Patent Publication Number: US-2007094964-A1

Title: Dynamically ventilated exterior wall assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This Utility patent application is related to commonly assigned and concurrently filed Utility patent application Ser. No. ______, entitled EXTERIOR WALL ASSEMBLY having Attorney Docket Number M420.101.101, and which is herein incorporated by reference. 
    
    
     BACKGROUND  
      Recent improvements in the construction of homes and buildings have resulted in the fabrication of highly energy efficient structures. New construction materials, improved construction methods, and more stringent local and state building codes have all combined to provide highly energy efficient structures. In particular, exterior walls that are insulated and sealed, made according to code, and with the latest construction materials, increase the energy efficiency of these structures.  
      Insulated and sealed wall structures (i.e., “airtight” structures) reduce heat loss by substantially preventing drafts that remove heat from the wall structure. In addition, insulated and sealed wall structures are constructed to prevent the passage of moisture through the wall. Thus, insulated and sealed walls are airtight and moisture resistant, and are highly energy efficient. However, since insulated and sealed walls do not “breathe,” breached or damaged insulated and sealed walls can harbor moisture and provide nearly ideal breeding grounds for mold and bacteria.  
      In addition, environmental climate changes can create temperature differences between the internal and external spaces of the insulated and sealed walls that can contribute to the formation of condensate on interior surfaces of the walls. For example, during northern cold winter months, the air outside of an insulated and sealed wall is cold and dry, and the air inside of the wall is warm and humid. Thus, a natural humidity gradient is formed where moisture vapor in the air of an interior of the wall structure naturally migrates to the exterior of the wall structure. Thus, large gradients in outside and inside air temperatures can lead to an accumulation of moisture within even an insulated and sealed wall.  
      The opposite conditions occur during the summer months, when the air outside the structure is warm and humid, and the air inside the structure is conditioned to be cooler and dryer. Thus, during summer months a natural gradient exists driving warm humid air toward an interior of an insulated and sealed wall. Consequently, moisture can accumulate within an insulated and sealed wall due to normal, climate-induced temperature and humidity gradients.  
      Moisture includes bulk liquid, such as rain or rain droplets, and moisture vapor, such as in warm and humid air. Moisture, whether bulk or in the form of moisture vapor, can accumulate on surfaces of an insulated and sealed wall, as described above. In some cases, moisture is the result of natural condensation, but may also be the result of wind driven water that enters the wall along a window or door seam. For example, forming a window or a door in an exterior wall provides locations where water can enter the wall assembly and accumulate behind the wall covering. In some cases, moisture entering in the form of water is the result of poor workmanship, or alternately, a deterioration of flashing or sealants around the window/door.  
      In general, moisture accumulation within a wall, whether in the form of bulk liquid or in the form of moisture vapor, structurally damages the wall and can lead to health and safety issues for the occupants of the structure. In particular, moisture within a wall is known to create a breeding ground for insects, and can form other health hazards, such as the growth of molds and/or bacteria. The deleterious effects of moisture accumulation within a wall are accelerated in hot and humid environments.  
      This undesirable moisture penetration and accumulation within a wall assembly in new building structures has created challenges for the construction and insurance industries. Thus, there is a need for a system and a method to prevent moisture from accumulating in a sealed exterior wall assembly of a building structure, and for the removal of moisture that potentially collects within an exterior wall assembly.  
     SUMMARY  
      One aspect of the present invention is related to a dynamically ventilated exterior wall system. The dynamically ventilated exterior wall system includes a sealed exterior wall assembly and a ventilation assembly fluidly coupled to the exterior wall assembly. The sealed exterior wall assembly includes an interior wall portion and an opposing exterior wall portion, and insulation and a flexible porous grid disposed between the interior and exterior wall portions. The ventilation assembly includes a head end unit coupled to at least one air supply conduit and at least one air return conduit, where each of the conduits communicates with the porous grid of the exterior wall assembly. The head and unit is configured to supply conditioned air through the air supply conduit(s) to the exterior wall assembly and remove humidity from the exterior wall assembly through the air return conduit(s).  
      Another aspect of the present invention relates to a method of dynamically ventilating a sealed exterior wall that includes an interior wall portion and an opposing exterior wall portion and insulation adjacent to the interior wall portion. The method includes disposing a porous grid between the insulation and the exterior wall portion to define an air space within the sealed exterior wall. The method additionally provides supplying conditioned air through the air space. The method ultimately provides for removing humidity from the air space.  
      Another aspect of the present invention relates to an exterior wall system. The system includes an exterior wall assembly and means for transporting moisture out of the exterior wall assembly. The exterior wall assembly includes an interior wall portion and an opposing exterior wall portion, and a flexible porous grid disposed between the interior and exterior wall portions. In this regard, means for transporting moisture through the flexible porous grid and out of the exterior wall assembly is provided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles of the invention. Other embodiments of the present invention, and many of the intended advantages of the present invention, will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts.  
