When constructing a greenhouse, one of the most important decisions is choosing the right type of glass. The glass is a crucial component that regulates temperature, humidity, and light transmission inside the greenhouse. With many options available, it can be challenging to determine which glass material will best meet the needs of your greenhouse plants and suit your climate conditions.
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How does greenhouse glass work?
Greenhouse glass serves several important functions:
- Traps heat from sunlight during the day to keep the greenhouse warm
- Retains warm air at night to prevent frost and cold damage
- Allows sunlight to enter to support plant growth
- Blocks some ultraviolet rays to prevent sun scorch
- Reduces heat loss through the glass surface
- Controls humidity levels inside the greenhouse
The optimal greenhouse glass maximizes heat retention, sunlight transmission, and durability. Key factors that impact these functions include thickness, tinting, coatings, and composition of the glass.
Main types of greenhouse glass
There are 5 main types of glass used in greenhouse construction:
Horticultural Glass
Horticultural glass is the traditional glass choice for greenhouses. It is transparent, allowing most light to pass through. Horticultural glass has these features:
- Thickness ranges from 2-6 mm
- Allows 85-95% light transmission
- Low iron content improves clarity
- Durable for impact resistance
- Typically in an annealed state for easy cutting during installation
Advantages of horticultural glass:
- Excellent light transmission for plant growth
- Good thermal insulation properties
- Long lifespan with proper installation
Disadvantages of horticultural glass:
- Higher rate of radiative heat loss at night
- Requires shade cloth or coatings in hot/sunny climates
- More fragile than polycarbonate and fiberglass options
Tempered Glass
Tempered glass is treated with a heating/cooling process to make it 4-5 times stronger than annealed glass. Key properties:
- Withstands higher wind, hail, and impact than horticultural glass
- Heat treating reduces light transmission to 80-85%
- Typically 4-6 mm thick
- Safer if broken – shatters into small fragments rather than sharp shards
Advantages of tempered glass:
- Very durable and resilient material
- Good insulation values for heat retention
- Safer breakage pattern than horticultural glass
Disadvantages of tempered glass:
- Higher cost than horticultural glass
- Lower light transmission than horticultural glass
- Difficult to cut during installation – requires specialized tools
Polycarbonate
Polycarbonate is a very strong plastic material used for greenhouse glazing. Key attributes include:
- Extreme impact resistance – does not crack or shatter
- Sheets are lightweight and easy to work with
- Blocks over 99% of UV radiation
- Excellent insulation value due to multiwall and double-wall options
- Light transmission around 80-85%
Advantages of polycarbonate:
- Very durable – resists hail, wind, and other impacts
- Great thermal insulation properties
- Lighter and easier to handle than glass options
- Blocks harmful UV rays
Disadvantages of polycarbonate:
- Can scratch easier than glass over time
- Higher rate of heat loss through the surface than glass
- Degrades over time, losing light transmission and strength
Acrylic
Acrylic glazing is another plastic option made from polymethyl methacrylate (PMMA). Key attributes:
- Transmittance of 85-92% of visible light
- Softer and more prone to scratching than polycarbonate
- Offers good strength but less impact resistance than polycarbonate
- Available in multiwall sheets for insulation
- Resists some UV radiation and weathering
Advantages of acrylic:
- Excellent light transmission
- Lightweight and easy to install
- Low cost alternative to glass
- Does not yellow over time like polycarbonate
Disadvantages of acrylic:
- Softer surface prone to scratches
- Loses strength over time
- Less impact resistant than polycarbonate
- Typically has a shorter lifespan than other glazing
Fiberglass
Fiberglass reinforced plastic (FRP) offers superior strength for greenhouse panels. Key properties include:
- Extremely high impact strength
- Light transmission around 85%
- Withstands caustic chemicals, saltwater, and moisture
- Rigid and won’t sag under heat
- Available in multiwall sheets for insulation
Advantages of fiberglass:
- Virtually unbreakable with very high impact resistance
- Blocks UV rays and resists yellowing
- Durable in hot and humid environments
- Long lifespan with minimal degradation
Disadvantages of fiberglass:
- Higher cost than plastic and glass panels
- Specialized tools needed for installation
- Brittle edges and prone to cracking if not installed properly
How to choose the best greenhouse glass
Consider the following factors when selecting the optimal greenhouse glazing material:
Climate conditions
The climate where your greenhouse is located will impact the ideal glass choice. Factors to consider include:
- Sunlight – Intense sun regions need glazing with UV resistance and/or shade cloths
- Temperature – Colder climates need materials with high insulation values to retain heat
- Weather – Areas with rain, hail, or snow should use durable impact-resistant glazing
- Humidity – Coastal tropical regions need materials that resist moisture corrosion and fogging
- Wind – Choose glazing able to withstand high wind speeds and debris
Light transmission needs
The amount of light your greenhouse plants require will determine the optimal light transmittance percentage of the glazing:
- Low light plants (ferns, orchids) need at least 80% transmittance
- Most flowering plants grow best with 85-90% transmittance
- High light vegetables (tomatoes, cucumbers) need 90%+ transmittance
Insulation and heat retention
Select glazing with high insulation values if heating bills and nighttime temperatures are a concern. Options include:
- Double-pane glass – two layers trap air for insulation
- Tempered glass – higher density glass retains more heat
- Multiwall polycarbonate or acrylic – multiple layers with air chambers provide insulation
Durability needs
Greenhouses in areas with potential vandalism, hail storms, heavy snow loads, or high winds require extra durable glazing resistant to breakage. Most durable options include:
- Tempered glass – 4-5 times stronger than standard glass
- Polycarbonate – extremely high impact resistance
- Fiberglass – very high impact strength
Budget
Greenhouse glass costs range widely depending on the material, thickness, coatings and quality:
| Glazing Material | Cost per Square Foot |
| Horticultural Glass | $2 – $4 |
| Tempered Glass | $4 – $8 |
| Polycarbonate | $1.50 – $3 |
| Acrylic | $2 – $5 |
| Fiberglass | $4 – $7 |
Horticultural and tempered glass have the highest upfront costs but can last over 25 years. Plastics like polycarbonate and acrylic are cheaper initially but need replacement every 4-10 years.
Conclusion
The optimal greenhouse glazing depends on many factors unique to your situation like climate, plants grown, budget, and desired features. For most growers, either horticultural glass or polycarbonate are good options providing high light transmission and good durability.
Horticultural glass is excellent for sunny regions where high light transmission is needed and impact resistance is less critical. Tempered glass is a top choice for colder climates where insulation and durability are essential.
Polycarbonate is ideal for greenhouses in extreme climates with its high impact strength, insulation, and UV resistance. It is also a very cost-effective option. Acrylic can be a lower-cost alternative but lacks the strength and longevity of polycarbonate.
Fiberglass panels provide maximum durability but have a very high initial cost. They are best suited for commercial-scale greenhouses in regions with extreme weather conditions.
With so many factors to balance like climate, budget, plant types, and glazing features – there is no single best greenhouse glass for every situation. Carefully weigh the pros and cons of each type to pick the right material that will maximize plant growth and stay within your budget constraints.