That sounds great, right? In theory, radiant barriers do seem useful for reducing the heat load during the summer. Theoretically, that would reduce your power bill. However, there is a lot more to air conditioning than keeping buildings cool, and radiant barriers can pose unique challenges to your comfort. Other problems may arise from improper installation.

If you plan to include radiant barriers in a new construction home (or already have them in your home), you may want to learn a bit more about them. This article will explain how they work, why improper installation renders them ineffective, and the challenges they pose for your HVAC system.

WHAT A RADIANT BARRIER DOES

Before we can understand what a radiant barrier does, we need to understand what heat is. Heat is the energy that causes molecules to move. Whenever you have something that has a temperature over -460 degrees Fahrenheit, there will be some heat. Heat moves in three different ways: conduction, convection, and radiation.

Conduction is how heat moves between two objects that are touching each other. When you hold a cup of hot coffee, your hands will start to warm up; that’s conduction at work. The thickness of walls and attic insulation help control heat in the attic via conduction.

Convection happens when heat transfers within a fluid (vapor or liquid). If you’re boiling water in a pot on the stove, you can hold your hand over it and feel warm. That’s because heat is traveling upwards through the air via convection. We can control convection in our homes by sealing gaps and cracks between rooms and unconditioned spaces (like the attic, crawlspace, or outdoors).

Radiation is what happens when heat travels on electromagnetic waves. The heat that comes in through glass windows has entered our home through radiation; the heat has passed through the glass via the sun’s electromagnetic waves, and we call it radiant heat. Radiant heat can either be mostly reflected by lighter objects or mostly absorbed by darker ones. The sun is just one example of radiant heat transfer; our bodies also radiate heat, as does everything around us.

Radiant barriers consist of a highly reflective material to control radiation; the idea is that heat will be absorbed into the dark roof tiles and be reflected by the barrier. Your roof will get hot, but the attic will be cooler than it would have been without the barrier.

WHAT ARE THE UPSIDES OF HAVING A RADIANT BARRIER?

When installed correctly, radiant barriers keep your attic cooler and significantly reduce your energy expenses during the summer. In a warm and sunny climate like ours in Central Florida, you could see a 5-10% reduction in your cooling costs (per the U.S. Department of Energy).

Also, if you happen to store things in the attic, you’ll be able to keep them cooler. Your attic won’t be climate-controlled (compared to a storage unit), but it won’t be nearly as hot.

That’s pretty much where the benefits end.

WHAT ARE THE DOWNSIDES OF HAVING A RADIANT BARRIER?

When radiant barriers keep your attic cooler, the duct surfaces get closer to the dew point. The dew point is the temperature at which water vapor in the air starts to condense and become water (like dew). The air at night tends to be cooler than during the day, and it can’t hold as much moisture; at night, objects radiate their heat and cool down, so the water vapor in the air around those objects condenses and becomes the dew that we see in the mornings.

We can get very high dew points in the summer, and if the attic air is cool enough to cause duct surface temperatures to drop below that dew point, then your ducts will start sweating.

Sweating ducts result in a wet attic, and fungal matter thrives in moist environments. So, you could very well get mold growth in your attic if it doesn’t stay warm enough during the summers.

That is the downside of having a correctly installed radiant barrier. What about those that incorrectly installed ones?

IMPROPER INSTALLATION: WHEN RADIANT BARRIERS ARE NO LONGER “RADIANT”

For radiant barriers to work properly, they need to face the source of radiant heat and have an air gap between the barrier and the source.

If the radiant barrier is sandwiched against the surface and doesn’t have that air gap, it becomes a means of conduction. The foil also happens to be a highly conductive material. As a result, it can transfer heat INTO your attic very effectively! Instead of reflecting radiant heat away from your attic, the barrier will conduct heat into the attic and achieve the opposite intended effect.

So, when radiant barriers are installed incorrectly, you could have even HIGHER cooling costs because of your hotter attic.

FINAL THOUGHTS

While correctly installed radiant barriers can yield some energy savings, they may also lead to fungal growth when the dew point is high. Mold can negatively impact your health. So, unless you have a dehumidifier in your attic (which uses energy and costs money to install and run), a radiant barrier may cause more trouble than it’s worth.

