RIMA on Radiant Barriers

The following information is provided by the Reflective Insulation Manufacturers Association (RIMA):

RADIANT BARRIERS

Definition: The generally accepted definition of a radiant barrier system specifies that the reflective material face an open air space. The idea is that a radiant barrier facing an enclosed air space is a "reflective insulation" with a measurable R- value.

Physics of Radiant Barriers

A "radiant barrier" is a reflective/low-emittance surface as defined by ASTM where the emittance is 0.10 or less on or near a building component, that intercepts the flow of radiant energy to and from the building component.

The aluminum foil shields that are commonly inserted behind radiators in older houses are radiant barriers, blocking radiant heat transfer from the radiator to the exterior wall.

It should be clearly understood that although a radiant barrier reduces heat loss and gain through the building envelope because it is installed in vented cavities (like attics), it is not an insulation material per se and has no inherent R-value.

Radiant Barrier Systems (RBS)

A "radiant barrier system" (RBS) is a building section that includes a radiant barrier facing an air space. An attic with a radiant barrier on top of the mass insulation on the floor, or under the roof is an RBS. A vent skin wall with a radiant barrier facing the vented air space is also an RBS.

The distinction between a radiant barrier "material" and radiant barrier "system" is not merely academic. In an attic, the effectiveness of a radiant barrier is significantly affected by the amount of attic ventilation. A vented attic with a radiant barrier is a very different system from an unvented attic with the same radiant barrier.

Types of Radiant Barrier Material

Several types of radiant barrier materials are available. Although they all have similar surface properties (and consequently similar performance), variations in materials and construction result in significant differences with respect to strength, durability, flammability and water vapor permeability.

Most products available commercially fall into two major categories:

1. Aluminum Foil Laminates - foil laminated to kraft paper, plastic films, or to OSB/plywood roof sheathing

2. Aluminized Plastic Films - a thin layer of aluminum particles deposited on film through vacuum process

Installing Radiant Barriers

Attics

The most common location for a radiant barrier system is in attics. Three basic configurations are used:

  1. Top side of truss under sheathing
  2. Under bottom of top cord
  3. Horizontal installation over existing ceiling insulation (This application is not recommended because it will be subject to loss of performance when dust accumulates on it.)

radiant barrier, attic insulation

RIMA-I acknowledges the placement of a radiant barrier on top of mass insulation in attic spaces subject to the following conditions:
  • The mass insulation and ceiling building materials should be checked for any evidence of moisture accumulation. Any existing moisture problem should be corrected before installing the radiant barrier.
  • Radiant barriers used for this application must have a water vapor transmission per of at least five (5), as measured by ASTM E-96.
  • Installation should be accomplished by laying the radiant barrier materials on top of the attic insulation without stapling or taping, so that it has very loose contact with the material below.
  • Radiant barriers for this application should meet a Class A, Class 1 flame spread and smoke development rating as determined by ASTM E-84.
  • The potential for contamination of the top surface by dust or dirt must be considered in specific applications where applicable.
  • As with all building materials, local building codes should be considered.

As noted before, a vented attic with a radiant barrier is a very different system from an unvented attic with the same radiant barrier. Common types of attic ventilation are:

  • Soffit to ridge
  • Soffit to gable
  • Soffit to soffit
  • Gable to gable

Most codes require at least a 1 to 300 ventilation rate. What this means is that for every 300 square feet of floor space, there should be one square foot of free vent area.

Walls

A very effective technique for walls is a vented skin wall using a radiant barrier. Furring strips are used to separate the outer skin from the internal structural wall. The wall is wrapped with a radiant barrier facing the vented air space. Vents are used at top and bottom to allow the heated air to rise naturally to the attic, where it is vented out through the roof vents.

radiant barrier, wall insulation     radiant barrier, wall insulation

TECHNICAL NOTE: Radiant barriers which are non-perforated are vapor barriers. Care should be exercised with placement!

Floors

Radiant barriers can also be used in floor systems above unheated basements and crawl spaces. The radiant barrier is either stapled to the underside of floor joists, creating a single reflective air space, or between the joists, followed by some type of sheathing, creating two separate reflective air spaces as shown below.

Radiant barriers are an ideal choice for this application because, in addition to their excellent thermal properties, they are also vapor barriers that prevent ground moisture from migrating into the living space above.

 radiant barrier, crawl space insulation

REFLECTIVE INSULATION

Definition: Thermal insulation consisting of one or more low emittance surfaces, bounding one or more enclosed air spaces (like bubbles).

Concept of Reflective Insulation

Standard types of insulation, such as fiberglass, foam, and cellulose primarily reduce heat transfer by trapping air or some type of a gas. Thus, these products or technologies reduce convection as a primary method of reducing heat transfer. They are not as effective in reducing radiant heat transfer, which is often a primary mode of heat transfer in a building envelope, in fact, these products, like most building materials, have very high radiant transfer rates. In other words the surfaces of standard types of insulation are good radiators of heat.

Reflective insulation uses layers of aluminum, paper, and/or plastic to trap air and thus reduce convective heat transfer. The aluminum component however is very effective in reducing radiant heat transfer. In fact, the metalized and foil materials commonly used in reflective insulation will reduce radiant heat transfer by as much as 97%.

