Improving Roof Quality
By Patrick L. Downey
- Improving Roof Quality with Life-Cycle Costing
- Specifying a top quality
commercial roofing system is not always easy in today's penny-pinching
economy. Many roofing contractors equate quality with expense, but are
reluctant to choose a roofing system that has a higher installed cost.
However, life cycle energy costing can show the true Cost of' a roofing system
over the life of the roof. Often times, money can be saved by specifying the
high caliber roofing system instead of a system with a lower installed cost.
Life-cycle costing, in Simple terms. is looking at the total sum of the installed and operating costs through the life of the roof. Usually the installed cost is the only number used when looking at roofing systems. However, it is not the only number when thinking, in terms of life-cycle costing.
Quality systems may come with a higher installed price, but over the long term they are often lower in cost because they require less maintenance, fewer repairs and last longer. More important, some quality systems offer substantially reduced energy costs over the life of the roof, which is critical since energy costs represent the largest of the "hidden" costs.
Yet, in some cases -- depending on the type of roof installed-it is possible for the total 10-year energy cost savings of a building to be greater than the roofing system's installed cost. With energy prices continually on the rise, selecting such energy efficient roof systems will become increasingly important.
- Energy Costs
- Why does the roof play such an
important role in energy costs? In non-residential buildings, the roof
typically represents the largest portion of the exterior building, surface. In
fact, for many buildings, such as manufacturing facilities, the roof area is
three times the Surface area of the walls.
The area directly under the roof, i.e. the top floor of a multi-story structure, is the area which typically experiences the most severe heat gain or loss. That means the roof is obvious choice for major savings in energy use.
Many different elements can influence the effect a roofing system may have on the building's energy cost, including the long-term performance and R-value in the roof insulation. However, one important factor that is often overlooked is the surface color of the roof.
- How color saves energy
- To understand the effect color
can have on a building's energy costs, think of two cars parked in the hot sun
for several hours. One car is black, the other white. You could not hold your
hand comfortably on the black car's hood because the surface temperature may
well be above 1700 F. The hood of the white car, however, will only have a
surface temperature of 110OF-a 600 degree difference.
Different colors of roofing membrane have the same effect. Just as the black hood on the first car absorbed the sun's heat, so will a dark roofing membrane. This heat will flow into the building through the roofing assembly. Conversely, a white membrane is reflective, so little heat enters the building
The typical building is maintained at an inside air temperature of 78F throughout the year. There will usually be a dramatic cost difference in maintaining this internal temperature if the roof's white Surface temperature is only 1100F, compared to the 170OF surface temperature of a black roof.
Even in buildings that aren't air conditioned, the white roof will help the building maintain a lower internal temperature and greater occupant comfort.
- Dramatic cost savings
- How great a savings can
a white roof represent? According to Steve Moskowitz, manager of technical
services for the Roofing Systems Division of JPS Elastomerics, a computer
study done by Du Pont in the mid- I 980s estimated that a 50,000 square-foot
roof structure in Chicago would save
$19,200 in energy costs over 10 years by using a white roofing membrane as
opposed to a black membrane. An identical building in Phoenix would save
$57,800 in the same period.
"This type of information really impresses all roofing system specifiers, who are amazed that something as simple as color can have such a dramatic effect on their operating costs." Moskowitz said.
"That's why we now, offer a free life-cycle cost analysis service that allows them to highlight the difference color can make. Once the difference in the life cycle energy costs for different colored roofing systems is understood, the post-bid discussion usually changes."
Another important factor of the roofing system's life-cycle energy cost is the assembly's R-value. Typically, as the R-value of a roof increases, you reduce the annual cooling and heating costs for the structure.
- Real-world examples
- Let's consider a 100,000
square foot building in Atlanta with R-4.16 insulation and a black roof. It
would have an estimated 10-year energy cost of $207,245.
Is it worth spending $35,000 for the extra insulation and white membrane in order to save $525,963 in long-term operating costs? Most would say "yes," if given a choice.
However, it is important to note that increasing R-VaIue does not always correspond to dramatic savings, especially when the savings possible with a white roof are factored into the equation.
A computer program developed bv Merik Marketing to perform energy cost shows that in some cases -- usually when the R-value is very high -- there is a law of diminishing returns. In these instances, adding more insulation may not make enough of a difference to justify its added expense
Therefore, it is important to know exactly how much of an energy cost savings the added R-value will offer with the particular color of the roof being selected.
