Small Greenhouse Thermal Losses:

All small greenhouses suffer from the geometric reality of a high surface area to volume ratio. To understand this a simple geometric sequence is useful. The sequence is analyzing ever larger cubes and the ratio of their exterior surface area to the enclosed volume. For example:

1-ft x 1-ft x 1-ft = 6-ft2 surface area; 1-ft3 volume = 6 ratio of surface area / volume

2-ft x 2-ft x 2-ft = 24-ft2 surface area; 8-ft3 volume = 3 ratio of surface area / volume

3-ft x 3-ft x 3-ft = 54-ft2 surface area; 27-ft3 volume = 2 ratio of surface area / volume

4-ft x 4-ft x 4-ft = 96-ft2 surface area; 64-ft3 volume = 1.5 ratio of surface area / volume

Further, regardless of greenhouse size, construction materials are typically the same. Hence the insulating value of an exterior square foot of a “standard” greenhouse of any size is equal. This leads to the unavoidable conclusion:

The smaller the greenhouse, the quicker it losses heat.The only way to combat this is through better insulation and reducing air leakage.

These are examples of a small 8-ft x 16-ft greenhouse size using various levels of construction to show the impact of improvements in insulation and air-leakage have on heating load on cold nights. The last column is a 15-ft diameter cold-climate geodesic dome greenhouse as another common option.The ACH(air changes per hour)factor covers the fact the greenhouses are not fully air-tight. High-tunnel plastic sheet covered greenhouses tend to leak more air (hence ACH 2). It is possible that the other two greenhouse models leak less than 1-ACH, but using this value is not unreasonable.

Final row: temperature inside greenhouse on 15F night using 1500W heater running continuously.

*see post on condensation and reduce R-value (2.3 for dome yields lower in-dome over night temperature of 44F).

The above table clearly shows why trying to keep a standard (fully-glazed) greenhouse heated in cold weather is impractical and electrically expensive (if electric resistance heat is used). The dome is much better for two reasons: 1) lowest surface area to volume ratio and 2) better glazing.

Coming Soon:

The TerraPoniK greenhouse DIY system plans. The feature covered in this article is the remarkable heat-retention of this 8-ft x 16-ft greenhouse.

TheTerraPoniK greenhouse attacks the high heat loss inherent in small greenhouses with selectively placed glazing balanced with opaque and highly-insulated surfaces. The greenhouse is available in two versions: Starter-tier and Pro-tier. The glazing on the Starter-tier is the same 8mm twin-wall polycarbonate used in the “Quality poly-glazed” example above. The Pro-tier adds a second layer of the same 8mm twin-wall glazing with an air space between for even better insulating properties. A Heat-Loss table for the TerraPoniK greenhouses is shown below.

Final row: temperature inside greenhouse on 15F night using 1500W heater running continuously.

From these results, it is apparent that the Pro-tier TerraPoniK is a viable cold-climate greenhouse that can be heated economically with just 1500W of heat.

Further, the TerraPoniK Pro-tier (full-spec model) adds a GAHT (ground-air heat-transfer) system. Many gardeners considering a greenhouse are aware of GAHT systems and dream of having one, but be forewarned, that using a GAHT system in a Quality poly-glazed greenhouse will not be rewarding. It will struggle to arrest the plummeting overnight temperature before it comes dangerously close to freezing (if not worse). For smaller greenhouse (< 200 sqft) it is imperative to have a highly-insulated and optimized greenhouse like the TerraPoniK greenhouse.

TerraPoniK - "Engineered for Growth"