QUALITY SERVICE SINCE 1989
Radiant Slab Insulation
There are new products making the rounds involving the use of multiple layers of bubble pack with foil placed on various surfaces as ground cover below radiant slabs. Some of these products are claiming R-10 in a ½ inch thick tarp. Be careful, these systems do not work.
Now, radiant barriers work well in space. Yes, NASA uses them, and they work great for their application. When I was a kid, we could buy “Space Blankets” for camping. They were basically scrim reinforced aluminum foil blankets. They made good tents, and if you set a fire near the tent the foil would bake you, but if you just wrapped yourself in the foil, you froze.
There are two problems with using radiant barriers as insulation. First, when you heat air, it wants to move. If you give it an escape path, it will escape and carry away heat and moisture. This is called convection, and it is a very efficient way of moving heat. In space there is no air, so radiant barriers are very effective.
The second is heat loss by conduction. Most people are familiar with the pitfalls of using steel studs in homes because the heat transfer through the studs is so large. And, we also know aluminum is much better at transferring heat than steel. Therefore, putting a sheet of aluminum in an area that is below ground (and usually damp) is not going to help prevent heat from moving out of the slab and into the ground.
In space, heat is developed by a transfer of energy. And, the only energy to transfer is radiant energy from the sun and various stars. Foil is very good at reflecting this type of energy and the heat it creates. Insulations developed to prevent heat loss by conduction or convection are ineffective in space because these heat transfer mechanisms rely upon the movement of energy through materials by transfer of molecules, or by moving the molecules themselves. In space, there is no material to move to transfer the heat. You have solids or a vacuum. Heat cannot move by conduction or convection in a vacuum. But on earth, we have water and air to fill all the spaces where we don’t have solids or other liquids. That means we have efficient means to move heat by conduction and convection. Therefore, radiant barriers that do not completely and permanently stop movement of air and water are ineffective.
Claims of R-10 in a ½ inch thick material are also less than accurate. R&D Services of Tennessee is a testing lab founded by Ron Graves and Dave Yarborough. Ron & Dave retired from Oak Ridge National Labs about 10 years ago. Their company is frequently referenced when manufacturers seek information or statistics through testing of their products. Dave Yarborough has gone on record saying the maximum theoretical R value of any material using air as an insulating medium is R-5.6 per inch of thickness. To go higher you need encapsulated gas, a vacuum, or nano scale powders. The bubbles in bubble pack do not constitute ‘encapsulated gas.’ He is referring to materials like argon or refrigerants suspended in cells so small the molecules of gas cannot move.
Lab results and actual field results are frequently different. In a perfect world, it is possible to use this system, in our world it is flying too close to the flame. Please reference Energy Design Update’s September 2005 issue, page 16.
The best materials for insulating below slabs is rigid foam sheets at R-10 or higher.