We've inheirated my grandmother-In-Laws house and I want to use propane gas logs in fireplace. There is about a 2" gap on both sides and top between the firebox and the bricks on the front. Cold air blows in every winter. I'm going to shoot Dap polyurethane fire block foam sealant in first. Then I plan on inserting firebricks within an inch of the edge of the firebox. Then finish the edge, bring it flush with the box with firebox mortor. Is there any suggestions or comments that anyone has? I want it sealed and insulated good to prevent cold air in an to prevent heat loss up the back side of the walls.