I have a brand new direct venting Heat n Glo fireplace(model SL-550) that is installed on an exterior "bump out". Since this is a direct vent it brings in unfiltered undampered air directly into the sealed chamber. The problem is this cold air is convecting itself through the metal housing and causing the surrounding area to be very cold when the temperature outside drops. I have already sealed all of the knockouts for gas and electrical to reduce this effect and had expanding insulation installed in the bump out which has helped but not resolved the issue.
I have never heard of that where I am and we are cold or colder than you. There are minimum clearances to that unit and hopefully your blown insulation has not decreased them and caused a fire hazard. If you have the model with the circulator fan then try run it to get rid of the effect. Most of those units can have that fan as an add on but need 120 volts to it. It can be ordered/added on later. Some have variable speed controls so you can slow it down. Try run the unit longer in cold weather otherwise you are going to get condensation in it and it will rust out and that can cause CO problems. You can adjust the flame size with the gas valve on some models and keep it low.
"Cut it twice and it is still too short".
It's a zero clearance fireplace which rests on a sheet of OSB(spray in insulation is beneath this).
Running the appliance with or without a fan, or keeping the pilot on bandaids but does not fix the issue.
I've been comparing my installation to others without the issue and the only thing I've really noticed is that I have a very short run to the outside therefore I get a larger volume of cold air pumping in....