Why is some freezer glass single-layer and some double-layer hollow?

In-depth analysis of single-layer and double-layer hollow design of refrigerated glass: a comprehensive explanation from technical principles to application scenarios

As an important part of modern commercial and domestic refrigeration equipment, the design of glass doors may seem simple, but it contains a comprehensive consideration of multiple disciplines such as materials science, thermodynamics, engineering and economics. There are two typical structures of freezer glass doors on the market: single-layer and double-layer hollow, and this difference is by no means accidental, but an inevitable choice based on different usage scenarios, performance needs and cost control. This paper will systematically analyze the design logic of refrigerated glass from eight dimensions, including structural differences, thermodynamic principles, cost composition, application scenarios, condensation control, energy-saving benefits, manufacturing process and safety performance, in order to provide a comprehensive cognitive framework for industry practitioners, consumers and related researchers.

1. Structural differences: the performance base from the physical structure
The single-layer and double-layer hollow design of freezer glass presents an essential difference in structure, which directly determines its thermal performance and scope of application.

(1) Structural characteristics of single-layer glass
Single-layer freezer glass usually uses ordinary float glass or tempered glass with a thickness of 3-6mm, which has a single structure and is composed of only one layer of glass. The core advantages of this design are simple structure, easy processing, and can meet the visual needs of the foundation. In specific applications, single glazing can be subdivided into:


Ordinary float glass
: Without special treatment, it has high light transmittance (about 91%), but poor impact resistance and thermal stability, and is mainly used in low-end household freezers or scenes with low performance requirements.
Tempered single glazing
: Tempered by physical or chemical methods, the impact strength is 3-5 times that of ordinary glass, the thermal stability is improved, even if it is broken, it will become obtuse angle fragments, the safety is significantly improved, and it is commonly found in mid-range household freezers.
(2) Composite structure of double-layer insulating glass
The double-layer insulating glass is composed of two layers of glass, with a 6-12mm air layer in the middle, and the edges are sealed by sealing strips to form a closed thermal insulation cavity. Its typical structure includes:


Glass substrate
: Usually tempered glass, mostly 4-5mm thick, to ensure strength and safety; Some high-end products use Low-E glass, which reduces heat radiation by coating a low-emissivity film.
Air layer
: The thermal conductivity of air (0.023W/(m・K)) is much lower than that of glass (0.78W/(m・K)), which can effectively block heat conduction. To further improve performance, some products fill the air layer with argon gas (thermal conductivity 0.017W/(m・K)) or krypton gas to reduce the thermal movement of gas molecules.
Sealing system
: Composed of aluminum spacer strip and sealant, aluminum strip filled with desiccant (such as molecular sieve) to absorb moisture in the air layer and prevent condensation; The sealant is typically sealed with a double layer of butyl glue and silicone glue, ensuring airtightness.
(3) Structural comparison and performance rehearsal

The following table visualizes the basic differences between the two structures:

Contrast dimensions	Single glazing	Double insulating glass
Number of glass layers	1 floor	2 layers
Total thickness	3-6mm	12-24mm (with air layer)
Heat Conduction Path	Single glass conduction	Glass-air-glass composite conduction
Light transmittance	Approximately 91%	Approx. 85%-88% (affected by air layer)
Impact strength	Ordinary glass is lower and lifted after tempering	On the basis of tempered glass, the structure is more stable
Manufacturing costs	low	High (material + sealing process cost)

2. Thermodynamic principles: adversarial design of heat transfer mechanisms
The core function of freezer glass is to provide a visual window while maintaining a low temperature environment inside, and heat transfer control is key to achieving this function. Single-layer and double-layer hollow design are essentially different strategies for heat transfer, convection and radiation.

(1) Heat conduction: the contest of the intrinsic properties of materials
Heat conduction is one of the main ways heat is exchanged inside and outside the freezer. The heat conduction of single-layer glass is carried out only by a single glass medium, and its thermal conductivity coefficient (U-value) is about 5.8W/(㎡・K), which means that 5.8W of heat per hour is transferred per square meter of glass at a temperature difference of 1°C. The U-value of double-layer insulating glass can be reduced to 2.8-3.2W/(㎡・K) due to the presence of an air layer, and if it is filled with inert gas and Low-E glass is used, the U-value can be as low as 1.8W/(㎡・K).


