The Annapolis Net Zero Home is the culmination of technical, aesthetic, and functional retirement home dreams, as envisioned by the homeowner couple and their architect, Peter Twohy.

The couple’s husband, who is a professional engineer, sought net zero energy performance for the home from project inception.

Goals.  With the target of net zero well defined in the preliminary project goals, the next decision was whether to be grid-tied or off-grid, while maintaining the comfort and reliability necessary in a retirement home. With the home being a half mile to the nearest utility pole, the decision deserved further analysis. 

Solution.  The solution combines a high performance building enclosure, very high efficiency mechanical systems, grid-tied photovoltaics, and an emergency back-up generator. The investment for connection to the grid plus a backup generator turned out to be cost effective when weighed against the necessary battery systems and the larger photovoltaic system required for an off-grid solution.

Enclosure.  The super-insulated air tight building enclosure includes a 2x6 stud wall system filled with 2” of closed-cell spray foam + 3.5” fiberglass batt, then 1” of rigid insulation outboard of the sheathing that provides continuous insulation for a total of R-33. An R-51 roof assembly combines 5” closed-cell spray foam and 4.25” fiberglass in the 2x10 roof framing. R-10 is under the concrete floor slab with R-20 on the foundation walls, paired with building details to mitigate all thermal bridging. High performance U-0.15 triple pane windows/doors offer a SHGC-glass of 0.50 to capture the sun's energy. The south window are covered by a deep overhang blocks direct solar gain in the summer.

Systems.  A ground source heat pump provides space conditioning, distributing heat through radiant flooring and cooling through ductwork. A heat recovery ventilation system completes the package with highly efficient ventilation and heat recovery. Domestic hot water is supplied by the combination of a solar thermal hot water system coupled with an auxiliary electric hot water tank. On the roof a grid tied photovoltaic system turns the meter backwards to offset consumption in the home. 




- PaSsive House Consulting

- Mechanical Design


pEUI: 0kBtu/sf/yr
Better Than Code: 100%


2x6 Stud walls
Truss roof
Basement foundation
U Value: 0.15
Closed cell spray foam, fiberglass, XPS
R10 Slab
R20 Foundation
R33 Above Grade Walls
R51 Roof


Ground source heat pump (3.5 COP)
HRV (80% efficiency)
Hot Water
Solar Hot Water & Electric Backup Tank


2E Architects
Architecture & Photos
ZeroEnergy Design
Energy Consultant
Mechanical Designer