There are a lot of definitions about “Thermal/Heat Insulation” and the common misinterpretation of “Thermal/Heat Insulation” = “Bulk/Mass/Batt Insulation” which is actually uses to resist Conduction Heat Transfer with certain R-Value.
Heat/ Thermal Insulation is a barrier material to resist / block / reflect the heat energy (either one or more of the Conduction, Convection or Radiation) to transfer from one side to another through the boundary between two systems which are at different temperatures from each other or from their surroundings. Heat transfer always occurs from the region of high temperature to low temperature
Radiant Barrier (= Reflective Insulation) is one of the Heat/Thermal Insulation to reflect Radiation Heat (Radiant Heat) transfer from one side to another due to the reflective surface (or low emittance).
As such Materials reflecting Radiant Heat with negligible “R-Value” should also be classified as “Thermal/ Heat Insulation”.
Roofs and Attics
Radiant solar energy strikes the roof, heating the shingles, felt paper and roof sheathing by conduction, and causing the underside of the sheathing and the roof framing to radiate heat downward through the attic toward the attic floor. When a radiant barrier is placed between the roofing material and the insulation on the attic floor, much of the heat radiated from the hot roof is reflected back toward the roof and the low emissivity of the underside of the radiant barrier means very little radiant heat is emitted downwards. This makes the top surface of the insulation cooler than it would have been without a radiant barrier and thus reduces the amount of heat that moves through the insulation into the rooms below the ceiling.
This is different from the cool roof strategy which reflects solar energy before it heats the roof, but both are means of reducing radiant heat. According to a study by Florida Solar Energy Center, a white tile or white metal cool roof can outperform traditional black shingle roof with a radiant barrier in the attic, but the black shingle roof with radiant barrier outperformed the red tile cool roof.
For installing a radiant barrier under a metal or tile roof, the radiant barrier may be applied directly over the roof sheathing. Then furring strips (1x4s) are applied over the radiant barrier before the metal or tile roof is applied. The furring strips ensure that the radiant barrier faces into a sufficient air space. If an air space is not present or is too small, heat may be able to conduct through the radiant barrier. Since the metal in the radiant barrier is highly conductive, the heat transfer would all be through conduction and the heat would not be blocked. According to the US Department of Energy, “Reflective insulation and radiant barrier products must have an air space adjacent to the reflective material to be effective.”
The most common application for a radiant barrier is facing into attics. For a traditional shingle roof, radiant barrier may be applied over the rafters or trusses and under the roof decking. This application method has the radiant barrier sheets draped over the trusses of rafters, creating a small air space above with the radiant barrier facing into the entire interior attic space below.
Another method of applying radiant barrier to the roof in new construction would be to use radiant barrier that is pre-laminated to OSB panels or roof sheathing. Manufacturers of this installation method often tout the savings in labor costs in using a product that serves as roof decking and radiant barrier in one.
To apply radiant barrier in an existing attic, a radiant barrier may be stapled to the underside of the roof rafters. This method offers all the same benefits as the draped method, in that dual air spaces are provided. However, it is essential that the vents still be allowed to remain open to prevent moisture from being trapped in the attic. In general, it is preferred to have the radiant barrier applied to the underside of the roof with an air space facing down to prevent the accumulation of dust, preventing the radiant barrier from conducting.
The final method of installing a radiant barrier in an attic is to lay it over the top of the insulation on the attic floor. While this method can be more effective in the winter there are a few potential concerns with this application, which the US Department of Energy and the Reflective Insulation Manufacturers Association International feel the need to address. First, a breathable radiant barrier should always be used here. This is usually achieved by small perforations in the radiant barrier foil. The vapor transmission rate of the radiant barrier should be at least 5 perms, as measured with ASTM, and the moisture in the insulation should be checked before installation. Second, the product should meet the required flame spread, which includes ASTM E84 with the ASTM E2599 method. Lastly, this method allows for dust to accumulate over the top surface of the radiant barrier, potentially reducing the efficiency over time.
According to a study by the Building Envelope Research Program of the Oak Ridge National Laboratory, homes with air-conditioning duct work in the attic in the hottest climate zones, e.g., the deep south of the U.S., could benefit the most from radiant barrier interventions, with annual utility bill savings up to $150; whereas homes in milder climates, e.g., Baltimore, could see savings about half those of their southern neighbors. On the other hand, if there are no ducts or air handlers in the attic, the annual savings could be even much less, from about $12 in Miami to $5 in Baltimore. Nevertheless, a radiant barrier may still help to improve comfort and to reduce the peak air-conditioning load.
One common misconception regarding radiant barrier is that the heat reflecting off the radiant barrier back out the roof has the potential to increase the roof temperature and possibly damage the shingles. Performance testing by Florida Solar Energy Center demonstrated that the increase in temperature at the hottest part of the day was no more than about 5 degrees F. In fact, this study showed that radiant barrier had the potential to decrease the roof temperature once the sun went down because it was preventing the heat loss through the roof. RIMA International wrote a technical paper on the subject, where they collected statements from the largest roofing manufacturers, and none said that radiant barrier would in any way affect the warranty of the shingles.
Attic Dust Accumulation
When laying a radiant barrier over the insulation on the attic floor, it is possible for dust to accumulate on the top side. Many factors like the dust particle size, dust composition and the amount of ventilation in the attic will affect how dust accumulates and the ultimate performance of radiant barrier in the attic. A study by Tennessee Valley Authority mechanically applied a small amount of dust over a radiant barrier and found no significant effect when testing for performance. However, TVA referenced a previous study which stated it was possible for the radiant barrier to collect so much dust that its reflectivity could be decreased by nearly half. It is not true that a double-sided radiant barrier on the attic floor is immune to the dust concern. The TVA study also tested a double-sided radiant barrier with black plastic draped on top to simulate a heavy dust accumulation, as well as a single-sided radiant barrier with heavy kraft paper on the top. The test indicated that the radiant barrier was not performing, and the small air spaces created between the peaks of the insulation were not sufficient to block radiant heat.
Radiant barrier may be used as a vented skin around the exterior of a wall. Furring strips are applied to the sheathing to create a vented air space between the radiant barrier and the siding, and vents are used at the top and bottom to allow convective heat to rise naturally to the attic. If brick is being used on the exterior, then a vented air space may already be present, and furring strips are not necessary. Wrapping a house with radiant barrier can result in a 10% to 20% reduction in the tonnage air conditioning system requirement, and save both energy and construction costs.
Radiant barriers are also quite effective in floor systems above unheated basements and crawl spaces. The radiant barrier may be either stapled below the floor joists, creating a single reflective air space, or between the joists, followed by some type of sheathing. Radiant barrier works extremely well in this application for two reasons. First, a radiant barrier that is not perforated for breathability acts as an excellent vapor barrier. This means that ground moisture will not be able to pass through the radiant barrier and enter the living space. Secondly, the floor is the only part of the building where the heat flow is always down, unlike a roof where the heat would be coming down during the summer and rising to escape in the winter. When the heat flow is down, 93% of the heat is radiant heat, which is exactly what the radiant barrier is designed to block.