Taking a freezer with a volume of 200L as an example, let's say the glass door area is 0.5㎡, the internal temperature is -18°C (freezer), the external ambient temperature is 25°C, and the temperature difference is 43°C:


The heat transmitted to the freezer per hour of single-layer glass is: 5.8×0.5×43=124.7W
The incoming heat per hour of double-layer insulating glass (U=2.8): 2.8×0.5×43=60.2W The heat transfer is reduced by more than 50%, which directly affects the cooling load and energy consumption level of the freezer.
(2) Thermal convection: the blocking effect of the air layer
In single-layer glass, due to the temperature difference between the two sides of the glass, the air near the surface of the glass forms natural convection, accelerating heat transfer. The air layer of double-layer insulating glass is confined to a small space (6-12mm), and the natural convection of air is significantly suppressed. When the thickness of the air layer is less than 10mm, the convective heat transfer effect is basically negligible, and the heat transfer is mainly carried out through the heat conduction of gas molecules, which is also the core thermodynamic principle of insulating glass design.


It is worth noting that the thickness of the air layer is not the better. When the air layer exceeds 12mm, new convection circulation may occur within the air layer, causing the heat transfer coefficient to rise. Therefore, the thickness of the air layer of double insulating glass for freezers is usually controlled between 6-12mm for optimal insulation.

(3) Thermal radiation: advanced application of Low-E technology
In addition to conduction and convection, heat radiation is also a non-negligible heat transfer method. The high transmittance of ordinary glass to far-infrared radiation will cause the low-temperature radiation inside the freezer to be lost to the outside world, and the external heat radiation will also enter the freezer. When double-layer insulating glass is used with Low-E glass, the metal or metal oxide film (only 0.1-0.5μm thick) coated on the glass surface can effectively reflect far-infrared radiation and reduce the radiative heat transfer coefficient by 30%-50%.


Compared with ordinary insulating glass (U=2.8), the radiative heat transfer of the former can be reduced by about 40% under the same temperature difference conditions, which is especially important in high-temperature environments (such as tropical regions) or in commercial scenarios where energy conservation is extremely high.

3. Cost analysis: consider the whole chain from materials to processes
Behind the single- and double-layer design of refrigerator glass, there is a rigorous cost-benefit analysis, which directly affects the technical choice of products with different positions.

(1) Material cost difference
The cost of the original glass sheet
Taking 5mm tempered glass as an example, the unit price is about 80 yuan/㎡, while the material cost of double-layer insulating glass (5+9A+5) is about 180 yuan/㎡, and the material cost increases by 125%.
Auxiliary material costs
: Double-layer insulating glass requires auxiliary materials such as aluminum spacers, sealants, and desiccants. Taking standard-sized (1000×600mm) insulating glass as an example, the cost of aluminum strips is about 15 yuan, sealant is about 8 yuan, and desiccant is about 2 yuan, totaling 25 yuan, while single-layer glass does not require these materials.
Inert gas cost
: The cost of insulating glass filled with argon gas will increase by about 30 yuan/㎡, and the cost of krypton gas will be higher, about 100 yuan/㎡, which is also a configuration used by high-end freezers.
(2) Processing process cost
The processing process of single-layer glass is simple: cutting→ edging→ (tempering) → cleaning → finished products, high single machine production capacity, low labor cost. The processing of double-layer insulating glass needs to go through:


Glass cutting edging→ tempering→ cleaning→ aluminum strip bending→ desiccant filling→ laminating sheets→ sealant coating→ (inflating) → curing, the process is more complex, requires insulating glass production lines, large equipment investment (a set of small and medium-sized production lines is about 50-1 million yuan), and the production efficiency is low, and the labor cost increases by 30%-50% under the same production capacity.
(3) Comparison of life cycle costs
Although the initial purchase cost of double-layer insulating glass is higher, its energy-saving benefits can offset the cost difference in long-term use. Take a commercial freezer (24 hours a day, 365 days a year) as an example:


Single-layer glass freezer: The average daily electricity consumption is about 8 kWh, and the electricity bill is calculated at 1 yuan/kWh, and the annual electricity bill is 2920 yuan
Double-layer insulating glass freezer: The average daily electricity consumption is about 5 kWh, and the annual electricity bill is 1825 yuan
Cost difference: Assuming that a double-decker freezer is 500 yuan more expensive than a single floor, saving 1095 yuan in electricity bills per year, the cost can be recovered in less than half a year

In the home scenario, due to the short use time (10-12 hours of daily operation), the energy saving benefits are relatively low, and it may take 1-2 years to recover the cost, which is also an important reason why household freezers use more single-layer glass.

