A very useful way of evaluating potential changes to a roofing system is to consider the “payback,” usually in years, of each roofing option.
This payback method has been co-opted lately by mandatory requirements – usually energy related.
An example of this is the recent revision to ASHRAE 90.1-2007, where R-values have been increased from 15 to 20, and R-values as great as 30 are being considered. A careful analysis of this requirement shows that we’ve reached the point of no payback.
From a newsletter from Holt Architects: “Although we’ve been urged for years to add as much insulation as possible, more recent integrated design methodologies, using building modeling analyses such as DOE 2.2, have demonstrated the diminishing return of increased R-values. This is due to the inverse relationship between R- and U-value, which is the factor used in quantifying thermal transmission. Many designers today see R-25 (U-0.04) as the cutoff point, after which added R-value continues to add cost at a more-or-less constant rate, but returns energy savings at a negligible rate because the improvement in U-value is inconsequential.
This fact was corroborated for us in the energy modeling for the Gateway Building, currently under construction at Ithaca College, and registered for LEED Platinum certification. We modeled for 4-inch (LTTR-25) and 6-inch (LTTR-36) polyisocyanurate roof insulation, and found that 6-inch saved only $31 per year in energy costs over 4-inch for the 58,000 square foot building, but carried an additional construction cost of $14,000. Clearly, in this case, adding insulation beyond R-25 was not at all cost-effective.”
Another example would be the requirement for highly reflective roofs in all climates and for all structures. Dr. James Hoff reported that, in some situations, reflective coatings would save little and might actually have a negative payback. Hoff also reported that well-insulated ballasted roofs were as effective as reflective coatings in reducing air-conditioning loads. (Ballasted roofs do not require cleaning or recoating, another factor in payback analysis.) Also, ballast can be removed and reapplied at the time of reroofing.
Heating & Cooling Comparison
Reflective Roof (0.50) vs. Black Roof (0.05)
We do have plenty of examples where payback is rapid and cost effective.
Degree Days Cooling Degree Days Solar Load (BTU/SF/Day) Annual Savings for Reflective Roof
Phoenix 1,154 3,815 1,839 $4,300
Los Angeles 1,291 470 1,579 $3,100
Atlanta 3,090 1,611 1,478 $1,400
Chicago 650 749 1,243 $0
Portland 4,461 279 1,127 (-$300)
Since the roofing industry has been involved with providing shelter and conserving energy for centuries, let’s look at some other successful options that may be lost among all the current mandated options:
The use of smooth-surfaced or cap-sheet roofs, especially on the West Coast. In areas of seismic activity, lightweight decks that form a diaphragm are very popular. While oriented strand board (OSB) has recently displaced plywood, both are good decking choices for low-slope use. (And wood is a renewable resource.)
In mild climates, such as Southern California and Arizona, it’s possible to use under-deck insulation, such as glass fiber batts. These are more cost effective than above-deck insulation, and need not be replaced at time of re-roofing.
Some manufacturers have developed techniques for factory-applying a reflective coating to granule-surfaced cap sheets. This will increase the albedo (reflectivity) of the granule-surfaced cap sheets. Durability of these coatings is not yet established, but when they erode, the granules will still be there, screening UV.
Smooth-surfaced BUR for low-slope applications is traditionally glass fiber-based, although there are still some smooth modified bitumen (MB) roofs around. Traditional roof surfacings have included just plain asphalt cutback coatings, but they’re not very durable and aren’t VOC compliant. Another option has been asphalt-aluminum roof coatings – fibrated and non-fibrated. While they’re moderately reflective when fresh, a greater benefit from a payback point of view is that the aluminum flakes screen ultraviolet rays, protecting the asphalt from aging. Most of these aluminum coatings are solvent-based and not VOC compliant. (There are some water-based aluminum roof coatings now available.) A better choice for these smooth roofs is the application of a fibrated asphalt emulsion, or emulsion combined with chopped glass fibers. This is quite durable, fire rated, VOC compliant, and can be topped off with a reflective coating.
Sprayed-in-place polyurethane foam roofs may not leak as the weather-protecting coating erodes, but the best time to recoat is when pinholes first appear in the coating. Meanwhile, the building owner can repair punctures and the like with just a rag and tube of compatible sealant. Even if the coating is in poor condition and the foam is eroding, it’s feasible to scarify (grind down) the foam until sound foam is reached, and then spray new foam and coating. Foam is highly efficient from an insulation point of view, and adds very little additional weight to a roof system.
For metal panel roof systems, highly durable, factory-applied coatings have consisted of polyvinylidene difluoride (PVDF) Until recently, the PVDF coatings needed to be oven-cured, and it was difficult to find a field-applied maintenance coating that would adhere to the factory coating. That has changed recently, with the introduction of air-curing, waterborne PVDF coatings.
For EPDM single-ply membranes, tape adhesives have replaced earlier neoprene and butyl solvent-based adhesives. New details address shrinkage problems where repairs consist of cleaning the EPDM surface, priming, and applying a rubber patch (a “target” piece). Life can be extended for many years this way.
Thicker membranes, such as 60 or 90 mil materials, cost very little more than the traditional 45 mil material, as labor is virtually unchanged. Some thicker membranes are being warranted for as long as 30 years.
For thermoplastic materials, most can be re-welded to effect patches. Progress has also been made in recycling PVC membranes.
Thicker membranes are also available for thermoplastics. Increased thickness will be worth the effort for all single-ply membranes when PV panels are to be installed, as there will be an inevitable increase in traffic on the roofs.
For BUR and MB roofs, the art of patching has been greatly enhanced by the use of torch-grade repair materials. Since flashing failure occurs years before the rest of the roof system, maintenance and repair of MB flashings has a significant payback.
If the old flashings are in horrible condition, it may be well worth the money to install new curbs separating the membrane from the wall. NRCA calls these “non-wall-supported flashings.”
It‘s also worthwhile to use two-piece counterflashing details so that, when it’s time to re-flash, the lower metal can be removed, flashing installed, and the lower metal piece reinstalled.
Protected roof membranes (PMRs) have proven to serve well, and the insulation and ballast can be reused when the roof is replaced.
The roofing industry has not been standing still during this time. We have self-adhering membranes, moisture curing adhesives and coatings, versatile cover boards, corrosion-resistant fasteners, and many other innovations that will improve payback and still meet environmental and energy issues.
To sum up, we have many time-tested options for roofing. While PV panels, vegetated roofs, cool roofs, and super-insulated roofs are interesting and may someday not require legislation or subsidies, we have many things we can do right now that we know will extend roof life with proven paybacks.