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Article # 0020

A Case for Radiant Barrier Systems Cost Effective Thermal Management

by  Michael J. Sowers, P.E.

Introduction

 

During recent and extensive remodeling of my residential property in Houston, Texas, I elected to install radiant barriers in the attic, and behind the brick veneer. This installation was facilitated by use of aluminum foil faced engineered wood panels for the replacement roof deck, and also use of the same type panels for exterior wall sheathing. The extra cost per each 4 X 8 X panel, for the foil facing, was approximately two dollars fifty cents ($2.50) per panel. This calculates to under eight cents ($0.08) per square foot. No additional labor costs were necessary, although handling the foil faced panels does require a slightly increased need for caution, to avoid damage to the thin foil facing on the panels, during movement and placement of the panels.

 

Subsequent experience with the radiant barriers created by these aluminum foil faced panels, has been absolutely stellar. However, several issues did come out of this installation effort, and will be discussed below. It was discovered that despite the fact instructions for placement and handling of these panels were printed on each panel by the manufacturer, some essential information was ignored, and other information was misunderstood.

 

Some Technical Background

 

Technical considerations, about thermodynamic processes pertinent for justifying the extra expense for radiant barriers, takes focus with some consideration of the Second Law of Thermodynamics. This Second Law describes the relationship of entropy to thermal equilibrium between isolated systems, and stipulates that systems with higher levels of thermal energy will tend to transfer thermal energy to systems with comparably lower levels of thermal energy, and not the reverse. Thus it is probably OK to say that in a closed system where something is hotter than another something that is comparably cooler, the hotter something will serve to transfer energy by conduction, convection, radiation (however it can transfer energy) to the something comparably cooler, until entropy achieves its highest level and both somethings are at thermal equilibrium, or perhaps the somethings achieve the same temperature (where entropy is now maximum for the closed system of somethings). For all of you thermodynamic purists, my apologies, I took basic thermodynamics during summer session, and that was a long, long time ago (in those bygone days it was thermos bottle theory, or thermo-gd-mics).

 

Back in the real world, the Second Law predicts that thermal energy in the electromagnetic (EM) spectrum, from nuclear processes on the sun, will transfer radiant thermal energy between the sun and earth through the void of space, resulting in a radiant thermal energy heat transfer into whatever terrestrial material the sunlight initially strikes. Using the assumption that sunlight radiant energy is delivered directly to the roof of your house, without being intercepted by tree leaves or anything else shading your roof, it should come as no surprise that radiant energy heat would be added to your roofing shingles. This added heat is subsequently conducted from the shingles toward the interior attic air space (also convected into hotter outside ambient air being heated by the shingles at the surface of the roof). Conducted heat flows, via the various layers of materials comprising the roofing system, and will typically result in a net temperature increase for all the roofing material directly above the interior air space of your attic.

 

Analytical Tools and Data from Research

 

Research data from a number of sources, including Oak Ridge National Laboratory Buildings Technology Center, offers insight into what temperature increases can be realized from radiant energy striking typical roof shingles, and how much heat from this sunlight radiant energy might be added to the house. Specific data is derived from measurements taken in test houses equipped with test sensors strategically positioned at designated locations throughout these houses. Measured data values, for the data pool, are based on actual radiant energy conditions at the times and locations where specific measurements are taken. From this pool of actual test data, realistic values suggest that temperature increases to roofing shingles and other components of the roofing system can range in temperatures up to one hundred ninety degrees Fahrenheit (190oF). Data from this same research suggests that radiant energy can impart as much as ninety percent (90%) of all heat being gained by your house, and the source for all of this radiant energy is essentially the lucky old sun.

 

At this point in the analysis, of Second Law effects, it is important to realize that now the relatively hot material in the roofing system (assumed to be at a temperature of approximately 190oF) is the new source of radiant energy to be considered, and this new source of radiant energy is poised to radiate from the lower boundary of the roof deck material, facing into the attic air space. In this example, the specific materials are foil faced engineered wood products (plywood or OSB panels), and to use this type material as a radiant barrier, the foil on each panel must, by design, face into the air space of the attic. Thus, if someone were to look through an opening into the attic from inside the house, you would actually see the silver colored aluminum foil facing, on the underside of each roof deck panel.

