What You Need To Know: Radiant Barriers
There are three ways that energy transfers from a hot attic to a room that you would like to keep cool. Convection, Conduction and Radiation. Radiant barriers address the third of these energy transfer methods. I will lay out the case for how to retrofit an attic to make rooms below it cooler, and show where radiant Barriers fit into that strategy.
When retrofitting your home for increased comfort and decreased energy consumption, focusing on improvements in the attic is your number one priority. Without going into a great deal of detail this results from the “Stack Effect”, temperature and pressure differentials that are always the highest on this plane of your property. Perfecting the air barrier between your unconditioned attic and conditioned rooms below is priority number one. This reduces transfer of energy through Convection, or direct air transfer between conditioned and unconditioned space.
Priority number two is to address Convection by increasing the amount of insulation to reduce molecule to molecule transfer of energy between the conditioned and unconditioned space. In our region, we recommend insulating to R60 levels of thermal performance when designing a traditional attic.
In order to reduce “Heat Flux” during summer from the attic to the conditioned rooms below you also need to address the thermal gain caused by Radiant Energy. Radiant Energy is transferred through space. Think wave lengths of light that hit a surface heating it to intense temperatures. During the heat of Northern Virginia Summers, radiant energy heats roof decks to temperatures in excess of 145 degrees on the top to 130 degrees on the bottom. This is a large part of the reason that the temperatures in your attic can rise to 130 degrees during a summer day.
Studies have shown that radiant barriers can deflect some of this thermal energy back through the roof deck. In a study done by the Berkley National Laboratories, peak attic temperatures were reduced from 125.7 degrees F to 104.3 degrees F when radiant barriers were deployed. This 21 degree reduction in temperature reduces the load or potential for heat to transfer from the attic to the rooms below. Consider that insulation has an R value that represents the “Resistance to Heat Transfer”. The greater the Heat Differential the greater the work that needs to be done by the insulation. By “Bouncing” the radiant energy back into the atmosphere, and reducing the heat in the attic, you make the other insulating systems more effective.
This factor is particularly important when the attic has duct work contained within the space. Insulated ductwork generally carries an R value of 8 or less. The radiant energy heats the air in these ducts up to an extreme temperature, causing the Air Conditioner to have to work to overcome this temperature rise. A study done by Hagman and Modera in 1996 on the effects of radiant barriers found that they decrease the amount of time that HVAC systems have to work to overcome this heat gain by up to 30% in these circumstances.
Radiant Barriers are manage heat gain in attics during the summer in Northern Virginia. They are an important component when attempting to