ABSTRACT Steep-sloped roofs are chosen for many reasons. Most homes in the U.S. have steepsloped roofs, primarily for their exterior aesthetic appeal. Churches and other audi¬ toriums most often have steep-sloped roofs. Steep-sloped roofs in general have a rep¬ utation of providing superior weather resistance. There are many factors is the design of steep-sloped roofs. Having the right tools can help ease the design process. Among the design issues are ice damming and the opposite, excessive high temperature. Ventilation is an approach that is used to alleviate these issues, and the research has provided guides on how to appropriately use ventilation. Consultants may be knowl¬ edgeable of the research, but the issue is finding the time and tools to apply this knowledge. The use of the new tools to determine if ventilation is adequate will be demonstrated. The scientific background for the tools is discussed, and suggestions on how the information is to be applied are provided. SPEAKER David L. Roodvoets is an independent consultant. He consults with ARMA and sev¬ eral manufacturers of roofing products. He served as technical director for SPRI; he is past chairman of RICOWI (Roof Industry Committee on Weather Issues) and board member of the Cool Roof Rating Council (CRRC) . Previously, he was employed as an associate development scientist for the Dow Chemical Company and technical direc¬ tor for the T. Clear Corporation. Mr. Roodvoets has been involved with research on all facets of roofing systems. He has worked with major research institutions conducting extensive wind tunnel testing of roofing systems. Mr. Roodvoets has published arti¬ cles in several journals and is active in International Building Code development. He recently spoke at the ASTM Roofing Research and Standards Development Sym¬ posium, the RCI Building Envelope Symposium, and the 2005 International Code Council (ICC) Hurricane Symposium. CONTACT INFO: ldlrconsul@charter.net or 231-893-1291 COAUTHORS Tony Malinger is chief operating officer of Metal Era Inc. He is actively involved in the Edge Metal Design Task Force at SPRI. Dr. David Banks has completed research related to trace gas analysis for his master’s degree in aerospace engineering from the University of Toronto, and award-winning published research relating to wind suction forces induced on roofs by conical vor¬ tices for his PhD from Colorado State University. He has ten years of experience in consulting engineering related to wind and air flow around and though buildings, including extensive use of both CFD and physical modeling. His current focus is the combination of CFD and wind tunnel modeling to improve natural ventilation design. He is the handbook subcommittee chairman for chapter 34 of the ASHRAE funda¬ mentals handbook, Indoor Environmental Modeling. Roodvoets, Malinger, and Banks – 168 Proceedings of the RCI 24th International Convention
INTRODUCTION This paper addresses two issues that are common to steepsloped roofs: ice dams and exces¬ sive heat buildup that can lead to roof cover deterioration. Ice dams occur in U.S. climate zones 5-8 and are caused by melting snow or ice that refreezes at the lower edge or eave of steep roofs. Excessive heat buildup can occur when the cooling effects of a roof deck and building interior are blocked by an underlying insula¬ tion. Excessive roof heating can occur in most climates but is more of a problem in hot and hot/ dry climates. Ice Dams Ice dams occur when snow or ice melts at the top of a roof slope and refreezes at the bottom of the roof slope or eaves. Tobiasson et al. 1 give detailed information on why ice dams occur. To para¬ phrase Tobiasson’s excellent re¬ search, ice dams occur because heat from the interior of the build¬ ing accumulates at the top of the roof slope and raises the roof sur¬ face material to a temperature above freezing, thereby allowing melted water to flow downslope and refreeze. The cause is not pri¬ marily the heating by the sun, as that typically is uniform across the roof surface, and therefore causes equal melting at the top and bottom of the slope. Random hot spots can also occur around chimneys, vent stacks, or directly above heating appliances. These hot spots may also be corrected with ventilation but usually need to be corrected by stopping air flow from the heated inte¬ rior. The primary cause of ice damming is that the heat com¬ ing from the interior is not removed quick¬ ly enough to prevent the temperature from rising above freezing. The quick response by some designers is that additional insu¬ lation needs to be Figure 1 – Typical ice dam. added. Although the additional insulation may be of value, it often is not the cure. The ice dams still occur because the roof surface remains above 32°F (0°C). Heat is entering the space beneath the roof deck because of heat transmission through insulation material and ,in the case