Sloped, standing seam, metal roofing provides a continuous surface that is designed to shed water efficiently while providing a long-lasting and great looking roofing solution. When the roof design and shape is simple, (i.e. continuously extending from ridge to eaves with no changes or interruptions) then all of the attributes of the metal roofing can be assured by using some very conventional and well-known details for design and construction. But in the real world, there are lots of conditions that require more specialized attention to detail. For our purposes here, we will simply refer to those as “specialty roof conditions.”
What types of roofing conditions warrant the special attention? Most are associated with changes in the shape or surface of the roof, say where the ridge line is interrupted or offset. Others could be a means to accommodate a roof feature such as a dormer, a dutch hip type of roof, or the intersection between a ridge and a parapet wall. Some might be related to the design of a valley, particular if it is a “dead valley” that doesn’t drain directly to the gutter but stops short, as around a dormer or elsewhere. Or some could be the result of some special conditions created by the roof design such as cascading water over an edge or heavy snow accumulation conditions. There are certainly many others too, but the point is that any of them are a potential source of water leakage and building damage if they are not properly addressed.
Most metal building manufacturers not only recognize the importance of such specialty roof conditions, but they also have lots of experience in developing very workable solutions for them. The key for success is found in the fundamental principles of properly overlapping (i.e. “shingling”) all materials to allow water to drain smoothly away where it is intended without getting diverted to places where it shouldn’t go. That means the metal roofing panels need to be cut, fit, and installed properly, but it also means that flashing, sealants, and fasteners need to be installed correctly too, all regardless of the slope of the roof. To communicate ways to achieve better results in the field for specialty roof conditions, manufacturers like MBCI make step-by-step details available for installers. The significance of using and following these details can not be overstated since they are a key component in getting a weathertightness warranty from the manufacturer.
As an example of how this might play out on a specific building, let’s look at a dead valley that occurs because a gable roofed dormer is installed in the main area of a roof. The first thing to recognize is that multiple layers of materials are involved in the transition around the dormer, all of which need to be installed in the proper location, following the proper sequence, and with the proper connections. A step-by-step process as detailed by the manufacturer might look like this:
With the substrate in place (rigid insulation over a metal deck), a special width panel will likely need to be installed and serve as the collection area for the dead valley to drain into. Then, plywood spacers and nailers are installed, and the main lower valley area is covered with “rubber” (EPDM) flashing.
Secure continuous eave trim over the plywood nailers and add and offset cleat on top to receive roof panels, all secured with tri-bead tape sealer and fasteners as shown.
Install extended valley trim across the valley with an offset cleat on either side secured as shown.
With all of the prior steps in place, then the installation of upper panels can begin to interface with the edge of the dead valley.
Continue cutting and installing panels to fit over and drain into the dead valley, which then drains without interruption onto the special width panel and the roof.
By following step by step details from the manufacturer for this or other specialty roof conditions, then the likelihood increases that everyone involved in the project is both proud and satisfied with the end results. The key is to start at the beginning with the proper planning and preparation by communicating with the manufacturer about all roof conditions that require special attention like this example.
Energy codes and increasing energy costs have prompted the installation of more roof insulation into metal buildings in recent years to make them more energy efficient. That is fundamentally a good thing and metal building manufacturers have developed ways to accommodate a variety of building enclosure packages that increase energy performance while still being engineered to meet the structural requirements of the building. This allows the whole building envelope to be designed and fabricated so it works as a complete, coordinated system.
The metal roofing or metal building suppliers typically don’t design the insulation systems. However, it is important to include them in the discussions or make them aware of what type of system is to be installed. It is not uncommon for a metal building to be ordered with the design stipulation of “insulation by others.” In that case, coordination is needed between the person ordering/designing the insulation system and the metal building manufacturer or roofing supplier. Since there are a great many variables in the way that insulation can be provided, it is not appropriate to think that the design of structural systems (purlins and roof bracing) and cladding systems (clips, fasteners, and metal roofing profiles) will necessarily accommodate all the same insulation in all conditions. Rather, unless the specific details of the insulation system being used in the building are communicated effectively at the time of the order, the manufacturer can not assure compatibility of the systems used with the insulation system that is to be installed.
In order to understand some of the variability in the options, let’s look at some of the common ways that metal buildings are or are not insulated.
