Moisture control within the roof assembly includes such factors as moisture migration, dew point condensation, ventilation, and vapor retarders. Some migration of interior moisture is allowed due to the natural downward drying cycle that occurs in most climates; and therefore, the criteria for determining the need for vapor retarders shall take this into consideration. The term vapor retarder refers to a waterproof membrane installed prior to the roof membrane so as to suppress the flow of moisture vapors from the interior of a building into the roof assembly. Moisture from the interior of a building generally comes in the form of water vapor from the following sources:
Construction processes which include the drying of interior concrete, cementious roof fills, plaster compounds, water-based finishes, and fuel burning space heaters.
Operational sources such as swimming pools, greenhouses, laundries, textile manufacturing, food processing, and paper plants.
Occupancy sources which include kitchens, shower facilities, exercise rooms, and even breathing from high-density occupancies.
Cool climate regions where moisture vapor generally flows upward through the roof system from a heated, more humid interior toward a colder, drier exterior during the winter months. Such a climate condition causes vapor to migrate into an insulated roof system where it can cool and condense into water.
Buildings with high interior relative humidity require vapor retarders, as well as buildings located in the Central and Northern regions of theUnited States. For such buildings, the designer shall calculate the roof assembly dew point so as to determine the correct location to install the vapor retarder within the roof system. Included in the calculations shall be the R-value of the roof assembly components, anticipated interior and exterior temperatures, regional temperatures, the building’s planned relative humidity, and any other pertinent data.
The Three Primary Moisture Control Factors
Ventilation is the exchanging of air in a given space. Passive or mechanical ventilation is designed to control moisture and reduce heat buildup. Exchanging the air in a given space is accomplished by venting moisture-laden air to the outside, while inducing fresh outside, or conditioned drier air to the inside.
Insulation provides R-value and aids in temperature control within a building. Insulating a roof reduces the amount of energy required to heat and cool a building and can also reduce the potential for condensation on interior surfaces or within the roof assembly.
A vapor retarder is a waterproof membrane generally installed somewhere between the deck and the insulation. Vapor retarders are installed to inhibit the flow of moisture vapor into the roof assembly from the interior of a building.
Moisture Control & Insulated Roof Assemblies: There are two types of insulated roof assemblies that are classified by the location of the insulation in relation to the roof deck: (1) cold ventilated roof assemblies or cold roof assemblies and (2) warm compact roof assemblies or warm roof assemblies.
Cold roof assemblies have a fixed air space under the roof deck such as an attic that is insulated below the deck and vented to the outside of the building.
Warm roof assemblies have all their components installed in direct contact with each other including rigid insulation boards installed above the deck, leaving no space for designed venting.
Cold Roof Assembly Moisture Control
Ventilation: Adequate ventilation of the attic space is required. The designer shall calculate the volume of ventilation required, and specify ventilation per the building’s requirements.
Insulation: Soft insulation, bats or blown, shall be installed below the roof deck. The designer shall calculate the thickness of the insulation so as to allow adequate air space for venting between the insulation and the roof deck.
Vapor Retarder Assemblies: With some cold roof assemblies, a vapor retarder can assist the control of moisture migration from the building’s interior into the roof assembly.
Warm Roof Assembly Moisture Control
Ventilation: Adequate ventilation under the roof deck is required. A ventilation system will exhaust the moisture generated from inside the building before it can migrate into the roof assembly.
Insulation: Rigid insulation boards shall be installed over the roof deck of warm roof assemblies. The required R-value shall be specified so as to facilitate the control of heating and cooling loads while at the same time protecting the primary insulation from the degrading affects of accumulated moisture. The designer shall specify the insulation R-value required per the local building codes, the building occupancy, and the climate location. The designer shall also specify the required moisture control for keeping the insulation dry so that R-value is not impaired through accumulated moisture in the insulation.
Vapor Retarder: A vapor retarder for most warm roof assemblies is required, especially for roof assemblies installed in the northernUnited States, and for buildings that have high relative humidity’s.
Cold & Warm Low-Slope Roof Assembly Ventilation
Moisture generated from within the interior of a building shall be suppressed from passing into the roof assembly by a vapor retarder, and the moisture shall be satisfactorily vented outside so that moisture does not accumulate and condense in the joist spaces and other cavaties under the roof system.
Cold Low-Slope Roof Assembly Ventilation: Cold low-slope roof assemblies have no high, attic type spaces. Therefore, these assemblies have no natural chimney effect created by rising, warm moist air as in steep slope roof attics. Installation of continuous soffit vents around the building’s perimeter may not remove sufficient amounts of moisture from the joist spaces in cold low-slope roof assemblies. Depending upon the depth of the joists and the thickness of the insulation put in the joist space, it may be necessary to raise the roof deck or shim it above the top of the joists so as to facilitate air movement along the underside of the roof deck. The underside of the roof deck is where moisture is most likely to accumulate and remain, causing wood to decay and steel to rust.
