Foil Radiant Barriers and Energy Savings
The topic we are talking about today is the use of radiant barriers made of aluminum foil or aluminum based paints. These products are usually used in new home attics and or in an existing attic as a retrofit. The product is known by several names including radiant barrier, foil insulation, reflective insulation, reflective paint, insulating paint and a few more. It is available in the form of aluminum foil on a paper backing, an aluminum coating on Mylar, as a foil with bubble pack attached and even in the form of aluminum paint.
Do They Work and If So, How?:
They all work on the same basic principle of emissivity. Emissivity is defined as the ratio of the energy radiated by a body to that of a material that is a perfect emitter or “black body.” Confused? I was the first time I read that description back in 1989. OK, most construction materials will absorb heat radiation and then emit or pass on most of it to the next surface in line. Remember, heat always flows from hot toward the cold side as per the Second Law of Thermodynamics. In other words, they conduct heat well, which is why we have to use an effective insulation layer to slow the movement of heat into or out of our homes if we want our utility bills to be less than the national debt and we wish to be comfortable in our homes.
A very few materials pass on only a very small amount of the heat energy they absorb. These materials include highly polished aluminum and gold. My friends who are physicist will dislike this because it is a simplification and not entirely technically accurate. A material that has a very low emissivity acts like a short circuit in the movement of heat energy. They absorb the heat, but they don’t allow it to move on just as an electrical short stops the flow of electric current.
On an interesting side note, I’ll bet you have seen a very high tech use of applied emissivity to control heat gain and not even realized it. Take a look at a satellite being prepared to go into orbit. In the direct rays of the sun, with no atmosphere to filter it, the heat reaches 250 degrees! NASA knew that this would cook our astronauts and the sensitive electronics in our satellites. There is no room for two feet or more of bulky insulation when every pound costs a fortune to get into space, so NASA has used radiant barriers to protect our astronauts and satellites for many decades. Notice the shiny gold foil wrapping the satellites in the shots below. Yeah, aluminum has an emissivity of 0.05 It stops 95% of the heat transfer) and gold is better at 0.02 (98% reduction). So, our attics get aluminum and our astronauts and multi-million dollar satellites get gold. Makes sense to me.
The attribute of low emissivity is what gives us our Low-E windows (E for emissivity) that block as much as 85% of the sun’s heat that hits them. This ability of Low-E coatings on glass to not pass along or as I described it “short circuit” the transmission of heat, greatly reduces the heat gain of a home in summer and in so doing drops the cooling load a great deal. The U.S. Department of Energy has stated that by simply changing the glass in a home from double pane clear glass to double pane Low-E glass will reduce the a/c sizing required from a 5.0 ton unit to a 3.5 ton unit! So if you are asking if radiant barriers work, the short answer is, Yes.
Can They Save Me Utility Dollars and If So, How Much?:
Now let’s talk about the big questions on your minds. How much can they save you on your monthly utility bills? Are the claims of “40% savings” true? “What kind of a payback am I looking at?” All I can say about many of the claims being made out there is “Wow!” I just got a mass mailing offering me and my wife a free steak dinner if we sit through a sales pitch that will show us how a radiant barrier retrofit on our home attic will, according to the flyer, save us 40% on our utility bills. I only wish it were true. However, as the old saying goes, “If the claims sound too good to be true, they probably are.”
First, let’s look at the potential for energy savings from any roof applied energy saving product in a general sense. The independently performed and funded studies done usually show that retrofitting an attic deck with a top quality 99.99% aluminum foil radiant barrier with proper ventilation saves around 5% to 10% of the total utility costs for the home. These savings are derived from a combination of the reduction in heat that flows from the roof deck to and then through the insulation and the reduction in heat gain experienced by the air conditioning ducts that are located in the attic. That’s a long way from 40%!
