Part 1: Common Mistakes in Roof Insulation and Membrane Selection
In the roofing industry, several everyday design practices and jobsite tasks can, and typically do, have a major impact on the proper completion of a roof system. These items include the following critical elements:
- Roof insulation assembly selection, storage, and installation
- Rooftop staging and loading points
- Insulation installation
- Heat-welded thermoplastic field seams
- Roof detailing
From the perspectives of a roof consultant specifying the roof assembly and an observer conducting a roof observation site visit, let’s take a closer look at each of these items to better understand their impact on a properly installed roof system and identify best practices for the design and installation of thermoplastic roofing systems. But first, we need some background information on roof insulation assembly selection.
ROOF INSULATION ASSEMBLY SELECTION
Specifying the correct insulation type, R-value, assembly, and attachment method for the project is an important step in the process of selecting a roof insulation assembly. The following are important questions to answer in the process of determining the appropriate insulation assembly and attachment criteria:
- What are the wind conditions for the area? It is important to refer to the wind speed map in the relevant standard from the Single-Ply Roofing Industry (SPRI) or Factory Mutual (FM) Global, or in the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7)1 because wind speed conditions can affect the wind uplift rating and fastening patterns. For any project that is to be insured or approved by FM, the design must conform to FM standards.
- What other local or regional factors are relevant? These might include:
- Proximity to coastal areas or hurricane-prone coastlines
- Building exposure
- Location in the central United States
- Location in a mountainous region
- Location in a large city or rural area
- The local building code
- What type of structural roof deck will the roof be installed over?
- What is the intended use for the building?
- How tall is the building?
- Are there any large openings in the walls of the building?
- What is the elevation of the building’s location?
- What are the roof system manufacturer’s minimum requirements and warranty prerequisites?
I will leave the details of this selection process for another article. Suffice it to say, there are numerous considerations and decisions that affect the insulation selection and attachment for any given roof assembly.
ROOF INSULATION STORAGE AND INSTALLATION
Roofing consultants and observers frequently find that roof insulation is stored directly on the ground or left unsecured, or they observe that the weatherproof covering is not secured and is only partially protected from sun, wind, and rain. The following are among the many reasons to make note of these scenarios and document them in the jobsite’s third-party roof observation reports:
- Insulation can be damaged or become wet from rain, which would render it unusable.
- Insulation can be blown around and be damaged or lost.
- If the insulation becomes wet or contaminated by oils, fuels, or chemicals, that could render the insulation unusable (see Fig. 1). A common, incorrect practice is to install wet insulation board so that visible damage is on the underside of the roof assembly, against the substrate. Whether the insulation is water damaged on one side or both sides, this is not an acceptable solution. If the insulation is damaged, wet, or warped by moisture, it should not be installed in the roof assembly.
- Because most roof observation site visits are periodic in nature, the observer may not be on site when all the materials are being installed.
- It is easy to overlook signs that damaged products have been incorporated into the assembly. Signs of moisture in the insulation may include curled corners or edges of the boards or arched/cupped boards, with the middle of the boards warped. Moisture in boards and facers can also cause delamination of the insulation facers and, consequently, the delamination of adhered membranes. In severe cases, the insulation may cup and pull up over the fasteners and insulation plates on assemblies in which the insulation is mechanically attached.
- Saturated polyisocyanurate (polyiso ) insulation can compress from foot traffic, the weight of heavy rains, or snow loads. Signs of this condition may include insulation that feels very soft and “squishy” when walking across it, fasteners and plates protruding upward, or insulation “tenting” upward and possibly protruding through the roof membrane.
Although warped insulation boards can be replaced after the roof is completed, the repair costs are high and large patches must be installed to accommodate the size of the repair or replacement. The best practice is not to install the damaged insulation in the first place, as stipulated in the roofing manufacturer’s specifications.
In recent years, a popular insulation and cover board installation procedure has been to adhere both the insulation and cover board with low-rise expanding foam adhesive. This adhesive can be applied directly to a concrete deck in some cases; alternatively, it can be applied over a vapor barrier or substrate board, or over a metal or wood deck. There are a number of other ways in which low-rise foam adhesive can be incorporated into the assembly, such as adhering subsequent layers of flat or tapered insulation adhered over base layers of insulation and cover boards, and crickets adhered over previously installed layers of insulation (see Fig. 2).
The bead size and spacing requirements can be based on a variety of design criteria, the geographic location, and wind-speed requirements. Bead size and spacing requirements vary by manufacturer, project-specified wind rating, and other design criteria. Roof manufacturers specify minimum size and spacing criteria for construction, which do not require specific wind ratings (see Fig. 3).