Radiant barriers aren’t necessarily bad, but they do raise a set of challenges in our climate. Unless you are willing to mitigate those challenges and know how you’d like to do that, a radiant barrier likely won’t be a worthwhile investment.

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There are three means of transferring heat: conduction, convection, and radiation. Conduction refers to the transfer of heat via direct touch. If you touch a hot pan, you’ll burn yourself because it transferred its heat to your hand via conduction. Convection relies on a fluid (like air) to move heat. Some of you may have convective ovens, which cycle warm air through the oven to heat your food. Radiation happens when heat “rides” on electromagnetic waves. For example, the sun’s radiation can pass through a glass window and heat a room.

All bodies radiate, absorb, and reflect heat to some extent. Theoretically, fully reflective or absorptive bodies may exist, but we don’t see them. So, everything in a building gives off heat, especially our bodies. When we wonder why our homes and businesses get so hot, we may have to think about radiant heat and its sneaky effects. Once we understand how radiant heat works inside buildings, we can understand how a building’s design affects our comfort. Then, we’ll be able to figure out how to get the most out of the HVAC equipment in our homes and businesses.

HOW HUMANS GIVE OFF HEAT

Just as we absorb heat from everything around us, we also give off heat via conduction, convection, and radiation. We also transfer some of our body’s heat to our sweat, which evaporates off our bodies.

Of all those heat transfer methods, radiation makes up about 60% of the heat we transfer from our bodies! That’s why you start to feel very warm in places with lots of people in a building, such as a church service, a large gathering with friends and family, or an indoor concert. When all of those people radiate their heat in a small space, it can start to feel warm and stuffy inside.

However, we can use our bodies’ heat to our advantage when it’s cold. Have you ever heard of a space blanket?

They look like giant tinfoil blankets and are a bit of a fashion disaster. But when we wrap them around our bodies, they reflect most of our heat back towards our bodies. They’re a must-have in cold environments with poor heating!

HOW A BUILDING’S DESIGN AFFECTS RADIANT HEAT TRANSFER

We’ve already covered one way that heat enters buildings: through windows.

The sun gives off several waves, including visible light and UV. Those waves travel to the earth and can slip past windows. The windows may reflect some of the heat and light, but most of it passes right through the glass. So, rooms like lanais heat up very quickly because they allow light and heat to pass through easily. That’s the same reason why curtains and blinds can keep a room cool; the curtains or blinds help reflect or absorb heat before it passes into a room. (If you have dark curtains, they can get pretty warm!)

Insulation also affects radiant heat transfer in buildings. Buildings with reflective insulation (like the radiant barrier in the picture below) don’t allow heat to pass into the building easily.

The barrier looks like a giant space blanket! Radiant barriers reflect the heat attempting to enter the building because they have low emissivity. Emissivity refers to the ability of an object to absorb and emit radiation; objects with an emissivity of zero reflect all radiation, and objects with an emissivity of one absorb all radiation and emit lots of it.

Radiant barriers have a very low emissivity, as do space blankets, tinfoil, and aluminum. Humans have an emissivity of around 0.95, and polished aluminum has an emissivity of around 0.05! Many houses in Florida have a wooden base, and wood has an emissivity of 0.86-0.9. On top of that, drywall has a similar emissivity, so there’s a lot of heat absorption and radiation going on within the structure of a building. So, a radiant barrier beneath the roof decking can reflect heat and prevent it from getting into spaces like attics and crawl spaces. (Although it doesn’t work if the radiant barrier has no air gap.) When a home has a hot attic, that heat can transfer into the occupied space, which isn’t very comfortable.

WHERE DOES THE A/C UNIT COME IN?

So, since radiation is a major source of heat transfer in the home, then the A/C unit must do something to control it, right?

Actually, the A/C unit does pretty much nothing to control radiation! It only controls heat transfer via convection, which is when heat energy moves via a fluid like air. The way in which an A/C or fan moves heat energy may make us feel more comfortable. After all, the evaporation of your sweat has a cooling effect on your body. In fact, ceiling fans don’t cool the space down at all! They just move the heat towards either the top or bottom of a room, depending on the direction they spin.

But where is most of the heat in a building coming from? It’s coming from radiation, whether the source is the sun, your body, or furniture. The best way to combat the sneaky effects of radiant heat transfer is by adding reflective insulation into building designs and making the HVAC system work with the design’s strengths and weaknesses.

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