There are two prime moisture considerations in roofing system design, rain penetration and the
condensation of water vapor within the roofing system (Handegord). Rain penetration is controlled
by trying to keep water off the roofing membrane with adequate sloping and drainage in conjunction
with carefully designed and installed flashing at roof edges and penetrations (Baker 1969, NRCA).
Water vapor condensation within the roofing system is controlled by preventing water vapor from
the building, or the outdoors in cooling situations, from entering the roof and reaching cold elements
within the system. The control of water vapor transport must address both diffusion and air
leakage. Diffusion can be controlled with a vapor retarder, but a vapor retarder is insufficient to
control the greater amounts of water vapor that can be transported by air movement. As in the
case with walls, the vapor retarder must be positioned in relation to the thermal insulation such that
it is maintained at a temperature above the dewpoint of the moist air.
The decision on the necessity for a vapor retarder is the source of much discussion. The basic
issue of concern is whether a sufficient quantity of water vapor will condense within the roofing
system beyond the absorptive capacity of the materials and whether these materials will have an
opportunity to dry out before any damage is done. An analysis of climate, conditions within the
building and the thermal resistance and moisture absorptive properties of the roofing system
elements is necessary to determine the need and appropriate position for a vapor retarder. Such
an analysis of the need for a vapor retarder and its position within the roofing system should be
conducted in all cases, following the examples contained in the NRCA manual. NRCA
recommends that a vapor retarder be considered when the average January temperature is less
than 5 C (40 F) and the interior relative humidity is at least 45% in the winter. While these general
guidelines are useful, Tobiasson points out that these guidelines will result in the use of vapor
retarders when they are not needed and their lack of specification when they should be used. He
instead recommends the consideration of condensation potential during the entire winter and the
drying potential during warm weather, and has developed a map of the U.S. that gives the relative
humidity above which a vapor retarder should be specified. This map allows for corrections based
on interior temperatures.
In order to control the great quantities of moisture transport due to air movement, a roofing system
vapor retarder needs to be as airtight as the roofing membrane is watertight (Condren). As in the
installation of an air barrier, extreme care must be taken to insure that the vapor retarder is fully
continuous throughout the roofing system, including all seams, penetrations and roof edges.
Condren stresses the need to maintain air-tightness at all seals and terminations through the
attention to detail during design and rigorous inspection during construction.
Regardless of how much care is taken in the design and construction of roofing systems, it is
inevitable that some moisture will migrate into the roofing system from precipitation and
condensation of water vapor. Some recommend the use of breather vents and air channels within
the roofing system to remove such moisture (Condren). Others state that it is extremely difficult to
ventilate a compact roof and that breather vents are apt to do more harm than good. Tobiasson
holds the latter viewpoint and has done experimental work that shows it can take decades to dry out
a compact roof with breather vents. He states further that he sees no evidence that unvented roofs
perform any worse than vented roofs.