Heat flow by radiation has been brought to the public’s attention with high efficiency windows, which commonly use the term "Low E" to advertise the higher performance ratings. The "E" stands for emittance and the values range from 0 to 1, with 0 being no radiation and 1 is the highest measure of emittance or radiation. Most building materials, including fiberglass, foam and cellulose have surface emittances or "E" values in excess of 0.70. Reflective insulations typically have "E" values of 0.03 (again, the lower the better). Therefore, reflective insulation is superior to other types of insulating materials in reducing radiant heat. The term reflective, in reflective insulation, is in some ways a misnomer, because aluminum either works by reflecting heat (reflectance of 0.97) or by not radiating heat (emittance of 0.03). Whether stated as reflectivity or emissivity, the performance (heat transfer) is the same. When reflective insulation is installed in building cavities, it traps air (like other insulation materials) and therefore reduces heat flow by convection, thus addressing all three modes of heat transfer. In all cases, the reflective material must be adjacent to an air space. Aluminum, when sandwiched between two pieces of plywood for example, will conduct heat at a high rate.

All insulation products including reflective insulation are measured by R-values, whereby the "R" means resistance to heat flow. The higher the R-value, the greater the insulating or thermal performance of the material.

Reflective insulation is a non-toxic, user and building owner safe, and environmentally safe building material. In addition, the products are typically recyclable and thus can be termed a Green Building Material.

Another benefit is that the reflective insulation can also serve as a high performance and thus effective vapor barrier.

Understanding a Reflective Insulation System (RIS)

Layers of aluminum or a low emittance material and enclosed air spaces, which in turn provide highly reflective or low emittance cavities adjacent to a heated region, typically form a reflective insulation system. Some reflective insulation systems also use other layers of materials such as paper or plastic to form additional enclosed air spaces. The performance of the system is determined by the emittance of the material(s), the lower the better, and the size of the enclosed air spaces. The smaller the air space, the less heat will transfer by convection. Therefore, to lessen heat flow by convection, a reflective insulation, with its multiple layers of aluminum and enclosed air space, is positioned in a building cavity (stud wall, furred-out masonry wall, floor joist, ceiling joist, etc.) to divide the larger cavity (3/4" furring, 2" x 4", 2" x 6", etc.) into smaller air spaces. These smaller trapped air spaces reduce convective heat flow.

Reflective insulation differs from conventional mass insulation in the following:

  • Reflective insulation has very low emittance values "E-values" (typically 0.03 compared to 0.90 for most insulation) thus significantly reduces heat transfer by radiation;
  • A reflective insulation does not have significant mass to absorb and retain heat;
  • Reflective insulation has lower moisture transfer and absorption rates, in most cases;
  • Reflective insulation traps air with layers of aluminum, paper and/or plastic as opposed to mass insulation which uses fibers of glass, particles of foam, or ground up paper;
  • Reflective insulation does not irritate the skin, eyes, or throat and contain no substances which will out-gas;
  • The change in thermal performance due to compaction or moisture absorption, a common concern with mass insulation, is not an issue with reflective insulation.

Types of Reflective Insulation Materials

Reflective insulation has been used effectively for decades and is available throughout the world. The following are the major types of reflective insulation currently available:

  • Layer or layers of aluminum foil separated by a layer or layers of plastic bubbles or a foam material;
  • Multiple layers of aluminum, kraft paper, and/or plastic with internal expanders an flanges at the edge for easy installation;
  • Single layer of aluminum foil laminated to a kraft paper or plastic material when encapsulated with an adjacent air space.

Applications for Reflective Insulation Materials

Reflective insulation materials are designed for installation between, over, or under framing members and as a result, are applicable to walls, floors, and ceilings. Applications for reflective insulation extend to many commercial, agricultural and industrial uses, such as panelized wood roofs, pre-engineered buildings, pole barns and other wood framed structures. A few representative applications are listed below:

Residential Construction, New and Retrofit - Walls, basements, floors, ceilings, roofs, and crawl spaces.

Commercial Construction, New and Retrofit - Walls, floors, basements, ceilings, roofs, and crawl spaces.

Manufactured Housing Construction, New and Retrofit  - Walls, floors, roofs, and crawl spaces.

Other Uses, New and Retrofit - Water heater covers, cold storage units, poultry, and livestock buildings, equipment sheds, pipe insulation and recreational vehicles.

Installing Reflective Insulation Systems

Reflective insulation products incorporate trapped air spaces as part of the system. These air spaces, which may be layered or closed-cell, can be included in the system either when the product is manufactured or while it is being installed. In either case, the advertised performance of the insulation requires that these air spaces be present after the product is installed. The labeled R-values will not be achieved if the product is not installed according to the instructions of the manufacturer.

The thermal performance of the reflective system varies with the size and number of enclosed reflective spaces within the building cavity. Most reflective systems range from one to five enclosed air spaces.

There are other beneficial considerations for using reflective insulation. Generally, these products have a very low water vapor and air permeance. When installed properly, with joints taped securely, reflective insulation materials are efficient vapor retarders and an effective barrier to air and radon gas.

Since reflective insulation materials are effective vapor retarders, care should be taken to ensure that they are installed correctly within the structure. Correct installation depends on the climatic conditions and moisture sources involved. An appropriate installation ensures that all joints and seams are butted against each other and taped, or overlapped and taped. This will reduce the possibility of moisture condensation within the cavity and improve performance.