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Size sun-facing glazing against floor area and thermal mass

Too little sun-facing glass wastes free heat; too much overheats the house and loses heat at night. US guidance links glass area to floor area and to how much thermal mass can absorb the sun.

How

1. Heating-led climates: start with sun-facing glass of about 7 percent of floor area (suntempering, no extra mass) per the NREL builder guide and DOE fact sheet.
2. Only go above 7 percent if you add thermal mass the sun can reach. The NREL guide adds about 1 unit of glass per 5.5 units of sunlit mass floor, per 40 units of shaded mass floor, or per 8.3 units of mass wall in the same room.
3. Cap direct-gain glass at about 12 percent of floor area, and all passive solar glass at about 20 percent (NREL guide).
4. Give each daylit room glass of at least 5 percent of its floor area (DOE fact sheet).
5. Cut window area on the pole-facing, east and west walls in cold climates (DOE).
6. Cooling-led climates: use smaller, well-shaded windows with a large openable area for ventilation (Your Home).
7. Use the south-glass calculator below and record results per room.

Sources

1. Passive Solar Design Strategies: Guidelines for Home Builders. Passive Solar Industries Council and National Renewable Energy Laboratory (US DOE), accessed 2026-11
2. Passive Solar Design (Technology Fact Sheet DOE/GO102000-0790). US Department of Energy and NREL, accessed 2026-11
3. Consumer Guide to Passive Solar Home Design. US Department of Energy, Energy Saver, accessed 2026-11
4. Orientation. Your Home, Australian Government, accessed 2026-11

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