Green Building Primer

Radiant Barrier
A radiant barrier is a layer of metallic foil that blocks radiated heat. An airspace must be adjacent to the radiant barrier for it to work. Radiant barriers are generally installed in an attic directly under roof rafters to reduce unwanted heat gain from the sun. They are more effective in hot climates than in cool climates, and aren't generally economical in areas where heating is the primary concern.



Emissivity is an indication of a surface's ability to emit heat by long-wave radiation. Thus the lower the emissivity, the better the radiant barrier qualities of the material. Emissivity is measured on a scale of 0-1. Most foil type radiant barriers have an emissivity of 0.05 or below, which means 95% of the radiant heat is being blocked.

Radiant barriers are available in several configurations:
1) Applied to rigid insulation
2) Applied to structural sheathing (The reflective side faces down into the attic)
3) Reinforced sheet radiant barrier material, with and without perforations, and with one-sided or two-sided reflective surfaces. It can be draped over the top of roof rafters or applied to the underneath side of the rafters. As an airspace will be present on both sides of the material, this can be combined with a ridge and soffit venting system
4) Multiple layers. An additional layer is added for the purpose of blocking the remaining 5% of radiant gain that a single layer of typical radiant barrier material fails to block. There is an added R-value due to dead air spaces between the layers.

As radiant barrier material can function as a vapor barrier, only perforated materials should be used in the application if radiant barrier material is to be applied over the top of attic insulation. Dust accumulating on the radiant barrier material surface inhibits its performance, and this is unavoidable in the flat location over insulation. Air tightness does not affect performance, and thus sealing seams is not necessary. A radiant barrier material should be 99% Al and the emissivity value should be 2-5%.

Ridge-and-Soffit Venting
A ridge-and-soffit venting system is a continuous, weather-shielded opening at the peak of the roof in combination with continuous screened openings along the eaves of the house. This system provides continuous air movement under the roof that washes the underside of the roof with air that is exhausted out the top of the roof through the ridge vent, reducing unwanted heat gain and moisture. This type of venting system can operate effectively regardless of wind direction and velocity.