Smart Homes LLC.

Radiant Barriers

What is a Radiant Barrier?

Building a new house?  Save money year round on your energy bills by asking your builder to install a radiant barrier.  If he is a smart builder, he may already be doing it.  Home builders and home owners alike are constantly looking for ways to cut heating and cooling costs, and one underutilized but extremely effective method to go green and cut a home’s energy consumption is the use of radiant barriers.  A “radiant barrier” is typically a thin sheet or coating of a highly reflective material (usually aluminum), which is placed on one or both sides of a supportive substrate which is used to reflect electromagnetic infrared energy back towards its source, typically the sun, much like a mirror reflects light waves.

How Radiant Barriers Work

Radiant barriers work with your existing HVAC system and insulation to reduce the amount of radiant heat gain or loss a structure exhibits by reducing the amount of energy transfer.  There are three primary types of heat transfer: conduction, convection and radiation(infrared).

1. CONDUCTION is direct heat flow through materials . It results from the physical contact of one body with another.  Hot attic fiberglass insulation resting atop a Sheetrock ceiling in a living space will transfer heat by conduction from the insulation to the Sheetrock.
2. CONVECTION is the transport of heat within a gas or liquid, caused by the actual flow of the material itself.  Cool air on the interior of a house in summer will pass over the hot Sheetrock enabling heat transfer through convection.  This is how a car’s radiator is cooled.
3. RADIATION is the transmission of electromagnetic, infrared rays through space.  Infrared rays reside in the electromagnetic spectrum between light and radar waves.  All materials that have a temperature above absolute zero emit infrared radiation, including the sun, icebergs, stoves or radiators, humans, animals, furniture, ceilings, walls, floors, etc. Infrared heat has no temperature until it strikes a material.  The interaction between the infrared energy and the material causes the material to become hot.  The is exactly the same mechanism by which  microwave ovens use to heat food.  Also Thermos containers are lined with a reflective radiant barrier on the interior to keep hot drinks hot.

When the sun heats a roof, it is primarily the sun’s radiant energy that makes the roof hot.  The hot roof material then conducts heat through to the attic side of the roof where convection plays a part in heating the attic air.  However a large majority of the attic heat is also directly attributable to radiation.  The hot roofing materials are emitting radiation, so is the insulation, the studs, the plywood decking, the nails and the drywall.  The hotter the materials, the more infrared rays they will emit.  Thus the roof material is also heating the attic air through radiation as well as through conduction and convection.  It is estimated that in summer a  up to 93% of a home’s summer heat gain can be directly attributed to radiation.  Only 7% of summer heat gain can directly attributable to convection and conduction combined. Thus radiation’s heat gain causes your HVAC unit to work harder and expend more energy costing you more dollars every single month.

With a radiant barrier in place, the heat gain is reduced because the the barrier reflects a majority of the infrared energy back towards the atmosphere.  Conventional insulation reduces the amount of heat transferred through convection and conduction by trapping still air within the insulation itself thereby limiting heat transfer through conduction and convection.  Thus a complete insulating solution must include both a radiant barrier and conventional insulation.

R-Ratings

While conventional insulations are rated by their R-value, the R-values have not been established for radiant barriers since their performance is dependant on so many variables (barrier application type, house size, single story or two story, amount of attic insulation, effectiveness of ventilation, efficiency of heating and cooling equipment, tightness of the building envelope, thermostat settings, and even roof color). So, radiant barriers are then measured by their ability to absorb/emit thermal radiation which is called the material’s emittance or “emissivity” (a rating between 0 and 1). The lower the emittance coefficient, the less energy is absorbed and re-emitted. Intuitively, emittance is the counterpart to reflectance and the sum of both coefficients always equals 1.00; hence what isn’t reflected is thus absorbed and potentially re-emitted into your home. In order to be effective, radiant barrier materials must face an open air space, have a high reflectance rating (.9 or higher), a low emissivity rating (.1 or less), and an optimal angle of reflection (perpendicular provides the greatest advantage). For example aluminum has an emittance of .03 and thus a reflectance of .97, meaning it reflects 97% of the thermal energy and only absorbs and re-emits the remaining 3%.

Cost of Radiant Barriers

Installation costs for an attic radiant barrier depends on several factors:

• Installed in a new home or in an existing home.
• Installed on the attic floor or on the rafters.
• Any retrofit measures such as adding venting (ridge, soffit, etc.).
• Any extra “features” desired (perforation, permeable, etc.)

Advantages of Radiant Barriers

• Field tests show that radiant barriers are effective at reducing 15%-25% of the total cooling load, translating to an average of 3%-10% (as high as 17%) overall cost savings on your heating bill.
• Up to 93% of summer heat gain is directly attributable to radiant energy.  A radiant barrier will reflect a good portion of that total back into the atmosphere.

Disadvantages of Radiant Barriers

• Surface dust reduces the effectiveness of the radiant barrier, being 50% effective after one to ten years.
• Potential for condensation (rectified by naturally permeable barriers, perforation, or adequate ventilation)
• Roof mounted barriers increase shingle temperature by 2°-10° (although effects on roof life are unknown).

More Information

• http://www.ornl.gov/sci/roofs+walls/radiant/index.html
• http://www.radiantbarrier.com/physics-of-foil.htm
• http://apps1.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11680