of attics, from air passing from the heated interior of the building directly though holes in the ceilings or walls adja¬ cent to the attic space. When the temperature is above 32°F (0°C), snow and ice melt; and the eave area will also be warm enough to prevent refreezing. Bright sunlight will help melt the dam, but it can also allow slippage of the entire sys¬ tem, creating a significant hazard for anyone in the area. There are several designs of snow and ice guards that reduce the potential for large pieces of ice to slide from the roof. Basic roof design that eliminates steep slopes near pedestrian areas or ice guards should always be considered when there is a potential for ici¬ cles or ice dams. System Design Residential structures with attics or cathedral ceilings built to meet the requirements of the International Residential Code may not have adequate insulation to prevent ice damming if the attic space or cathedral ceiling system does not have ventilation. Cur¬ rently, the International Resi¬ dential and International Building Codes do not require attic ventila¬ tion. The International Residential Code requires R-49 attic insula¬ tion in climate zones 6, 7, and 8. Commercial buildings de¬ signed using ASHRAE 90.1 -2007 are required to have R-20 insula¬ tion. This is unlikely to be enough insulation to prevent ice damming in most areas where snow or ice Proceedings of the RCI 24th International Convention Roodvoets, Malinger, and Banks – 169 storms are likely. Tobiasson’s and the author’s research show that the most effective way to reduce the potential for ice damming is to add ventilation. System design then calls for the code-required insulation to be installed and ventilation to be provided – either using the code¬ ventilation requirement of one sq ft of ventilation per 150 sq ft of floor space for attics, or by using the ventilation designs in Tobi¬ asson’s tables or the computer program discussed in this paper. Correcting and Preventing Ice Dam Problems The first step when an attic space is involved in a structure with existing ice dam problems is to find the source of air entering the attic from within the heated building. It could be duct leakage or holes around vent stacks, light¬ ing, or wiring fixtures. Don’t over¬ look the possibility that the warm air could be coming from the basement or crawl space through the walls. All holes should be closed using an appropriate method, such as sealing large holes with an air-and-moisture barrier and using expanding foam for smaller cracks or penetra¬ tions. Stopping the air movement will also stop moisture coming from the interior. Moisture can be a significant problem in attics. The next step is to make sure that ventilation is installed and that it is operating, not painted shut or plugged with insulation. Also determine that the code min¬ imum ventilation of one sq ft of net-free ventilation for every 150 sq ft of ceiling area when the attic space is ventilated at both the eave and ridge is installed. Roofs that have the code-required mini¬ mum of one sq ft of eave ventila¬ tion for every 300 sq ft of ceiling area may not have adequate ven¬ tilation to prevent ice damming, as heat can be trapped at the ridge area. 2 The third step is to add insu¬ lation to bring the attic up to cur¬ rent standards. The new Interna¬ tional Energy Code and ASHRAE Standard 90.1 require more insu¬ lation in attic spaces than previ¬ ous versions of the energy codes. The minimum insulation in attics of commercial buildings for cli¬ mate zone 5 is R-30 and for cli¬ mate zone 6 is R-38. For residen¬ tial buildings in climate zone 5, R- 38 is required, and for climate zones 6 to 8, R-49. Having this much insulation will be most helpful if there are no direct air leaks into the attic space from within the building. ASHRAE is a good source for information on controlling heat and moisture. Chapter 25 of ASHRAE Fundamentals provides the science and some design information that can be applied. Chapter 43 in the ASHRAE Applications Handbook provides examples of good design practice. Several tests, such as blower door tests, tracer gas tests, and infrared analysis can also be used to determine where the warm air is leaking into the attic space. Solving the air leakage problem is also likely to provide energy sav¬ ings to the building owner. Ice Damming and Cathedral Ceilings The cause of ice damming when cathedral ceilings are used differs from the attic system in that there is less likelihood that air is leaking from the interior into the space just below the deck. This includes heating ducts, which could leak but are not often found in the cathedral ceiling space. Cathedral ceilings / roofs are primarily installed for aesthetic reasons, both for the outside and inside appearance. Historically cathedral ceilings provided both the surface for the attachment of roofing materials and the interior finish. Many