Jimmy;

I agree with wes , underlayment - no air space - no problem

Rain water infiltration is the largest source of material deterioration in buildings.  The control of rain water is best achieved if some simple principles of drainage are followed.  The fundamental design looks to create a means to drain water off the building, out of the assemblies and components, and away from the building.  The design uses a strategy referred to as a rain screen approach.  In a rain screen approach, the exterior primary plane of water shedding (cladding, shingles, metal roofing, etc) is not relied upon to be completely watertight.  A secondary drainage plane (usually a housewrap or taped insulating sheathing), [or roofing felt], is installed behind the main exterior water shedding surface.  This drainage plane in combination with flashing details allow any water that may penetrate through the exterior water shedding plane to drain back out to the exterior.

After liquid water intrusion, air leakage is the second most common mechanism for depositing moisture in wall assemblies.  Air leakage occurs due to air pressure differentials causing air to flow through or within the building assembly.  In order to control air leakage a continuous plane of air seal must be created.  This air seal must be continuous not only for each building assembly, but at the connection between adjoining building assemblies.  Uncontrolled air leakage can also impact the energy efficiency of the building as infiltrating air will need to be conditioned or through the loss of exfiltrating conditioned air.  The Building America goal is to achieve an infiltration rate equivalent to 2.5 square inches per 100 square feet of building enclosure area.  Creating a continuous air seal is possible; however, special attention is often needed at transition details between different assemblies and systems.  

Vapor transport through diffusion can be a benefit or a detriment.  In some circumstances, vapor diffusing into a wall assembly can condense and accumulate resulting in problems with material deterioration.  On the other hand, vapor diffusion can also be used as a drying mechanism that will allow assemblies to dry to either the exterior or the interior or both.  In general, the vapor control strategy used should maximize the drying potential of the assembly while minimizing the potential for wetting.  With vapor diffusion being affected by both permeability of building components and temperature gradients across assemblies, the vapor control strategy is often related to, and integrated in, the insulation system design as well.  For cold climates, walls are generally designed to dry to the exterior, with the vapor permeability of the exterior of the wall being 5 times more permeable than the interior; or, they are designed with insulating sheathing in order to control the temperature of the condensing surfaces. The thickness of the insulating sheathing is determined by calculation based on the severity of the climate.

Roof Design

The roof is designed with asphalt shingles installed over a predominantly cathedralized ceiling.  [The same principles apply to metal roofing]  While the shingles will ensure that the vast majority of the liquid rain water and snow melt sheds off the surface, an SBS roof membrane (similar to a W.R. Grace Ice and Water Shield) fully adhered to the roof sheathing is installed at the eave locations and completely over the low slope roof areas to protect the roof from ice damming and potential water penetration from wind driven rain.  The overhangs from the roof are designed to extend a minimum of 12 inches from the exterior wall.  This amount of overhang will provide some protection for the wall elements such as windows and doors that are traditionally common sources of water leakage.  With the overhangs preventing the wall systems from getting wet, the risk of water intrusion through these elements is greatly reduced.

Figure 8: Roof Drainage

The vented cathedral ceilings and attics are designed with the interior plane of air tightness is located at the plane of the interior gypsum board.  All the joints in the gypsum boards must be taped and sealed.  In addition, any penetration through the gypsum must be air sealed, and all light fixtures should use air tight electrical boxes.  In order to maintain the continuity of the air seal between the roof and the wall, the interior gypsum board is sealed to the wood framing at the top of the wall assembly at band joist locations or to the wall gypsum board.  In order to maintain the interior air seal at the band joist location, sealants or gaskets are used to seal between the framing members. 

Figure 9: Roof Air Barrier

The vapor control strategy of the assembly is to promote drying primarily to the exterior and to reduce the amount of vapor able to diffuse from the interior environment into the roof structure.  This roof assembly has continuous back-venting from eave to ridge of the structural roof deck, providing higher drying potential of the assembly to the exterior. This, in combination with the low vapor permeability of the rigid insulation on the interior of the catherdralized ceilings and the latex paint finish on the ceiling of the vented attic portions that keeps interior moisture out of the roof assembly, makes for a robust, cold-climate roof assembly.

Figure 10: Roof Vapor Management