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TECHNICAL FIELD 
   The invention relates to doors and, more particularly to threshold and doorsill assemblies for entryways and seals therefore. 
   BACKGROUND 
   Entryway systems used in building construction generally include a pair of vertically extending door jambs and a head jamb that frame the entryway and receive a hinged door. An elongated threshold assembly is attached at its ends to the bottoms of the door jambs and spans the bottom of the entryway atop a sub-sill and/or sub-floor. Many modern threshold assemblies include an extruded aluminum sill frame. These extruded frames often include an upwardly open channel from which a sill slopes outwardly and downwardly. A threshold cap, which may be made of plastic or wood, is disposed in the upwardly open channel and underlies a closed door mounted in the entryway. The threshold cap usually is vertically adjustable to meet and form a seal with a flexible seal attached to the bottom of the door. 
   Threshold assemblies are typically mounted atop a wooden or concrete sub-sill and/or sub-floor. The forward portions of the threshold assemblies often overhang a front face of the sub-sill and/or sub-floor and outwardly extend above an adjacent underlying structure such as sheathing, finished siding material or brick masonry. One important design parameter for threshold assemblies is to minimize if not eliminate intrusion of moisture under the front edges of the thresholds. Intruding water may damage an underlying wooden sub-floor and sub-sill. 
   In addition, the forward edges of extruded aluminum sill frames often are provided with functional yet unattractive configurations. For example, extruded sill frames may include one or more recesses, notches, grooves, or other features for optional mating engagement with extruded sill extensions or other attachments. 
   Accordingly, there is a need for a seal that reduces or eliminates the possibility of moisture reaching the wooden frame members underlying a threshold assembly, and also provides an attractive finished appearance to an irregularly shaped front edge of a doorsill. 
   SUMMARY 
   The invention includes a threshold for installation atop a sub-sill of a building wall frame having a front face. The threshold includes a sill frame with a sill deck having a forward deck portion and a forward edge. The forward deck portion and forward edge are configured to overhang the front face of the sub-sill by an overhang distance when the threshold is installed in the wall frame. A sealing fin is removably connected to the sill frame. The sealing fin includes an upwardly extending upper panel substantially covering the entire forward edge of the sill frame. The sealing fin further includes a downwardly extending lower panel that is inwardly offset from the upper panel by an offset distance that is substantially equal to the overhang distance. A web connects the upper and lower panels of the sealing fin. 
   The invention also includes a sealing fin for a door sill frame having a front edge. The sealing fin includes an upwardly extending upper panel having a lower edge. The upper panel is configured to engage in a mating fashion the front edge of the door sill frame, and to substantially cover the entire front edge of the door sill when the upper panel is engaged with the front edge. A substantially horizontal web rearwardly extends from the lower edge of the upper panel and has a rear edge. A lower panel downwardly extends from the rear edge of the web portion. The front edge of the door sill frame and the lower panel of the sealing fin are located a substantial distance behind the front edge of the sill frame when the upper edge of the upper is engaged with the front edge of the sill frame. 
   The invention further includes an entryway comprising a sub-sill having a top surface and a front face. A sill frame is mounted atop the top surface of the sub-sill, and includes a forward deck portion that overhangs the front face of the sub-sill, a front edge, and a bottom. A sealing fin has an upper panel substantially covering the entire front edge of the sill frame, a web portion rearwardly extending from the upper panel and underlying at least a portion of the bottom of the sill frame, and a lower panel downwardly extending from the web portion. The lower panel is closely proximate to the front face of the sub-sill and is affixed to the sub-sill by a plurality of fasteners. The fasteners can be removable or non-removable. The sealing fin provides a substantially water-resistant barrier between the sill frame and the sub-sill. 
