Fix direct vent FP draft from outside?
I have searched through the forums, but I haven't really seen how to fix this problem after the fact. The builder who built my house went out of business and cannot help me. I have a Heatilator ND 4842IL Propane direct vent fireplace installed in a bump out/chase. I have always had a dramatic draft coming out the bottom of the fireplace when not in use. It was too the point I ended up just covering the whole fireplace over from the inside with plastic and not using it. You could see the plastic suck in and out from the draft on really cold and windy days.
Anyway, I am putting on an addition on the house and had the new different general contractor take a look at it and try to see if it was insulated. He took some siding off the outside and opened up a small whole near the roof of the bump out. Reaching and looking inside, it seems there is no insualation. He said it does not appear to be sheet rocked either from what he can tell. He is not really familiar with these types of fireplaces, but is willing to help me out and pull off the siding and plywood back of the fireplace chase from the outside to add some insulation and whatever else is needed. He thought some "foam insulation board" installed might work better than traditional insulation between the studs to prevent heat loss and air infiltration. He said it doesn't have as high an R value, but it would stop air infiltration better than fiberglass batts. He said air infiltrations seems to be the main issue. Air is entering through the soffit on the roof and just drooping down into the chase. He said he does not see how he could do any sheet rock from the back end on the outside. The only way he could do that is removing the fireplace from the front which would be a bigger deal since it is already tiled/mantled and finished around it on the inside.
What is the best way to proceed with correcting this install from the outside if possible? Any advice from you experts I can give the contractor who is doing me a huge favor here?