cathedrals used wood planking for the deck and provided a roof cover – typically lead, copper, tern, or slate – for water shedding. In the Middle Ages, cathedrals were unheated or had small stoves that provided a small amount of warmth. The relatively low R-values of the wood decks were not an issue, as there was little heat in the building. Prior to the energy crisis in the 1970s, cathedral ceiling roofs were still built primarily with wood planks with R-values of 3.5 to 7. Because of energy costs and code-mandated higher energy effi¬ ciency, building owners have used many techniques to minimize the heat loss from the roof. Several have built structures inside of the roof deck. This may be an insulat¬ ed cathedral ceiling suspended from the roof or a flat or designer ceiling. Many have maintained the slope, but not the aesthetics of the natural wood interior. Batt insula¬ tion with an air gap between the top of the insulation and the deck is often added in the space. Some have made the space an attic by putting a flat ceiling at a lower level and insulated above the ceil¬ ing. Other techniques include applying spray foam under the deck or to the top of new interior finished ceilings. All of these approaches save energy. Now almost all buildings with cathe¬ dral ceilings are heated and cooled. Preserving aesthetics and pro¬ viding more insulation can be done economically when the roof cover is replaced. This involves adding insulation on top of the structural deck. The system becomes the same as compact roofs used in low-slope construc¬ tion. Although this adds R-value, insulation levels typically speci¬ fied are not adequate to prevent ice dams. Tobiasson found that insulation R-values of R-45 or greater are required to prevent ice damming when there is one foot of Roodvoets, Malinger, and Banks – 1 70 Proceedings of the RCl 24th International Convention snow on the roof. Current building codes (ASHRAE 90.1, 2007) require R-20 insulation for low-slope roofs on commercial buildings, churches, and other large auditoriums (increased from R-15, in the 2004 edition of ASHRAE 90.1). It there¬ fore is possible to meet the latest energy code and still have ice dams. Also, getting the insulation level to R-45 or greater is an ex¬ pensive proposition. However, because there is a need to save energy and reduce carbon emis¬ sions, codes will be aiming at increased R-values in the future. Techniques to alleviate ice damming have been covered quite thoroughly by Tobiasson. The basic solution for cathedralized roof systems is to create an air space over the insulation under the deck that supports the roof cover. A key require¬ ment of the air space is that the air must move freely from the eave to the ridge of the roof and not be heated to a temperature greater than 32°F (0°C). Systems should also pro¬ vide for lateral movement of the air to avoid obstruc¬ tions in the air space or limited ridge ventilation. One choice for providing insu¬ lation and ventilation is to remove the existing roof cover, recover the deck with new roofing felt, and add 2-in x 4-in or 2-in x 6-in purlins and 1-in x 2-in cross purlins above the existing deck to provide ventilation and a cold roof deck. The cross purlins allow interconnected airflow in the sys¬ tem. The space between the main purlins is filled to within 1 in of the top of the main purlins with closed cell insulation to the designed predetermined depth. A new supporting roof deck and roof cover are installed over the new deck. This design does an excel¬ lent job and can be built by any competent contractor; however, it may not provide enough insula¬ tion/ air gap for all buildings. Several sources suggest the space between the main support purlins should be filled with closed-cell spray foam or a combi¬ nation of rigid board foam and spray foam to provide a secondaiy moisture barrier. 3 For roofing contractors, prod¬ ucts generically known as venti¬ lated nailbase insulation sheath¬ ing provide both insulation and ventilation. Ventilated nailbase insulations consist of closed-cell foam, spacers, and top surface. Typical products use either closed-cell polyisocyanurate foam or extruded polystyrene foam insulation. When using shingled or metal roofs, OSB is typically used as the top surface of the ven¬ tilated nailbase. Other materials may be used for adhered single¬ ply or modified-bitumen roofs. All commercial ventilated nail¬ base products use spacers to enable the airflow to rise up and/or across the slope when the vertical path is impeded. When the length of the run is less than 20 ft, the use of a 1-in air gap has proven adequate in most cases, however, when the distance from the eave to the ridge is greater than 20 ft, calculations using the program developed by CPP show that a 1-in air gap height is insuf¬ ficient. (See Chart 3.) Nailbase insulation with a 1-in air gap height that is traditionally used for