We are building an earth-sheltered home concrete walls x3 buried with conventional front wall. R-49 insulation front wall and a standing-seal steel roof (for water collection) which is at ground level in back and most of way to front. This includes the normal soffit & facia needed for air circulation into the rafters etc so it does not rot. Question is with air circulating from the front wall through ceiling and out through soffit and spinners on top of roof do we need HVAC? We are going with a steel roof for 2 things: cost savings over concrete dome roof and water collection. In a well-insulated (i.e. earth) sheltered home including the roof no HVAC is needed, and will this work the same for us? R-49 insulation front wall and ceiling. Any comments welcome. We are doing a Rocket Mass heater at 1st opportunity, house situated SE (so we can see pond) with 4 foot overhang in front and double pane, non-low-e windows to allow heat in in winter.