ABSTRACT

This paper investigates the application of superinsulation to manufactured housing in hot, arid climates. It includes an overview of potential benefits and problems that would arise from the adoption of these principles, along with a description of a 100% passive heating and cooling system that can be developed as a response to these considerations.

The thermal performance of this superinsulated system, which utilizes a roof pond for radiant cooling, is analyzed using a computer simulation model and radiant cooling algorithms to determine the potential energy savings.

1. INTRODUCTION

In spite of all the advances in construction techniques and the availability of performance data, superinsulation has mainly been limited to cold climate applications using conventional, on-site construction practices. The aim of this study is to explore the potential thermal benefits and problems that would arise from the application of superinsulation to hot, arid climates. Manufactured housing is chosen to provide a case study for this process, due to its poor thermal performance history and low income owner profile. The construction details and specifications are developed to integrate the superinsulated envelope and the roof pond system into the manufacturing and transportation requirements of the existing framework of this industry.

The thermal performance of this passive heating and cooling system is analyzed and compared to a base case which was modeled after the minimum thermal properties recommended by the California Code of Regulations [1].

This analysis is repeated to asses the compatibility of the system with different climate zones in Southern California.

2. BACKGROUND

2.1. Superinsulation

Although superinsulation was developed in early 1960s [2], it did not become popular until the energy crisis in 1974. The principles of superinsulated conservation systems have been tested initially on test models and prototypes [3,4] in mid 1970s. As the performance results were made public and construction technology was better understood, superinsulation became more popular, especially in cold climates. It is estimated that over 10,000 houses had been built in Northern America by 1985 [2].

Since superinsulation has mainly been a cold climate system, the construction technology developed for its application to existing traditional construction practice has centered around increasing the wall and ceiling thickness to accommodate very high levels of insulation. However, it should be recognized that the other principles of superinsulated systems, namely restricted infiltration and controlled ventilation, are just as crucial to the performance of the system as the increased envelope resistance., and therefore be carefully designed for different construction and climate applications.

2.2. Superinsulation in Warm Climates

A number of organizations have conducted extensive research in hot, humid climate applications, including the Oak Ridge National Laboratory in Tennessee [5] and Florida Solar Energy Center [6]. The technology developed and tested by these research centers revolves around the use of radiant barriers to reduce the sol-air effect, rather than using very high levels of insulation.

Among the very limited research on hot, arid applications is the study conducted by the Swedish Council of Building Research [7],which simulated the thermal performance of a superinsulated modular in a number of U.S. cities. Although it was determined that the system required no auxiliary heating in the Los Angeles climate, the cooling conditions were not assessed and the system was not specifically designed for a hot climate application. Another interesting study was conducted by Kuwait University, which concentrated mainly on the cooling load and electric consumption of the air conditioning units [8]. Computer analysis for this projects showed over 65% reduction in both peak and annual cooling loads. However, this system was not a totally passive system and it operated on cooling the space by mechanical air conditioning and preserving the cool indoor air using the superinsulated envelope.

2.3. Potential Problems

The single biggest problem arising from hot, arid climate applications, is the internal loads, which become a summertime liability in superinsulated houses. As the building envelope becomes tighter and more resistive to external conditions, the buildup of intrinsic heat from people, lights and equipment becomes the major component of the cooling load and the house becomes internal load dominated. This might increase not only the annual cooling load, but the peak load itself, which in return eliminates potential savings from a smaller air-conditioning unit.

The first possible solution to this problem could be modifying the ventilation scheme and schedule. The air-to-air heat exchanger still operates successfully in exchanging heat between incoming and outgoing air streams preserving, cooler indoor conditions when the ambient air temperature is higher. However, as the internal loads increase during the day and peaks between 7-10 p.m. in a typical household [2] using natural ventilation at these hours when heat gain from the ambient air is less of a threat could provide an efficient solution.

The concept of using night time natural ventilation works together with the addition of thermal storage mass to the superinsulated envelope. This is called convective cooling and it operates on the principle of creating a heat sink during the night by cooling the mass to the minimum ambient temperature and using this sink to store internal and external heat gains during the day. For this strategy to work, it is extremely important to keep the building closed and airtight during the day to minimize heat gain from the ambient air [9].
 

3. CONCEPTUAL DEVELOPMENT

3.1. Manufactured Housing as a Case Study

Manufactured housing is chosen to provide a case study for

the application of superinsulation to hot, arid climates. The

thermal performance of manufactured housing has been analyzed by a number of government funded projects [10,11] which have indicated that the 1976 HUD thermal standards were not adequate in establishing energy-efficient design. The construction of super energy-efficient houses of tomorrow through mass production becomes even more important when the low-income profile of the mobile home occupants and their poor energy performance is considered.

3.2. Superinsulation in Manufactured Housing

The first problem arising from the application of superinsulated principles to manufactured housing is the limitation in size which eliminates the use of double wall, truss wall or strapped wall superinsulated systems and dictates the use of a high R-value per inch material such as polyurethane in an exterior sheathing wall system.