residential applications pro¬ vides a good solution where there are limited roof heights and eaveto- ridge runs. However, roofs of commercial buildings, churches, and other auditoriums feature much longer slopes that require different strategies for determin¬ ing the needed air gap height and insulation. CPP Wind Engineering and Air Quality Consultants of Fort Collins, CO, conducted a study using known airflow and heat transfer relationships. The goal of this program was to provide guid¬ ance on the insulation Rvalue and air gap height required to prevent ice dams. As a result of the study, a program was devel¬ oped to do the necessary calculations. This program is based on an assumed worst-case scenario of no wind. Research shows that wind will generally augment the airflow through the air gap and provide the required temperature reduc¬ tion to keep the snow from melting; however, when there is no wind, the change in air density as it is heated becomes the primary driver for the airflow. In all cases, heat will be transferred from the interior of the building through the insula¬ tion to the air space. As the air space is heated, the air density will decrease, and the lower-den¬ sity air will move upwards (i.e., the buoyancy effect used in hot air balloons). This creates airflow that brings the cooler, heavier air into the air space at the bottom or eave; and the warmer, lighter air flows out at the ridge or top of the air space. The constriction for the airflow is the air gap between the insulation and the cover board. The more the space is restricted, the less airflow can occur, and the Photo 2 – Nailbase insulation. Proceedings of the RCI 24th International Convention Roodvoets, Malinger, and Banks -171 Figure 3 – Airflow is restricted through this complex site-built design. air temperature in the air gap will increase. The design of the air gap requires that the height of the air gap does not impede the flow of the air, allowing the basic thermal dri¬ ver to work. The fundamental assumptions of the program are that heat is transferred from the interior of the building through interior sur¬ faces and deck to the air space at a rate dependent upon the insulation level and the temperature differ¬ ence between the building interior and the gap. The roof tem¬ perature is determined based on the reflectance and emittance of the roof covering. The insulating value of the nailbase material is also included. For winter condi¬ tions, it is assumed that the roof is covered with snow, providing high reflectance, and that the temperature at the roof surface is 32°F. Once the temperature in¬ creases beyond 32° F, the snow or ice will melt and flow toward the eaves. Another key assumption is that there is no wind. This is the worst-case situation, as in most cases when there is wind; it will create suction at the peak and some pressure at the eaves, dri¬ ving more air through the air gap space and keeping the deck cool. The program combines known relationships between air flow rates and viscosity; heat transfer and conductance, radiation and emittance; to predict the airflow rate and air temperature rise in the ventilated nailbase airspace. The equations used in this analy¬ sis can be found in the complete report available online at www.metalera.com. 4 By combining all of the factors that drive the airflow though the air gap, the program iteratively solves the equations and plots a graph that shows the temperature in the air gap. The goal of the designer, then, is to achieve an air temperature in the air gap of less than 32°F at the top of the slope. Go to the Metal Era Web site to use the program. There is a need to know several factors for the roof being designed. These are: roof slope, distance from eave to ridge, thickness of material used as an attachment base (the top plywood/OSB layer), ceiling insu¬ lation R value, eave vent length, ridge vent length, the basic color and type of the preferred roof cover material, and outside tem¬ perature for summer conditions. (For winter, the program assumes a snow-covered roof at a tempera¬ ture of 32°F.) An air gap is chosen and entered into the program. Clicking “calculate” results in a graph showing the temperature of the top surface material and the temperature of the air in the gap. It also provides a direct reading of the air temperature at the ridge vent. Using the pre¬ mise that air tem¬ perature less than 32° prevents the nailbase sheath¬ ing from exceeding the melt tempera¬ ture of the snow, there should be no water to run down the roof slope and freeze. If the insu¬ lation/ air gap cho¬ sen allows the temperature of the roof cover to heat up to greater than 32°F, a red line appears on the graph at a length down the slope where the temper¬ ature of 32°F is exceeded. The most effective way to decrease the tem¬ perature below 32°F is to increase the air gap height. Tobiasson found 22°F to be a