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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Vapor Retarder, Air Barrier Contributes to Thermal and Waterproofing Performance

Duro-Last Inc. has added to its Duro-Guard product line with Duro-Guard SOPRAVAP'R, a vapor retarder and air barrier membrane.

Duro-Last Inc. has added to its Duro-Guard product line with Duro-Guard SOPRAVAP’R, a vapor retarder and air barrier membrane.

Duro-Last Inc. has added to its Duro-Guard product line with Duro-Guard SOPRAVAP’R, a vapor retarder and air barrier membrane. This product is composed of a self-adhesive SBS modified bitumen with a polyethylene facer on the top surface and a silicone release film on bottom. Duro-Guard SOPRAVAP’R addresses the movement of air in roof assemblies as required in the 2012 International Energy Code.

When correctly engineered into a roof assembly, it will contribute to long-lasting thermal and waterproofing performance in all climate zones for all colors including white membranes provided by Duro-Last. Duro-Guard SOPRAVAP’R can also be used as a temporary roof in new construction or roof tear offs. Duro-Guard SOPRAVAP’R can be installed on steel, plywood, gypsum, concrete board, asphalt panel or concrete deck. The addition of the Duro-Guard SOPRAVAP’R provides Duro-Last customers with another Edge-to-Edge & Deck-to-Sky solution.

Air, Vapor and Water Barrier Products for All Six Sides of the Building Envelope

Garland Aero-Block product line offers solutions for all six sides of the building enclosure

The Garland Aero-Block product line offers solutions for all six sides of the building enclosure.

As the push for more energy-efficient buildings continues, numerous building codes are now including air barrier requirements. To address these changing standards, Garland has engineered a new product line of high-performance solutions that prevent unwanted air, vapor and water from penetrating the building envelope. The Garland Aero-Block product line offers solutions for all six sides of the building enclosure, as well as for gaps in walls or between sections of walls.

This new polymer-modified-asphalt technology is available in three formats. The fluid-applied solvent-based polymer and fluid-applied water-based polymer versions can be applied by brush, spray or roller. There is also a pre-fabricated, self-adhering multi-layer membrane. The entire Aero-Block family create vapor-closed protection, meeting the requirements for a Class I air/vapor barrier.

A companion vapor-open product line, Aero-Perm, will become available later this year. Aero-Perm will offer the same capabilities as Aero-Block systems, while providing permeability to water vapor.

Attention Roof System Designers: Numerous Roof Components Work Together to Affect a Building

There has been a great deal of opinion expressed in the past 15 years related to the roof cover(s), or the top surface of a roof system, such as “it can save you energy” and “it will reduce urban heat islands”. These opinions consequently have resulted in standards and code revisions that have had an extraordinary effect on the roofing industry.

The building type should influence the type of roof system designed. Some spaces, like this steel plant, are unconditioned, so insulation in the roof system is not desired.

The building type should influence the type of roof system designed. Some spaces, like this steel plant, are unconditioned, so insulation in the roof system is not desired.

Let’s say it loud and clear, “A single component, does not a roof make!”. Roofs are systems, composed of numerous components that work and interact together to affect the building in question. Regardless of your concern or goal—energy performance, urban heat-island minimization, long-term service life (in my opinion, the essence of sustainability) or protection from the elements—the performance is the result of an assembled set of roof system components.

Roof System Components

Energy conservation is an often-discussed potential of roofs, but many seem to think it is the result of only the roof-cover color. I think not. Energy performance is the result of many factors, including but not limited to:

Building use: Is the building an office, school, hospital, warehouse, fabrication facility, etc.? Each type of building use places different requirements on the roof system.

Spatial use and function be low the roof deck: It is not uncommon in urban areas to have mechanical rooms or interstitial spaces below the roof—spaces that require little to no heating or cooling. These spaces are typically unconditioned and unoccupied and receive no material benefit from the roof system in regard to energy savings.

Roof-deck type: The type of roof deck—whether steel; cast-in-place, precast and post-tensioned concrete; gypsum; cementitious wood fiber; or (don’t kill the messenger) plywood, which is a West Coast anomaly—affects air and moisture transport toward the exterior, as well as the type of roof system.

Roof-to-wall transition(s): The transition of the roofing to walls often results in unresolved design issues, as well as cavities that allow moisture and vapor transport.

Meanwhile others, like this indoor pool, require extreme care in design and should include a vapor retarder and insulation.

Meanwhile others, like this indoor pool, require extreme care in design and
should include a vapor retarder and insulation.

Roof air and/or vapor barrier: Its integration into the wall air barrier is very important. Failure to tie the two together creates a breach in the barrier.

Substrate board: Steel roof decks often require a substrate board to support the air and vapor barrier membranes. The substrate board also can be the first layer of the roof system to provide wind-uplift resistance.

Insulation type: Each insulation type—whether polyisocyanurate, expanded polystyrene, extruded polystyrene, wood fiber, foam glass or mineral wool—has differing R-values, some of which drop with time. Many insulation types have differing facer options and densities.

The number of insulation layers: This is very important! A single layer of insulation results in a high level of energy loss; 7 percent is the industry standard. When installing multiple layers of insulation, the joints should be offset from layer to layer to avoid vapor movement and thermal shorts.

Sealing: Voids between rooftop penetrations, adjacent board and the roof-edge perimeters can create large avenues for heat loss.

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