Secondly, due to the extensive use of adhesive materials, the buildup of indoor pollutants becomes a bigger problem in superinsulated manufactured housing. A possible solution to this problem is to increase the amount of air volume used in ventilation, which could be achieved by using a higher efficiency air-to-air heat exchanger.

However, a more important problem arises when thermal mass needs to be added to a superinsulated manufactured home. Limitations in width, wall cavity, and transportable weight, prevent the use of high mass materials in factory-built houses. The solution to this problem is adding mass on-site, that is adding water. At 62.26 Btuh/ft^{2 O}F, water can store about three times as much heat per unit temperature as concrete for equal volumes. Although water can be used in a water wall system as thermal mass, the only possible application of water as a storage material is in a roof pond configuration.

3.3. Roof Ponds as Thermal Storage

Although roof ponds have been used as thermal storage systems for years in different configurations the breakthrough development in this field came in 1971 with the incorporation of movable insulation into the roof pond system by architect Harold Hay [12]. Commercially marketed as SKYTHERM^R , Hay’s system has been evaluated through computer simulation and monitored data [13,14] and was determined to be the most effective radiant roof cooling system in maintaining comfort conditions within the space.

Among all the movable panel systems developed, Hay’s

sliding system, along with the bifold scheme, provides optimum heating and cooling conditions due to the amount of clear sky expose they provide to the pond area. Additionally, Hay’s system has the advantage of allowing more solar access to the surroundings comapared to a bifold configuration.

Construction details for both systems are developed and integrated into the roof system of the superinsulated mobile home.
 
 

4. SYSTEM DESIGN

4.1. Case Study

The base case chosen for the application of the heating and cooling system is the standard two bedroom, single-wide mobile home (Fig. 2.) unit. This model is 12 feet wide, 54 feet long, with a gross floor area of 648 square feet. The total wall area of the mobile home is 990 square feet which includes 81 square feet of single-glazed window area (14 % of floor area). The southern and northern sides has almost equal amount of glazing. The west and east walls carry only about 30 % of the total window area with the three exit doors located on the north and south walls.

The thermal properties of the base case were modeled after the California Code of Regulations, Title 25 minimum requirements, with R-8 walls, R-11 floor, R-16

roof resistances, single pane glazing, and an infiltration rate of 1.4 air changes per hour.

4.2. The Superinsulated Envelope

The base case was modified with no architectural changes, to accommodate a superinsulated envelope. This Envelope design includes a wall section of 1/2 in. plaster, air/vapor barrier, 3.5 in. batt insulation within the wall cavity, 1.5 in. exterior polyurethane sheathing, 3/4 in. air space with a radiant barrier and exterior siding., giving an overall thermal resistance of R-25.

The floor system consists of vinyl floor board, an air/vapor barrier, 1/2 plywood floor boards, 6 in. batt insulation within the floor beams, 1/4 in outside asphalt board, and 2 in. insulation on the outside to reduce thermal bridging from the floor beams to the undercarriage, resulating in an R-25 floor assembly resistance.

Other improvements to the thermal envelope include double-pane windows with exterior shutters, infiltration rate improved to .4 air -changes-per-hour, higher resistance doors and a Mitsubishi-Loosnay air -to-air heat exchanger with a capacity of 50 - 80 cfm. and a heat recovery efficiency of 80 %.

4.3. Roof Pond System

The first details developed belong to a bifold movable insulation system ( Fig. 4) that can be retracted to a vertical position either manually or by a 1/4 HP reversible motor. The force in the pulling cable (Fig. 5.), activates the polyurethane panels (5) which are moved on wheels (4) to a closedposition to be sealed by a neoprene gasket. The water

An alternative to the bifold movable system is the flat horizontal tracks system. For this configuration, the problem of space needed for panels can be solved by using the trellis structure, which is quite popular in mobile home parks, or by providing series of brackets attached to the roof and wall edge to support the extended tracks. The suspension of the top panels reduces the number of wheel brackets (6), sealing plates (3) and panel encasements

(Fig. 6.) , making the design more cost efficient. To reduce the costs further, the flattop plate on the deck is removed and water bags are allowed to fill in the gaps of the deck, which additionally increase the thermal conductivity of the ceiling.

The depth of the water bags should be kept between 4 and 6 inches, so that the roof live load from the water bags does not exceed 30 lbs/sq.ft.. However, it should be recognized that the water depth is a resultant of a number of climatic factors, which will be examined in the thermal analysis.

5. THERMAL PERFORMANCE ANALYSIS

5.1. Methodology:

The thermal performance of the superinsulated mobile home is analyzed using DOE2.E hour-by-hour simulation

program, which calculates the heating and cooling load of a building assuming a fixed indoor air temperature. This analysis is carried step-by-step by modeling the base case, then the superinsulated envelope and finally, the night ventilation of mass of the superinsulated system. Due to the inability of DOE2 programs to calculate the radiant cooling capacity of roof ponds, a verified mathematical model [15] is used to calculate the minimum depth and capacity of the pond.

