Understanding Radiant Heat Transfer and Radiant Heat Barriers

Understanding Radiant Heat Transfer and Radiant Heat Barriers

The Sun is our greatest source of heat and we are always in a battle to control that heat in our homes. The heat we experience from the sun is made up of different parts ultraviolet heat visible light heat and infrared heat. When you’re out in the hot sun, you feel it’s 3% from ultraviolet light, 40% from visible light, and 57% from infrared heat. This same heat is what strikes the roof of your home and heats up the inside. When the Sun hits your roof, most of the heat energy is absorbed by the roof and the heat is conducted through the roof assembly towards the cool interior of your home. When the heat come no longer conduct because it hits the air space on the underside of your roof deck. It changes back to radiant heat and transfers down to the next object in its path. Years ago, when we did not insulate homes, the majority of that radiant heat would conduct through the ceiling and then into the home’s interior. We we start to improve the home, we are able to work on the different types of heat transfer to keep the heat out. When you use white shingles or a white coating on your roof, you are cutting down the heat that is gained from the visible light heat, the top 40% of the heat spectrum, but white roofs have no impact on the radiant heat gain of the home. When we insulate the home, we are slowing down the conduction of the heat into the home by adding a high-R layer between the heat and our home’s interior. The R means resistance to conductive heat. Note that insulation will slow down the transfer heat, but ultimately it will not stop the heat, that is why a home is not air-conditioner in summer will get extremely hot inside, the heat works its way in by adding ventilation to our attics. We can cut down on the heat gain from the air itself getting hotter in the closed addict. Ventilation works on the conductive heat gain by allowing the hot air to escape from the ridge vince or gable ends of attic, but even whit insulation and ventilation, your roof is still letting in all that radiant energy. The additional of a radiant heat barrier on the underside of the roof deck will have a dramatic effect on the performance of the home. A radiant heat barrier like heat block ultra will actually stop over 80% of the radiant heat that is trying to enter the attic from your roof. This is very important, because studies show that over 80% of the summer heat gain of a home in the south is directly from radiant heat. The radiant heat barrier accomplishes this by stopping the heat energy when it is trying to change from conducting through the roof to radiating from the underside of the roof deck. Even though we have blocked the heat from coming in, the shingles are not damaged.

By stopping the heat transfer, we cut down the heat load that is trying to get through the insulation into the house, and we are also keeping that heat from heating up air conditioner equipment that may be in the attic and the ductwork sending the cold air through the attic area. To understand the radiant heat transfer, we need to discuss several topics. Radiant heat is measured by using E factor which stands for Emittance or Emissivity. Radiant heat always moves from the hotter to the colder. And radiant heat doesn’t do anything until it strikes an object, when it strikes something, the energy does only a combination of two things, and that how we come up with its E factor. The E-factor scale is from 1.0 to 0.0, if radiant heat strikes something, and all bounces off and goes elsewhere, the E factor of that object is 0 (zero). If the radiant heat energy is totally absorbed by the object, the e-factor is 1.0. From data, we can note that most amid over 90% of the radiant heat that strikes them, the low E material like heat block ultra and low E window emit very low heat.

Radiant barriers work in the infrared portion of the heat spectrum we mentioned earlier, it is important to understand that a radiant barrier is a surface effect, that means we must have air on one side for it work, if you have something come in physical contact with any radiant barrier, it just becomes a conductive layer, and the heat passes right through, it is also important to note that it only takes a very thin layer about 1/1000 inch to create the radiant barrier. Radiant barrier has little effect on air temperatures, to demonstrate this, imagine that you are standing in the Sun and you are hot, we all know that if you step two feet and get under the shade of a tree, you will feel cooler, the air didn’t get any cooler, you actually get cooler, because the building or tree forms a pretty good radiant barrier. This is the same effect a radiant heat barrier has on your home. It puts it in the shade all day and every day.


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