Traditionally, attics have been insulated at the flat ceiling with deliberate venting between the attic and outside. It is also increasingly common to place the HVAC system and ductwork in this attic space. This exacerbates the thermal and air leakage losses for the HVAC system. One solution is to seal and insulate the attic at the sloped roof surface, with no insulation between the home and attic. This makes the attic more like conditioned space, so that HVAC losses are minimized. However, this is offset by increased building envelope area and, sometimes, less insulation, leading to increased building loads. If the attic contains older, leaky duct systems, then the net savings could be significant. But what about a modern, insulated duct system, built to current code? Does a sealed and insulated attic add value to a system with R8 ducts and 5% leakage? Can we identify what makes this approach successful? When and where should it be used? How do we limit moisture risk? We will answer these questions through reviewing past literature, exploring data from current sealed attic monitoring studies, and through detailed simulations addressing these very questions in new California homes.