   In addition, the invention includes a method of sealing a juncture between a sub-floor having a front edge and a door sill having a bottom and a forward edge overhanging the front edge of the sub-floor by a substantial distance. The method includes obtaining a one-piece moisture-resistant polymeric sealing fin having a first connecting means for connecting a front portion of the sealing fin to the front edge of the sill and a second connecting means for connecting a rear portion of the sealing fin to the front edge of the sub-floor. The method further includes connecting the first connecting means of the sealing fin to the forward edge of the sill, and connecting the second connecting means of the sealing fin to the front edge of the sub-floor such that the front portion of the sealing fin substantially covers the forward edge of the sill and the sealing fin substantially conforms to the juncture between the sub-floor and the sill. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of one embodiment of a threshold assembly according to the invention; 
       FIG. 2  is an enlarged perspective view of one end of the threshold assembly shown in  FIG. 1 ; 
       FIG. 3  is an end view of an embodiment of a sealing fin according to the invention with its lower panel in a first position; 
       FIG. 4  is an end view of the sealing fin of  FIG. 3  with the lower panel in a second position; 
       FIG. 5  is a cross sectional view of the threshold assembly of  FIGS. 1 and 2  installed in an entryway; 
       FIG. 6  is a perspective view of one end of a second embodiment of a threshold assembly according to the invention; 
       FIG. 7  is an end view of another embodiment of a sealing fin for use with the threshold of  FIG. 6  with the lower panel of the sealing fin in a first position; 
       FIG. 8  is an end view of the sealing fin of  FIG. 7  with the lower panel in a second; 
       FIG. 9  is an end view of another embodiment of a sealing fin according to the invention; 
       FIG. 10  is a cross sectional view of a threshold assembly installed in an entryway with the sealing fin of  FIG. 9  conforming to an overhang distance L 1 ; 
       FIG. 11  is a cross sectional view of a threshold assembly installed in an entryway with the sealing fin of  FIG. 9  conforming to an overhang distance L 2 ; and 
       FIG. 12  is a perspective view of the sealing fin of  FIG. 9  attached to a forward portion of a door sill. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows an embodiment of an inswing doorsill or threshold assembly  10  with a detachable sealing fin  30  in accordance with the invention. The threshold assembly  10  includes a doorsill  20  having a sill frame  22  and an upwardly open threshold channel  24 . The threshold channel  24  is configured to receive a threshold cap  70  like that shown in  FIG. 5 . The sill frame includes an outwardly extending sill deck  21 . In a preferred embodiment, the sill deck  21  also is downwardly sloping. As shown in  FIG. 1 , the sealing fin  30  is affixed to the front edge  25  of the sill frame  22 , and includes an upper panel  34  that substantially covers the entire front edge  25  of the doorsill  20 . The sealing fin  30  also includes a downwardly extending lower panel  32  having a plurality of fastener openings  39  therein. 
   As shown in  FIG. 2 , the threshold assembly  10  includes an outside dam  26  and a nosing piece  28  on opposed sides of the channel  24 . Preferably, the sill frame  22  is extruded aluminum. A suitable substrate  29  underlies and supports the extruded sill frame  22 . Suitable substrates include wood and rot-resistant synthetic materials. The front edge of the sill frame has a height “a” as shown in  FIG. 2 . 
   A configuration for the detachable sealing fin  30  is shown in  FIGS. 3 and 4 . The sealing fin  30  is specifically configured to correspond to the shape and mating features of the matching sill frame  22 . The sealing fin  30  includes a substantially planar upper panel  34  and a substantially planar lower panel  32 . A substantially planar horizontal web  36  connects a lower edge of the upper panel  34  to an upper edge of the lower panel  32 . The sealing fin  30  may include a flexible hinge portion  35  at the intersection between the web  36  and the lower panel  32 . In a preferred embodiment, the flexible hinge portion  35  may be integral, i.e., formed with by co-extrusion or otherwise, the sealing fin  30 . The hinge portion  35  permits the lower panel  32  to pivot between a first position (as shown in  FIG. 3 ) and a second position (as shown in  FIGS. 4 and 5 ). The lower panel of course may pivot to any desired position, and is not limited to the first or second position shown. Preferably, the lower panel  32  is in the first position in a relaxed state. Accordingly, the lower panel  32  is tucked in its first position prior to installation of the sealing fin  30  in an entryway, and can be downwardly pivoted into the second position during installation. 
   As shown in  FIGS. 3 and 4 , the upper edge  31  of the upper panel  34  may be tapered. An upwardly curved tongue  38  inwardly extends from the rear of the upper panel  34 , and includes a downwardly depending tab or sealing lip  37 . A catch  33  upwardly extends from the web  36 . 
   Preferably, the sealing fin  30  is extruded in one piece from a non-metallic material. For example, the sealing fin  30  may be an extruded polymeric material such as polyvinyl chloride (PVC). In a preferred embodiment, the sealing fin  30  is extruded in a single piece, and comprises multiple co-extruded polymeric materials. For example, the upper panel  34 , tongue  38 , web  36 , catch  33 , and lower panel  32  may be a rigid PVC material, while the tab  37  and hinge portion  35  may be a co-extruded flexible PVC material. This combination of materials provides a durable sealing fin with select portions that are highly flexible. The sealing fin also may be formed of varying colors, to match surrounding environmental elements. 
