LEARN ABOUT :
Glazings properties 
Water as thermal mass 
Greenhouse conservation
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© copyright 2000, 2006 Reproduction or printing of material from this web site is prohibited. This material is excerpted from the book Greenhouse Gardener's Companion by Shane Smith. 

Materials used in building a greenhouse have many differing characteristics. This section might come in handy when you are working with, retrofitting or constructing greenhouses. Especially if you are interested in creating a solar heated or energy conserving greenhouse. This section includes properties of glazings, thermal mass and suggested gallons of water (thermal-mass) to assist with solar heating of greenhouses and more . . . read on!

Glazing and insulation R values for energy conservation.

Many of the materials you’ll use in a greenhouse have different, noteworthy characteristics. This section might come in handy when you are working with retrofitting, or constructing greenhouses.

 R values for energy conservation.

“R-value” is a commonly used term to measure a material’s energy conserving (or insulating) properties. It is a measure of resistance that a material has to the flow of heat. The below R-values reflect real world outdoor situations. Specific brand-name products may vary from these figures. It is always good to check on the stated R-value of a product as you evaluate a glazing. Also, consider other important attributes when evaluating glazings or insulation materials (see discussion in chapter 1).
The more energy conserving a material is, the higher its R-value.

Note: The term R-Value is a common measure of insulation (hr°Fsq.ft/BTU).

Note: You may also find products sold with a U-value instead of an R-value. A U-value is the inverse of an R-value. To convert a U-value to an R-value do the following: 1 divided by the U-value equals the R-value.  For example:   fiberglass has a U-value of 1.20 when it is divided into 1 it equals a .83 R-value.

Remember a greenhouse glazing is only as good as its warranty!

Consider when thinking about new greenhouse glazings:
- Life span
- Hail & rock resistance
- Cost (over the life of the glazing)
- Ease of installation
- Strength: snow load support
- Fire resistance
- Light transmittance
- Energy efficiency

For further description of glazings see the book: Greenhouse Gardener's Companion

Thermal storage values
Solar greenhouses have long used the principle of thermal mass to enable the structure to store the day's heat and then re-radiate that heat as the greenhouse cools. It creates a buffer that reduces the days high temperatures while raising the night-time low temperatures. 

Thermal mass is often simply drums filled with water. These drums are usually painted black and placed in the back of the greenhouse (usually the north side). Water helps store heat and has a built-in frost prevention system in the chemistry theory called "the latent heat of water"- where water actually gives off heat when ice is formed. Use it all to your advantage in a home greenhouse!

The chart below illustrates why water is a preferred storage medium for retaining the heat in greenhouses. This is especially common in solar greenhouses. The higher the number, the better the material will store heat. Heat storage is also a function of how much of the material you have in the greenhouse.

Material      Value BTU/Sq. Ft./degree F.

Water or other thermal mass alone doesn't make a greenhouse solar heated, there are other important considerations.

See the book Greenhouse Gardener's  Companion for further description of using the properties of thermal mass and other important features to make your greenhouse more energy conserving.

 

Suggested Gallons of Water (thermal-mass)
to Assist with Solar Heating of Greenhouses
Note: This assumes that you have already provided common greenhouse energy conservation measures to maximize the use of the incoming solar heat such as: 

• Insulated the perimeter of the exterior foundation (placed vertically)

• Weather-stripped and/or caulked vents, doors and cracks. 

• Double or triple south facing glazing.

• Insulated north wall and roof.

• Possibly have an insulated west and east wall (depending upon your climate and wind patterns).

HERE ARE THE VALUES. . . .
Attached greenhouse:
2.5 gallons per sq. ft. of south facing glazing area for cool climates (4 month winters)
2    gallons per sq. ft. of south facing glazing area for temperate climates (3 month winters)
1    gallon per sq. ft. of south facing glazing area for warmer climates (2 month winters)

Free standing greenhouse:
3    gallons per sq. ft. of south facing glazing area for cool climates (4 month winters)
2.5 gallons per sq. ft. of south facing glazing  for temperate climates (3 month winters)
2    gallon per sq. ft. of south facing glazing for warmer climates (2 month winters)

Note: what do I mean by "winters"? I mean most nights are well below freezing (24 degrees F or -5.5 degrees C) for each of these months

See the book Greenhouse Gardener's Companion, for further description of using water as a method of capturing solar energy and making your greenhouse more energy conserving.

© copyright 2000, 2004 Reproduction or printing of material from this web site is prohibited. This material is excerpted from the book Greenhouse Gardener's Companion by Shane Smith.