       FIG. 1  illustrates a cross-sectional view of a structure including a dynamically ventilated exterior wall system according to one embodiment of the present invention.  
       FIG. 2  illustrates a cross-sectional view of an above-grade exterior wall assembly according to one embodiment of the present invention.  
       FIG. 3  illustrates a cross-sectional view of a below-grade exterior wall assembly according to one embodiment of the present invention.  
       FIG. 4A  illustrates a cross-sectional view of a flexible moisture grid according to one embodiment of the present invention.  
       FIG. 4B  illustrates a perspective view of another flexible moisture grid according to one embodiment of the present invention.  
       FIG. 4C  illustrates a cross-sectional view of another flexible moisture grid according to one embodiment of the present invention.  
       FIG. 5  illustrates a perspective view of the flexible moisture grid illustrated in  FIG. 4C .  
       FIG. 6  illustrates a flexible grid coupled to a construction board according to one embodiment of the present invention.  
       FIG. 7  illustrates a perspective view of a head end unit including air supply and return conduits according to one embodiment of the present invention.  
       FIG. 8A  illustrates a structure end of an air supply/return conduit including a single row of orifices formed in a conduit wall according to one embodiment of the present invention.  
       FIG. 8B  illustrates a structure end of an air supply/return conduit including a plurality of orifices disposed helically about a circumference of the conduit according to one embodiment of the present invention.  
       FIG. 8C  illustrates a structure end of an air supply/return conduit including a plurality of orifices disposed in parallel columns along the conduit according to one embodiment of the present invention.  
       FIG. 9  illustrates a system flow chart directed to the removal of moisture from a zoned structure according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
      In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.  
       FIG. 1  illustrates a structure  20  including a dynamically ventilated exterior wall system  22  according to one embodiment of the present invention. Structure  20  includes a first sealed exterior wall assembly  24 , and a second sealed exterior wall assembly  26 . Sealed exterior wall assemblies are structures that are sealed against the passage of moisture and air and include, for example, finished exterior wall structures having caulked seams, sealed seams, fitted flashing, and/or exterior claddings configured to prevent the transmission of air and moisture through the wall.  
      In one embodiment, the first sealed exterior wall assembly  24  is an above-grade exterior wall, and second sealed exterior wall assembly  26  is a below-grade exterior wall. The ventilation assembly  22  is fluidly coupled to the exterior wall assemblies  24 ,  26 , and in one embodiment, includes a head end unit  28 , air supply conduits  30 ,  32 , and air return conduits  34 ,  36 , where the conduits  30 - 36  extend from head end unit  28  into an interior of the sealed exterior wall assemblies  24 ,  26 .  
      For example, in one embodiment head end unit  28  supplies conditioned dry air through air supply conduits  30 ,  32  into above-grade exterior wall assembly  24  and below-grade exterior wall assembly  26 . Air return conduits  34 ,  36  remove air, for example relatively humid air, from the sealed above-grade exterior wall assembly  24  and below-grade exterior wall assembly  26 , and deliver the return air to head end unit  28 . In one embodiment, a humidity sensor  40  is coupled between air return conduit  38  and head end unit  28 , although other suitable locations for humidity sensor  40  along a return path from exterior wall assemblies  24 ,  26  to head end unit  28  are also acceptable.  
      In one embodiment, desired structural openings, such as a window  50  and a door  52 , are formed in the exterior wall assemblies  24 ,  26  that provide a pathway for the ingress of moisture into structure  20 . While it is desirable to have window  50  and door  52  formed in structure  20 , such openings provide a potential pathway for the entrance of moisture into the sealed exterior wall assemblies  24 ,  26 .  
      In one embodiment, air supply conduit  30  is disposed in a zone adjacent to window  50 , and air supply conduit  32  is disposed in a zone adjacent to door  52 , to supply these potential moisture entry areas with conditioned, dry air. In another embodiment, air supply conduit  30  surrounds window  50 , and air supply conduit  32  surrounds door  52 . In any regard, air supply conduits  30 ,  32  supply conditioned, dry air to exterior wall assemblies  24 ,  26 , and air return conduits  34 ,  36  remove air (at a typically higher humidity) from exterior wall assemblies  24 ,  26  and deliver the humid air back to head and unit  28  to cyclically condition exterior wall assemblies  24 ,  26 .  
       FIG. 2  illustrates a cross-sectional view of above-grade exterior wall assembly  24  according to one embodiment of the present invention. Exterior wall assembly  24  includes an interior wall portion  60 , an opposing exterior wall portion  62 , insulation  64 , and a flexible grid  66 . In one embodiment, insulation  64  is disposed adjacent to interior wall portion  60  and defines an opening  68  between insulation  64  and exterior wall portion  62 . In one embodiment, flexible grid  66  is disposed within opening  68  to form an air passageway between exterior wall portion  62  and insulation  64 .  