 

With the foil facing, (at a temperature of approximately 190oF) on the roof deck panel, as the new source of EM radiant energy into the attic space for transfer of heat by Second Law considerations, what is the real deal? Well, now it is time to look into the blackbody properties of this foil, and consider the reflectivity and emissivity of this material.  From blackbody definitions and mathematical relationships, it is considered that a perfect blackbody will radiate all energy possible using all wavelengths possible based on the temperature of this blackbody, and such a perfect blackbody radiator has an emissivity value of unity (1) and a reflectivity value of zero (0). With consideration of the conjoined blackbody relationship of the sum of emissivity and reflectivity, for any specific material, being equal to unity (1), it is easy to identify both the emissivity and reflectivity of this aluminum foil facing material. Using a table of emissivity for various materials, it is possible to look up experimentally derived emissivity data for the aluminum foil panel facing (also usually provided by the panel manufacturer as part of the panel specifications). From these sources it is possible to determine an emissivity value of approximately 0.06 for the foil. This means the reflectivity of this foil is approximately 0.94, or the foil will reflect about 94% of any radiant energy impinging on this particular material (as compared to 0% for a perfect blackbody reflector), and the foil will only emit 6 percent as much energy as a perfect blackbody emitter at the same temperature (190oF).

 

It should be noted, however, the radiant barrier foil material at 190oF will heat the air it faces in the attic area, and as a result, heat will be transferred from the hot foil into the attic area by convection. Fortunately, convection is a relatively puny mechanism for bulk heat transfer, and the hot air will simply rise to the top of the attic air space from along the surface of the hot foil, and hopefully, with the foresight to have soffit and ridge vents installed, the hot air will simply exhaust into the outside atmospheric air plenum via the ridge vent along the peak of the roof. There is no conduction path into the attic space from the hot foil interface, and the radiant barrier foil (a low-e value material) has done its job to essentially minimize heat transfer by radiation, which surprisingly is frequently the dominant heat transfer mechanism in these Second Law thermodynamic processes (consider the sun to earth situation, where radiation is the only thermodynamic process possible for heat transfer).

 

The foil faced panels used for exterior wall sheathing are deployed in the same manner as the roof deck, with the only difference being the aluminum foil on the panels is oriented facing toward the exterior of the house, in other words facing toward the moisture barrier and brick veneer. As in the case described above for the roof deck radiant barrier, in order for the radiant barrier to properly function (true for any and all radiant barriers), it is essential that any low-e material (in this case, as per above, the aluminum foil panel facing) be positioned to face an air gap. However, in the case of the foil faced panels being used for exterior wall sheathing, when Second Law considerations are applied to the thermodynamic processes actually imbedded in the walls of this house, the new hotter source of radiant energy will typically be the exterior brick veneer and the moisture barrier material immediately opposite the foil on the exterior wall sheathing panels. With this configuration in summer months, when the temperature of the outside air, along with the bricks and moisture barrier are hotter than the interior of the house, and Second Law heat flows are typically from the house exterior toward the house interior, it is the reflective properties of the radiant barrier foil that serves to reflect approximately 94% of the radiant heat energy (as compared to the perfect blackbody reflection of 0%) back toward the moisture barrier and brick veneer. In the winter months, when outside air, along with the brick veneer and moisture barrier are the cooler areas as compared to the heated interior areas of the house, and your energy bill is being paid to maintain a typically hotter temperature in the interior of the house as compared to the exterior of the house, the low-e foil on the exterior wall sheathing now functions to only radiate at an emissivity level of approximately 6%, as compared to the perfect blackbody rate of 100%. As for the radiant barrier in the attic during the winter months, any radiant energy flow from the now hotter interior vs. exterior areas, is reflected back toward the interior of the house at the 94% reflectivity level (as compared to the perfect blackbody 0% reflectivity). From this analysis it is possible to see how the energy you are paying for to heat the interior of the house is prevented from being lost to the exterior of the house, except by the relatively puny thermodynamic process of convection, now the dominant heat transfer mechanism in the attic, and also the dominant heat transfer mechanism in the air gap space next to the foil on the exterior wall sheathing panels.

 

Also from this analysis, it should be evident that if you are anticipating using radiant heat effects, with Second Law heat transfer from exterior to interior in the winter months, the radiant barriers will minimize this process. However, do not forget the thermodynamic properties of your windows, also available as possibilities for tuning in favor of specific radiant energy flows (mainly IR) for passive winter heating. But, that is another story for consideration using Second Law heat flow analysis.

