Cleaning of space around chimney...

Hi Everyone, I have a fireplace (metal) with a metal vent chimney. It has a space built around it that is sheetrock all around except the front which is brick. Critters have got into this space and made a mess of my insulation and I need to get into that space to put it back into place. I feel it could be a fire hazard to have a fire going with the insulation resting against the chimney pipe. Do building codes allow for an access door to be cut into the existing sheetrock or would that possibly violate some sort of fire code having a possible opening there if there happened to be a fire? Id like to put a door there that would remain closed except for service. Has anyone run up against something like this before? I live in the Ft. Worth, Texas area. Thanks. Brian
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