Radiant Barrier

DEFINITION
CONSIDERATIONS
COMMERCIAL STATUS
IMPLEMENTATION ISSUES
GUIDELINES

CSI Numbers:

07194 Radiant Barrier


DEFINITION:

A radiant barrier is a layer of metallic foil, reflective paint, or other reflective material that blocks radiated heat, assisting in the energy performance of a building


CONSIDERATIONS:

A radiant barrier is used in our area principally as a cooling strategy. Buildings gain heat in three ways – conduction, convection, and radiation. A radiant barrier is useful in preventing heat gain from radiation.

Emissivity is an indication of a surface’s ability to emit heat by radiation. The lower the emissivity, the better the radiant barrier qualities of a material. Emissivity is measured on a scale of zero to one. Most foil type radiant barriers have an emissivity of 0.05 or below, which means 95% of the radiant heat is being blocked.

A radiant barrier is typically placed beneath roofs in our area to block the heat gain radiating from hot roofs. Temperature reductions of 10 degrees or more are typical during peak summer days. The reduction of attic temperatures is considered less important in highly insulated attics (R-30 and higher), in respect to conduction through the ceiling. If air conditioning ductwork is located in the attic, lowering the attic temperature reduces heat gain on the ductwork.

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Radiant Barrier Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory in most conditions Satisfactory
Satisfactory Satisfactory
Satisfactory in most conditions Satisfactory in most conditions
Satisfactory in Limited Conditions Satisfactory in Limited Conditions
Unsatisfactory or Difficult Unsatisfactory or Difficult


COMMERCIAL STATUS

TECHNOLOGY:

Well-developed.

SUPPLIERS:

Well-developed.

COST:

Radiant barrier varies according to the type of radiant barrier material selected, with lower costs around $0.10 per square foot. These costs do not include installation.


IMPLEMENTATION ISSUES

FINANCING:

Not an issue.

PUBLIC ACCEPTANCE:

There is moderate interest in radiant barriers among energy conscious buyers.

REGULATORY:

Not an issue.


GUIDELINES

Radiant Barrier

An airspace must be adjacent to the radiant barrier for it to work. (This is true for all radiant barrier applications.)

1.1 Radiant barriers are available in several configurations:

Pre-applied to rigid insulation

Pre-aApplied to structural sheathing

  • The reflective surface is on one side and faces downward into the attic.

Reinforced sheet radiant barrier material – with and without perforations, and as one-sided and two-sided reflective surfaces

  • Can be draped over the top of the roof rafters. In this application, allow the material to drape downward 2 inches or so between the rafters to create an airspace on both sides of the radiant barrier material.The radiant barrier material can be applied to the underneath side of the rafters. In this location, an airspace will be present on both sides of the material and can be effectively combined with a ridge and soffit venting system.

Multiple layers.

  • One layer of a typical radiant barrier material will block 95% of radiant heat gain. A second layer for the purpose of blocking additional radiant gain blocks less than 5%.There is an added R-value due to dead air spaces between the layers.

Radiant barrier paints

  • Typically applied on the underside of existing roof decking.

1.2 Radiant barrier material applied over the top of the attic insulation.

There are several precautions for this type of installation.

  • Radiant barrier material can function as a vapor barrier. Therefore, use only perforated material in this application.
  • Dust accumulation on the radiant barrier material’s surface inhibits its performance. This is unavoidable in the flat location over insulation.
  • When the material is placed flat on the attic insulation, it can also be punctured or torn during any service work that may need to be done in the attic (i.e. ductwork).

1.3 Radiant barrier installation and characteristics:

Air tightness does not affect performance. Sealing seams is not necessary.

The radiant barrier material should be 99% aluminum and the emissivity value should be 2% to 5%.

Radiant barrier paints are available with reflectance as high as 85% (as of 2007), far superior to versions available just a few years earlier.