 

Hopefully Helpful Radiant Barrier Tips

 

As promised above, here are some things to consider to improve the functional life cycle of any radiant barrier system. This includes some hopefully helpful information for front side planning to: a) assist in reducing installation degree of difficulty, and b) to facilitate installation forward progress, with first pass success, for new implementation or retrofit of an efficient, cost effective, long life cycle radiant barrier, with a focus on using foil faced OSB or plywood panels:

 

 

 

        It is possible to purchase foil faced engineered wood panels with either a perforated or an unperforated foil facing (this is completely based on manufacturer supply side options). In the case of using an unperforated foil facing, it may be stated by the manufacturer that this foil facing material is suitable for use as a vapor barrier. Some people mistakenly make a leap to assume this is the same as rating the foil facing as a moisture barrier. This erroneous assumption, for all the reasons described above, often results in an attempt to use the foil facing for applications which will eventually or immediately ruin the foil for use as a radiant barrier. If the low-e foil material is actually used as a barrier against bulk moisture (without an actual intermediate moisture barrier between the foil and a source of bulk moisture, such as directly behind a brick veneer) bulk moisture will ultimately migrate via mortar joints to wet the foil facing, with bulk moisture and mortar chemicals providing damaging results, also as described above.

 

        Requirements to have an air gap immediately adjacent to the low-e foil facing material, to successfully use the foil as a radiant barrier, also presents the requirement to include additional space for this air gap. Information available from the Reflective Insulation Manufacturers Association (RIMA), and other sources explains how to use furring material to provide spacing for a moisture barrier immediately adjacent to the foil facing material in walls or siding, and also how to employ this furring as a structural member to secure brick ties for a brick veneer (which is also typically a building code requirement). The use of a brick veneer also requires suitable air gap spacing for drainage and ventilation, between the moisture barrier, on the exterior wall, and the brick veneer. Brick Industry Association (BIA) technical recommendations offer specific information about requirements for tolerances to apply in stacking bricks used as a non-load bearing brick veneer, i.e. on a preexisting slab-on-grade brick ledge. With a requirement for two (2) air gaps, in a brick veneer situation, adjustments and new spacing may need to be considered for positioning of wall studs and sheathing with respect to requirements for stacking bricks on a preexisting brick ledge. It is possible to widen a brick ledge, but this does present a degree of difficulty and the potential for addition costs, that may be resolved by early planning.

 

        For my particular implementation I did some tests, using a small battery powered TV set, to try to determine if antenna reception had been impacted by installation of the radiant barriers. I was not able to identify any new problems with direct from the air reception of radio or TV signals, cell phone usage, or WiFi, WiMax, or whatever. These matters are most certainly something to give serious consideration to, and also possibly attempt to examine for direct benchmark data for any specific application. All panels in my radiant barriers are not bonded or connected together, nor are they bonded to an earth ground. Before I went into the implementation of the radiant barriers, I had already decided I would add an external antenna system if necessary, and determined that life cycle energy savings would cover the costs of such an antenna system.


Article # 0020       TEST QUESTIONS:

1.   Approximately, what was the extra cost for the radiant barrier foil?

  1. $2.50 per square foot

  2. $0.90 per square foot

  3. $0.08 per square foot

  4. $0.025 per square foot

2.   Radiant energy can impart as much as ___ of all heat being gained by your house.

  1. 50%

  2. 70%

  3. 90%

  4. 99%

3.   According to test data, the temperature increases to roofing shingles and other components of the roofing system can range in temperatures up to ___ degrees Fahrenheit.

  1. 150

  2. 170

  3. 190

  4. 200

4.   What was the approximate emissivity for the aluminum foil panel facing?

  1. 0.06

  2. 0.32

  3. 0.94

  4. 0.98

5.   After the installation of the radiant barrier to the attic and walls, the dominant heat transfer mechanism is ____?

  1. radiation

  2. convection

  3. conduction

  4. none of the above

6.   If aluminum foil gets damp or wet repeatedly the effectiveness of the radiant barrier will be _____ .

  1. reduced

  2. improved

  3. unaffected

  4. doubled

7.   Where the radiant barrier material comes into direct contact with another building material, a ____ will occur.

  1. conductive path for heat flow

  2. electrical short

  3. convective path for heat flow

  4. vapor barrier fault

8.   What does RIMA stand for?

  1. Radiant Insulation Makers of America

  2. Reflected Imaging Material Association

  3. Reflective Insulation Manufacturers Association

  4. None of the above

9.   What does BIA stand for?

  1. Better Insulation for America

  2. Brick Industry Association

  3. Brick Insulation Application

  4. Bureau for Insulation Applications

10.   A perfect blackbody radiator has an emissivity value of ____. 

  1. 1.0

  2. 0.5

  3. 0.0

  4. 100

 

 

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