      Insulation  64  is a thermally insulating filler configured for placement in an exterior wall. In one embodiment, insulation  64  is a fiberglass insulation. In another embodiment, insulation  64  is a blown fibrous insulation. In general, insulation  64  is disposed between studs used to frame exterior wall assembly  24 , and can include rolls or sheets of insulating material.  
      In one embodiment, interior wall portion  60  includes a sheathing board  70  and an air barrier sheeting  72  attached to sheathing board  70 . In one embodiment, and is best illustrated in  FIG. 2 , air barrier sheeting  72  contacts insulation  64 .  
      Sheathing board  70  is generally a structural board suited for construction of new homes and commercial buildings. In one embodiment, sheathing board  70  is an oriented strand board, although other structural boards suited for the construction of walls are also acceptable.  
      Air barrier sheeting  72  is generally a single layer of polymeric film suited for adhering to sheathing board  70 . In one embodiment, air barrier sheeting  72  is a polyethylene film, although other films and construction fabrics suited for covering sheathing board  70  are also acceptable.  
      In one embodiment, exterior wall portion  62  includes a second sheathing board  80 , a water barrier sheeting  82  attached to sheathing board  80 , and exterior cladding  84  attached to the water barrier sheeting  82 .  
      Sheathing board  80  is highly similar to sheathing board  70 . Water barrier sheeting  82  is attached to an exterior face of sheathing board  80  to provide a level of weather resistance for exterior wall portion  62 . In one embodiment, water barrier sheeting  82  is a flash-spun polyethylene nonwoven fabric that is adhered, for example by stapling, to the exterior face of sheathing board  80 . Exemplary materials for water barrier sheeting  82  include Tyvek® house wrap, wax coated fabrics, tarpaper and the like, although other suitable materials and/or fabrics are acceptable.  
      Exterior cladding  84  includes suitable exterior insulation and finish systems (EIFS) such as, for example, stucco finishes, shakes including cedar shakes, vinyl and metal siding, plastic and wood siding, and the other suitable exterior wall coverings.  
      In one embodiment, flexible grid  66  is disposed within opening  68  and bounded by sheathing board  80  on one side and by insulation  64  on an opposing side. In this manner, flexible grid  66  provides an air passageway between insulation  64  and exterior wall portion  62 , and is configured to transport moisture that accumulates within exterior wall assembly  24  along opening  68  and away from insulation  64  and exterior wall portion  62 .  
       FIG. 3  illustrates a cross-sectional view of below-grade exterior wall assembly  26  according to one embodiment of the present invention. In one embodiment, exterior wall assembly is a below-grade wall assembly forming a portion of a foundation of structure  20  (shown in  FIG. 1 ). Exterior wall assembly  26  includes an interior wall portion  90 , an opposing exterior wall portion  92 , insulation  94 , and a flexible grid  96  disposed within an opening  98  formed between insulation  94  and exterior wall portion  92 .  
      In one embodiment, interior wall portion  90  includes a sheathing board  100  and an air barrier sheeting  102  attached to the sheathing board  100 . Sheathing board  100  and air barrier sheeting  102  are highly similar to sheathing board  70  and air barrier sheeting  72  described with reference to  FIG. 2 . With this in mind, air barrier sheeting  102  is attached to sheathing board  100  and contacts insulation  94 .  
      In one embodiment, exterior wall portion  92  forms a foundation of structure  20  (shown in  FIG. 1 ) and includes concrete blocks  104 ,  106 ,  108 . In another embodiment, exterior wall portion  92  is formed of a continuous concrete wall, although other suitable below-grade foundation materials can also be employed.  
      Insulation  94  is highly similar to insulation  64 . As illustrated in  FIG. 3 , flexible grid  96  defines an air passageway between insulation  94  and exterior wall portion  92  and is configured to transport moisture along opening  98  and away from insulation  94  and exterior wall portion  92 .  
       FIG. 4A  illustrates a cross-sectional view of a flexible grid  110  according to one embodiment of the present invention. Flexible grid  110  is representative of flexible grid  66  (shown in  FIG. 2 ) and flexible grid  96  (shown in  FIG. 3 ). In this regard, flexible grid  110  includes a first surface  112 , an opposing second surface  114 , and a core  116  disposed between first surface  112  and second surface  114 . Flexible grid  110  is, in general, pliable and porous to air flow. In this Specification, porous to air flow means that air and moisture vapor, and air containing moisture vapor, can be transported (dynamically and/or passively) through the flexible grid.  
