I was asked to comment on a pit-style solar greenhouse because I had one for many years.
I built it in 1979, and this Brace-style greenhouse had a solid, metal-coated north roof with R-40 and a reflective coating (heavy-duty shiny construction wrap) stapled to it’s inner surface.
The south facing wall was air-inflated double polyethylene but always greenhouse-grade with UV coatings.
This solar greenhouse was 75-feet long and 20-feet wide for 1500 square feet of growing area.
Where I differed from the basic greenhouse construction was that I had a large pit dug (approximately 10 feet deep) and a solid block wall built up to a block above ground level .
The solar greenhouse was constructed on top of this block wall.
Pit style solar greenhouse circa 1979 at Simple Gifts Farm, Athens, Ontario, Canada
The pit was filled to within 40-inches of the top of the block wall with baseball sized rock but only after drainage pipe had been laid on the bottom of the pit in an extensive grid. The theory was that we’d exhaust some of the extra greenhouse daytime heat into the ground pit where it would be stored in the rock and provide some night buffering.
Good News and Bad News
The greenhouse was quite energy efficient and used only 30% of the fuel of my other regular-greenhouses on a square foot basis.
This resulted in me using this greenhouse as a propagation house and it was the only greenhouse to run all winter.
It performed very well as a propagation house and stock plant house.
The solar heat storage was not a success. The costs of moving air (we needed very large fans to handle the pressure of moving air through this network of pipes and back out to the stone) and the size/noise of the fans simply defeated this. Plus we didn’t get much more of a heat sink effect with the fans running versus just allowing passive collection.
Heat for the growing plants was therefore provided by 2 small oil furnaces (one primarily for backup as we didn’t want failure at this critical time) and I was easily able to maintain an 80F differential between indoor and outdoor temperatures no matter the outside temperature.
The bad news from a growing point of view is that light levels inside this greenhouse are lower than light levels in conventional greenhouses.
This is fine for propagation but not acceptable for crops such as tomatoes that demand a higher light level.
Without those higher light levels, crop yields are not high enough to justify the extra costs of both construction and crop management in smaller spaces provided by these solar greenhouses.
You *can* grow tomatoes but simply not at commercially viable levels compared to larger conventional houses (or you need a very, very good price)
Ventilation was absolutely critical for this unit.
You can see in the picture above that I have 2 36-inch fans and one 18-inch fan (sitting on top of the big fans).
Even on the coldest day of the year (normally sunny during the day) we’d have to vent air at some point.
The small fan would push out enough air but not too-much at any given time. Too much and we’d have thermal shock on tender plants. Not enough, and they’d bake.
The big fans would pump out too much too fast in the winter but come March, they’d be unpacked (all were insulated during the winter to prevent too much heat loss) and put into service.
These two big fans were needed to cool the greenhouse down until the middle of June when all the plants were outside and then we’d shut it all down for the summer. You could fry an egg in there during a hot summer day.
You can also see a plastic tube running along the top of the greenhouse. This circulated air and heat within the greenhouse – the fan ran constantly to push air around, eliminate damp spots and provide good air circulation around the tender starter plants.
Home Greenhouse Use
I’d recommend this kind of pit-style greenhouse for home use and if I were needing a greenhouse, I’d build another in a heartbeat.