation back into the atmosphere instead of allowing it to enter the attic where it would be absorbed by the insulation and re-radiated down into the living area. The increased indoor comfort and energy savings gained by using a radiant barrier make it a must in warmer climates.

Metal building roofs are insulated with either reflective insulation or batt insulation laid under the metal roof decking. In addition, spray foam can be added from underneath after the roof decking is in place. In colder climates, a combination of batt insulation and reflective insulation can be used with the reflective insulation on the warm side of the building. Placing the reflective insulation on the warm side will allow it to double as a vapor barrier and prevent moisture from getting into the batt insulation or condensing on the ceiling. Any of these insulation types will help dampen noise levels both inside and from the outside like the sound of rain or hail.

Condensation can cause major problems in a steel building. Water in contact with metals contributes to corrosion and can lower the service life of the building. Collection of water on the ceiling can lead to dripping, staining, mold, mildew and odors. In addition, water or ice collected in fiberglass, paper or wool insulation seriously degrades both thermal performance and insulation service life.

All of these problems can be solved with a properly installed insulation/vapor barrier system under the roof. Even in only moderately humid climates, condensation can cause serious problems in an unprotected metal building. Building experts advise against using fiberglass insulation at all in humid climates. Instead, they recommend reflective foil insulation - it will not get wet and have to be replaced like fiberglass batts. Reflective insulation acts as a vapor barrier or vapor retarder by limiting the passage of water vapor thereby preventing condensation within the insulation or on the interior surfaces of the building.

Most homes use a combination of reflective and bulk insulation to create a total insulation system. In an unheated attic, bulk insulation is used on the attic floor while reflective insulation is used as a radiant barrier under the roof. If the attic is heated, or if there is no attic space above the ceiling such as a cathedral ceiling, then both types of insulation must be placed in the roof. Using insulation in combination will allow you to achieve the desired roof R-value, particularly where the cavity between the rafters is shallow. Most roofs have enough room between the rafters to maintain the required 2 inches of airspace to allow for circulation under the roof and install a both reflective insulation and foam insulation. The reflective insulation will act as a vapor barrier here as well.

Cathedral ceilings

Roof insulation is particularly important in cathedral ceilings where there is no attic space above the conditioned living space to trap warm rising air or ventilate incoming radiant heat and moisture. The illustration below shows how cathedral ceilings employ both reflective and bulk insulation to create a complete insulation system that controls convective and radiant heat, as well as moisture and condensation.

Combination of rigid and reflective insulation in a cathedral ceiling

Proper installation is key to the effectiveness of an insulation system. Reflective insulation can be installed either over the trusses or under the roof decking leaving at least 2" of airspace between the roof decking and the insulation for proper ventilation. As the space available for insulation is usually small, a combination of rigid insulation and reflective insulation fits much better than using blankets. Both rigid and reflective insulation have much higher R values per inch than other forms of mass insulation. In retrofit applications, install reflective insulation under the attic rafters.

Look for insulation that is ICC-ES recognized and Energy Star Qualified. ICC-ES is a nonprofit organization that does technical evaluations of building products, components, methods, and materials. ICC-ES evaluation reports provide evidence that products and systems meet code requirements. Their reports on code compliance are available free of charge to code officials, contractors, architects, engineers, and anyone else with an interest in the building and construction. Request a copy of ICC-ES reports at their website: www.icc-es.org/Evaluation_Reports/index.shtml.

ENERGY STAR is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy designed to help people save money and protect the environment through rating energy efficient products and practices. Products that are rated by this program will be marked with an Energy Star.

Germer, Jerry
Quick Guide Insulation & Ventilation
Creative Homeowner Press, Upper Saddle River NJ, 1995.

Metal Roof Insulation
Roofing/Siding/Insulation (RSI), Sept 1, 2007

Roof Insulation Guideline
New Buildings Institute, November, 1998

US Department of Energy
"Insulation Fact Sheet", Table 2. Evaluating the R-value of Insulation Previously Installed in Existing Homes
2002

Florida Solar Energy Center
Radiant barriers: a question and answer primer.
http://www.fsec.ucf.edu/en/publications/html/FSEC-EN-15/index.htm