Passive Solar Greenhouses, Solariums, & Pineapple Pits

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As an addendum to our earlier article on passive solar house design basics, I thought that it would be worth providing an overview of some of the possibilities when it comes to the design of attached passive solar greenhouses and/or solariums.

Such additions to a passive solar home can be quite effective when it comes to improving solar gain during the day in cold-weather climates and/or seasons, and also when it comes to improving daylighting. The addition of a greenhouse or solarium, of course, brings with it other benefits as well — which aren’t much available any other way (to be discussed below).

To start things off here, I’ll provide a differentiation of the terms “greenhouse” and “solarium” — and also “sunroom” as well.

A greenhouse in this context is a building featuring large-amounts of transparent glazing of some kind and utilizing passive solar building design principles, with the intent being to allow for the out-of-season growth and/or survival of vegetative life. An attached greenhouseis the same, but shares one or more walls with the main building or home.

A solarium then, in the same context, is usually a portion of a building that utilizes the same passive solar design principles with the aim of maximizing solar gain during the cooler times of the year (e.g. the winter). Such building additions often utilize open roofs (glazed, that is, rather than covered in opaque material) and large amounts of thermal mass (such as stone floors to absorb the Sun’s heat during the day and discharge it into the air at night). Solariums are much more common in high-latitude regions than elsewhere — with the aim being to provide residents with an insulated room where they can enjoy the sunlight even in the depths of winter. Often, window quilts are utilized to reduce thermal losses when the sun isn’t out in conjunction with solariums.

A sunroom by contrast is usually simply a room oriented so as to receive high levels of light at the desired times of year, and featuring a large array of windows. If paired with high levels of thermal mass, such rooms can be useful in passive solar homes, but they are fundamentally different than solariums (also sometimes known as conservatories), and also from greenhouses.

With that all in mind, I’m going to provide some more specialized information in the sections below.

Passive Attached Greenhouse Design Ideas & Elements

A passive greenhouse or sunroom, if it is meant to be used to help increase solar gain during the winter months and to benefit from shared walls, should include a few features. The basic elements it should include are: a large amount of insulated glazing; effective insulation in the non-glazed parts of the external structure; large amounts of thermal mass to absorb solar radiation; a sealable pathway/doorway between the greenhouse and the main building; and good weatherization.

Optional elements include: the use of a body of water as thermal mass and to provide moisture; radiant heating; ground-coupled heat exchangers; composting systems (which generate heat); and removable shades and/or night-insulation for the glazed surfaces; amongst other things.

Venting systems are of course a necessity as well, but these can be as simple as windows that can be opened or as involved as a ground-coupled heat exchanger (a heat-recovery ventilation system).

The goal with regard to a passive greenhouse attached to a passive solar home is to absorb large amounts of solar radiation in thermal mass in the adjoining building. Even when the pathway between this greenhouse and the main house is kept closed at all times, such a passive greenhouse will still work to reduce the thermal losses of the building — due to the greenhouse acting as a buffer, and also (ideally) thermal mass on the connecting wall itself. When the pathway is kept open, the greenhouse can also function as a means of increasing the solar gain of the structure during winter months.

To elaborate further here on the means of meeting those aims, an attached passive greenhouse does well to incorporate large amounts of dark stonework. Raised beds created from such materials is a particularly effective means of doing so, as the soil in these structures is thus kept warm by the heat stored in the retaining walls (e.g., one of the many techniques utilized by the Inca to grow crops in places they otherwise wouldn’t survive).

Alternately, an artificial pond or fountain can be effective as well — or a combination of the two (a raised fountain created from dark stone, or painted to be dark). A cheaper option is simply to utilize a large number of large buckets filled with water, black buckets.

While the heat gain achieved by such design elements can be highly welcome at some times of year (winter, for instance), at others, it can be a hindrance — which is where the need for cross-drafts via windows or ground-coupled heat exchangers (e.g. cool tubes) come in. Alternately, one could simply employ shade curtains or alternating shutters during the warmer months (in some climates, not all). Ventilation is highly helpful for other reasons as well, though, which is what makes the potential use of ground-coupled heat exchangers as attractive as it is (the potential being there for fresh-air year-round without much heat loss). The use of a solar chimney venting system is another option during the warm months as well.

Image by Crsmithscotland • some rights reserved

It should also be noted here that in this sort of application the use of double-glazed or triple-glazed windows is highly preferred. While the costs of such windows is much higher, in the context of a passive greenhouse sharing walls with a passive solar home they are pretty much a necessity in my opinion.

A final note: There are systems and designs available whereby large amounts of thermal energy can be built up during the warmer/sunnier months and then released slowly through the winter. Such designs/tech could perhaps make good sense in the creation of the sort of passive attached greenhouse that we are discussing here, but I’m not aware of many implementations as such. Deep heat pumps are another possible option, depending upon location.

Using Compost Heat To Raise Interior Temperatures (e.g., Victorian Pineapple Pits)

During the Victorian period in the UK, there was a strong demand for exotic foods of all kinds, some of which were/are perishable and thus not easily shipped over long distances (without spoilage). Owing to this reality, gardeners in the UK with wealthy clients devised a number of interesting solutions to the growing of foods out of climate and/or season.

As an adjunct to the earlier discussion, it seems worth giving a full section to the topic of so-called pineapple pits, and to the use of composting systems as a means of heat generation in greenhouses as a whole.

Image via the Lost Gardens of Heligan

Owing to the long use of rotting domesticated animal manure as a means of generating low-level heat in various parts of the world (by pre-modern chemists, amongst others), and a ubiquitous supply of horse manure at the time, the most cost-effective solution devised for the growing of pineapples in the UK during the Victorian era was a simple compost-box style arrangement.

To put it simply, 3 trenches were dug a bit below grade, outfitted with accompanying boxes connected internally via 2 “cavity walls” (walls with empty internal-space and alternate holes at the tops and bottoms), and the outside trenches were then filled and kept full of horse manure. The middle chamber was of course then planted up with a pineapple head. Owing to the slow passage of warmed air generated by the composting process (in the adjoining chambers) through the interior cavity walls, the interior chamber was kept fairly warm.

Such an approach to pineapple cultivation was of course labor intensive, and required large amounts of horse manure, but labor was cheap in the UK during the period and horses were ubiquitous. And, for that matter anyway, pineapples were mostly just for the wealthy. Unsurprisingly, though, this method was eventually supplanted by large-scale importation from foreign territories (the scale allowing for large degrees of spoilage without that being a problem, aided by the lower “costs” of foreign labor).

I chose to highlight this old tech for the purpose of showing how effective composting systems can be at generating heat. When in a very well-insulated greenhouse, such a system can aid in maintaining high-temperatures year round (with the composting process itself benefiting from the high temperatures in the greenhouse) — effectively complementing large amounts of thermal mass and good insulation/weatherization. Something to keep in mind.

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James Ayre

James Ayre's background is predominantly in geopolitics and history, but he has an obsessive interest in pretty much everything. After an early life spent in the Imperial Free City of Dortmund, James followed the river Ruhr to Cofbuokheim, where he attended the University of Astnide. And where he also briefly considered entering the coal mining business. He currently writes for a living, on a broad variety of subjects, ranging from science, to politics, to military history, to renewable energy.

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