Installation of raised area dividers and curbs with mechanical vents aid the ventilation of cold low-slope roof assemblies. Air shall move along the underside of the roof deck for adequate ventilation to be achieved. If air is restricted from free movement beneath the roof deck, each joist space shall be individually vented so as to allow for the movement of sufficient volumes of moisture-laden air within the joist spaces.
Cold low-slope roof designs require adequate free air movement immediately beneath the roof deck, allowing for the exhausting of warm moist air to the outside. Mechanisms that facilitate positive ventilation such as raised area dividers with ventilators, and continuous soffit vents are beneficial for the movement of air through the roof joist spaces.
Warm Low-Slope Roof Assembly: Venting interior moisture with a heating, ventilation, and air conditioning (HVAC) system is the primary means of minimizing humid air from migrating into warm low-slope roof assemblies.
The moisture-laden air entering warm low-slope roof assemblies without an effective vapor retarder generally enters between the joints of the insulation boards, and accumulates on the bottom side of the cold, impermeable roof membrane. The moisture can then move laterally dampening the top surface of the insulation boards. The extent of lateral moisture migration depends on the method of attachment of the roof membrane.
Completely adhered roof membranes to the top surface of roof insulation boards prevents any appreciable lateral migration of moisture beneath the roof membrane, while mechanically or partially attached roof membranes allow lateral migration of moisture to the top surface of the roof insulation boards.
One way surface vents are prohibited from being installed on warm low-slope roof assemblies because of the large number of flashed penetrations necessary in the BUR membrane, and because vents into a warm low-slope roof assembly may allow entry of moisture into the roof assembly as weather changes occur. The U.S. Army Corps of engineers has demonstrated that commonly available one way roof vents are ineffective in drying wet insulation boards.
Ventilation of warm low-slope roof assemblies with one way roof vents is impractical, marginally effective at best, and in some instances detrimental to the long-term performance of the roof assembly. Moisture control for warm low-slope roof assemblies is most effective with a well designed HVAC system, an effective vapor retarder, and the installation of moisture resistant components.
Direct Ventilation of Building Interiors
If building temperature control is unimportant, and if process heat generated within the building is such that it requires venting, then ventilation ducts straight through walls near the bottom of the roof deck, or ventilation ducts straight through the roof system shall be the most positive means of eliminating excessive quantities of moisture from inside the building. The installation of structural components resistant to moisture is also required.
However, if energy conservation is important, or if interior temperatures must be maintained within specified tolerances, heat exchangers may be installed in the HVAC exhaust system to warm make up air taken from outside.
Self-Drying Roof Assemblies
A self-drying roof is any roof assembly that is installed without a vapor retarder. However, there may be other forms of secondary protection for the roof assembly from moisture originating inside the building or migrating upward from damp soil conditions. These forms of protection may be well designed HVAC systems, or a combination of adequate HVAC and dehumidification equipment.
Self-Drying Roof Assembly Performance
Most successful self-drying roof assemblies have been warm roof assemblies where the roof membrane has been completely adhered. Another factor that contributes to the successful performance of fully adhered self-drying roof systems is that many of these systems have included low permeable rigid roof deck insulation that has also been adhered.
Mechanically attached self-drying roof systems have a greater potential for air leakage or the transport of moisture vapor from the interior of the building into the roof assembly, and therefore, a greater potential for condensation to form within the roof assembly.
It has been proven that air can move laterally under a membrane that is not completely adhered. During times of wind and certain building pressure conditions, a pumping action can occur that actually suctions air into the roof Assembly. During these conditions, warm moist interior air can be transported into the roof assembly, where cold surfaces can cause the moisture to condense. Beads of moisture have been found on the underside of a number of mechanically attached self-drying roof systems.
Solar Warming of Self-Drying Roof Assemblies
In order for self-drying roof assemblies to function correctly, there shall be adequate solar warming of the roof membrane that promotes downward drying. Heat energy absorbed from the sun causes the evaporation of winter accumulated moisture from within the roof system, and drives it downward through a vapor permeable roof deck into the building’s interior. The building’s ventilation system must be such that moisture evaporating from the self-drying roof assembly is vented, preventing excessive build-up of interior moisture. The moisture evaporating from the roof assembly shall be vented to the outside or absorbed into the interior air where it is eventually vented outside the building. Downward drying of winter accumulated moisture is required in order to prevent damage from moisture accumulation in a building.