Buildings that do not have any heat or air conditioning in them may not need for an insulated roof. This could be true for outdoor shelters, some agricultural buildings, or vehicle storage buildings. However, uninsulated metal roofs have the potential for “roof rumble” as they move due to thermal expansion and contraction, wind, or weather as there is no insulation to mask or deaden this noise. Absence of insulation can also lead to condensation during certain times of the year if temporary heat is added to the building. This condensation builds up and can drop or fall onto whatever is below. Many times condensation issues are mistaken for roof leaks when in fact it’s a mechanical design issue of the building envelope that’s not been properly addressed. If neither sound nor potential condensation are a concern, then there’s no problem. But if either or both need to be avoided, then some basic level of insulation may be prudent.
Over the Purlin Systems:
One of the most common insulation systems for metal buildings and/or open framing systems is to simply install rolls of blanket insulation. In this case, fiberglass insulation with a reinforced liner is draped over structural beams and purlins. The rolls are supplied to length by the insulation supplier based upon the roof structural layout and the required “R” value necessary for the building envelope in thicknesses that can vary from 3″ to 12″. Is is this thickness to be installed over open framing that the metal building/roofing supplier must be made aware of. Based on this thickness, the panel profile can be verified to determine if it can be used as well as confirmation of the correct clip heights and screw lengths for installation. Keep in mind that the supplier will offer a guide to the installer based upon insulation thickness. As insulation can vary by manufacturer, it will be up to the installer to make adjustments as needed in the field to ensure proper placement and hold modularity of the steel system. (See Respect the Module: Metal Roofing Panels are Modular for Good Reason)
Cavity Fill Insulation Systems:
When higher “R” values are required for roof insulation, a single layer over the open framing system may not be sufficient. When that occurs, the designers of the building envelope may need to employ the framing cavity to add more insulation. There are also variation on the cavity fill approach.
One means is to simply introduce a second layer of unfaced blanket on top of the faced insulation. Sometimes referred to as a “sag and bag” approach, here the first layer of insulation over the purlins is ordered to accommodate larger amounts of drape between the roof structure to permit another layer of unfaced insulation to be added on top. This increases the insulation thickness between the purlins but keeps it thin enough to be compressed to accommodate the roof panel installation. For coordination purposes, the thickness of this upper insulation over the purlins needs to be known by the building manufacturer so the clips and fasteners can be properly sized. Likewise, the amount of insulation draping between the purlins needs to be known to determine if purling bracing or other accessories may potentially interfere with the insulation installation.
Other types of cavity fill system may include a faced batt or face roll insulation with long tabs, which are secured to the tops of roof purlins and nest fully into the purlin cavity to fill the space more effectively. This helps in eliminating greater compression of multiple layers of insulation on top of the purlins and permits an additional layer of unfaced insulation on top of the roof structures and/or a thermal spacer block. This system may also require some intermediate banding to support the insulation between the primary supports.
A liner system may be installed that employs a continuous vapor retardent material. This liner is secured to the bottom of the roof structure and additionally supported with metal banding allowing the cavity to then be filled with unfaced insulation between the purlins. More unfaced insulation can also be added on top of the purlins as well. In all of the cases where cavity fill systems are used, it is important to advise the building manufacturer/roof supplier which type is being used to ensure proper panel clip heights and screw lengths. This is important because these systems can and will interfere with the roof structural bracing making them more difficult to install. The metal building supplier may be able to offer bracing alternatives or remedies to eliminate some or all of the bracing that would otherwise be in the way when installing the roof insulation. There may also be suggestions on how to avoid impeding or penetrating the vapor barriers which could lead to condensation issues. Overall, it is best to discuss and coordinate all of these items ahead of time.
Rigid Board/ Composite Systems:
In this insulation approach, rigid foam insulation board is used to achieve the sought after energy performance. Commonly, these use metal deck panels over the roof structure thus supporting the insulation and a vapor retardant material on top of the deck. The insulation and the metal roofing can then be secured to the framing substructure or to the metal deck itself, which means the details of attachment need to be reviewed and engineered to avoid adverse affects on the roofing system.
All of the above systems typically require attention to providing additional air and vapor barriers and proper cutting and fitting during installation so as not to cause unwanted infiltration or to prevent condensation from occurring. For these reasons and more, some people will consider the use of closed cell spray-on foam insulation, which can continuously provide all of these features in one product. It can also be installed after the roof is completed and structure is weathertight.
In the case of metal buildings, spray-on insulation is typically applied in the field onto the inside face of installed roof panels and sometimes wall panels too. There are, however, a few concerns with this approach in metal buildings. First, if conditions are not right and the panels are not properly prepared, then the spray foam can, in fact, trap moisture between the insulation and the metal components it is sprayed onto. That can lead to corrosion of the metal or deterioration of the insulation. Secondly, not all spray foams on the market are intended for this type of use so they don’t always adhere well to some metal panels, meaning it could become loose and fall away. Finally, continuous spray foam in this application will not always be able to expand and contract at the same rate that metal does. In some cases, that could mean that the foam suffers from differential movement causing it to break or lose adhesion.
