I've gleaned a lot of good info here on the subject issue. Now coming back for more specific advice to see if my sweep's counsel is based on thermodynamics or ... not. Situation: Insert (good quality Regency unit) installed w/ masonry chimney on exterior wall. Chimney isn't lined. No insulation installed between insert and fireplace; i.e. insert just backs up into fireplace. Insert was installed with about an 18" length of 6" stainless pipe that extends through the old fireplace's damper "bracket" into the chimney, then a bunch of insulation shoved up around the cracks/crevices. My sense is this is NOT good, not if I want heat to heat the house. I know what I've read here but here's my sweep's advice: No need for a liner - it'll be a lot of money for no heat gains. No need for a separate block off plate - shoving the insulation around the damper bracket serves the same purpose. Putting some insulation between the insert and fireplace brick may make some difference. What I want to know: I'm obviously skeptical of the sweep's advice. He's the only game in a three-county area so please don't tell me to find another guy ... I'm rural enough where there aren't choices. I think maybe he's just trying to save me money. I think I want a different answer though. I'm thinking the liner & block off plate could make a BIG difference in the insert's performance. The block off plate is something I can do but the insert looks to be comparatively expensive; is the consensus that the liner isn't as important as the block-off plate? My chimney is 25' high coming through a steep-pitched roof ... can liners be installed from below - from inside the house - or do I need to rent a cherry-picker and install from above? Thanks folks; appreciate all the advice that's shared here.