Thursday, December 29, 2011
Thermal Radiation Barriers
During the dog days of summer when the temperature passes 100 degrees Fahrenheit here in Virginia my split system heat pump struggles to try to cool the master bedroom. I have both draperies and window films, have retrofitted insulation in the attic, sealed the ducts, regularly service the heat pump and blower, but still the best my system can do on those hot days is 78 degrees Fahrenheit in the southern facing master bedroom, though other rooms are several degrees cooler. Before I consider equipment solutions and additional ducts, which will have to wait until the current system serves its useful life, I have been looking at radiant barriers and interior radiation control coatings as a possible method to shave a couple of degrees off the maximum daily temperature.
Radiant barriers and radiation control coatings have low Emittance, typically below 0.25. Infrared Emittance is measured between 0 and 1 with highly polished stainless steel at less than 0.1 and wood and sheetrock approaching 0.8-0.9. Infrared Emittance measures the ability of a warm or hot material to shed some of its heat in the form of infrared radiation. A material with an emittance of 1.0 emits about 3.4 watts per square meter, for each degree F above ambient temperature. Radiant barriers are designed to work in your attic to prevent some of the heat from the roof from being transferred into the attic space. The idea is to have the radiant barrier or coating not allow all the heat from the roof to move into the attic space. Oak Ridge National Laboratory, ORNL, found in field experiments that radiant barriers installed in the attic could reduce air conditioning bills in the hottest parts of the country. For homes that had both air-conditioning ductwork in the attic and were located in the Deep South, radiant barriers were found to reduce utility bills by as much as $150 per year using average residential electricity prices (for the late 1990’s) and an average size house with a single peaked rectangular rood. For more moderate summers, like those in Atlanta and Baltimore, annual energy savings were about half those of their southern neighbors. In the northern climate zones, the savings drops further, going from about $40 to $10 per year as you go from Chicago to Fairbanks, Alaska.
If there were no ducts or air handlers in the attic, the savings were found to be much less, and a radiant barrier may not be worthwhile from a cost benefit basis, but ORNL states that a radiant barrier may still help to improve comfort and to reduce the peak air-conditioning load on occasion. In northern climates where winter heating is the largest cost, radiant barriers can potentially reduce indoor heat losses through the ceiling during winter nights, but they may also reduce beneficial daytime heat gains due to solar heating of the roof. ORNL had no data measuring the heating benefits, but climate, orientation of the home, level of attic insulation, number of winter sunny days and other factors, can determine if the net winter effect of a radiant barrier to be positive or negative. The measured and tested benefit field studies were performed with air conditioning. It is to be noted that the field testing showed that the radiant barriers produce less energy savings when used in combination with high levels of insulation since the fraction of cooling load that comes from the ceiling is larger when the amount of insulation is small.
ORNL’s field testing showed that a new application of a radiant barrier on the attic floor, does work better than applying the radiant barrier to the roof rafters. Most of the field tests have been done with clean radiant barriers, and laboratory measurements have shown that dust on the surface of aluminum foil increases the emittance and decreases the reflectivity. This means that dust or other particles on the exposed surface of a radiant barrier will reduce its effectiveness. Radiant barriers installed in locations that collect dust or other surface contaminants will have decreasing performance over time. Though initially better, the attic floor accumulates dust resulting in the radiant barrier losing its effectiveness. Predictive modeling results, based on the ORNL testing, indicate that a dusty attic floor application will lose about half of its effectiveness after about one to ten years. Applying the radiant barrier to the floor of the attic is also not effective when a large part of the attic is used for storage, since the radiant barrier surface must be exposed to the attic space to work and applying a radiant barrier with the reflective surface touching the insulation is not effective.
In addition a radiant barrier installed on the attic floor directly on top of insulation can create a condensation, moisture and ultimately a mold problem. During cold weather, water vapor from the interior of a house moves into the attic through bathroom and kitchen vents and other openings. In most cases, this water vapor is not a problem because attic ventilation allows the vapor to dissipate. But, during cold weather, a radiant barrier on top of the insulation could cause water vapor to condense and even freeze on the barrier's underside. A radiant barrier used in the attic floor application must allow water vapor to pass through it. Some allow water vapor passage through holes or perforations, while others are naturally permeable.
Due to the above factors it is usual to install a radiant barrier to the interior of the attic roof, but that installation may cause other problems. The testing showed that radiant barriers can cause an increase in roof temperatures. Roof mounted radiant barriers may increase shingle temperatures by 2 to 10 degrees F. Radiant barriers on the attic floor may cause smaller increases of 2 degrees F or less. The effects of these increased temperatures on roof life, if any, are not known, but should be considered with asphalt shingles. Attic ventilation helps to cool your attic in the summer and to remove excess water vapor in winter and should not be blocked by a radiant barrier. After installing a radiant barrier always check that existing ridge vent systems are not blocked by a radiant barrier and there is free flow of air, check the soffit vents to ensure that they have not been covered with insulation or the barrier, and check gable vents to make sure that they have not been blocked.
The attic is a system consisting of many components that work together. Radiant barriers are only a small element and possibly the least important. The radiant barriers reduce radiant energy transfer. Insulation on the attic floor reduces conductive and convective heat transfer. The duct insulation reduces conductive and convective heat transfer at the duct surface. Duct sealing reduces the energy losses caused by increased air exchange between the inside and outside of your home. Attic ventilation in the gables, ridges or soffit can reduce the amount of energy that enters the attic from the outside. Overall, as you can see in the chart above, derived from the ORNL research, shows that the most energy savings come from having adequate insulation and sealed and insulated ducts in the attic, not the radiant barriers, and radiant barriers are most effective with less insulation in the air conditioned south. Nonetheless, that small savings might improve comfort on a very hot day. Finally if you install a radiant barrier make sure the product label indicates that emittance is less than 0.25 as measured by ASTM C1371 and the product is designed to work in your attic.