Hybrid Insulation Systems for RVs

Rising energy costs has given attention to improving both fuel efficiency and energy conservation for heating and cooling in recreational vehicles. The thin wall and roof construction inherent in RV design is a major restraint to increasing thermal performance. Consumer demand for energy efficient models have forced manufacturers to seek solutions that could get the job done within the same dimensional constraints.

Until the 1960's, energy had been so cheap that there was little incentive to insulate homes and commercial buildings. As dwindling supplies of oil in the US ground caused increases in costs in getting supplies to the markets, insulation was seen as a way to conserve energy. This gave impetus to the growth of a new product, fiberglass, which provided good insulation against heat transfer as well as sound attenuation properties. Fiberglass could be made in blankets of various thicknesses that could easily be customized to all manner of applications. Two-inch thick fiberglass batts became a natural solution for the thin walls and roofs of recreational vehicles and soon became a standard in the industry.

A test method to define the thermal performance, (ASTM C 518), was refined and is widely used to evaluate homogeneous materials. The scale that came into use was the R-value system. The higher the R-value, the greater the insulating value. As an example, two-inch thick fiberglass batt commonly used in the RV industry has a thermal resistance of R 7. By comparison, a 1" dead air space is equivalent to R-4, which is the maximum R-value that can be achieved with standard mass insulation products, and as a practical matter is about the limit of performance in that amount of space.

Reflective insulation and radiant barriers, consisting of low emittance surfaces, (highly reflective to infrared radiation, usually aluminum foil), have been around since the early 1920's. They predate fiberglass and were often used in homes built in the 20's and 30's. Unlike fiberglass, they do not "resist" heat transfer. Instead, they reflect radiant heat waves, (which have no temperature, only energy), from their surfaces before they can be absorbed by the surface. Since reflective insulation products reflect rather than resist heat transfer, testing their performance by the same method as is used for mass insulations like fiberglass will not result in an accurate measure of their thermal performance.

In order for reflective products to perform at their maximum, they need to face air spaces. The air spaces can be small, 1/4" or even less, to much larger, and depending on the direction of the heat flow, will result in different rates of performance. The American Society of Testing and Materials, (ASTM), has established industry standards for evaluating the thermal performance of reflective insulation and radiant barriers. ASTM 1363 has been adopted by the U.S. building code agencies as well as the scientific community as providing accurate representations of the performance capabilities of reflective products in a wide variety of applications and conditions. In addition thermal calculations can be made using industry accepted calculation programs such as were used in the example shown here.

Owners of RVs that have hybrid insulatin equipped units report improved the comfort inside the RV unit in all climates. Cooling down faster in hot weather conditions, warming up faster in cold weather conditions is common testimony. The problem is-- how do we quantify that? With mass insulation we can say that 4" is equal to a R-14. Adding a reflective barrier that does't have any air spaces as part of its structure doesn't add any significant R-values, but thermal performance does improve, especially in extremely hot temperatures. However, testing to make an evaluation has not been done because there isn't a standard method that would render a reliable report.

The function of a radiant barrier (RB), is to interrupt infrared energy rediating across air spaces. Infrared energy (radiation) is converted to conductive heat when it strikes a surface such as a wall or roof or any surface inside or outside of an RV. The energy transmits through material by exciting the molecules, until its path reaches the next air space where it then rediates from that surface to the next object(s) surface. A RB stops infrared energy that would otherwise be absorbed by the mass insulation. Its inclusion in the wall or ceiling cavity will reduce the thickness 01 the mass insulation required andlor improve the thermal performance, of the total walilceiling system, reducing the amount of energy needed to maintain comfort within the unit.

Some calculations have been made at an independent lab that specializes in thermal evaluation of insulation products, which show that a 4" deep roof truss with 1 layer of 2" fiberglass and a radiant barrier laid across the top of the truss with the roof deck then installed, will outperform the same truss with 2 layers of 2" fiberglass.

Background

R-values have been calculated for two roof assemblies consisting of combinations of fiberglass batt insulation and reflective air spaces. The program REFLEC2D.FOR was used to calculate the thermal resistances of the enclosed air spaces [ref.1]. The R-values for compressed fiberglass batts were calculated using previously published correlations [ref.2]. Both summer and winter conditions have been included in the analysis since the R-values for air spaces depend on the heatflow direction. All calculations are based on a temperature difference of 30°F across the roof cavity with a roof deck temperature of 100°F in the summer and 40°F in the winter. Variation of the thermal resistance of fiberglass batts with temperature was not included in the analysis. The fiberglass batt material was taken to be R 7 ft" h °FIBtu at thickness 2.25 inches and density 0.6Ib,lft'. The variation of roof cavity depth along the perimeter of the camper was taken into account using a parallel­path approximation.

Two layers of R 7 fiberglass batt insulation.

The lower R-value, (less than R-14), is the result of compression where the truss tapers to less than 4". It should be noted that roof temperatures that vary from the 750 F temperature tested value can significantly reduce the effective R-value of the mass.

One layer of R 7 fiberglass batt below radiant barrier foil installed to form two enclosed reflective air spaces. Conclusion

Reflective insulation or radiant barriers can improve the thermal performance of an RV, especially in the roofline. As the roofs heat up, (temperatures can reach 120° F and morel, the effect of radiant heat transfer greatly increases, thereby reducing the ability of mass insulation to resist transfer to the inside of the unit. The radiant barrier wilI effectively reflect the radiant heat resulting in much cooler temperatures inside. The R-value of the insulation system might not increase significantly with the addition of the RB, but because the radiant heat transfer is interrupted, the performance will, and that's what the customer looks for-comfort in all weather conditions. The enhanced performance will be most noticeable in hot sunny conditions. For year around living comfort, hybrid insulation systems make a lot of sense.

References

[1] Desjarlais, A.O. and D.W. Yarbrough, "Prediction of the Thermal Performance of Single and Multi-Airspace Reflective Insulation Materials", AASTM STP 1116(1991) pp 24-433.

[2] Graves, R.S. and DW. Yarbrough, "The Effect of Compression on the Materiel R-Value of Fiberglass Batt Insulation", Fire Safety and Thermal Insulation, St. Petersburg, Florida (1990) pp. 268-286.