RIMA on Radiant Barriers
The following information is provided by the Reflective Insulation Manufacturers
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
The most common location for a radiant barrier system is in attics. Three
basic configurations are used:
- Top side of truss under sheathing
- Under bottom of top cord
- 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.)
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.
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.
TECHNICAL NOTE: Radiant barriers which are non-perforated
are vapor barriers. Care should be exercised with placement!
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
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
- 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