4. Differentiation of application scenarios: the gap between commercial and household demand
The design and choice of freezer glass are highly dependent on its application scenarios, and the difference in demand between commercial and household fields has formed a clear technical division.

(1) Commercial freezers: strict requirements for performance priority
The performance requirements for refrigerator glass in commercial scenarios such as supermarkets, convenience stores, and restaurants are much higher than those for household scenarios, mainly reflected in:


Show needs
: Commercial freezers need to keep the glass clear and transparent for a long time so that customers can view the goods. Due to the small temperature difference between the inside and outside (air layer insulation), the surface temperature is closer to room temperature, which can effectively reduce the generation of condensation and avoid fogging of the glass. Studies have shown that under the condition of ambient humidity of 60% and the internal temperature of the freezer is 4°C, the surface temperature of the single-layer glass is about 10°C, which is lower than the dew point temperature (about 12°C), and obvious fog will occur. The double-walled insulating glass surface temperature can reach 18°C, which is higher than the dew point temperature, and remains clear.
Long run
: Commercial freezers usually operate continuously 24 hours a day, and the energy consumption expenditure is huge. The energy-saving advantages of double-layer insulating glass (30%-40% power saving compared to single-layer) translate into significant cost savings in long-term operation, and the freezer cluster of large supermarkets can save tens of thousands of yuan in electricity bills every year.
Durability requirements
: The commercial environment has a large flow of people, the refrigerator door is opened and closed frequently, and the impact resistance and structural stability of double-layer tempered insulating glass can better meet the needs and reduce maintenance and replacement costs.

Typical application scenarios include:


Supermarket open freezer (air curtain cabinet): Double insulating glass must be used, otherwise the condensation cannot be controlled, and the energy consumption is extremely high
Convenience store roundabout freezer: requires 360° display of goods, large glass area, double-layer design is standard
Kitchen freezer: Although there is no need to display, the freezing temperature is low (below -18°C), and the double glazing can reduce the loss of cold capacity and maintain temperature stability
(2) Household freezers: cost-effective and balanced choice
In home use scenarios, the frequency and duration of freezers are low, and consumers pay more attention to purchase costs and space utilization, so single-layer glass has become the mainstream:

Cost sensitive
1: The price of household freezers is usually 1000-3000 yuan, and single-layer glass can reduce the cost by 10%-15%, which is attractive to price-sensitive consumers.
Gentle use environment
2: The indoor temperature of the home is relatively stable (18-26°C), the internal temperature of the freezer is mostly refrigerated (2-8°C) or lightly frozen (-12°C), the temperature difference is smaller than that of commercial scenarios, and the heat transfer of single-layer glass is acceptable.
Space constraints
3: Household freezers are usually embedded in cabinets or placed in small spaces, and the thickness of double-layer insulating glass (12-24mm) will take up more space, while single-layer glass (3-6mm) is thinner and more adaptable.
Exceptions:


High-end home freezers: Some high-end brands (such as Sub-Zero) use double-walled hollow Low-E glass or even vacuum glass for ultimate performance
Special environment households: For example, in tropical homes or kitchens without air conditioning, double-glazed freezers may be chosen to reduce energy consumption and condensation
5. Condensation control: precise management of glass surface temperature
Condensation control is one of the core considerations in the design of freezer glass, as condensation not only affects visibility, but can also lead to frost on glass edges, rust on cabinets, and even electrical safety.