For all of these reasons, be certain to research all options before considering or selecting a foam spray-on insulation that will not negatively impact your roof performance. If a foam insulation is preferred, it may be worth considering the use of insulated metal panels (IMPs) that are designed, engineered, and fabricated to be compatible with metal building construction.
Recognizing all of the above variations and options, the key point to remember about insulating metal buildings is the importance of communication between those designing and ordering an insulated metal building and those who are manufacturing and fabricating it. To find out more about the best ways to do that, contact your local MBCI representative.
One of the most misunderstood aspects of a metal roofing system is the proper use of a vapor barrier. There are many sources of information about this topic – some of which are based on science, some based on anecdotal field experience, and some based on journalism. Here, we will try to break it down to the basic principles that can be used to understand the latest options for a metal building roof system today.
What is Vapor?
The observed science tells us that water can take three forms, depending on temperature and its ability to interact with other things around it. Water can be a liquid that we drink, solid ice that we can skate on, or a gaseous vapor that is part of the makeup of the air we breathe. We can’t see water vapor in the air but we can feel it – we call that humidity. High humidity means a lot of water vapor is in the air, typically coupled with higher air temperature – and both can make us feel uncomfortable and “sticky.” Low humidity means the air is dryer – more typical in lower-temperature air – but this may also be uncomfortable for our breathing, skin dryness, etc.
Why is Vapor a Concern?
As long as the gaseous water vapor stays in the air at a moderate or comfortable level, there is no real concern. However, since water vapor responds quickly to temperature, it can turn back into water as soon as it encounters a surface that is cold enough for it to make the transformation. We know this phenomenon as condensation, and anyone who has seen a cold drink collect water on the outside of a glass on a humid summer’s day has experienced it. It is the same phenomenon that shows up on the surface of windows in a building when there is a big difference between inside and outside temperatures. We know that the amount of water vapor (i.e., humidity) present and the air temperature can both be variable at any given time, but there is always a predictable point at which water vapor will condense and form water drops – this is called the dew point. When vapor in the air encounters a temperature at or below the dew point, condensation occurs.
What Does This Have to do With Metal Roofing?
Metal roofing systems and condensed moisture are not a good combination. If airborne moisture seeps into a metal roof assembly, finds a cool surface, and condenses on any surface there, it likely won’t be visible from inside or outside of the building. That trapped water can then cause rust and corrosion of metal parts, resulting in real damage. It can also collect and saturate building insulation, rendering it ineffective. If enough water condenses, it can cause visible staining or grow mold, causing concerns for people inside the building.
Do Building Codes Address This?
Absolutely – they require that the building be protected from the possibility of damage caused by water vapor. Since the concern is to restrict the flow of airborne moisture in relatively warm air from reaching a cooler surface to condense on, they call for something to be installed on the “warm” side to prevent that flow. For most buildings across the United States, the warm side is the interior face of the roof and walls. However, if the building is kept cold as in a refrigerated warehouse or storage building, then the warm side is likely on the exterior. The same is true in southern climates where warmer, humid air is the exterior condition and cooler interiors are common.
What is the Best Solution?
Manufacturers of insulating products have been involved in addressing the best ways to provide not only insulation to keep building temperatures warmer, but also vapor barriers to restrict the flow of airborne moisture. After literally decades of trying different types of vinyl and polyethylene facings over fiberglass insulation, most have realized that those membranes simply don’t provide enough protection to be effective. Instead, most are now offering a choice of laminated facings over the insulation that can be installed so they are exposed to the appropriate warm side of the roofing system. These fairly sophisticated laminations include:
Polypropylene-scrim-kraft consisting of layers of white or metalized polypropylene, fiberglass reinforcing, and white kraft paper on the order of 11-30 lb. weight
Polyprolene-scrim-kraft consisting of aluminum foil, fiberglass reinforcing, and 30 lb. kraft paper
All of these latest advancements in vapor barriers can provide comparable, high levels of protection, but their selection can depend on a variety of other building factors. Therefore, it is always best to engage an architect or engineer in the design to review the needs of the entire building to select the most appropriate, specific solution for an given project. It will also be important that all seams, connections, and penetrations of the vapor barrier are addressed in the design and construction, which are similarly best addressed by an architect or engineer to assure there are no breaches in the protection provided by the barrier.