Services & Maintenance: Low Slope Roofs And Energy Efficiency
By Andrew S. Hoover
Published in the November 2009 issue of Today's Facility Manager
Roof systems are often overlooked as an area for potential energy and cost savings. However, considering the significant impact roofs have on multiple aspects of a facility’s performance, these overhead systems should be evaluated when energy efficiency and service life are areas of focus. Facility managers (fms) can have their roofs assessed in order to identify improvements that will increase performance in terms of energy efficiency.
A wide variety of roof system types, such as built up, modified bitumen, liquid applied, EPDM, TPO, PVC, metal, and spray foam may be installed onto facilities based upon climate, building use, and local environmental, regulatory, and ownership goals. Still, there are common elements to be evaluated. While it is not mandatory, fms may choose to use experts in the field during these exercises. This is due to the factors such as potentially complex calculations and an expert’s knowledge in working with manufacturers and contractors to ensure any warranties remain in effect.
Identifying Problem Areas
Fms and their staff members would never walk by an open door or window where conditioned air was escaping. With roofing, the equivalent of this loss is not so readily recognized. It is not unusual to find “openings” in a roof system that, collectively, are equivalent to an open 3'x6' window through which conditioned air and/or energy are escaping.
Other situations, such as wet insulation caused by leaks (which provides minimal to zero R-value), also contribute to energy loss. Insulation is measured in R-values—the higher the R-value, the better the roof will resist the transfer of heat.
Evaluation by a competent professional should identify short-, medium-, and long-term work that can be performed on existing roof systems to increase energy efficiency. Fms should not presume that adding more insulation is the only, or even the best available, action, or that it can only be undertaken when installing a new roof system. (See “Roof Evaluation Checklist” sidebar to view what every evaluation should include.)
Typically, there are several actions required in determining the best way to save energy (and money) through changes to the roof and related areas. These actions are: studying construction documentation; consulting with the manufacturer (if system is under warranty); performing “core cuts” and other system disassembly at various components; and examining from below the roof system.
Low Hanging Fruit
The vast majority of roof systems have low hanging fruit for improving energy efficiency. This work should coincide with regularly scheduled maintenance. Generally speaking, these opportunities include:
- correcting the fit/closure at hatches and skylights;
- replacing small areas of wet insulation;
- repairing/replacing open and deteriorated sealants;
- insulating/sealing at penetrations such as drains, pitch pans, conduits, and vents where there is good access from underneath the deck (rarely is the air/vapor/thermal barrier sealed at penetrations);
- insulating/sealing at curbs where possible from underneath the deck (rarely is the air/vapor/thermal barrier run into units as designed and sealed at curbs); and
- insulating/sealing at roof/wall interfaces from underneath (hollow parapet walls are just chimneys if there is no thermal/air/vapor barrier).
When evaluating a roof from below, if a person can see up into the roof system at penetrations or curbs, it is virtually certain there is minimal or no R-value in place and that there is no air or vapor barrier where needed. Wet insulation provides extremely low (if any) R-value, which means that if it is replaced, the return on investment (ROI) through increased energy efficiency would most likely be short-term.
Higher Cost Improvements
Higher cost actions that carry a longer ROI include insulating and installing air/vapor seals tight around penetrations and other roof details where the work must be done from above. Still, implementing these types of improvements at larger penetrations, such as those for power vents, air exchangers, HVAC equipment, grease traps, and skylights, typically have a shorter ROI.
Consider the following example: There is a 4'x6' curb, 8" tall surrounding an HVAC unit. While this is a relatively small HVAC unit, it represents an approximately 13 square foot area (over a 2'x6' opening) that has inadequate or missing insulation and air/vapor barrier. Even worse, conditioned air may be escaping or even blowing underneath the roof membrane.
These constructions are seldom installed with the proper combination of insulation and sealing materials. Perhaps this is due to a lack of skill or concern regarding insulation value and air/vapor tightness by those performing the installation.
Here is another scenario: A hollow parapet wall—4' high and 1' wide—without proper insulation is situated around a building rooftop. If this is a 50,000 square foot building with a rooftop area of 200'x250', the hollow parapet wall represents 8,100 square feet of uninsulated/unsealed area.
If the sides of the parapet walls are insulated but the tops are not, this leaves a roughly 6" wide un-insulated area underneath the coping or wall top around the building perimeter. The result is a 450 square foot area that is uninsulated/ unsealed. It is not uncommon to find parapet walls that are not insulated at all along the top and that have poor/missing insulation on the sides. Seldom is there no insulation at all; however, often 25% to 35% of the insulation is missing or has been dislodged, which results in significant uninsulated areas in existing buildings.
While there are specific ways to shore up an existing roof system, fms can also make sure energy efficiency is addressed during initial construction or when a roof recover or replacement occurs. Items to be considered include: recognizing current energy codes (in most municipalities, recovering, replacing, or repairing more than 25% of a roof area requires the entire assembly to meet current code); minimizing penetrations; and tying in new insulation and air/vapor barrier materials at exterior and interior walls. Even if a new roof is well insulated with appropriate air/vapor barriers, significant energy losses will occur if those insulating barriers are not tied into the existing structure.
White (Cool) Roofs
In the case of white (cool) roofs, a common issue is that when they are installed, the amount of insulation is often not increased and proper vapor/air sealing is not done. These types of roofs (e.g., TPO, PVC, liquid applied, and acrylic surfaced modified bitumen) are sometimes sold by manufacturers and installers as energy efficient and/or compliant with applicable energy codes (and sometimes even approved) without an increase in insulation and appropriate vapor/air sealing. Insulation is not increased based upon the roof’s ability to reflect energy (principally radiant heat) from the sun rather than absorbing it.
Fms should be aware of several issues with this approach. It is true that a white roof surface reflects the sun’s energy and that the roof membrane does not heat up and retain and/or transfer as much energy to any insulation underneath and perhaps into the facility interior. This can be very positive in reducing the urban heat island effect. In a properly installed white roof system with adequate insulation and vapor/air sealing, very little energy is transferred through to the interior of the facility.
However, without adding appropriate insulation and vapor/air barriers, these roof systems have no impact on keeping conditioned air inside the structure or on the transfer of energy from exterior ambient air to and from the conditioned air inside. In this scenario, a white roof does not increase insulation R-value, and operating budgets do not realize the cost savings provided by additional insulation and proper air/vapor sealing.
Whether fms employ knowledgeable in-house staff members or external sources to evaluate their roof systems, they should insist upon a report that identifies low, medium, and high cost opportunities based upon ROI requirements. These reports should provide calculated R-values along with other meaningful information including overall conditions and individual components. A comprehensive evaluation will enable an fm to decide what areas to address first to improve energy efficiency.
Hoover is principal of The BEST Consultant, Inc., a Suwanee, GA-based firm providing complete building envelope consulting services. He has more than 25 years of experience in real estate including consulting, property management, general construction, roofing, and waterproofing. Hoover is a member of numerous industry groups, including RCI (the Institute of Roofing, Waterproofing, and Building Envelope Professionals), U.S. Green Building Council, Green Building Initiative, Sustainable Building Industry Council, the National Roofing Contractors Association (NRCA), and several ASTM committees (including Roofing and Waterproofing, D-08).
Other posts by