The portion of a total house cooling load (ACCA Manual J) attributable to the roof heat gain usually falls between 10% and 15% of the total cooling load on a home. When you add in the heat gain of the a/c ducts if they are also in the attic and you can get another 5% to 10% at most. That sets the upper limit on the potential savings since you can’t do more than reduce the total heat flow to zero. Think about it. If the attic is only responsible for at most 20% of the heat your a/c has to remove, even if you could completely eliminate it, your savings would only be 20% of the portion of your electric bill that is the result of your air conditioner. We all understand that the a/c is only responsible for a portion of our summer electric bills and that portion is the only part that is being reduced. Your overall reduction would be around 5% to 10% at most.
The next thing to understand is that if you have a decent level of insulation on the attic floor, the savings will be on the small side of that 5% to 10% outcome. The better your attic floor is insulated, the less the addition of a radiant barrier will save. Why? When your attic floor is insulated to R-19 or better, the heat flow down into the house has already been reduced very significantly. Any additional measure taken can only take a bite out of the small portion that is left. So, the better your attic is insulated, the less the addition of radiant barrier or for that matter, any type of insulation can potentially save.
Another factor is the fact that the largest portion of the energy and dollar savings will come from the reduction in heat gain experienced by the air conditioning/heating ducts in the attic! Yes, the ducts only cover a small portion of your attic floor, but they are far less well insulated than the attic floor (hopefully) and they contain the coldest and hottest air in your home. The table below comes from a U.S. Department of Energy study and if you are interested, you will find a link to it at the end of this story. The key fact to take note of is that if the attic is insulated between R-19 and the IECC code level (R30-42) or if there are no ducts in the attic, your possible energy savings drop to between $50/yea(purple arrow) and $5/year (red arrow)! As you can see, adding more insulation is by far the bigger winner for you with savings of up to several hundred dollars per year.
When I ran Austin Energy’s conservation division our programs were analyzed on an annual cycle by a team of outside economists to determine if all measures and the programs as a whole were cost effective to the both the utility and our customers. The economic analysis consistently found that the cost of installing a radiant barrier only had a positive Return On Investment (ROI) if the attic floor was insulated at less than R-13 or 3-4 inches of insulation and the a/c ducts were in the attic, too.
Most people who aren’t physics and heat transfer geeks will find the chart below to be very informative and I’ll wager to bet, surprising, too. The straight blue line running from corner to corner is the R-value of the insulation as shown on the right side of the chart. The curved red line that starts at the upper left hand corner and runs down to the lower right hand corner shows the U-value of the insulation as shown on the left side of the chart.
What’s a U-value? The U-value is the actual rate of heat flow through the material in standardized units of heat called British Thermal Units or BTU’s. Stated another way, the red U-value line shows us how much energy is being saved as we increase the R-value of the insulation. The U-value is also the inverse of the R-value. Just to give you a scale, a BTU is about the amount of heat given off by burning a wooden kitchen match completely. Most people think that R values and energy savings are directly proportional, but that is far from the truth. When you double the R-value, you don’t cut heat flow and the bill in half as many think and the midpoint for energy savings between R-0 and R-30 is not R-15 as a constant relationship would yield. As you can see, the R-value is a straight line at a 45-degree slope. The U-value showing energy savings has a very different shape. The vast majority of the savings achieved by any insulation is found in the first R-5 or R-10. After that, the rate of reduction in heat transfer flattens out and after R-25 it becomes almost a flat line across the bottom of the graph. In other words, insulation experiences a rapidly diminishing rate of return.
Glancing at the graph you will see that the green R-13 line points to a heat flow (on the left axis) of just under one tenth of a BTU per hour per square foot of surface area for each degree of temperature difference across the material. The purple line shows us the reduction in heat flow at R-30 and as you can see, that lines up very close to the green R-13 heat flow line. The reduction in heat flow by going from R-13 to R-30 is approximately 0.05 BTU/hr/sf/degree difference. Take a look at the far larger difference between the green R-13 and the yellow R-5 line. The difference for these values is about 0.2 BTU’s/hr/sf/degree of temperature difference. In science we often find that very small difference in the beginning conditions of an experiment greatly affect the outcome. Insulation is an example of this reality. The energy saved by going from R-5 to R-30 is four times (400%) greater than the energy saved by going from R-13 to R-30! This fact of science has a big impact on your return on investment!