critical temperature for ice dam¬ ming. His work shows that at tem¬ peratures above 22°, ice damming rarely occurs, and there is less of a problem at colder temperatures. Therefore, 22°F is a good design outdoor temperature for ice dam prevention.5 This is not to imply that the design temperature of the roof should be other than the code-required design temperature for the building. But when consid¬ ering all the factors, including the insulation and materials used, a calculation at 22° is likely to pro¬ vide data for the worst case for ice dams to occur, and the minimum insulation/ air gap should be de¬ termined using 22°. The air gap height required is also a function of the slope of the roof and the distance between the eave and ridge or top of the roof section. Ice damming is a greater problem on roofs that have a 3 /12 slope than those with greater slopes, and the air gap heights in Roodvoets, Malinger, and Banks – 1 72 Proceedings of the RCI 24th International Convention the ventilated nailbase insulation need to be larger to adequately remove enough of the heat to prevent the ice damming. Of course, the sys¬ tem needs free airflow through the roof vents greater than or equiva¬ lent to the airflow through the ventilated nailbase. The design of the eave and ridge vents is critical to getting the air through the system. Therefore, if the design of the airspace requires a 2-in-high airspace, then the eave and ridge vents will need to have at least 2 inches per lin¬ ear inch of net free area. It is also important that there is adequate air entering the eaves. There must be a clear air path from the eave Figure 4 – Airflow is not restricted in this commercially available, pre¬ vents to the base of the manufactured edge system. insulated nailbase air gap. Some soffit designs create complex paths that are ineffective in providing the air required to meet the air¬ flow requirements of the system. In cold regions, sloped roofs should have overhangs. Tobiasson recommends at least one foot of overhang. 6 This overhang can include a soffit, or the underside of the deck may be exposed if there is sufficient blockage at the wall. Ventilated edge systems that incorporate the details required by adequate airflow design can now be purchased as premanu¬ factured products specifically designed for the building. The key requirement of the eave vents is that they provide an unobstructed air gap at least as large as the ventilated nailbase air gap, and that the airflow is not restricted. Building Codes Insulation Requirements Residential buildings up to three stories typically are built to the International Residential Code (IRC) that includes energy design requirements and also references the International Energy Code. The IRC code requires R-30 or greater insulation levels for most areas of the U.S.; and for the heaviest snow areas (Climate Zones 6, 7, and 8), R-49 is required. For typical residential applications, the 1-in gap height, with adequate eave and ridge ven¬ tilation, works (typical 6/12 slope and 1-story height increase). This additional insulation provides a greater margin of safety before ice damming occurs on residential roofs. However, the additional insulation is not as effective in preventing ice dams as additional ventilation is. Most U.S. commercial build¬ ings, churches, and schools are designed using the International Building Code (IBC). The IBC code references ASHRAE Standard 90.1 for energy design require¬ ments. ASHRAE Standard 90.1 now requires an R-20 roof for most of the U.S. Commercial buildings, churches, and schools, when constructed with cathedral ceilings, may have lower slopes, and almost always have much longer distances between the eave and ridge. Thus, using less insu¬ lation and longer slopes can lead to ice damming issues in the northern, heavy snow areas. The solution is a combination of more insulation and a larger air-gap height, so that the nailbase insu¬ lation remains cool. Proceedings of the RCI 24th International Convention Roodvoets, Malinger, and Banks – 1 73 Changing Cathedral Construction Most cathedral ceilings for large buildings are now construct¬ ed with the same steel decks as used in low-slope roofs. They have an interior finish, generally con¬ structed of gypsum board; are insulated on the exterior with foam board insulation; and have a wide variety of roof coverings. In most cases, the heavy planking is gone, the supports are steel con¬ struction, and the interior finish¬ es vary and may be directly attached to the bottom of the deck or attached with an attic space between the ceiling and the struc¬ tural deck. All designs should have the dewpoint fall in the insu¬ lation; if possible, there should be no condensing surfaces. A vapor barrier may be required to accom¬ plish this in some situations. Figure 5 – Potential solution for hip roof ventilation. Designs incorporating venti¬ lated nailbase