      In one embodiment, flexible grid  110  is a single layer structure formed of a random distribution of fibers in a matt or fabric-like sheeting. In one exemplary embodiment, flexible grid  110  is a nonwoven sheeting including a fibrous core  116 . For example, in one embodiment flexible grid  110  is a nonwoven web of randomly distributed polyolefin fibers where first surface  112  and second surface  114  are thermally treated (e.g., by embossing, or calendering, or by hot can treating) to define a relatively smooth and flat surface.  
      Generally, core  116  defines a plurality of chambers that form a network, or air space, between first surface  112  and second surface  114 . In one embodiment, core  116  defines a “dead” air space. In another embodiment, core  116  defines an air space configured to permit air and moisture transport.  
      In one embodiment, flexible grid  110  is permeable to moisture vapor and impermeable to liquid water, and includes a surface energy-reducing additive, such as a fluorochemical, added to fibrous core  116 . The surface energy-reducing additive is melt-added to the fibers during formation in one embodiment. In another embodiment, the surface energy-reducing additive is added topically to the fibers after formation.  
       FIG. 4B  illustrates a perspective view of a flexible grid  117  according to one embodiment of the present invention. Flexible grid  117  includes strands  118   a - 118   e , and strands  119   a - 119   f  overlapping and contacting strands  118   a - 118   e  to define a core  121 . Strands  118  and  119  overlap to form voids between the strands, where the voids permit airflow through core  121 . In addition, the overlapping strands  118  and  119  defining air channels M 1 -M 5  longitudinally along core  121 , and air channels N 1 -N 4  laterally along core  121 . In one embodiment, strands  118  and  119  are each approximately 0.125 inch wide and 0.125 inch thick, such that overlapping strands  118 / 119  combine to form a core  121  having a 0.250-inch thickness. Other suitable dimensions for strands  118 / 119  are also acceptable.  
      In one embodiment, strands  118  are aligned in a first direction, for example a horizontal orientation, and strands  119  are aligned in a second direction not equal to the first direction, for example, a vertical orientation. In this manner, air channels M 1 -M 5  and N 1 -N 4  are defined in at least two orientations. In one embodiment, the voids formed by the overlapping strands  118 / 119  provide air passageways extending through core  121 , and air channels M 1 -M 5  and N 1 -N 4  provide air passageways that are approximately orthogonal to the air passageways through the core defined by the voids.  
      In one embodiment, air channels M 1 -M 5  are vertical air channels and air channels N 1 -N 4  are horizontal air channels. In one exemplary embodiment, and with reference to  FIG. 2 , strands  119   a - 119   f  are aligned along respective wall studs (not shown) and define vertical air channels M 1 -M 5  configured to aerate, for example, an above-grade exterior wall assembly  24 . Strands  118   a - 118   e  in this embodiment are aligned horizontally relative to strands  119   a - 119   f  and define horizontal air channels N 1 -N 4  that are configured to transport air and moisture along, for example, insulation  64 .  
       FIG. 4C  illustrates a cross-sectional view of another flexible grid  120  according to one embodiment of the present invention. Flexible grid  120  is representative of one embodiment of flexible grid  66  (shown in  FIG. 2 ) and flexible grid  96  (shown in  FIG. 3 ). In this regard, flexible grid  120  includes a film layer  122 , an opposing porous backing  124 , and a reticulated core  126  disposed between film layer  122  and porous backing  124 . In one embodiment, flexible grid  120  is a three-layer composite structure that is pliable. However, it is to be understood that flexible grid  120  can include a single core layer, or multiple layers (i.e., two, three, or more layers) including more than one core layer.  
      Film layer  122  is generally a substantially continuous surface and is suitable for contact and/or adhesive attachment to a solid construction surface. In this regard, film layer  122  is in one embodiment a polymeric film that is permeable to moisture vapor and impermeable to liquid water. In another embodiment, film layer  122  is a polymeric film that is mechanically perforated to permit the passage of air, moisture vapor, and water. In another embodiment, film layer  122  is a mesh netting permeable to air, moisture vapor, and bulk moisture.  
      As described above, film layer  122  is permeable to moisture vapor and impermeable to liquid water, according to one aspect of the present invention. In one embodiment film layer  122  includes a surface energy-reducing additive, such as a fluorochemical, a wax, a silicone, or an oil. In one aspect of the present invention, the surface energy reducing additive (for example, a carbon-8 fluorochemical) is applied as a topical additive to film layer  22 ; in another embodiment, the surface energy reducing additive is a melt additive added to film layer  122  during processing of film layer  122 .  
      Porous backing  124  is generally configured for contact with insulation  94  (shown in  FIG. 3 ). In this regard, porous backing  124  generally defines a highly open structure that permits free air exchange. In one embodiment, porous backing  124  is a plastic mesh netting. In another embodiment, porous backing  124  is a woven fabric. In another embodiment, porous backing  124  is a nonwoven fabric formed of, for example, a polyolefin material such as polyethylene or polypropylene. In any regard, porous backing  124  is highly porous to air flow and is configured to abut against insulation  94  and impede an entrance of insulation  94  into flexible grid  120 .  
