PASSIVE DESIGN
Heating and cooling the building gets for free, from the sun it lets in and the sun it keeps out.
Passive design is the part of the mechanical system that has no moving parts. A south window admits winter sun and a roof overhang blocks the summer sun — same glass, opposite results, because the sun sits at a different height in the sky. It is geometry, decided on the drawings, and it costs nothing to run for the life of the building. Move the two sliders below and watch it work.
Noon sun · Dec 21
21.6°
Low enough to reach almost twenty feet into the room through a south window, straight onto the floor.
Noon sun · Jun 21
68.4°
High enough that a thirty-inch overhang stops all of it before it touches the glass.
At the equinox
45.0°
Plymouth sits at almost exactly 45° north, so the equinox sun is at 45°. The geometry here is unusually easy to draw.
Why this comes before the equipment
Passive design is not an alternative to a heat pump. It is what decides how small the heat pump can be.
NO PARTS, NO POWER, NO SERVICE
An overhang has no compressor, no filter, no refrigerant, no warranty and no failure mode. It works identically in year one and year forty, during an outage, and when nobody is home. Nothing else in the building can say that.
FREE ON THE DRAWINGS, EXPENSIVE LATER
Orientation, glass area and eave depth cost nothing while they're lines on a plan. After the foundation is poured, the south wall faces where it faces. This is the cheapest decision in the project and the shortest window to make it in.
SMALLER LOAD, SMALLER EVERYTHING
Shading the July sun cuts the cooling load, which shrinks the equipment, which shrinks the circuit, which shows up in the service size. Passive work is the only kind that makes the rest of the mechanical system cheaper instead of more complicated.
Frequently Asked Questions
Won't all that south glass cook us in July?
That's the question the overhang answers, and the drawing above is how we answer it in writing before anything gets built. Summer sun at this latitude comes in at 68 degrees, which a modest eave stops completely. Winter sun comes in at 22 and walks straight under it. The glass doesn't change; the sun does.
Checked at three dates for every project
Our lot doesn't face south.
Almost none do exactly. Within about thirty degrees either side of south you keep most of the benefit and the overhang still works. Past that we stop chasing solar gain and shift the effort to shading, glass placement and the ventilation path — which is a different design, not a failed one.
Orientation is a spectrum, not a pass/fail
Isn't this just a 1970s solar house?
The seventies version failed for a specific reason: enormous glass areas and trombe walls bolted onto buildings that leaked. The aperture had to be huge because the envelope threw the heat away overnight. Build the envelope properly first and the glass area needed drops to something that looks like an ordinary window.
The envelope does the work; the glass is the aperture
Moving air without parts
Sun control is the half you can draw. Cooling a Minnesota summer night is the half that depends on how the openings are arranged.
CROSS VENTILATION
-
Operable openings on opposite walls, not adjacent ones
-
Inlet smaller than outlet, so the air moves faster where people are
-
A clear path between them — the plan decides this, not the windows
WHERE IT STOPS
-
Humid stretches when night air brings moisture in, not relief
-
Wildfire smoke days, now a normal part of a Minnesota summer
-
A rental, where nobody is home to open and close anything
-
Which is why the heat pump is still in the package
STACK EFFECT
-
Low inlet and high outlet, so warm air leaves at the top
-
Works with no wind at all, which is when you need it most
-
A single high operable unit is usually the whole intervention
NIGHT FLUSH
-
Open at night, closed and shaded by mid-morning
-
Minnesota summer nights usually drop far enough to make this work
-
Exposed slab or tile holds the coolness into the afternoon