White Paper Identifies Appropriate Mean Reference Temperature Ranges and R-values of Polyiso Roof Insulation within this Range

A number of recent articles have explored the relationship between temperature and R-value with an emphasis on the apparent reduction in R-value demonstrated by polyisocyanurate (or polyiso) roof insulation at cold temperatures. The science behind this apparent R-value decrease is relatively simple: All polyiso foam contains a blowing agent, which is a major component of the insulation performance provided by the polyiso foam. As temperatures decrease, all blowing agents will start to condense, and at some point this will result in a marginally reduced R-value. The point at which this occurs will vary to some extent for different polyiso foam products.

a mean reference temperature of 40 F is based on the average between a hot-side temperature of 60 F and a cold-side temperature of 20 F.

A mean reference temperature of 40 F is based on the average between a hot-side temperature of 60 F and a cold-side temperature of 20 F.

Because of this phenomenon, building researchers have attempted to determine whether the nominal R-value of polyiso insulation should be reduced in colder climates. Because of the obvious relationship between temperature and blowing-agent condensation, this certainly is a reasonable area of inquiry. However, before determining nominal R-value for polyiso in colder climates, it is critical to establish the appropriate temperature at which R-value testing should be conducted.

TO DETERMINE the appropriate temperature for R-value testing of polyiso, it is important to review how R-value is tested and measured. Figure 1 provides a simplified illustration of a “hot box” apparatus used to test and measure the R-value of almost all thermal-insulating materials. The insulation sample is placed within the box, and a temperature differential is maintained on opposing sides of the box. To generate accurate R-value information, the temperature differential between the opposing sides of the box must be relatively large—typically no less than 40 F according to current ASTM standards. The results of this type of test are then reported based on the average between these two temperature extremes, which is referred to as mean reference temperature. As shown in Figure 1, a mean reference temperature of 40 F is based on the average between a hot-side temperature of 60 F and a cold-side temperature of 20 F. In a similar manner, a mean reference temperature of 20 F is based on a hot-side temperature of 40 F and a cold-side temperature of 0 F.

NOW THAT we’ve had an opportunity to discuss the details of R-value testing, let’s apply the principles of the laboratory to the real-world situation of an actual building. Just like our laboratory hot box, buildings also have warm and cold sides. In cold climates, the warm side is located on the interior and the cold side is located on the exterior. If we assume that the interior is being heated to 68 F during the winter, what outdoor temperature will be required to obtain a mean reference temperature of 40 F or 20 F? Figure 2 provides a schematic analysis of the appropriate mean reference temperature.

As illustrated in Figure 2, the necessary outdoor temperature needed to attain a 40 F mean reference temperature would be 12 F while an outdoor temperature as low as -28 F would be needed to obtain a 20 F mean reference temperature. And herein lies a glaring problem with many of the articles published so far about the relationship between temperature and R-value. Although a 20 F or 40 F “reference temperature” may sound reasonable for measuring R-value, average real-world conditions required to obtain this reference temperature are only available in the most extreme cold climates in the world. With the exception of the northernmost parts of Canada and the Arctic, few locations experience an average winter temperature lower than 20 F.

schematic analysis of the appropriate mean reference temperature.

A Schematic analysis of the appropriate mean reference temperature.

To help illustrate the reality of average winter temperature in North America, a recent white paper published by the Bethesda, Md.-based Polyisocyanurate Insulation Manufacturers Association (PIMA), “Thermal Resistance and Temperature: A Report for Building Design Professionals”, which is available at Polyiso.org, identifies these average winter temperatures by climate zone using information from NOAA Historical Climatology studies. As shown in Table 1, page 2, the PIMA white paper identifies that actual average winter temperature varies from a low of 22 F in the coldest North American climate zone (ASHRAE Zone 7) to a high of 71 F in the warmest climate zone (ASHRAE Zone 1).

In addition to identifying a realistic winter outdoor average temperature for all major North American climate zones, Table 1 also identifies the appropriate mean reference temperature for each zone when a 68 F indoor design temperature is assumed. Rather than being as low as 40 F or even 20 F as sometimes inferred in previous articles, this mean winter reference temperature varies from a low of no less than 45 F in the coldest climate zone to above 50 F in the middle climate zones in North America.