      Reticulated core  126  generally separates film layer  122  and porous backing  124  to form an air passageway configured to fit within opening  68  (shown in  FIG. 2 ) or opening  98  (shown in  FIG. 3 ). In one embodiment, reticulated core  126  defines a honeycomb lattice that includes a plurality of chambers  130   a ,  130   b  . . .  130   z  defined by walls  131 . In this regard, chambers  130   a - 130   z  extend between film layer  122  and porous backing  124 . Generally, reticulated core  126  defines a plurality of chambers that form a network, or air space, between film layer  122  and porous backing  124 . In one embodiment, the network defines a “dead” air space. In another embodiment, the network defines an air space configured to permit passive and/or dynamic air and moisture transport.  
      In one embodiment, reticulated core  126  is an expanded polymeric film that is porous to air and liquid. In another embodiment, reticulated core  126  is a felted network of fibers. In general, reticulated core  126  provides a measurable degree of separation between film layer  122  and porous backing  124  to form an air spacing therebetween. In this regard, in one embodiment reticulated core defines a thickness D of between 0.05 inch and 2.0 inches, preferably reticulated core  126  defines a thickness D of between 0.1 inch and 1.0 inch, and more preferably reticulated core  126  defines a thickness D of between 0.25 and 0.75 inch. To this end, a thickness of flexible grid  120  is compatible with insertion of grid  120  into an exterior wall assembly such that the wall assembly will comply with building and construction codes.  
      In one embodiment, each of the flexible grids  110 ,  120  is sufficiently flexible to be rolled onto a core and suitable for delivery to a construction site in, for example, roll form. In another embodiment, each of the flexible grids  110 ,  120  is sufficiently flexible to be folded multiple times and suitable for delivery to a construction site in, for example, a folded sheet form.  
       FIG. 5  illustrates a perspective view of flexible grid  120  according to one embodiment of the present invention. Film layer  122  forms a substantially continuous surface against which one end reticulated core  126  is supported. In one embodiment, film layer  122  is porous to air and moisture vapor. For example, in one embodiment film layer  122  includes macroporous holes or orifices that enable the grid  120  to be “breathable” and transport air and moisture vapor between film layer  122  and porous backing  124 .  
      Porous backing  124  is secured over another end of reticulated core  126 . In one embodiment, film layer  122  and porous backing  124  are thermoplastically sealed to reticulated core  126 . In an alternate embodiment, film layer  122  and porous backing  124  are adhesively adhered to reticulated core  126 . As illustrated in  FIG. 5 , in one embodiment reticulated core defines a honeycomb lattice  132  including the plurality of chambers  130   a - 130   z  that extend between film layer  122  and porous backing  124 . Film layer  122  is suitable for adhesively sealing to construction boards, such as oriented strand boards. As illustrated in  FIGS. 4 and 5 , in one embodiment walls  131  are porous to airflow and enable air and moisture vapor to flow longitudinally and laterally along core  126 .  
       FIG. 6  illustrates a perspective view of an exterior wall portion  140  according to one embodiment of the present invention. Exterior wall portion  140  includes a sheathing board  142  and a flexible grid  144  attached to sheathing board  142 . In this regard, sheathing board  142  is highly similar to sheathing board  80  (shown in  FIG. 2 ), and flexible grid  144  is highly similar to flexible grid  120  (shown in  FIG. 5 ). Thus, optionally, sheathing board  142  includes a water barrier sheeting, for example a plastic film, attached to a side of board  142  opposite flexible grid  144 .  
      In one embodiment, flexible grid  144  is adhesively attached to sheathing board  122 . In this manner, exterior wall portion  140  is suitable for use in the construction trades in forming a sealed exterior wall assembly, for example exterior wall assembly  24  (shown in  FIG. 2 ). Similar to flexible grid  120  (shown in  FIG. 5 ), flexible grid  144  includes film layer  146 , an opposing porous backing  148 , and a reticulated core  150  disposed between film layer  146  and porous backing  148 .  
      In one embodiment, reticulated core  150  includes a honeycomb lattice of chambers defined by walls  151  that extend away from sheathing board  142 . In a manner analogous to  FIG. 5 , the honeycomb chambers permit airflow through core  150  such that air and moisture vapor is transported away from sheathing board  142 . In one embodiment, walls  151  are porous to air and moisture vapor and are configured to permit airflow longitudinally and laterally through core  150  and along sheathing board  142 .  
