Question: I am designing a log home and would like to achieve the most efficient fireplace design. I am considering a 48" Rumford masonry fireplace and have located the flu and smoke chamber suppliers on the internet. My design calls for the fireplace to be on the outside wall. In building the fireplace, would it be worth my effort to I incorporate a foil faced rigid insulation (2-4" thick) sandwiched, between the outer masonry surfaces of the fireplace and chimney? I understand that the masonry area would have be tied together with straps and that the finished fireplace would be substantially deeper. Also, if I install an outside air make that is ducted through the insulated crawl space ( in hopes that the make up air would absorb some heat along the 40' run), and terminating in a floor register near, or in front of the hearth, will this create any problems? What diameter metal duct and register would you recommend? I appreciate any input. Answer: I'm stumped by question #1, because I'm unsure of the installation process of what you are buying. Insulating the outside of an exposed masonry fireplace is helpful because masonry fireplaces will indeed transfer heat to the great outdoors. But sandwiching the insulation as you have proposed is an area I'm not familiar with. Perhaps Craig has further input on this, but I must pass on this. Question #2: Providing outside air to help decrease the amount of heated room air used for combustion will be beneficial. A six inch round duct, transition to a standard 4" x 12" register, or slightly longer should adequate. Will the air be warmed as it passes through the insulated crawl space? Perhaps to a small degree. It is a good thought, though. I highly recommend that you also consider glass doors for your fireplace as well. You'll the have a screen for protection while the fireplace is in operation thus receiving the radiant heat benefits of the Rumford design. But, you can also close the doors when you go to bed or are not using the fireplace, and will not lose heat up the flue. Good luck.