Do Radiant Barriers Actually Work? Head to Head Comparison Test!
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3 سال پیش
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There is a lot of
There is a lot of debate online as to whether radiant barriers actually work, and if so - how. In this video, Jordan builds two identical doghouses using LP TechShield Radiant Barrier roof decking. The only difference in the two is that one dog house is built CORRECTLY - with the shiny side in towards the attic , and the other is built INCORRECTLY - with the shiny side facing out towards the sky. The results are impressive with the LP TechShield installed correctly beating the incorrect install by 18-20 degrees F. This translates directly into less heat load on cooling equipment and a lower energy bill, especially in hot climates.
So how does a radiant barrier work?
Heat transfers in three ways: radiation, conduction, and convection. The sun's rays radiate to the earth through space via, you guessed it, radiation, and require no medium like air, water, metal ect. to transfer. Some of the suns energy is radiated directly onto our homes roof where it is absorbed and transferred via conduction. Conduction is the transfer of heat through material. Think of a frying pan on a stove. The hot flame transfers heat into the frying pan, and the frying pan conducts through itself and into the food that is in direct contact. A cast iron frying pan's handle will get hot even though it has not come in direct contact with the flame. The heat has conducted through the pan and into the handle. The same thing happens on your roof. The suns radiant energy is absorbed into the roof and the heat is conducted through the roof materials to the bottom side of your roof deck. When the roof deck is hot, the energy will try to find equilibrium with the surroundings. Some of the energy will conduct into the surrounding rafters. Some of the heat will conduct into the adjacent air, which will in turn become buoyant and move, resulting in convection (the movement of heat through the movement of a medium) and some of the heat will re-radiate into the attic space heating the insulation below, which will in turn conduct heat towards your conditioned space.
Now, there are very effective strategies to combat the movement of heat from the roof to the conditioned living space. A well ventilated attic with allow heated air to escape through the ridge vent and allow cool air to enter through soffit vents limiting convection, and good insulation will limit conduction into the living space. But what about the third mode of heat transfer - radiation? This is where a radiant barrier comes into play. The aluminum backing on LP TechShield is a low emissivity material. This means that it is not a very good radiator of energy. So the energy that has been absorbed by the roof and conducted through the decking is not easily radiated off the underside of the roof decking and must find another way out - either by conduction, convection, or re-radiating back into the sky.
The real-world results for installing LP TechShield radiant barrier is about a 20 degree Fahrenheit cooler attic temperature, which is significant.
However, to achieve this advantage, it is critical that the product be installed correctly! This means that the low emissivity backing (the shiny side) is installed inward toward the attic. It seems intuitive to place to shiny side our to "reflect" the sun's rays, and this might have some effect if it weren't for the pesky need to put some sort of roofing in direct contact with the decking rendering the reflectivity pointless and changing the heat transfer mode to conduction.
Now that we understand how radiant barriers work, and have seen in real world testing that there is a significant difference in attic temperatures, the remaining question is that of cost. Is there an ROI for installing radiant barrier? In a renovation situation, I am not sure if a add-on product, whether foil or paint, is worth the time and expense. However, in a new build or complete re-roof of a ventilated attic where you must put down a roof deck anyway, I think it would be a missed opportunity to improving the comfort and performance of your home by using LP Techshield.
JUST MAKE SURE YOU INSTALL IT CORRECTLY!! SHINY SIDE IN!!!
So how does a radiant barrier work?
Heat transfers in three ways: radiation, conduction, and convection. The sun's rays radiate to the earth through space via, you guessed it, radiation, and require no medium like air, water, metal ect. to transfer. Some of the suns energy is radiated directly onto our homes roof where it is absorbed and transferred via conduction. Conduction is the transfer of heat through material. Think of a frying pan on a stove. The hot flame transfers heat into the frying pan, and the frying pan conducts through itself and into the food that is in direct contact. A cast iron frying pan's handle will get hot even though it has not come in direct contact with the flame. The heat has conducted through the pan and into the handle. The same thing happens on your roof. The suns radiant energy is absorbed into the roof and the heat is conducted through the roof materials to the bottom side of your roof deck. When the roof deck is hot, the energy will try to find equilibrium with the surroundings. Some of the energy will conduct into the surrounding rafters. Some of the heat will conduct into the adjacent air, which will in turn become buoyant and move, resulting in convection (the movement of heat through the movement of a medium) and some of the heat will re-radiate into the attic space heating the insulation below, which will in turn conduct heat towards your conditioned space.
Now, there are very effective strategies to combat the movement of heat from the roof to the conditioned living space. A well ventilated attic with allow heated air to escape through the ridge vent and allow cool air to enter through soffit vents limiting convection, and good insulation will limit conduction into the living space. But what about the third mode of heat transfer - radiation? This is where a radiant barrier comes into play. The aluminum backing on LP TechShield is a low emissivity material. This means that it is not a very good radiator of energy. So the energy that has been absorbed by the roof and conducted through the decking is not easily radiated off the underside of the roof decking and must find another way out - either by conduction, convection, or re-radiating back into the sky.
The real-world results for installing LP TechShield radiant barrier is about a 20 degree Fahrenheit cooler attic temperature, which is significant.
However, to achieve this advantage, it is critical that the product be installed correctly! This means that the low emissivity backing (the shiny side) is installed inward toward the attic. It seems intuitive to place to shiny side our to "reflect" the sun's rays, and this might have some effect if it weren't for the pesky need to put some sort of roofing in direct contact with the decking rendering the reflectivity pointless and changing the heat transfer mode to conduction.
Now that we understand how radiant barriers work, and have seen in real world testing that there is a significant difference in attic temperatures, the remaining question is that of cost. Is there an ROI for installing radiant barrier? In a renovation situation, I am not sure if a add-on product, whether foil or paint, is worth the time and expense. However, in a new build or complete re-roof of a ventilated attic where you must put down a roof deck anyway, I think it would be a missed opportunity to improving the comfort and performance of your home by using LP Techshield.
JUST MAKE SURE YOU INSTALL IT CORRECTLY!! SHINY SIDE IN!!!
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