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The Benefits of Above-sheathing Ventilation

We know proper ventilation of the attic space is an important part of construction. But what is “above-sheathing ventilation”?

Most roofing materials lay directly on the sheathing. Heat from solar radiation and interior heat loss from the conditioned space are easily transferred through the deck and roof system. This can increase energy costs and cause ice damming. The build-up of heat and extreme temperatures wings can also reduce the life of underlayment and other system components.

Tile roofs have an air space between installed roof tiles and the roof sheathing. This space reduces heat transfer and allows heat buildup to dissipate from the sheathing and roofing materials. This above-sheathing ventilation, or ASV, inherent to tile roof installations can be enhanced using counter battens, shims or manufactured systems to raise the horizontal battens above the roof deck. The system design will vary with the environmental challenge and goals. Specific examples are described below.

The Elevated Batten System by Boral Roofing uses treated 1 by 2s with high-grade plastic pads, a vented eave riser flashing and vented weather blocking at the ridge. With these components in place, heat transfer is minimized and heat buildup is dissipated, which reduces energy costs.

The Elevated Batten System by Boral Roofing uses treated 1 by 2s with high-grade plastic pads, a vented eave riser flashing and vented weather blocking at the ridge. With these components in place, heat transfer is minimized and heat buildup is dissipated, which reduces energy costs.

Energy Conservation in Hot Climates

In hot and dry climates, the natural ASV and thermal mass of the tile provide a layer of insulation when exterior daytime temperatures are greater than the conditioned space in the home. Vertical counter battens or shims that raise the horizontal battens increase this space and the corresponding benefit. The addition of vented eave riser flashing and ridge ventilation completes an energy-saving ASV system. The system shown below is the Elevated Batten System made by Boral Roofing, which uses treated 1 by 2s with high-grade plastic pads, a vented eave riser flashing and vented weather blocking at the ridge. With these components in place, heat transfer is minimized and heat buildup is dissipated, which reduces energy costs. The upgraded ASV reduces temperature extremes that shorten the life of the underlayment and other roofing components. These benefits are achieved with no mechanical or moving parts.

Cool and Humid Climates

The same installation can provide a different benefit in cool and humid regions. The Tile Roofing Institute and Western States Roofing Contractors Association’s Concrete and Clay Tile Installation Manual for Moderate Climate Regions says that in areas designated “Cool/Humid” zones, “Batten systems that provide drainage/air-flow (shims, counter battens or other approved systems) are required.” The area designated “Cool/Humid” in the current manual runs from approximately Eureka, Calif., to the Pacific Northwest, west of the Cascade Mountains. In this climate, moisture-laden air can migrate under the tile and condense in the space between the tile and roof deck. The underlayment is there to protect the sheathing but if the battens are raised above the deck, condensation will be reduced. Raised battens also allow moisture under the tile to escape to the eave. When roof tiles are fastened to a raised batten, underlayment penetrations are minimized.

Cold and Snowy Regions

Ice dams are one of the most damaging phenomena roofing contractors face. Snow movement on roof surfaces can cause damage to people and property. The goal in cold and snowy environments is to prevent ice dams by enhancing the ASV under the tile roof. Typically, a more substantial air space is created using larger vertical battens. A well-designed “cold roof” system that includes proper snow retention is the solution.

The TRI/WSRCA Concrete and Clay Tile Installation Manual for Moderate Climate Regions refers installers to the TRI/WSRCA Concrete and Clay Roof Tile Design Criteria Installation Manual for Cold and Snow. Regions “in locations where the January mean temperature is 25 deg. F or less or where ice damming often occurs”.

For more information and to download the Tile Roofing Institute’s installation manuals, visit the Tile Roofing Institute at TileRoofing.org.

ILLUSTRATION: Boral Roofing

Vapor Retarders

The need for, use and design of a vapor retarder in the design of a roof system used to be a hotly debated topic. It appears now—when vapor retarders are needed more than ever—the design community seems to have lost interest, which is not good, considering how codes and standards (altered through concerns for energy savings) have changed how buildings are designed, constructed and operated. Most notably, positive building pressures are changing the game.

If not controlled, constructiongenerated moisture can have deleterious effects on new roof systems.

PHOTO 1: If not controlled, construction-generated
moisture can have
deleterious effects on new
roof systems.

