Radiant barriers
Radiant barrier products are sometimes recommended for attics to decrease the amount of heat entering the living space. This note explains what radiant barriers are and why they may not be a cost-effective choice for you.
What Are Radiant Barriers?
Heat energy can be transferred between your living space and your attic in three ways: radiated energy, conduction, and convection. Conduction (why the handle of a pan on the stove gets hot even though it is not in the fire) and convection (how hot air rises) are best addressed with attic insulation. Radiated energy is why your skin feels warm in sunlight, and this form of energy can pass, though attenuated, through attic insulation. In theory it can be completely suppressed with radiant barriers.
Radiant energy takes the form of heat, or infrared light. You can’t see it, but you can feel this energy being emitted from blacktop on a hot day, or from direct exposure to the sun. Because it is a form of light, it can be blocked by a shiny surface in the same way that a mirror reflects visible light.
The radiant barrier is a continuous shiny sheet material that is installed in your attic to reflect heat energy away from the conditioned living space.
Radiant Barriers are usually installed on the underside of the rafters in your attic. This position minimizes the amount of dust and dirt that accumulates on the surface. Dust or dirt of any kind absorbs the radiant energy and lowers the beneficial effects of the barrier.
The best candidate homes for radiant barriers are new construction, where the radiant barrier can be stretched on the rafters before the decking is installed, or attics with clear access to the rafters. Low-pitched roofs or roofs with lots of trusses are not good candidates, because the installation is more expensive and coverage is not ideal. Dusty attics are not good candidates because dust degrades the effectiveness of the barrier.
When are Radiant Barriers a good investment?
A Department of Energy bulletin states that radiant barriers have been shown to be effective in southern climates for reducing summer cooling bills. However, the barriers do not help much with winter heating bills. Where attic insulation is below R-19, radiant barriers can reduce the heat transfer through the attic by 16 to 42%, representing a potential reduction in home energy bills of 2% to 10% (since only 15%-25% of the cooling load is due to heat coming through typical ceilings.) Winter bills will see an even smaller savings as will houses with higher insulation R-values. Furthermore, all the testing has been done with clean barriers; older barriers will suffer degradation, predicted to be as much as half of their initial effectiveness in one to ten years (depending on how dusty the attic is and how the barrier is installed.)
For Atlanta, the DOE study calculated likely savings of $2-$8 per 100 square feet of attic over a 25 year period, but costs of installation can be over $20 per 100 square feet. For these reasons, radiant barriers may not be right for you, and it may be more cost-effective to have a thick blanket of blown-in fiberglass insulation installed instead.
