The requirement for sealed doors in fireplaces has been eliminated in the 2012 codes, if you are under a prior code the suggestion is to print out this letter from a testing laboratory to the ICC together with the ICC response requesting the elimination of the sealing requirement.

Quote Originally Posted by Response letter from the ICC
Accordingly, and solely for "factory-built fireplaces," I recommend pursuing relief from the proviso for "gasketed doors" administratively vis-à-vis IECC Sections 101.3 (". this code is not intended to abridge safety . requirements contained in other applicable codes or ordinances.") and 102.1 (". provided that such construction [or] design . has been approved by the code official as meeting the intent of this code.").