insulation are a way to capture the needed Rvalue and ventilation for steeldeck- based construction, and they minimize ice damming. Decks constructed with wood planking may provide the aesthet¬ ics and strength required, but they do not meet the code-mandated R-values; therefore, these roofs also need extra insulation, and the most effective system is a ventilated cathedral design. The ventilation also serves to keep the base of the roof covering surface cooler when the roof is exposed to direct sun in cooling mode. Hip roofs, where the ridge vent is less than 20% of the length of the eave vent, present a special challenge, as the ridges adjoining the other sections of the roof are somewhat difficult to ventilate with adequate waterproofing, and there is resistance to blowing or drifting snow. One of the more practical solutions is to extend the main ridgeline of the roof a few feet over the hipped section and to then install a dormer-type vent in the triangle section creat¬ ed between the roof slopes. To be most effective, the air must be able to move laterally to the pri¬ mary ridge, the ventilation from the opposing slopes should be interconnected, and the exhaust ventilation should exceed that required for a gable-end roof. The Dutch hip roof is one way to obtain additional ventilation in a hip roof design. Vents are added in the A section. These can be powered, if required, to achieve the amount of ventilation desired. When hip roofs converge at a sin¬ gle point, a cupola or modified cupola may provide the required ventilation. Venting to Minimize Cooling Costs Oak Ridge National Labora¬ tory has been evaluating tile and metal shingle roofs, which have a natural airflow. They have found that the ventilation from tile and other roofs where there is an air gap between the roof covering and the deck, provides additional cool¬ ing to the building. Tile and simi¬ lar products have air flow up the slope and also into the systems at many points. The moving air dis¬ sipates the heat from the surface and results in less cooling load for the building. If there is adequate air flowing up the surface with ventilated cathedral ceilings, there should also be measurable building cooling effects.7 The computer program devel¬ oped by CPP provides a recom¬ mended air gap height for roofs that are in cooling-dominated cli¬ mates. The same parameters are used as in the design for heating climates. In the cooling climates, the air is warmed by the sunlight that falls on the roof surface, and the maximum temperature of the roof surface is driven by the reflectivity or albedo of the roof surface material. The interior of the air gap is cooled by the roof deck, but in this case, the cooling is reduced by the insulation between the air gap and the build¬ ing. The same phenomenon of the air heating and rising due to buoyancy creates the driving force to remove the hot air from the building. The program ignores the Roodvoets, Malinger, and Banks – 174 Proceedings of the RCI 24th International Convention effects of wind, as the wind is like¬ ly to improve the airflow through the system. In all cases, the program includes the fundamentals of heat transfer, conduction, convection, and radiation, along with the effects of the air films and fluid dynamics. To calculate the airflow rate through the space, the amount of temperature increase is needed. This is found by taking into consideration such variables as airflow rate, temperature increase, and heat transfer coeffi¬ cients. These calculations occur in the background, so the user of the program only deals with the final answer. Constrictions creat¬ ed by eave and ridge ventilation, as well as the air gap height between the insulation and the deck will affect the performance of the system. The program shows that less air gap height is needed to cool the deck in the summer¬ time than what is needed to pre¬ vent ice dams in the winter. Therefore, a design that avoids ice dams will also avoid excessive heating of shingles and other roof coverings. Program Use and Results The program developed in this study can easily be accessed and is available for no charge at www.metalera.com. The Web site provides a list of inputs that can be changed by the user. In order to use the program the user should know the following infor¬ mation about the roof being designed: Roof Shape Details • Pitch on 12 • Length of passage from eave vent to ridge vent (ft) • Thickness of OSB/plywood (in) • Height of gap (in) • Ridge length (ft) • Eave length (ft) Thermal Information • Roofing composition • Full sun – roofing material (this is a drop-down menu) * Cement – Dark * Cement – Medium * Cement – Light * Ceramic – Red * Ceramic – White * Shingle – Dark * Shingle – Medium * Shingle – Light * Wood – Dark * Wood – Medium • Or snow-covered. (When the roof is snow-covered, the expected rooftop temp¬ erature is 32° F.) • Ceiling/wall insulation