- Dew point analysis
- The energy savings effect of
the white membrane and insulation can be significantly reduced if the
insulation becomes damaged or wet. This is caused by moisture entering the
roofing assembly from either the inside or outside of the building. This
moisture cannot only damage the insulation, but may also shorten the life of
the roofing membrane.
To avoid this, and provide owners with a quality, long-lasting, energy efficient roof, it is also important to understand dew point analysis.
Picture a glass of iced tea on a hot summer day. After sitting for 30 minutes, there may be beads of water on the outside of the glass. This occurs because moisture in the air has condensed against the cold surface of the glass. The temperature at which moisture in vapor form condenses into a liquid is called the dew point.
Two factors that affect this condensation are the temperature of the glass surface and the relative humidity of the air surrounding the glass. If the glass surface is sufficiently insulated or if the air contains little moisture, condensation will not occur against the glass.
Just as with the iced tea glass, moisture within the roofing assembly will be drawn toward the cooler surface. When the colder temperature is outside the building, (winter or night) moisture will attempt to condense against the underside of the roofing membrane.
Conversely, when the interior of the building is cooler than the membrane surface temperature (summer or day), moisture will be drawn toward the building interior.
It is important to remember that the movement of moisture within the roofing, assembly is dynamic. It is continuously changing from vapor to liquid and back, moving between the roofing, membrane and the roof deck.
While water vapor condenses into a liquid at points in the assembly where the dev,, point temperature is found. In changing from a liquid back to a gas, till moisture also expands significant in volume, exerting pressure within the assembly. One drop of water will occupy one gallon of space as a gas.
- Solving dew point dilemmas
- There are two factors that can
be implemented to mitigate dew point problems: proper design of the roof
assembly and careful selection of materials.
The National Roofing Contractors Association (NRCA) recommends that dew point related problems be considered in the design of the roofing When the outside mean January temperature is below 40F, and/or the expected interior relative humidity is 45 percent or greater.
One way to combat dew point problems is through the proper use of vapor retarders that restrict the amount of moisture drawn into the roofing assembly from the interior of the building.
Vapor retarders are rated by their resistance to the flow of water vapor. Look to use vapor retarders with a perm rating of .01.
During installation of this component, all penetrations, seams and termination details must be carefully constructed or the effectiveness of the vapor retarder will be adversely affected.
In order for a vapor retarder to prevent condensation from occurring within the roofing assembly, it must be warmer than the dew point temperature. If you refer back to the example of the iced tea glass the underside of the vapor retarder is similar to the outside Surface of the glass.
Prudent roof design requires calculating the dew point and installing insulation above the vapor retarder with sufficient R-value to insulate it from the outside temperatures.
It is particularly important to take into account any insulating materials under the retarder when rnaking these calculations. For example, a base board against metal decks, ceiling tile or insulated decks can dramatically increase the R-value needed above the retarder. Refer to NRCA's Roofing and Waterproofing Manual for a detailed discussion of this issue.
Note that many local building codes base R-value on energy conservation requirements, and do not consider dew point temperatures.
For instance, in Atlanta the building code requires an R-10 roof insulation in non-residential construction. This is often inadequate for dew point considerations. Therefore, it may fall upon the roofing contractor to determine the R-value needed to prevent condensation within the assembly.
Reroofing considerations
Reroofing presents special challenges. Tear-offs down to the deck allow the roof designer to "start fresh," and eliminate any pre-existing dew point problems. If a decision is made to recover the existing membrane, remember that the old roof now becomes a vapor retarder in terms of the new assembly.
Murphy's law dictates that adding insulation between the new and old membrane is likely to be insufficient in preventing condensation. It is important to be careful not to design a brand new problem into the system.
It is crucial to select roofing materials that have good resistance to moisture degradation. Avoid insulation materials that easily absorb moisture and tend to turn to "mashed potatoes" when subjected to sufficient moisture vapor drive or tend to warp and curl.
There are trade-offs in the selection of each roofing membrane system; some are more at risk than others in extreme moisture conditions. For instance, adhesive seams are more susceptible to moisture degradation than hot-air welded seams. In other systems, blistering can occur across the entire roof surface, increasing with each cycle of condensation.
- Quality life-cycle costing
- Using energy cost analysis to
determine the life-cycle costs of any roof has several benefits. First, it
shows that the real bottom line cost is not necessarily the same as the
roofing system's installed cost. Second, it allows specifiers to select a high
quality roofing system, while still being sensitive to cost.
We all believe there are advantages to quality, and with life-cycle costing all roofing professionals can clearly see the dollar value of a high quality roofing System. RS1
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Source: RSI - July 1993