Homestead · under plastic
The greenhouse
A walipini: a sunken orchard under polycarbonate, 30 × 60, its floor 8 ft down, east of the house and joined to it by a walkway above ground and a tunnel below.
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The orchard floor fills the pit. Along the north wall at grade, a 12 × 30 dining gallery looks down into the canopy, the walkway from the house arriving at its west end and a straight stair going down to the trees. A 4-ft walk carries on at grade to the bath house, with a changing room, a shower and a sauna, and the hot tub on an open deck south of it. The tunnel from the house comes out on the orchard floor at the north-west corner.
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A 10-ft glazed south wall, roof glazing at 30° to a ridge at +19'-3", and an insulated north roof down to a 12-ft north wall bermed with earth outside. In midwinter the south part of the pit is in shadow, which is the walipini's trade; the low sun lands on the north retaining wall, which carries espaliers. Dwarf apple, pear, sour cherry and peach grow in the middle beds, figs and citrus by the warm north wall.
How it holds heat
- Night curtains drawn at dusk along the 10-ft line and inside the south wall cut the design load from ~89,000 to ~57,000 Btu/h at −25 °F, holding 40 °F at night.
- A climate battery: a fan pushes the day's warm air from the peak through pipes under the beds, and the soil gives it back at night and keeps the roots warm.
- A fan coil under the ridge sends the afternoon's surplus by water into the greenhouse's own warm store, about 3,000 gallons in an insulated concrete tank under the dining gallery, charged to about 80 °F, and draws it back through the night. The hub's storage tank runs too hot to take much of it, and carries the coldest nights instead. Water, not air: greenhouse air is too humid for a vapour-open house.
- A glycol loop in the beds, and staged venting when it overheats: ridge and low vents, then two exhaust fans high in the insulated north roof that draw the air up from the low south vents across the whole orchard, then a shade screen on the curtain's track. Foggers along the ridge for humidity and summer cooling.
The glazing is plastic: cheaper than glass, tougher in hail, and one panel comes out when its cap comes off. 16 mm clear triple-wall polycarbonate, made in Wisconsin, one sheet from the eave to the ridge, floating in aluminium base-and-cap bars on northern white cedar purlins over bur oak posts and beams. It passes about 74 % of the light, diffused rather than clear, and is budgeted for a new skin at about 15 years. The purlins wait on the maker's span at the dwelling code's 40 psf, and the insulated north roof is R-24, the code's floor for a sunroom.
The frame, Homestead Greenhouse: 1,241 timbers of bur oak and northern white cedar, no pine, 21,054 board feet and 371 oak pegs, with the bath house and the tub deck. Six king-post bents with 29-ft ties carry the glazing bars at 30°.