Radiant barriers are one component in the attic system
Your attic is a complex system where many components work together to maintain comfortable (and dry!) indoor conditions, so you can increase your savings by considering the whole attic system. 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. Special shingles or ventilation arrangements can reduce the amount of energy that enters the attic from the outside.
|Reducing any air leaks between your house and the attic, that is, through the ceiling or inside any wall cavities, is also important and should be the first step in any attic improvement effort. (see http://www.engext.ksu.edu/henergy/envelope/airsealing.pdf)|
So, when you consider a radiant barrier, look at the whole attic system. We calculated the first year savings for a 1,540 square foot attic for several combinations of attic improvements: adding a radiant barrier, reducing duct energy losses, and upgrading insulation from R19 to code levels. The results, shown in Figure 1 and Tables 1 and Table 2, were based on a number of assumptions and won’t represent your house exactly. For example, if your home heating and cooling system has a lower efficiency than the efficiency used in these calculations, then your savings will be greater (until you replace the air conditioner). Comparing the different system improvement options is easier if you look at this bar chart. The greatest savings are available to homes with uninsulated, or poorly insulated, ducts in the attic (the values are also given in Tables 1 and 2 for folks who don’t like graphs). Research has shown that any duct system that hasn’t been subjected to a formal testing process leaks, it’s just a question of how much. These homes with the greatest savings potential are also likely to be homes with higher energy bills. If your ducts are already well sealed, or – even better, not located in the attic at all, your savings potential is less because you’re starting with an attic that is already more efficient.
Selecting your best approach will depend on your house’s configuration, including physical attic constraints, and your budget. For a shallow attic, it may not be possible to add conventional insulation, so a radiant barrier alone may be the best choice. If you already have a higher level of insulation in your attic (R-30 or more in the south, R-38 or more in the middle and northern climates), then adding attic insulation would not be your first choice. If your ducts are not in the attic, or if they are already well-sealed and insulated, then improving the ductwork will not be an effective choice. But in any case, addressing and improving the whole attic as a system will give you the greatest savings.
The annual savings are calculated by comparing the hourly energy consumption of attics with two different configurations. Another way to look at the issue is to look at the calculated annual cost for the total heating and cooling energy lost through the attic floor AND ducts located in the attic. Because we calculate the energy exchange for every hour during a year, we can also show the impact of Time-of-Day rates that are available to some homes in Miami and Austin. Figure 2 shows the cost we calculated for that energy for a 1,540-ft2 house for both time-of-day (from October 2010) and 2008 state average energy prices. In Miami, the savings due to installing a radiant barrier were about 35% greater with time of day pricing than with average pricing; in Austin they were only about 5% greater. (Also, if you’re considering participation in a time-of-day rate plan, remember that this only represents a portion of your total utility bill, and the effect of your appliance use schedule will also play an important role.)
Figure 2: Calculated annual cost for energy lost through the attic floor PLUS the energy lost due to ducts located in the attic.
Looking at this figure, the most important message is for anyone building a new home – even if it costs more, don’t put the ducts in the attic!
|Conventional insulations are usually rated by their R-value. Since the performance of radiant barriers depends on many variables, simple R-value ratings do not apply.|
For all the attic system elements, you need to compare the potential savings over many years to the installed cost.
You can use the Radiant Barrier Savings Caculator, or Table 3 (for homes with uninsulated or poorly insulated ducts in the attic) and Table 4 (for homes with well sealed and insulated ducts, or no ducts at all, in the attic) give you the annual savings per square foot. You can estimate a reasonable purchase price by multiplying those values by the size of your attic floor to and then multiplying that by ~15. (This simple payback is approximately the same as the present value of your energy savings over the next 20 years, for a reasonable rate-of-return value of 2 to 3%, and assuming that energy prices don’t go up faster than inflation. You could multiply the annual savings by ~20 if you want to consider the present value of savings over the next 30 years.) We don’t know what will happen to energy prices in the future, but if you think your energy prices will increase above the general inflation rate, then your savings in future years will be even greater than the first-year savings shown here.
For example, if you lived in a 2000 ft2 house in Miami, had R19 attic insulation, and the ductwork in your attic was poorly insulated, then Table 3 shows you could save ~$0.28/ft, or $560 per year by adding a radiant barrier and sealing and insulating the ducts. The present value of those savings over a 20-year period would then be about $8,400. If the price to make these modifications were less than that value, then this would be an economical choice.
Radiant barrier, coatings, and insulation costs vary widely. As with most purchases, some comparison shopping can save you money. Get prices from multiple contractors, or multiple stores if you’re going to do the installation yourself. You should always check with your local or state energy office or building code department for current insulation recommendations or see the Insulation Fact Sheet or the ZipCode Calculator. Costs will depend on several factors, including:
- Homeowner- or contractor-installation.
- New home or existing home.
- What extra "features" are desired; e.g., a radiant barrier with perforations and reinforcements may be more expensive than a "basic" radiant barrier.
- Any necessary retrofit measures such as adding venting (soffit, ridge, etc.)
- Type of radiant barrier selected, or type of insulation selected (loose-fill insulation is usually lower in price than "batt" insulation or blown foam insulation)
- Installation location and coverage
- Region of the country