Limitations of Self-Drying Roof Assemblies
Self-drying roofs are not applicable for buildings where there is significant interior moisture generated. Furthermore, many roof assembly components currently installed may not be capable of tolerating repeated wet and dry cycles. A number of low-slope roof assembly components, such as some types of roof decking and rigid insulation are intended to remain relatively dry. If these components are exposed to repetitive wet and dry or freeze and thaw cycles, the components break down and degrade to the extent of being incapable of functioning as intended.
Moisture Evaporation from Self-Drying Roof Assemblies
To qualify as a correctly functioning self-drying roof, enough moisture must evaporate from the roof assembly during the drying season so as to reestablish a maximum acceptable moisture content within the roof assembly. At the end of each drying season the roofing components shall not be degraded and the R-value of the insulation shall not be impaired. Also, there shall not be a trend toward long term accumulation of moisture within the building or roof assembly.
Self-drying roof assembly design requirements:
No vapor retarder shall be installed because downward drying would be surpressed.
A highly permeable roof deck is required to maximize downward drying.
The entire roof system shall be completely adhered, which includes the rigid insulation shall not be mechanically fastened.
An insulation is required that can tolerate seasonal wetting levels without loosing R-value, degrading, molding, or mildewing.
Vapor Retarders
Buildings with relatively high interior humidities require a vapor retarder as well as buildings located in the central and northern regions of theUnited States. For such buildings, the designer shall calculate the roof assembly dew point so as to determine the need for and the placement of a vapor retarder within the roof system. Included in the calculations shall be the R-value of the roof system components, interior and exterior temperatures, climate, the building’s relative humidity, and other pertinent data.
A vapor retarder’s effectiveness depends upon the following factors:
The vapor retarder’s perm rating is as close to zero as possible.
The adequacy design of the vapor retarder membrane.
The integrity of the vapor retarder’s seals at perimeters and penetrations.
The integrity of the vapor retarder’s membrane after other tradesmen finish their projects.
The vapor retarder’s location within the insulated roof assembly.
Within the typical low-slope roof assembly, the vapor retarder is usually located at or near the surface that is exposed to the higher water vapor pressure. Therefore, for most heated interiors, this means placing the vapor retarder near the winter warm side of the insulation or between the deck and the insulation.
On cold storage or freezer facilities, the roof membrane becomes the vapor retarder since the outside temperature and humidity will usually be higher than the interior temperature and humidity, creating inward vapor drive.
Vapor retarders are installed because water vapor causes several types of roof assembly failures such as:
Reduced R-value, since wet insulation becomes a conductor of heat rather than an insulator.
Deterioration of the BUR membrane, insulation, deck, and associated building components.
Delamination of roof components from trapped moisture, which freezes and thaws, eventually evaporating under solar heat with the resulting vapor pressure causing blisters and delamination of the roof assembly components.
Vapor Retarder Precautions
If water penetrates a roof membrane that has a vapor retarder, it may go undetected for an extended period of time because the vapor retarder may prevent the water from actually entering the building. This can result in damage occurring to other components, such as the rigid insulation.
Finding the source of water penetration into a roof assembly that has a vapor retarder can be much more difficult because water may not enter the building at the same location as it may enter the roof membrane. The vapor retarder can prevent the water from leaking into the interior of the building where it can be noticed until it migrates to a location where the vapor retarder can be penetrated. This may occur a great distance from the location where the water penetrated the roof membrane and thereby complicate the effort to find the point of water entry for repair.
Vapor retarders shall be specified for low-slope roof membranes when both the following conditions exist:
The outside average January temperature is below 40° F.
The expected interior winter relative humidity is 45% or greater.
Various types of insulation may be installed over a vapor retarder, however, the chemical and physical compatibility with a specific type of insulation and vapor retarder is important to confirm from the insulation manufacturer.
Vapor Retarder Attachment to Substrates
Cast in place structural concrete roof decks require the vapor retarder to be completely adhered to the roof deck.
Nailable roof decks shall have the vapor retarder base sheet mechanically fastened. The vapor retarder ply sheet shall be completely adhered to the vapor retarder base sheet.
Steel roof decks shall have the first layer of insulation board, or a water resistant rigid board, mechanically attached. The vapor retarder shall be solidly adhered to the board substrate Vapor retarders are prohibited from being installed directly over steel roof decks because the vapor retarder is vulnerable to puncture damage between the ribs of the steel panels.
The temperature at the vapor retarder shall be warmer than the dew point temperature for the vapor retarder to perform its designed function.