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What Drives Cooling Savings in Mobile Homes?

Home Energy Magazine Online July/August 1996

What Drives Cooling Savings
in Mobile Homes?

By Wendy Hawthorne and Rob deKieffer

Wendy Hawthorne is an energy consultant for Enermodal Engineering Incorporated in Denver. Rob deKieffer is the executive director of Sun Power Incorporated in Wheat Ridge, Colorado.

Much attention has been given to keeping mobile homes warm in winter, but most mobile homes are parked in hot climates where people are trying to stay cool.

It's cooling season, and if statistics are any indication, utilities, energy technicians, and HVAC contractors will be spending more time than usual dealing with mobile homes this summer.

Mobile homes represent the fastest-growing housing type in the United States, and air conditioning the fastest-growing residential energy load. This combination of trends should capture the attention of utilities trying to contain peak loads, especially in the South, where the majority of mobile homes are located.

Add to these statistics the fact that mobile home households have lower-than-average incomes and include one-quarter of the families who qualify under the Weatherization Assistance Program, and we can see why housing organizations and government agencies have been stepping up efforts to cut the cooling costs for mobile home occupants (see "A Warm-Up for Better Cooling").

Different by Design

Technicians accustomed to working on site-built homes will find mobile homes a new challenge to audit, weatherize, and retrofit. Designed for transport over highways, a mobile home has a long narrow frame bolted to a steel chassis; a low ceiling; and a shallow truss roof cavity. The floor, exterior walls, and roof are factory-built as components and then assembled on the chassis. Roofs and siding are generally metal. The inside wall panels provide added structural support, and steel strapping reinforces joints between the floor, walls, and roof. In older mobile homes, insulation in all of these components is minimal. Windows and doors are made with lightweight metal frames that screw into the wood frame from the exterior. The water heater is located in a closet that is vented, and is usually accessed from the outside.

The age of a mobile home is a good indicator of how energy-efficient it is. New mobile homes have to meet a U.S. Department of Housing and Urban Development (HUD) energy code adopted in 1994 that is quite stringent (see "Checking Out HUD's Proposed Mobile Home Performance Standards," HE Nov/Dec '93, p. 21). In fact, new mobile homes are often more energy efficient than new site-built homes, because HUD inspections lead to greater adherence to the energy code. HUD codes were first adopted in 1976 and improved the efficiency of mobile homes dramatically over pre-1976 homes.

Two important cooling techniques are visible in this mobile home in Mesa, Arizona. The home is equipped with awnings to cut down on solar gain, and the large trellis on the left provides excellent shade, while reducing ambient temperatures.

Unfortunately, many people (particularly low-income families) live in these older mobile homes. Imagine conditions inside one of these homes when temperatures hit the nineties and above. The low ceilings restrict the installation of ceiling fans. Typically, window air conditioners are undersized for the cooling needed. In mobile homes with central air conditioning, it is common to find ducts poorly designed and installed, and connections from external cooling units improperly fitted to the duct system. Forced-air systems sometimes have ductless returns, using the roof and belly cavities as passageways (see "Duct Improvement in the Northwest: Mobile Homes," HE Jan/Feb '96, p. 27). In double-wide mobile homes, which are transported in two sections, ducts may be loosened and damaged by transit.

The unique features of mobile homes seriously affect energy use and comfort levels in warm climates. Previous research has mostly focused on decreasing mobile home energy use in cold climates (see "CMFERT: Training and Testing of Mobile Home Retrofits," Jan/Feb '90, p. 23, and "Mobile Home Retrofits Revisited: CMFERT Phase II," Jan/Feb '91, p. 21). The following guidelines for achieving warm-weather comfort are based on years of experience, scientific studies, and preliminary results from computer simulations conducted by the National Renewable Energy Laboratory. NREL modeled an older (pre-1976 code) air conditioned mobile home in hot, dry climates (Tucson and Albuquerque) and hot, humid climates (Miami and Memphis), and modeled various retrofits to estimate reductions in cooling load.

A team of weatherizers applies an elastomeric coating, or "white roof," to a mobile home in a climate with both a heating and cooling season. The square patches cover holes drilled in the roof for blowing insulation. The holes are first plugged with a plastic cap that is sealed with caulk. The patches are made of an aluminum roof coating with an asphalt undersurface that adheres when warmed-by sunlight in summer, or by blow-torch in winter. This procedure ensures that when the elastomeric coating is applied, there are three sealants covering the insulation holes to prevent leakage.

A Comprehensive Approach

To assess energy use in any home, it's always helpful to look at the occupants' utility bills first, to find out how much they have been spending in each season of the year. There should be some obvious correlation between the bills and the type of mechanical equipment, or lack of it. In warm climates it is particularly important to distinguish between dry and humid regions, and to note whether the home has a heating load in winter. With this information it's easier to prioritize appropriate, cost-effective measures in the context of the occupants' comfort needs.

A comprehensive approach involves addressing all of the following elements: solar heat gains, conduction, infiltration, internal heat gains, and mechanical system efficiency.

Because mobile homes are small and have little thermal mass, heat can build up quickly inside. The first strategy for cooling should be to prevent heat from getting in; then mechanical equipment can be sized properly to run more efficiently-and less frequently.

Sun Block

Solar gain reduction offers the biggest opportunity for cost-effective cooling and comfort. The orientation of the mobile home and the existing shading conditions, throughout the day and at different times of the year, have to be assessed to achieve the greatest benefits. If any windows will be naturally shaded by a deciduous tree in summer, or by neighboring homes when the sun is lower in the sky in the winter, there may be no need for awnings, sun screens, or new low-e windows. Simple sun path diagrams can be used to make this assessment.

If the windows are unshaded for much of the cooling season, several of the measures described below may be cost-effective. Mobile homes have one advantage over site-built homes in this case: if the orientation of a home is responsible for serious heat gain, it may be cost effective to just move it!

Weatherizers cut holes in only one side of the roof of this mobile home to blow in insulation. They used a 60-foot long PVC pipe to reach the back of the roof cavity to start the insulation process, pulling the pipe further out as the area filled.

Shading

Exposed mobile homes in both hot-and-dry and hot-and-humid climates benefit from window shading. Exterior shading of windows is always more effective than internal shading-once the heat is in, it's in. Awnings or trellises can shade south-facing windows in summer while maintaining views and winter solar gains. Carports and porches can provide excellent shading as well. Exterior sun screens (simple meshlike fabric in metal frames attached to the outside of windows) can effectively shade east and west windows, but they also block some light. Technicians in Arizona report that clients like the way the screens look and appreciate the reduction in bright sunlight. (The screens also double as insect screens and provide a degree of privacy.) If the home is likely to be moved and reoriented, sunscreens can be moved to other windows.

Tree planting can be an effective shading strategy, and it can also reduce ambient temperature (see "Urban Heat Islands," HE May/June '94, p. 16). If fast-growing trees are chosen, a significant effect may be detected within four years. For about $50-$100, three fast-growing trees can be planted on the west, southwest, or east side of a home. In field experiments, air conditioning savings from tree planting have been measured from 10% to 50% in Miami, Tucson, and Phoenix (see "Shade Trees as a Demand-Side Resource," HE Mar/Apr '95, p. 11, and "Strategic Planting," EA&R July/ Aug '87, p. 7). In Arizona, the Tucson Electric Power Company is currently sponsoring a planting program in which homeowners can get two trees for shading their homes.

Window Replacements

Window replacement is rarely cost-effective based on energy savings alone. The only significant cooling savings from window replacements result from a reduction in solar transmissivity (not from conduction or infiltration reduction). If an east-, west-, or south-facing window must be replaced because it is broken, low-e coated glass, which further reduces transmissivity, will be more cost-effective than regular double-pane clear glass. But if the window is shaded by a porch, for example, there is no need for a low-e replacement.

Cleaning the coils on an air-conditioner is important for cooling efficiency, but must be repeated regularly for continued savings.

Reflective Coatings

A reflective roof coating can reduce cooling load and is a popular measure, because waterproof coatings provide an extra seal for the roof (although coatings should not be used to patch leaks). Based on energy savings alone, roof coating is not as good a choice as roof insulation, especially when the site has at least some heating load. But insulating requires cutting, or otherwise disturbing, the roof or shell, and many technicians working in wet climates are fearful of causing, or being blamed for causing, subsequent water damage. As a general rule, if the mobile home has little heating load, has roof leaks, or won't withstand drill and blow insulation, apply a reflective roof coating.

Reflective roof coatings may have to be re-applied after 5-7 years to maintain effectiveness (see "Saving Energy with Reflective Roof Coatings," HE May/June '94, p. 15).

Insulation

Though measures that reduce conduction are common in heating-dominated climates, few are justifiable in terms of cooling-load reduction alone. Roof insulation, however, is usually cost-effective even in hot climates. Most technicians agree that this is probably the most effective measure, and-unlike roof coatings-one that will continue to be effective if it is properly installed.

Mobile home roofs can be insulated using several techniques. One method is to insulate from the rim. The technician unscrews and bends up the trim strip at the top of the mobile home walls. Insulation can then be blown into each cavity via a long pipe, typically made from steel muffler pipe. Ceilings can also be insulated from the interior: the ceiling tiles are drilled and insulation is blown in.

With proper training, attics can also be insulated easily from the roof without damaging the home. If the roof does not leak and can withstand drilling and blowing, insulate it rather than apply a reflective coating. The two measures combined are rarely cost-effective.

Belly (underfloor) insulation, which is quite cost-effective in cold climates, may take decades to pay back in cooling savings. Wall insulation will pay back somewhat more quickly in hot climates. However, if the region also has a significant heating load, payback times will be shorter for both measures, so insulating the belly and walls should be considered.

Calculating Air Sealing Cost-Effectiveness

A simplified method for estimating the cost-effective air sealing cost for air conditioned mobile homes in cooling-dominated climates is as follows:

Cost per 100 CFM50 reduction =
110 x CDD/(SEER x 1,000) x $/kWh x payback time

where: CDD = cooling degree-days for the location (^oF)

SEER = air conditioning system seasonal energy efficiency ratio in Btu/Wh (7 to 9 would be typical for older systems)

$/kWh = average cost per kWh for electricity

payback time = the desired maximum payback time in number of years (five years would be typical).

For example, to get a five-year payback in Miami, weatherization technicians can spend about $20 in labor and materials to reduce infiltration by each 100 CFM50 in a fully air conditioned mobile home (SEER 9). Under the same conditions in Tucson, the cutoff would be less than $15 per 100 CFM50 saved.

Note that this method does not account for reductions in heating load (if any) or latent heat load (in humid climates). Latent heat can add up to 30% to the cooling load, so it may be appropriate to spend up to 30% more per 100 CFM50 reduction in humid climates.

Infiltration

Reducing infiltration is not as cost-effective in hot climates as it is in cold climates, but it can still produce savings. The best approach is to determine the allowable cost per 100 cfm50 reduction, based on desired payback time, potential cooling savings, and energy costs (see "Calculating Air Sealing Cost-Effectiveness").

Technicians should use a blower door and focus on large holes, including those in the ductwork, to get the best payback. Check the blower door numbers regularly in order to ensure that air sealing is cost-effective, and quit when cost-effective air sealing cannot be achieved.

Internal Heat Gains

Internal gains can be reduced with simple measures that also offer additional benefits, such as direct electricity or water savings. Hot water tanks and pipes, lights, refrigerators, stoves, clothes dryers, and showers are some of the major sources of indoor heat.

Because the water heater tank sits in a closet in the living space it is important to wrap it and all accessible pipes, and/or to insulate the sides of the water heater closet that border the inside. Remember that the closet itself is open to the outside, due to vents in the exterior wall for combustion air. Turning down the water heater thermostat will also help reduce internal gains.

Replacing inefficient appliances, like old refrigerators, reduces internal gains, and results in direct electricity savings. Less costly replacements include low-flow showerheads and compact fluorescent lamps, which tend to pay back quickly. Energy specialists should also suggest to the occupants that they consider moving some heat-producing activities outside (they might cook on a grill or line dry clothes, for example), or rescheduling them to noncooling hours.

Mechanical
System Efficiency

Cooling energy savings are very dependent on mechanical system efficiency, which varies widely with the age, type, and condition of the system. Although a standard evaluation and tune-up practice for mobile home cooling systems has yet to be developed, some efficiency improvement measures are described below.

Replace Air Conditioner

According to NREL's computer models, replacing a SEER-6.5 with a SEER-10 1-ton window air conditioner saves up to $175 per year. The cost is about $650, for a payback of less than four years. Replacing three SEER-6.5 1-ton window air conditioners with a SEER-13 3-ton central air conditioner saves about $300 per year in Miami. The cost of such a switch is approximately $2,000, for a payback of less than seven years. It makes sense to replace an air conditioner with a more efficient unit if the existing unit is SEER-8 or less.

Several utility companies offer incentives for purchasing high-SEER central and split units. Note that in order to achieve expected savings, duct repair and sealing must be an integral part of any central air conditioner replacement.

In humid climates, air conditioners help dehumidify the air and control moisture problems. But if the mobile home is in a dry climate, consider replacing an air conditioner with an evaporative cooler. This switch should produce savings of 60%-90% of the cooling energy.

Window units should be replaced if the refrigerant charge is low and cannot be adjusted.

Adjust Refrigerant Charge

A 20% over- or undercharge in refrigerant in a central residential air conditioning system can reduce its SEER by about 25%. Undercharge can be a result of a leak, but improper charging often occurs during installation of a central unit. Window units typically come factory charged. If the charge is low in a window air conditioner, it probably has a leak, in which case it should be replaced rather than recharged.

In a field study in Fresno, California, about a quarter of the air conditioners examined were overcharged and another quarter were undercharged. Correcting the refrigerant charge produced an average 12% reduction in cooling energy for single-family houses (see "An Ounce of Prevention: Residential Cooling Repairs," HE May/June '91, p. 23).

Clean Coils

Dirty evaporator coils reduce air conditioner efficiencies by up to 8%. Dirty condenser coils and blocked air intakes or outlets may cause similar drops in efficiency, or compressor failure. Coils can be cleaned for a low-cost efficiency upgrade, but they are often difficult to access in older window air conditioners. The payback can be less than one year. However the effectiveness of this measure degrades over time. To achieve long-term savings, the resident must be educated to repeat the measure regularly.

Evaporative Cooler Improvements

Pad replacement. In typical evaporative coolers, air blows through a wetted cellulose or aspen pad to produce the evaporative cooling effect. Cleaning or replacing pads improves effectiveness and can decrease fan power requirements by reducing the pressure drop through the pad (see "Installing and Maintaining Evaporative Coolers," HE May/June '96, p. 23).

Motor replacement. Many evaporative coolers are equipped with a very inefficient, aluminum-wound, split-phase motor. Replacing the existing motor with a high-efficiency motor will result in immediate energy savings.

Controls. Improved effectiveness can also reduce the evaporative cooler runtime if the cooler is routinely turned off either automatically or by the occupant when a comfort level is reached.

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