(1) The thermodynamic mechanism of condensation
When the surface temperature of the glass is lower than the dew point temperature of the ambient air, the water vapor in the air will condense into water droplets on the surface of the glass, which is the phenomenon of condensation. Dew point temperature is related to ambient temperature and humidity, such as:


At ambient temperature 25°C and humidity 60%, the dew point temperature is approximately 15.5°C
If the internal temperature of the freezer is 4°C, the surface temperature of the single layer of glass (thermal conductivity 5.8W/(㎡・K)) can be calculated by the formula:
\(T_s = T_{inside} + (T_{outside} - T_{inside}) \times \frac{R_{glass}}{R_{total}}\)
thereinto
\(R_{glass} = \frac{0.005}{0.78} = 0.0064 m²· K/W\)
(5mm glass thermal resistance),
\(R_{total} = R_{glass} + R_{air}\)
(Air thermal resistance is negligible), calculated\(T_s ≈ 7.8℃\), below the dew point temperature of 15.5°C, it will inevitably fog up.
Total thermal resistance of double-layer insulating glass (U=2.8).
\(R_{total} = \frac{1}{2.8} = 0.357 m²· K/W\)
, calculated surface temperature\(T_s ≈ 22.4℃\), much higher than the dew point temperature, will not fog up.
(2) Condensation control advantages of double-layer insulating glass
Double-walled insulating glass enables efficient condensation control through the following mechanisms:


Air layer insulation
: Greatly reduce the temperature difference between the inner and outer surfaces of the glass, so that the temperature of the outer surface is close to the ambient temperature, and avoid being lower than the dew point.
Edge heating technology
: Some high-end commercial freezers will install electric heating wires (power about 5-10W/m) on the edge of the insulating glass to further increase the edge temperature and prevent condensation in the corners.
Sealing system is moisture-proof
: The desiccant continuously absorbs moisture in the air layer, avoiding condensation inside and extending the service life of the glass.
(3) Condensation countermeasures for single-layer glass
In scenarios where single-layer glass must be used, the following compromises are often employed:


Surface coating
: Coating anti-fog film to reduce the surface tension of water, so that the condensate forms a thin water film instead of water droplets, keeping the line of sight clear, but the validity period is limited.
Ventilation design
: Set up a small fan near the freezer glass to speed up the air flow and increase the surface temperature, but it will increase energy consumption and noise.
Wipe regularly
: Manually wipe the condensate regularly, suitable for domestic or small commercial scenarios, but increase maintenance costs.
6. Energy saving benefits: look at the technical value from energy consumption data
In the context of the global energy crisis and carbon emission control, the energy-saving performance of freezers has become an important technical indicator, and the energy-saving advantages of double-layer insulating glass show great value in large-scale applications.

(1) Quantitative analysis of energy consumption comparison
Take a brand of 250L freezer as an example, under standard test conditions (ambient temperature 32°C, humidity 55%):


Single-layer glass freezer: 1.2 kWh of electricity per day and 438 kWh per year
Double-layer insulating glass freezer: daily electricity consumption of 0.8 kWh, annual electricity consumption of 292 kWh
The annual electricity saving is 146 kWh, which is calculated based on the commercial electricity price of 1.2 yuan/kWh, and the annual electricity bill is 175.2 yuan

For large commercial users, such as a supermarket with 50 freezers, the annual electricity bill can be saved: 175.2×50=8760 yuan, and the difference in freezer purchase costs can be recovered in 3 years (assuming that each freezer is 1000 yuan more expensive, 50 units are 50,000 yuan more expensive, 8760×3=26280 yuan, which has not been fully recovered, but the net income starts in the 4th year).

(2) The system effect behind energy conservation
The energy savings of double-walled insulating glass are not only reflected in the glass itself, but also lead to the overall optimization of the freezer system:


Compressor load reduction
: Reduced cooling loss means shorter compressor start-up frequency and run time, longer compressor life and lower maintenance costs.
Refrigerant dosage optimization
: The reduction of the cold load can appropriately reduce the amount of refrigerant charge, save material costs, and reduce the risk of leakage.
Thermal insulation design adjustment
: In the overall thermal insulation design of the freezer, the glass door is the thermal bridge part, and the double-layer insulating glass can reduce the requirements for the thickness of the insulation layer in other parts and save the cost of cabinet materials.
(3) The promotion role of policies and standards
Energy efficiency standards in various countries have a direct impact on the choice of refrigerator glass:


EU Energy Efficiency Label
: From A+ to G, double-wall insulating glass is necessary to achieve a high energy efficiency rating (A+++).
US ENERGY STAR certified
: The heat loss rate of freezers is required to be lower than a certain threshold, promoting the popularization of double glazing.
China Energy Efficiency Label
: The updated standards in 2021 have increased energy efficiency requirements, and it is expected that double-walled insulating glass will be commonly used in commercial freezers in the future.
7. Manufacturing process evolution: from simple cutting to precision sealing
The manufacturing process of freezer glass has evolved with technological demands, and the production of single and double glazing reflects different process complexities.