   The threshold assembly  10  is shown in a typical installed configuration in  FIG. 5 . The threshold  10  is installed atop a sub-floor or sub-sill  80  in an entryway. A forward portion of the doorsill  20  overhangs a front face  82  of the sub-sill  80  by an overhang distance “L”. In a typical installation, the overhanging front portion of the doorsill  20  is either free-hanging or further supported by brick masonry or other structure (not shown). The sealing fin  30  may be connected to the front edge  25  of the sill frame  22  in a variety of ways. Preferably, the connection is achieved by an inwardly protruding tongue  38 . The tongue  38  extends inside an elongated recess  23  in the front edge  25  of the sill frame  22 . The flexible sealing lip  37  on the bottom of the tongue  38  is wedged in the recess  23  and the upwardly curled tongue  38  interlocks in the recess  23  to securely retain the upper panel  34  of the sealing fin against the front edge of the sill frame  22 . The upper edge  31  of the upper panel  34  covers the recess  23  in the sill frame  22 , thereby providing the front edge  25  of the sill frame  22  with a finished appearance. The tapered upper edge  31  is held in close contact with the front edge  25  of the sill frame  22  by the engaged tongue  38 , thereby making the upper edge  31  of the installed sealing fin  30  difficult to snag or crush. In a preferred connection, therefore, no tools or separate fasteners are required either to connect the sealing fin  30  to the sill frame  22 , or to remove the sealing fin from the sill frame  22 . 
   As shown in  FIG. 5 , the width of web  36  of the sealing fin  30  is substantially equal to the overhang distance “L”. This permits the lower panel  32  to be closely proximate to the front face  82  of the sub-sill  80  in the second or installed position. The catch  33  on the web  36  engages a mating notch in the bottom of the sill frame  22 . The catch  33  cooperates with the engaged tongue  38  to securely hold the upper panel  34  against the front edge  25  of the sill  20 . The hinge portion  35  nests in the corner formed by the juncture between the substrate  29  of the doorsill  20  and the sub-floor  80 . The lower panel is secured to the front face of the sub-floor  80  by a plurality of suitable fasteners  90  such as screws, nails, or staples. A plurality of spaced fastener openings  39  may be provided in the lower panel  32  for receiving the fasteners  90  (see  FIGS. 1 and 2 ). 
   As shown in  FIG. 5 , the sealing fin  30  provides a substantially watertight seal at the juncture between the sill  20  and the sub-floor/sub-sill  80 . The sealing fin  30  minimizes the amount of moisture that may seep past the sill  20  and reaching the wooden sub-floor/sub-sill  80 . The upper panel  34  helps prevent water migration. The sealing lip  37  also helps substantially reduce or eliminate the water that may penetrate and eventually migrate to the sub-floor. 
   An alternative sealing fin configuration  60  is shown in  FIGS. 6-8 . Though the sealing fin  60  is similar to the sealing fin  30  described above, this second embodiment  60  is configured to engage the front edge  75  of a deeper inswing threshold assembly  40  like that shown in  FIG. 6 . The longer downward slope of the sill deck  51  causes the front edge  75  to have a height “b” that is shorter than the height “a” of the front edge  25  of the threshold  10  (see  FIG. 2 ). Accordingly, a different sealing fin  60  is required for the deeper threshold  40 . 
   As shown in  FIGS. 7 and 8 , the sealing fin  60  includes a substantially planar upper panel  64  and a substantially planar lower panel  62 . A substantially planar horizontal web  66  connects a lower edge of the upper panel  64  to an upper edge of the lower panel  62 . The sealing fin  60  may include a flexible hinge portion  65  at the intersection between the web  66  and the lower panel  62 . In a preferred embodiment, the flexible hinge portion  65  can be integral, i.e., formed with, the sealing fin  60 . The hinge portion  65  permits the lower panel  62  to pivot between multiple positions, including a first position (as shown in  FIG. 7 ) and a second position (as shown in  FIGS. 6 and 8 ). The lower panel of course may pivot to any desired position, and is not limited to the first or second position shown. Preferably, the lower panel  62  is folded in a compact arrangement prior to final installation of the sealing fin  60  in an entryway, and can be downwardly pivoted into the second position during installation. 
