Am installing a gas/direct vent fireplace in my LR (1st floor over full poured concrete basement) and need to build a 12" bumpout so only half the fireplace projects into the room. How should this be framed, insulated and enclosed....exterior is vinyl siding? Really want to avoid dealing with concrete footing.......since bumpout will be 12-24" above ground and about 12" deep, think that should be OK. Checked this site and Fine HomeBuilding mags I have for How-To-Do articles but came up empty. Please help asap. Thanks.