(1) Standardized production of single-layer glass
The production of single-layer freezer glass is highly automated, and the main processes include:


Glass cutting
: Adopts CNC cutting machine, with an accuracy of ±0.5mm, suitable for mass production.
Edging
: Chamfering by linear edging machine to prevent edge scratches and improve safety.
Tempering (optional)
: Heating the glass to 620-650°C, then cooling it quickly to form a compressive stress layer and increase the strength. The glass size shrinks by around 0.3% during tempering, requiring precise control.
Wash and dry
: Surface stains are removed by ultrasonic cleaning machine to ensure transparency.
(2) Precision manufacturing of double-layer insulating glass
The production of double-layer insulating glass requires more complex equipment and processes, and the core links include:


Aluminum spacer processing
: The aluminum strip is processed into a rectangular frame using a press brake, and the corners are spliced at 45° to ensure the regularity of the sealing cavity, and the air permeability holes are evenly distributed on the spacer strip for easy desiccant filling.
Desiccant filling
: Fill 3A or 4A molecular sieve inside the aluminum strip to absorb moisture and impurities in the air layer, the filling amount is calculated according to the volume of the air layer, usually 5-8g per meter of spacer strip.
Laminated and sealed
: Superimpose two layers of glass with aluminum spacers, first apply butyl glue (hot melt sealant, thickness 0.3-0.5mm), and then seal the outer layer with silicone glue to form a double-channel sealing system. The curing time of sealants typically takes 24-48 hours, and temperature and humidity have a significant impact on the curing effect.
Inert gas filling (optional)
: Argon or krypton gas is injected into the air layer through the inflatable equipment, and the inflation pressure is slightly higher than the atmospheric pressure to prevent air infiltration in subsequent use, and the air tightness test is required after inflation.
(3) Process difficulties and quality control
The manufacturing difficulties of double-layer insulating glass are:

The seal fails
: Uneven sealant coating or incomplete curing will cause air leakage in the air layer and reduced thermal insulation performance, so it is necessary to test the sealant thickness and continuity online.
Desiccant failure
: If the desiccant filling is insufficient or the adsorption is saturated, condensation will appear in the air layer, which will affect the service life, usually verified by the dew point test (no condensation below -40°C).
The glass stress is uneven
: The difference in tempering stress between the two layers of glass may cause the insulating glass to warp and affect the sealing, and the tempering process parameters need to be strictly controlled.
8. Safety and durability: hidden requirements beyond function
In addition to performance and cost considerations, the design of refrigerator glass also needs to meet safety specifications and durability requirements, which is especially important in public use scenarios.

(1) Impact resistance and explosion-proof performance
Safety benefits of tempered glass
: Whether it is single-layer or double-layer insulating glass, freezers usually use tempered glass, which has an impact strength (≥50MPa) much higher than ordinary glass (≤20MPa), and is broken into honeycomb-like obtuse angled fragments, which meets the safety requirements of GB 15763.2 "Safety glass for construction - Part 2: Tempered glass".
The enhancement effect of the double-layer structure
: The two layers of tempered glass are bonded by sealant to form a composite structure, and the impact resistance is improved by 20%-30% compared with the single-layer tempered glass, which can effectively resist accidental impacts (such as shopping cart collisions).
(2) Weather resistance and service life
Aging resistance of the sealing system
Sealants for double-walled insulating glass need to withstand temperature changes (-40°C to 70°C) and UV exposure, with silicone adhesives typically lasting up to 10-15 years and butyl adhesives longer (more than 20 years) to ensure long-term airtightness.
Resistant to frost and corrosion
: If the single-layer glass is frequently frosted, the melted water may seep into the edge of the cabinet, causing rust on metal parts; Double-walled insulating glass can reduce such problems and extend the overall life of the freezer due to good condensation control.
(3) