Energy Gains: The Envelope, Please
Measures to reduce energy losses through a building’s windows, walls and roof are often worth investigating, as four examples show.
Cutting a building’s energy bill often revolves around changes to its mechanical and electrical operations or system. But a building’s roof, walls and windows may also provide savings. Options fall into three broad categories: keeping outdoor heat out, keeping indoor heat in and controlling the flow of moisture through surfaces.
A variety of technologies are available, but whether any one is cost-effective depends on climate, building orientation and utility rates. Most of these technologies are mature, though a few have entered the market recently.
Simple heat loss and gain calculations, while appropriate for residential properties, may not yield accurate results when applied to commercial buildings. Interactions with internal loads like lighting and mechanical systems may require a more sophisticated analytical approach using computer software.
The federal government and some states offer financial incentives, but payback periods vary widely. Many envelope options are best designed into new buildings or incorporated into repair or renovation projects rather than treated as stand-alone efficiency investments.
To better understand options, consider four types of buildings in varying climates:
- High-tech industrial plant in the Southwest.
- Glass box office building near the Great Lakes.
- Turn-of-the-century courthouse in the deep South.
- Low-rise lecture hall at a New York college.
Each facility exhibits characteristics that make some choices worthwhile but may eliminate others. As will be seen, correctly evaluating some options may also require an understanding of a facility’s operations and mechanical and electrical systems, as well as relatively sophisticated analysis methods.
Don’t Let The Sun Shine In
The sun provides both light and heat — that is, infrared radiation. The Southwest high-tech industrial plant has few windows, but does have a large flat roof. The plant manager is looking for ways to cut the cooling bill — there is essentially no need for heat — but the facility has very little money to spend. Installing insulation would cut the conductive load — the heat transferred from the hot desert air — but a reflective roof coating that keeps solar radiation from heating the roof, and thus the plant, could reduce total heat transfer even further.
Such coatings can cut in half solar radiation typically absorbed by an uncoated roof. Doing so significantly lessens the amount of heat then conducted through the roof. Reductions in roof surface temperatures of 45 to 80 degrees have been reported, cutting peak cooling loads by 10 to 15 percent. The cost to add the reflective coating was only a fraction of the cost of insulating the roof, and a coating would allow heat produced by industrial equipment in the building to be conducted through the roof during cool weather, reducing the need for mechanical cooling.
Roof coatings vary considerably in their reflective properties, and some may not be compatible with existing roofing. Check with the roofing contractor before proceeding, and use a coating that carries the Energy Star label. Such products reflect at least 65 percent of solar radiation.
People Who Work in Glass Houses...
A building engineer in the Great Lakes area took pride in how much the energy bill had been cut through improved HVAC controls and operations but was told to find ways to cut it even further. The glass tower office building received a lot of sun, but it also lost a lot of heat during the winter through its glazing. Adding a second layer of glazing would cut heat loss and gain but would cost a small fortune.
Various types of reflective and low-emissivity — “low-e” — films could instead be applied to windows at a much lower cost while significantly cutting both heat loss and gain. Reflective film keeps solar radiation from entering, while low-e film keeps heat inside; films are also available that exhibit both properties. One electrically heated and cooled facility in the area of the office building measured net savings of more than 8 percent, achieving a three-year payback on the cost of the film. Because such installations are labor-intensive, payback may be influenced by whether in-house or contract labor is used.
A wide range of window films is available, and making the right choice involves analysis of building orientation, the relative costs of heating and cooling, and possible changes to a building’s appearance. In one case, the best solution involved one type of film for the north and east sides of a facility, and a different type for the south and west sides. A professional should determine which film is appropriate, and a small sample space should be fitted to verify that the film is visually acceptable before putting it on the entire building.
Insulation Not Always A Good Idea
When trying to cut heating costs, more insulation is not always better. Managers of a low-rise lecture hall built in the 1950s in New York were trying to cut the heating bill and improve comfort on very cold days. The roof was not well insulated; the U value was .16, whereas newer buildings may have U values of .05 or lower. Contractor bids were required to include installation costs and a calculation of expected energy savings. All claimed that they would achieve savings based on improved roof U value that would cut heat transfer in both winter and summer, as might be expected for a residential property.
To evaluate other energy-efficiency options for this facility, a DOE-2 computer simulation had previously been created and was used to verify contractor savings claims. To the facility executive’s surprise, adding more insulation had practically no impact on the total annual fuel bill. Close examination of the simulation’s output found that winter heating costs would indeed be lower, but summer cooling costs rose significantly.
Because of the lecture hall’s dimmable incandescent lighting and long hours of operation for both day and evening classes, adding insulation would hold much more heat from internal loads in the space, requiring more cooling even when outdoor temperatures were moderate and — to dump heat built up during the day — even after classes were over. The cooling system used a relatively inefficient single-stage steam absorption chiller that, like the heating system, used steam from the campus central plant. The simulation showed that extra steam used by the chiller in the summer nearly balanced the reduced steam usage for heating in the winter. Because upgrading the lighting or chiller would have added enormous cost to the project, the insulation idea was dropped.
Those same long operating hours, however, caused major loss of both heated and cooled air as students passed through the hall’s swinging doors. Creating a vestibule and adding revolving doors significantly cut that infiltration, making the space more comfortable year-round.
Keeping Those Mint JulEps Cool
A century-old uninsulated brick courthouse in the deep South often smelled musty as a result of poor air circulation and humidity control. Often chilly in the winter, its unshaded windows let in too much sun during the summer, but there was no money in the county budget to upgrade the HVAC systems. Exterior insulation could not be added underneath new siding or a wall treatment because the building was a local landmark. The same limitation applied to insulated interior wall paneling.
The inside of the facility had not been repainted in more than a decade, however, so the facility executive opted to use a primer that acted as a vapor barrier to cut moisture penetration in the summer, reducing the latent load on the air-conditioning system and cutting the humidity that previously fostered the musty smell. To the underside of the attic ceiling, he applied a low-e paint to reflect heat away from the interior while retaining heat in the winter. Many buildings with existing roof insulation may, however, already have radiant barriers in the aluminum foil attached to fiberglass batts. In such cases, low-e paint may have little or no additional impact.
In the back office and records storage areas where few people would see the change, low-cost storm window kits were added to cut infiltration and heat gain through leaky single-glazed windows. All other windows and framing were examined for leakage, with weatherstripping added as needed.
While rummaging through the records storage area, the building engineer saw mounted on the wall an old sepia photo of the courthouse when it first opened, long before mechanical air conditioning was available. The building had awnings to keep out the sun. A chat with the county historical society showed that, indeed, awnings were common at the time the courthouse was built. Therefore, it was acceptable to add them now. A local benefactor was found to cover the cost of the “historical” awning addition, and solar load through the windows was drastically cut. The building was cooler in the depths of summer while the low winter sun still shone in.
Paying For Envelope Upgrades
Envelope upgrades tend to be labor-intensive, so it often makes sense to pursue them in conjunction with routine maintenance or major renovations. If a roof is to be replaced, for example, the incremental labor to add insulation at the same time is small compared to the overall cost. Adding window film may involve disruption of an office space, so performing that task during remodeling, when a space has been emptied of all contents, could also cut its incremental cost. When analyzing an upgrade’s costs, be sure to use a life-cycle cost analysis that captures reductions in maintenance that may also result.
In some states, money is available for insulation and window upgrades under weatherization programs, but most are limited to residential properties. If such work can be shown to cut peak electric air-conditioning loads in nonresidential facilities, however, some programs seeking to reduce strain on electric grids may financially support such work. To learn more about such programs, start with the electric utility account representative. Also review programs posted at the state energy office’s Web site.
Even the federal government may sweeten the pot a bit, if the improvements are part of a larger upgrade that significantly cuts total energy cost. The Energy Policy Act of 2005 (EPAct) offers tax incentives for some energy-efficiency upgrades, thus improving the payback for commercial facilities. Further detail on EPAct’s funding impacts may be found in the December 2005 issue of Building Operating Management.
Measuring How Cool is Cool
Most roofing industry experts agree that a cool roof is one that exhibits a combination of high reflectivity and high emissivity. But the questions have always been how high is high and what combination of the two yields the most benefit? Specifications that either mandate cool roofing — like California’s Title 24, the Georgia Energy Code’s White Roofing Amendment and ASHRAE 90.1 — and those that make it an optional element of overall green building design — like the U.S. Green Building Council’s LEED rating system or ENERGY STAR — still don’t jibe when it comes to the exact emissivity and reflectivity numbers. Title 24 specifies an emissivity of .75 and a reflectance of .7, the Georgia White Roofing Amendment says that extra insulation must be used if a roof exhibits less than a .75 emissivity and reflectivity, and Energy Star certified products have at least a .65 reflectivity but there is no specification for emissivity.
All this has led to confusion about the combination of reflectivity and emissivity that would bring the most benefit to a building owner.
LEED version 2.2, released last October, is the first national specification to use a relatively new measure of reporting a cool roof’s properties. LEED 2.2 sustainable sites credit 7.2 states that to receive one point, building owners should use a roof with a Solar Reflective Index (SRI) of 78 over at least 75 percent of the roof’s surface for roofs with slopes less than 2:12. The new twist is SRI, a unit developed by scientists at Lawrence Berkeley National Laboratory. SRI incorporates reflectivity and emissivity properties into one, easy-to-read, standardized measure so that roof buyers won’t have to scratch their heads and try to figure out if a high reflectivity and low emissivity is better or worse than a medium reflectivity and high emissivity.
SRI is calculated with a complex formula spelled out in ASTM E 1980 and is a scale of 1 to 100 that is a measure of a roof’s combined thermal properties. It is defined so that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100. But some hot roofs can have negative values, and some white thermoplastics and white roof coatings have scored as high as 104 to 110.
SRI as a method for reporting cool roof data will probably take a little while to catch on. Most manufacturers still report separate emissivity and reflectivity data in their literature, but the Cool Roof Rating Council, an organization that verifies and labels cool roofing products has begun using the measure, while retaining reflectivity and emissivity measurements.
— Greg Zimmerman, managing editor
A Brief Lesson in Thermodynamics
Sensible heat, the kind measured with a thermometer, is transferred in four ways: conduction, convection, radiation and flow of a medium, for example, pumped hot water or air infiltrating through a crack. Convection and forced flow are typically associated with boilers, chillers, ducts, piping and other components of a mechanical system, while conduction, radiation and infiltration occur through a building’s skin.
Envelope conduction involves heat passing through a material like brick. It’s controlled by insulation, the capability of which is quantified by a coefficient of transmission known as a U value. A lower value means a slower rate of heat loss. Insulation comes in a variety of forms: fiberglass, cellulose, plastic foams or even sealed layers of air.
Radiation is the movement of infrared energy through a space, even across a vacuum. It may be controlled with surface coatings or architectural treatments that reflect or block infrared rays. Quantified by coefficients of reflectivity (r) or emissivity (e), higher reflectance or lower emissivity means less radiant energy — for example, heat from the sun, radiators, lights, office equipment — entering or leaving a space. Surface coatings include specialized paints and films while architectural treatments may include exterior baffles, awnings, curtains and blinds.
Infiltration is the uncontrolled movement of air into or out of a space through cracks or permeable surfaces. It may be limited with weatherstripping, door sweeps, caulking and impermeable barriers.
Latent heat, on the other hand, is energy held in the moisture content of air, which, in the summer, may be considerable. Just as heat tends to move from a warm to a cooler location, humidity tries to move from a moist place to a dryer place. Latent heat transfer is therefore blocked with a vapor barrier and various types of paints and sealants. Care must be taken, however, not to trap moisture where it could cause damage or foster mildew.
— Lindsay Audin, contributing editor
Lindsay Audin is president of EnergyWiz, an energy consulting firm based in Croton, N.Y. He is a contributing editor for Building Operating Management.