A vapor retarder is a material or system that is designed as part of the roof system to substantially reduce the movement of water vapor into the roof system, where it can condense. Everyone knows that water in roof systems is never a positive. Typically, a vapor retarder has to have a perm rating of 1.0 or less to be successful. Through my recent observations, the lack of or poorly constructed vapor retarders contribute to ice under the membrane, soaked insulation facers, destabilized insulation, rusting roof decks, dripping water down screw-fastener threads, compromised fiber board and perlite integrity, mold on organic facers and loss of adhesion on adhered systems, just to name a few. Oh, and did I fail to mention the litigation that follows?

The codes’ “air-barrier requirements” have confused roof system designers. Codes and standards are being driven by the need for energy savings and, as a consequence, buildings are becoming tighter and tighter, as well as more sophisticated. This article will discuss preventing air and vapor transport of interior conditioned air into the roof system and the need for a vapor retarder. The responsibility of incorporating a vapor retarder or air retarder into a roof system is that of the licensed design professional and not that of the contractor or roof system material supplier.

It should be noted that all vapor retarders are air barriers but not all air barriers are vapor retarders. In so much that the roof membrane can often serve as an air barrier, it does nothing to prevent this interior air transport.

WHEN TO USE A VAPOR RETARDER

So the question arises: “When is it prudent to use a vapor retarder?” This is not a simple question and has been complicated by codes, standards, costs and building construction, changing roof membranes and confusion about air barriers. Then, there is the difference in new-construction design and roof removal and replacement design. Historically, it was said that a vapor retarder should be used if the interior use of the building was “wet”, such as a pool room, kitchen, locker shower rooms, etc.; outside temperature in the winter was 40 F or below; or when in doubt, leave it out. In my experience, changes in the building and construction industry have now made the determination criteria more complex.

I find there are typically three primary scenarios that suggest a vapor barrier is prudent. The first is the interior use of the building. The second is consideration for the control of construction-generated moisture, so that the roof can make it to the building’s intended use (see photo 1). The third consideration is the sequence of construction. In all three situations I like to specify a robust vapor retarder that “dries in” the building so that interior work and construction work above the vapor retarder can take place without compromising the finished roof. Consider the following:

BUILDING USE

This characteristic is often the most determinant. If the interior use of the building requires conditioned air and has relative-humidity percentages great enough to condense if the exterior temperatures get cold enough, a vapor retarder is needed to prevent the movement of this conditioned air into the roof system where it can condense and become problematic.

Most designers consider building use only in their design thinking, and it is often in error as the roof system can be compromised during construction and commissioning (through interior building flushing, which can drive moist air into the roof system) before occupancy.

To seal two-ply asphaltic felts set in hot asphalt on a concrete roof deck, an asphaltic glaze coat was applied at the end of the day. Because of the inherent tackiness of the asphalt until it oxidizes, Hutch has been specifying a smooth-surfaced modified bitumen cap sheet, eliminating the glaze coat.

PHOTO 2: To seal two-ply asphaltic felts set in hot asphalt on a concrete roof deck, an asphaltic glaze coat was applied at the end of the day. Because of the
inherent tackiness of the asphalt until it oxidizes, Hutch has been specifying a smooth-surfaced modified bitumen cap
sheet, eliminating the glaze coat.

CONTROL OF CONSTRUCTION-GENERATED MOISTURE

I have seen roof systems on office buildings severely compromised by construction- generated moisture caused by concrete pours, combustion heaters, block laying, fireproofing, drywall taping and painting. Thus, a simple vapor retarder should be considered in these situations to control rising moisture vapor during construction, which includes the flushing of the building if required for commissioning.

CONSTRUCTION SEQUENCING AND MATERIALS

Building construction takes place year round. It is unfortunate decision makers in the roofing industry who are pushing low-VOC and/or water-based adhesives do not understand this; problems with their decisions are for another article. If the roof is to be installed in late fall (in the Midwest) and interior concrete work and/or large amounts of moisture-producing construction, such as concrete-block laying, plastering, drywall taping or painting, are to take place, a vapor retarder should be considered.

How will the building, especially the façades, be constructed? Will they be installed after the finished roof? This creates a scenario for a damaged “completed” roof system.

PHOTOS: Hutchinson Design Group Ltd.

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