there is a couple of differnt sources that could be letting in cold
air. If this is a direct vent fireplace that is mostly sealed off from
access and "breaths" through tubes or pipes that go through your
outside wall, then it is likely bad or no insulation around the gas
lines or venting pipes. This can be remedied easily once you find the
If this fireplace has a regular stack chimney and the hearth is
accessable to you, it could be that the fireplace damper is clipped
open or disabled in some way. Gas log fireplaces in many counties have
to have disabled or clipped open dampers due to fire code. The reason
for this is the county doesnt want you lighting a gas fireplace
without opening the damper, since this can start a fire or asphexiate
you and everyone in the home. Another possibility in this case could
just be a worn out damper.
If you find that the cold air source is comming from a clipped open,
disabled, or old and crusty damper and inexpensive insulation option
is a chimney balloon. They can be bought on Northline Express or
through the manufacturer at http://www.chimneyballoon.us . They are a
inflatable plug that stops air passage. They can be used with a gas
log since if you light a fire under them they melt and release (unlike
a metal damper).
Good luck finding the cold air entry point, i know from experience
that cold drafts like that can be uncomfortable.