Let’s Talk Differences in Types of RB:
There are big differences in the emissivity of different types of radiant barriers. Radiant barriers are available as almost pure aluminum foil, foil on bubble pack (the bubble pack is worth about an additional R-1) and as a spray applied paint. The best and purest of the aluminum foils have emissivity rates of 0.05-0.04. That means that they block about 95%-96% of the heat transfer through the roof deck. This usually equates to a temperature reduction in your Texas attic of around 10 to 15 degrees.
The typical paint applied radiant barrier has an emissivity of around 0.20-0.25. The paints therefore allow about four or five times more heat transfer than the products made from foil. Technically, these paints do not even meet the ASTM definition of a radiant barrier. Radiant barriers must reject at least 90% (0.10 or less emissivity) of the heat absorbed and the paints simply can’t do this. I talked to the technical director at one of the major radiant barrier paint manufacturer’s about this once. He explained that it would be impossible to spray pure aluminum through an airless sprayer and that paints required solvents and carriers to form a matrix, which inherently diluted the purity of the aluminum and reduced its effective emissivity. Bottom line, no paint equals the performance of the pure 99% aluminum foil products. I must also hasten to point out that most applications of radiant barrier paint that I have inspected looked like a light over spray on the roof deck and were not an effective application.
Just take the bid cost you have and divide it by the 5% realistic savings on your average summer monthly utility bill. Let’s say that your bid cost is $1,700 and your average summer eclectic bill is $250.
- $250 x 5% = $12.50 or a best case scenario: $250 x 10% = $25
- $12.50 x 6 months of cooling = $75/year saved at 5% and $150/yr. saved at 10% reduction
- $1,000 est. cost / $75/yr saved = 13.3 year payback at a 5% reduction overall or 6.6 year payback at 10%
Other Options to Reduce Your Cooling Bills:
There are several measures that have been found by building scientists and Texas utilities to have better paybacks than radiant barrier retrofits. Among them are:
- Adding insulation to the attic floor is usually the best and most cost effective way to reduce attic heat gains into the house and often less costly and not as difficult to do when using blown cellulose, blown fiberglass, or blown rock wool.
- Sealing the leaks in the a/c ducts is almost always very cost effective - the average US home looses 25%-35% of all of the conditioned air produced by your a/c-furnace before it gets to your rooms due to leaks in the ducts.
- Installing some type of solar shade screen, film or awning over windows that receive more than one hour of direct sun through them in the summer often ranks right up there with sealing the ducts in short payback times.
Radiant barriers do work and the physics of how they work is well understood. The question is how much can you reasonably expect to save by retrofitting one into your attic. The answer is about 5%-10% of your cooling costs. It is up to you to determine if that rate of payback is sufficient.
In new construction the marginal cost of radiant barrier roof decking does make sense especially if the air conditioning equipment and ducts are in the attic. Radiant barriers make the most sense in uninsulated or very poorly insulated buildings where adding more insulation is not possible. You should always consider adding more insulation before you spend your money on radiant barriers. Oh yeah, one more thing, don’t let anyone talk you into placing the radiant barrier foil on the floor of the attic. The foil will shortly lose its effectiveness due to the buildup of dust on the surface that will greatly reduce the emissivity of the product.
This is a good product when used correctly. It is unfortunately being oversold and often it is being very poorly installed, too. Do your homework, don’t buy into the exaggerated estimates of savings, always check the references of your contractor and never release the final payment (30%) for a job until you or a qualified person has performed a final quality assurance inspection.