   As shown in  FIGS. 7 and 8 , the upper edge  61  of the upper panel  64  may be tapered. A tongue  68  inwardly protrudes from the rear of the upper panel  64 , and includes a downwardly depending tab or sealing lip  67 . Sealing lip  67  functions similarly to sealing lip  37 . A clip portion or hook  63  upwardly extends from the top surface of the web  66 . Preferably, the upper panel  64 , tongue  68 , web  66 , hook  63 , and lower panel  62  are made of rigid or semi-rigid polyvinyl chloride (PVC), while the tab  67  and hinge portion  65  are made of a co-extruded, flexible PVC. This combination of materials provides a rigid or semi-rigid sealing fin with select portions that are highly flexible. 
   As shown in  FIG. 6 , the sealing fin is connected to the sill frame  52  by engaging the hook  63  onto a flange  59  on a lower portion of the sill frame  52 , and by inserting the tongue  68  in an elongated recess  51  in the front edge  55  of the frame  52 . The flexible tab  67  on the top of the tongue  67  (see  FIGS. 7 and 8 ) is wedged in the recess  51 , and the catch  69  on the top of the tongue  68  grabs a rib  57  protruding from the roof of the recess  51  (see  FIG. 6 ). When installed in an entryway, the sealing fin  60  is closely proximate to the bottom of the sill and the front face  82  of the sub-floor  80  as shown in  FIG. 5  for sealing fin  30 . 
   Another embodiment of a sealing fin  100  according to the invention is shown in  FIGS. 9-12 . The sealing fin  100  is similar to embodiments  30  and  60  described above, but the web portion  106  includes an extendable and contractable portion  110 . The extendable and contractable portion  110  permits the horizontal distance between an upper panel  104  and a lower panel  102  of the sealing fin  100  to be selectively lengthened or shortened to accommodate variations in the overhang distance of a mating door sill. The extendable and contractable portion  110  preferably is constructed of a flexible material such as flexible PVC. In the embodiment shown in  FIGS. 9-12 , the extendable and contractable portion  110  has a corrugated or fanfold configuration including a series of folds  111 . As shown in  FIGS. 10 and 11 , the sealing fin  100  can be used with a sill  50  that overhangs the outer face  82  of a sub-floor or sub-sill  80  by a long overhang distance “L 1 ”, and can also be used with a sill  50  that overhangs the outer face  82  of a sub-floor or sub-sill  80  by a short overhang distance “L 2 ”. In  FIG. 10 , the extendable and contractable portion  110  is stretched to permit the lower panel  102  to be flush with the outer face  82  of the sub-sill and/or sub-floor  80 . In  FIG. 11 , the extendable and contractable portion  110  is compressed to permit the lower panel  102  to be flush with the outer face  82  of the sub-sill and/or sub-floor  80 . Accordingly, the sealing fin  100  can be used with door sill installations that have variable sill overhang distances, while providing an effective moisture barrier between the sill  50  and sub-floor or sub-sill  80 ,  82 . The sealing fin  100  can also be used in a door sill installation having a overhang distance that varies from one end of the sill  50  to the other end. 
   As shown in  FIG. 12 , the sealing fin  100  preferably is constructed in a single piece. In a preferred construction, the upper panel  104 , tongue  108 , web  106 , hook  103 , and lower panel  102  are made of rigid or semi-rigid polyvinyl chloride (PVC), while the tab  107  and extendable and contractable portion  110  are made of a co-extruded, flexible PVC. The forward portion of the sealing fin  100  shown in  FIGS. 9-12  is configured like the forward portion of sealing fin  60  shown in  FIGS. 6-8  for use with a deep or extended sill  50  as shown in  FIGS. 10-12 . Alternatively, the forward portion of the sealing fin  100  can be configured like the forward portion of the sealing fin  30  shown in  FIGS. 1-5  for use with a shallow door sill  20  like that shown in  FIGS. 1 ,  2 , and  5 . 
   The invention has been described herein in terms of preferred embodiments and methodologies. It will be understood by those of ordinary skill in the art, however, that a wide variety of additions, deletions, and modifications might be made to the illustrated embodiments without departing from the spirit and scope of the invention as set forth in the claims.

Summary:
A threshold for installation atop a sub-sill of a building wall frame comprises a sill frame and a sealing fin including an upwardly extending upper panel substantially covering the entire forward edge of the sill frame. The sealing fin reduces moisture penetration and provides an attractive finished appearance.