The base case and superinsulated envelopes are modeled according to the thermal profiles described earlier. Occupancy, lighting and other internal loads and schedules were modeled using a number of sources [16,2].The thermal analysis was repeated for 4 main climate zones in Southern California, which include Southern Coastal (Los Angeles),Warm Inland (Riverside), Central Valley Climate(Fresno) and Hot, Desert Climate (China Lake).

5.2. Analysis Results: Heating

The thermal performance of the superinsulated system showed significant savings in annual heating loads in all of the four climate zones. The savings have ranged from

19.3 MMBTU ( $168.3 in annual heating bills) in Los Angeles to 29.1 MMBTU ( $218.2) in Fresno. Similarly the peak loads have decreased significantly, resulting in reductions that ranged from 12 to 16.7 KBTU/h This

demonstrates that the roof pond system will not be operating too much for heating purposes.

5.3. Analysis Results: Cooling

Under the cooling conditions, the system performed quite successfully compared to the base case. Even though the thermal savings from the cooling conditions could not be compared to the heating load reductions, the economical savings become even more significant considering the price of electricity. The annual cooling load reductions have ranged from 20.9 MMBTU ($275 electricity cooling) to 31.5 MMBTU ($415) and the peak loads were reduced to

under 12 KBTU/h in all climates. Naturally this indicates that the roof pond will be sized according to the cooling conditions.

It was also observed that, the convective cooling capacity for hot desert climate has diminished by 40 % compared to

the rest of the zones. This was determined to be the result of high night ambient temperatures in the cooling season.

5.4. Roof Pond Sizing:

To provide additional heating and cooling in relieving the remainder of the thermal loads, the roof pond system is analyzed using a simplified algorithm, developed by Fleishhacker et al.. Although the depth required in Riverside and Los Angeles was less than 4 inches due to their smaller peak loads and lower minimum dB temperatures, this system required the use of an electric fan to reduce the pond depth to under 6 inches.

TABLE 1. POND DEPTHS FOR VARIOUS CLIMATES

Depth Depth with

CITIES without fan 115 fpm fan

Los Angeles 3.57" 2.25"

Riverside 3.71" 2.30"

China Lake 11.40" 5.94"

6. CONCLUSIONS AND RECOMMENDATIONS

After a review of the design development process, system construction details and the thermal performance analysis of the superinsulated mobile home, it is concluded that superinsulation can be applied to manufactured housing in hot, arid climates to yield significant energy savings. However, this research needs to be taken one step further by the construction and monitoring of a prototype unit, which will provide a more accurate assessment of energy savings and cost effectiveness of the system.

7. REFERENCES

[1] California Code of Regulations, Title 25, Chapter 3, Barclays Publishing, San Francisco, 1994.

[2] Nisson N.N.D., Dutt G., The Superinsulated Home Book, John Wiley & Sons, New York, NY, 1985

[3] Besant, et al., " The Saskatchewan Conservation House :Some Preliminary Results", Energy and Buildings, 2, 1979

[4] Shurcliff, W.A., Superinsulated and Double-Envelope

Houses, Brick House Publishing Co., 1981

[5] Levins, W.P., Karnitz, M.A., " Cooling Energy Measurements S.F. Houses with Attics Containing Radiant Barriers", ORNL/CON-200, Oak Ridge Natl. Labs.,1986

[6] Fairey, P.W., " The Measured Side-by-side Performance of Attic Radiant Barrier Systems in Hot, Humid Climates "‘

Florida Solar Energy Center, Cape Canaveral, FL, 1985

[7] Mills, E., "Measuring the Energy Effectiveness of Mobile Homes", Energy and Buildings, 8, 1985

[8] Fereig, S.M., Younis, M.A., "Effects of Energy Conservation Measures on the Life Cycle Cost of Kuwaiti Residential Buildings", Energy and Buildings, 8, 1985

[9] Givoni, B., " Passive Cooling - State of the Art", 12th Passive Solar Conference Proceedings, ASES, 1987

[10] Hutchins, P.F., Hirst, E. "Analysis of Mobile Home Thermal Performance", Energy and Buildings, 3, 1978

[11] Krigger, J.T., Mobile Home Energy and Repair Guide, Saturn Resource Management, 1992

[12] Hay H.R., "A Passive Heating and Cooling System from Concept to Commercialization", Proc., Annual Meeting of the American Section of ISES, 1977

[13] Clark, G., Allen, C., " Comfort Conditions in Roof Pond Cooled Residences without Air-conditioners", Proc. Fifth Nat. Passive Solar Conference, ASES, 1980 Balcomb [14] Clark, G., et al., " Results of Validated Simulations of Roof Pond Cooled Residences", Proc. Eighth Nat. Passive Solar Conference, ASES, 1983

[15] Fleishhacker, P., et al., " A Simple Verified Methodology for Design of Roof Pond Cooled Buildings", Proceedings of the 7th National Passive Solar Conference, ASES, 1982.

[16] ASHRAE Handbook of Fundamentals, American Society of Heating Cooling and Air-Conditioning Engineers, Inc., 1986.