      Flexible grids  110  and  120  provide for a passive transportation of moisture away from interior surfaces of exterior wall assemblies  24 ,  26 . In one embodiment, flexible grids  110  and  120  are disposed in an interior opening, for example opening  68  (shown in  FIG. 2 ) or opening  98  (shown in  FIG. 3 ), to form a moisture-transporting air passageway inside the sealed and insulated exterior wall assemblies  24 ,  26 . Moisture is transported along the air passageway formed by flexible grids  110  and  120 , thus removing moisture from interior wall portions, exterior wall portions, and insulation inside the assemblies  24 ,  26 .  
      In another embodiment, and as best illustrated in  FIG. 6 , an entire exterior wall portion  140  includes sheathing board  142  and flexible grid  144  attached to sheathing board  142 . During the construction of an exterior wall assembly, exterior wall portion  140  can be erected in one step, such that upon finishing the interior portion of the wall assembly, insulation is simply unrolled over flexible grid  144  and interior wall portion  60  (shown in  FIG. 2 ), for example, is fixed in place. The exterior wall portion  140  can provide one-step erection of a sheathing board  142  and moisture-transporting flexible grid  144 .  
       FIG. 7  illustrates a perspective view of head end unit  28  according to one embodiment of the present invention. Head end unit  28  generally supplies conditioned air through air supply conduits, for example air supply conduits  30 ,  32 , and receives air removed from a structure, for example exterior wall assemblies  24 ,  26  (shown in  FIG. 1 ). In one embodiment, head end unit  28  is a stand-alone unit configured to supply dry, conditioned air to exterior wall assemblies  24 ,  26 , and configured to remove relatively humid air from exterior wall assemblies  24 ,  26 . In another embodiment, head end unit  28  is electrically coupled to an existing forced air heating and cooling system (not shown) within structure  20 , such that head end unit  28  cooperates with the existing forced air heating and cooling system to supply dry, conditioned air to exterior wall assemblies  24 ,  26 , and remove relatively humid air from exterior wall assemblies  24 ,  26 .  
      With this in mind, in one embodiment head end unit  28  is a heating ventilation air conditioning (HVAC) unit including a compressor (not shown) maintained in a compressor side  160 , a blower and a blower motor (neither shown) maintained within a blower housing  162 , air return ducts  164 , and humidity sensors  166  aligned with air return ducts  164 .  
      As illustrated in  FIG. 7 , air return conduits  34 ,  36  couple with air return ducts  164 , and humidity sensors  166  fluidly communicates with air return conduits  34 ,  36 . A plurality of controls  170  is provided on head end unit  28  to enable an automated control of air conditioning delivered through supply conduits  30 ,  32  and moisture removal pulled through return conduits  34 ,  36 . In one embodiment, a programmable controller (not shown) is coupled to controls  170  (internal to head end unit  28 ) to permit a computer/logic-controlled operation air supply and return. Controls  170  can be selectively adjusted to cycle conditioned air through air supply conduits  30 ,  32  in response to a humidity level sensed by humidity sensor  166  for air returned through air return conduits  34 ,  36 .  
      In one embodiment, controls  170  are set to a desired set point to maintain a relative humidity level within exterior wall assemblies  24 ,  26  (shown in  FIG. 1 ). For example, in one embodiment controls  170  are set to maintain a relative humidity within exterior wall assemblies  24 ,  26  of approximately 70%. In this embodiment, controls  170  cycle head end unit  28  to an on configuration where dry, conditioned air is supplied to exterior wall assemblies  24 ,  26 , and relatively more humid air is removed from exterior wall assemblies  24 ,  26  by air return conduits  34 ,  36  of head end unit  28 . Head end unit  28  remains in the on configuration until humidity sensor  166  communicates a relative humidity in the return air of less than the desired humidity set point (i.e., 70%).  
      Thereafter, a blower within head end unit  28  continues to remove air from exterior wall assemblies  24 ,  26  to enable humidity sensor  166  to continue sensing a relative humidity within the exterior wall assemblies  24 ,  26 . In one embodiment, consecutive readings of the relative humidity by the humidity sensor  166  indicating that air extracted from exterior wall assemblies  24 ,  26  is below the desired humidity set point will activate head end unit  28  to an off position.  
      In one embodiment, head end unit  28  is programmed to cycle between on and off positions over a set time interval (e.g., every 30 minutes). In another embodiment, head end unit  28  is programmed to cycle between on and off positions based upon a relative humidity reading from within exterior wall assemblies  24 ,  26  by a separate humidity sensor (not shown) within exterior wall assemblies  24 ,  26 . One aspect of the present invention provides for a continuous operation of head end unit  28  in continuously supplying dry, conditioned air to exterior wall assemblies  24 ,  26 , useful, for example, in drying exterior wall assemblies in tropical climates.  
      As illustrated in  FIG. 7 , air supply conduits  30 ,  32 , define a respective head end side  180   a  and  180   b , and a structure side  182   a  and  182   b . In a similar manner, air return conduits  34 ,  36 , define a respective head end side  190   a  and  190   b , and a structure side  192   a  and  192   b.    
       FIG. 8A  illustrates a perspective view of structure side  182   a  of air supply conduit  30  according to one embodiment of the present invention. Structure side  182   a  defines a closed end  200  and a plurality of orifices  202  formed in a wall  204  of structure end  182   a . In one embodiment, the plurality of orifices  202  defines a single column of orifices aligned along a longitudinal axis of structure end  182   a  that is useful in delivering conditioned air into exterior wall assemblies  24 ,  26 . Orifices  202  are formed through wall  204  and communicate with an interior portion of air supply conduit  30 . That is to say, in one embodiment conduit  30  defines an annular structure and a single column of orifices  202 .  
      Structure  182   a  defines an outside diameter O.D. and an inside diameter I.D. In one embodiment, the O.D. of structure end  182   a  is between 0.1 inch and 1.0 inch, preferably the O.D. of structure end  182   a  is between 0.2 inch and 0.5 inch. For example, in one embodiment a 0.25 inch thick flexible grid  120  is secured within exterior wall assembly  24 , and a structure end  182   a  of air supply conduit  30  having a 0.25 inch O.D. is coupled to flexible grid  120 . Wall  204  defines a thickness that is suited for supplying air through conduit  30 .  
      Orifices  202  are configured to deliver a flow of air, for example conditioned air from structure end  182   a  of air supply conduit  30  into an exterior wall assembly, such as exterior wall assembly  24  (shown in  FIG. 1 ). It is to be understood that although structure end  192   a  (shown in  FIG. 7 ) of air return conduit  34  is not illustrated, structure end  192   a  of air return conduit  34  is, in one embodiment, similar to structure end  182   a  of air supply conduit  30  illustrated in  FIG. 8A .  
       FIG. 8B  illustrates another embodiment of a structure end  210  of an air supply conduit  212  according to one embodiment of the present invention. Structure end  210  defines a closed end  214  and a plurality of orifices  216  formed circumferentially in a wall  218  of air supply conduit  212 . In one embodiment, orifices  216  are formed in wall  218  in a helical pattern about a circumference of structure end  210 . Structure end  210  defines an outside diameter O.D. and an inside diameter I.D. that are highly similar to the outside diameter and inside diameter described above in  FIG. 8A .  
       FIG. 8C  illustrates yet another embodiment of a structure end  220  of an air supply conduit  222  according to one embodiment of the present invention. Structure end  220  defines a closed end  224  and a plurality of orifices  226  formed in a wall  228 . In one embodiment, orifices  226  are formed in parallel columns along structure end  220  of air supply conduit  222 . In another embodiment, orifices  226  define a pair of staggered, parallel columns of orifices formed in wall  228 . Structure end  220  defines an outside diameter O.D. and an inside diameter I.D. that are highly similar to the outside diameter and inside diameter described above with reference to  FIG. 8A .  
       FIG. 9  illustrates a system flow chart  250  directed to the removal of moisture from a zoned structure according to one embodiment of the present invention. With additional reference to  FIG. 1 , a zone is defined by at least one air supply conduit, at least one air return conduit, and at least one humidity sensor communicating with the air return conduit. For example, air supply conduit  30 , air return conduit  34 , and humidity sensor  40  combine to define one zone in structure  20 .  
      Structure  20  can include a plurality of zones, for example a zone directed to removing moisture from around a window, and a separate second zone for removing moisture from around a door. In another embodiment, an entire exterior wall assembly, for example exterior wall assembly  26 , is serviced by a single zone. It is to be understood that structure  20  can include multiple zones within multiple exterior wall assembly structures, all controlled by head end unit  28 . Reference is made to  FIG. 1  in the following description where air supply conduit  30 , and air return conduit  34  combine to define a zone around window  50 .  
      During use, and with additional reference to  FIGS. 1 and 8 A, air supply conduit  30  is extended away from head end unit  28  and positioned to drive moisture away from a potentially moist area, for example window  50 . Orifices  202  are positioned to fluidly communicate with reticulated core  126  of flexible grid  120  (shown in  FIG. 4C ). Dry, conditioned air exits orifices  202  and transports moisture along an air passageway formed by opening  68  (shown in  FIG. 2 ). Air return conduit  34  draws the transported moisture away from window  50  and delivers the relatively humid air back to head end unit  28 .  
      With additional reference to  FIGS. 1 and 7 , humidity sensors  166  sense a humidity level in a zone of an exterior wall structure, for example exterior wall structure  24 . Controllers  170  in combination with humidity sensors  166  sense a relative humidity of air returned from exterior wall assembly  24 . The sensed humidity level within exterior wall assembly  24  is compared to a desired relative humidity level set point, as controlled by controls  170 . The process for comparing the sensed humidity level within exterior wall assembly  24  to the relative humidity set point is provided by process  252 .  
      Process  254  queries whether the relative humidity level within a zone of exterior wall assembly  24  is acceptable. If the relative humidity level is acceptable, process  256  provides for sensing a humidity level in a next zone of the exterior wall assembly  24  or of structure  20 . In an iterative manner, process  258  provides for sensing a humidity level in a last zone of an exterior wall assembly  24 /structure  20  where prior zones of the structure were evaluated to have an acceptable relative humidity level. In the case where each zone of structure  20  has an acceptable relative humidity level, process  260  provides for a timed out wait period prior to cycling system  250 .  
      With additional reference to process  254 , in the case where the relative humidity level within a zone of exterior wall assembly  24  is not acceptable, process  262  provides for cycling head end unit  28  to supply conditioned dry air through air supply conduits  30 ,  32 . Thus, head end unit  28  supplies conditioned air to the zone having a relative humidity level that is above the set point, and process  266  provides for sensing the relative humidity of air returning through air return conduits  34 ,  36  extracted from the too humid zone. A further query is made of the zone in process  254 , consistent with one drying cycle of system  250 .  
      In one embodiment, and in particular during periods of relatively dry weather, process  260  signals to head end unit  28  that conditioned air is not called for by any zone. Thus, head end unit  28  does not cycle between the on and off positions, but rather is maintained in an off position, but ready for subsequent cycling.  
      In addition, and with reference to  FIG. 2 , during periods in which head end unit  28  does not cycle, flexible grid  66  provides for a continual passive transport of moisture vapor away from interior wall portion  60  and exterior wall portion  62 . In other words, flexible grid  66  forms an air passageway within opening  68  that permits the transport of moisture vapor away from the interior surfaces of exterior wall assembly  24  without cycling head end unit  28 .  
      In contrast, winter seasons and summer seasons can create a natural humidity gradient across surfaces of structure  20  that results in frequent cycling of head end unit  28 . For example, during winter months associated with cold and dry exterior air temperatures and relatively warm interior air temperatures, the large temperature and humidity gradients between the interior air of structure  20  and the environment outside of structure  20  combine to cause moisture vapor in the air to condense upon surfaces of exterior wall assemblies  24 ,  26 . Thus, during winter months, humid air within structure  20  will condense on, for example, sheathing board  70  and air barrier sheeting  72 .  
      This condensation can lead to moisture accumulation along air barrier sheeting  72  and insulation  64 . Aspects of the present invention provide for humidity sensors  166  that sense a relative humidity associated with exterior wall assembly  24 . When the relative humidity within exterior wall assembly  24  exceeds a desired set point, head end unit  28  is activated to an on condition, supplying condition dry air through air supply conduits  30 ,  32 , and removing moisture from within exterior wall assembly  24  via air return conduits  34 ,  36 . Thus, moisture within exterior wall assembly  24  is driven to opening  68  and transported through flexible grid  66 , to be conditioned by head end unit  28 .  
      With the above in mind, in one embodiment head end unit  28  cycles between on and off settings periodically (e.g., every fifteen minutes) to maintain the desired relative humidity within wall assembly  24 . In contrast, during relatively dry months, head end unit  28  might not cycle to the on position for periods of greater than one week.  
      Aspects of the present invention have been described that provide for dynamically venting an exterior wall assembly to remove moisture from inside a sealed and insulated exterior wall. In particular, sealed exterior wall assemblies have been described that can accumulate moisture either through natural condensation processes or through a failure in weather proofing or sealing of, for example, doors and windows in an exterior wall assembly. Embodiments of the present invention provide for dynamically ventilating conditioned air through the flexible grid within the exterior wall assembly to displace humid moisture within the exterior wall assembly with conditioned dry air.  
      Other aspects of the present invention provide for a flexible grid that provides an air passageway within the exterior wall assembly for the passive removal of moisture. Embodiments of the present invention provide for statically ventilating the exterior wall assembly via the flexible grid to remove humidity from the exterior wall assembly.  
      A sealed exterior wall assembly that is highly energy efficient and in compliance with local and state housing codes has been described that provides for dynamically, and/or passively (statically), venting moisture from the sealed exterior wall assembly.  
      In one embodiment, the dynamic, and/or passive, venting of moisture from a sealed exterior wall assembly improves the overall energy efficiency of the wall assembly and its associated structure. The removal of moisture from a wall assembly results in increasing the “R-value,” or insulation value of the wall assembly. Since the wall assembly does not retain the potentially harmful moisture, the insulation performs better, the insulating quality is improved, and moisture that otherwise might conduct heat out of the wall assembly is reduced or eliminated, thus increasing the energy efficiency of the wall assembly. Embodiments of dynamically, and/or passively vented exterior wall assemblies as described above will remain warmer in winter, cooler in summer, and can cost-effectively satisfy even the most stringent building codes.  
      Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.