R-value • Outside temperature in degrees Fahrenheit. (From Tobiasson’s research, an outside temperature of 22°F creates the worst case, so that is a good bottom of the nailbase) will reach 32°F just about 3 ft from the eave, creating an ideal condition for ice damming. The first question that may be answered is, what is the effect of insulation R-value? Can increas¬ ing insulation R-value avoid ice damming? Chart 2 shows the out¬ let temperature when a high-slope roof is insulated. It can be seen that doubling the R-value of the insulation does not provide an outlet temperature that prevents ice damming, as the air tempera¬ ture is still above freezing. The R- 50 roof does not bring the outlet temperature below freezing and would not prevent ice damming on this roof with a 50-ft air pas¬ sage length. The air passage length is the distance from the inlet or eave vent to the outlet or ridge vent. It is assumed to be a straight line for this program. Increasing the air gap height to 1 in for the R-19, 12-in x 12-in sloped roof does not adequately supply cool air to avoid ice damming, as the airflow is restricted. However, as seen in tempera¬ ture to use.) As always, it is interesting to change the in¬ puts to deter¬ mine the effects of changes in the basic parame¬ ters. The results from several iter¬ ations follow. Starting with a minimum air gap height, it is easy to see that with an R-19 roof, there will be ice dams formed as the tempera¬ ture in the gap (and hence, the Proceedings of the RC1 24th International Convention Roodvoets, Malinger, and Banks – 175 Chart 2 – Slope 12 x 12; air gap, 0.5 in. Roof Shape Details Thermal Information PLietncght ho no f1 2P:a ssage; 5102 feet CReoiolfiinngg R C-oVamlpuoes:i 1t9i on: Snow-covered HTehiigchktn eosf sG aopf :O SB: 01 .I5 nicnhcehse s Outside Temperature: 22°F REiadvgee LLeennggtthh:: 5500 ffeeeett Chart 3 – Air gap height, 1 in; slope, 12 in x 12 in; R-19. Roof Shape Details Thermal Information PLietncght ho no f1 P2:a ssage. 5102 feet CReololfiinngg R C-Voamlpuoes:i t19io n: Snow-covered THheiigchktn eosf sG aopf :O SB: 02. i5n icnhcehse s QutsideTemperature: 22°F REiadvgee LLeennggtthh:: 5500 ffeeeett Temperature by Distance Along Roof * Gap Air Temperature at the endt ^osfi arru*n#. 31.1“R tF K t! Chart 4 – Air gap height, 2 in; slope, 12 in x 12 in; R-19. Gap VS Temperature Chart 5 Airgap increase decreases outlet temperature Outside Temperature 22° F R-19 Passage Length 50 ft Ridge = Eave 0 UtlCt temperature Chart 3, the point where the air gap temperature exceeds 32°F is at about 25 ft. This justifies the use of this standard 1-in air gap height for most residential build¬ ings, as the passage length of most of the cathedral ceilings is less than 25 ft. Increasing the air gap height to 2 in for the R-19, 12-in x 12-in sloped roof provides adequate air¬ flow to avoid ice damming. The airflow is not restricted, and it continues to remove the heat from the interior of the building, avoid¬ ing the melting at the top of the slope. Chart 4 is the desirable shape of a chart from the comput¬ er program. When the tempera¬ ture in the air gap is less than 32°F at the end of the air passage, ice dams are not likely to occur, as the building is not heating the underside of the wood-sheathing surface of the nailbase. The air gap height has a major influence on the outlet air temper¬ ature, and in most cases, it is likely to be the simplest and least expensive to change. Roodvoets, Malinger, and Banks – 176 Proceedings of the RCI 24th International Convention Slope /Temperature Stope Chart 6 Chart 9 – R-19 insulation. Chart 7 – Outside temperature, 8S°F; dark concrete tile roof; air gap height, 1.5 in. a vertioccurs air gap deck surface is 151°F, so the air in the gap continues to cool the underside of the deck. Chart 8 – Outside temperature, 85°F; dark shingle roof; air gap height, 1.5 in. Steeper stope reduces temperature Next, let’s examine what hap¬ pens when we are attempting to keep the roof surface cool to avoid premature damage to the roof covering in a cooling climate. In Gap l.S ” Outside Temperature R-19 —♦- o utlet Temperature When the roof cover is changed to a darker colored this case, cal line when the Although steeper slopes reduce the outlet temperature, increasing the slope is not likely to avoid ice dams unless the out¬ let temperature is already very close to 32°F. air temperature reaches 150°F. This an arbitrary default number in the program. The critical factor is that the air gap continues to cool the underside of the roof deck. Chart 7 shows that the air exiting the roof is at 134°F and the roof Proceedings of the RCI 24th International Convention Roodvoets, Malinger, and Banks – 177 Chart 9 – R-l 9 insulation. Slope vs Temp, Slope Gap = 1.5 ” Outside Temp 85° Passage Length 50′ Ridge = Eave —♦—Outlet Temperature Chart 10 – R-19 insulation. asphalt shingle, the starting tem¬ perature of the roofing material in bright sunlight exceeds 150°F, so the arbitrary limit of effectiveness is not meaningful. The critical fac¬ tor shown in Chart 8 is that the air temperature at the outlet is 22°F less than the temperature of the roof covering; so the air gap continues to provide cooling to the roof deck and roof covering mate¬ rials. In cooling climates, the air gap is very effective in cooling the underside of the roof deck as shown in Chart 9. When there is little gap (0.5 in), the temperature is 166°F, and with the 3-in gap, the temperature at the outlet is 116°F. This lower outlet tempera¬ ture is expected to have a major positive effect in reducing cooling load. Increasing slope also reduces the outlet temperature and increases the cooling effect. Insulation has little effect in the air gap temperature in cooling climates. Major drivers of the roof temperature are the color and reflectivity of the roof. Highly reflective roofs have much lower surface temperatures than dark¬ er, nonreflective roofs. The pro¬ gram has predetermined set points for reflectivity based on the material chosen. This consists of a drop-down menu that offers many of the options that may be considered. CONCLUSIONS Adding an air gap to a steepsloped roof may have some signif¬ icant benefits in avoiding ice dams and keeping the roof cover cooler. There also may be energy¬ saving benefits in cooling domi¬ nated climates. FUTURE RESEARCH Several questions and oppor¬ tunities still need to be explored. Does the program adequately address the systems that are not constructed with commercial nail¬ base insulation? Because of the surface roughness of field-con¬ structed systems, increasing the air gap height from that recom¬ mended by the computer program will be the more conservative solution. Another subject for fur¬ ther exploration is the effect on building cooling by the air gap. Is this effect equal to that achieved with tile and other systems that are installed on spacers above the primary roof deck? Are there ben¬ efits from an air gap on a reflec¬ tive roof? All of the roof designs derive some cooling benefit from Roodvoets, Malinger, and Banks – 178 Proceedings of the RCl 24th International Convention the air gap, but the value of that cooling in relation to energy sav¬ ings may be the focus of a future paper. FOOTNOTES 1. W. Tobiasson, J. Bruska, and A. Greatorex, “Roof Ventil¬ ation to Prevent Problematic Icings at Eaves,” Transac¬ tions v. 104, American Soci¬ ety of Heating, Refrigerating, and Air-conditioning Engi¬ neers (ASHRAE), 1998. 2. The 2006 International Building Code (IBC), Section 1203.2. 3. “Unvented Roof Assemblies for All Climates,” Building Sci¬ ence Corporation BSD- 149, Westford, MA, 2007. 4. The research serving as the basis for this program is available from the coauthor, Tony Malinger 5. W. Tobiasson, J. Bruska, and A. Greatorex, “Guidelines for Ventilating Attics And Cathe¬ dral Ceilings to Avoid Icings at Their Eaves,” Proceedings of Buildings VIII, ASHRAE, 2001. 6. Tobiasson, 1998. 7 W.A. Miller et al., “Natural Convection Heat Transfer in Roofs with Above-Sheathing Ventilation,” Proceedings of Thermal Performance of the Exterior Envelopes of Build¬ ings X, ASHRAE, 2007. OTHER REFERENCES “Advanced Energy Design Guides for K-12 School, Small Retail, and Small Office Buildings,” www.ashrae.org /publications /page/ 1604. A.O. Desjarlais, T.W. Petrie, and T. Stovall, “Comparison of Cathedralized Attics to Con¬ ventional Attics: Where and When do Cathedralized Attics Save Energy and Operating Costs?” Proceedings of the Performance of the Exterior Envelopes of Whole Buildings IX International Conference, ASHRAE, December 2004, Clearwater, FL. W. Elenbaas, “Heat Dissipation of Parallel Plates by Free Convection,” Phsica 9(1):2- 28, 1942. K.G.T. Hollands, T.E. Unny, G.D. Raithby, and L. Konicek, “Free Convection Heat Trans¬ fer Across Inclined Air Lay¬ ers,” Journal of Heat Transfer, May 1976, 189-193. W.A. Miller, W.M. McDonald, A.O. Desjarlais, J.A. Atchley, M. Keyhani, R. Olson, and J. Vanderwater, “Experimental Analysis of the Natural Con¬ vection Effects Observed Within the Closed Cavity of Tile Roof,” Proceedings of Cool Roofs: Cutting Through the Glare. RCI Foundation con¬ ference, May 12-13, 2005, Atlanta, GA. W.A. Miller, J. Wilson, and A. Karagiozis, “The Impact of Above-Sheathing Ventilation on the Thermal and Moisture Performance of Steep-Slope Residential Roofs and Attics,” Proceedings of the 15th Sym¬ posium on Improving Building Systems in Hot and Humid Climates, Orlando, FL, July 24-26, 2006. A.F. Rudd and J.W. Lstiburek, “Vented and Sealed Attics in Hot Climates,” Proceedings of the ASHRAE Symposium on Attics and Cathedral Ceilings, Toronto, ON, June 1997. The seminar is available on ASHRAE Transactions TO- 98-20-3. Proceedings of the RCI 24th International Convention Roodvoets, Malinger, and Banks – 1 79
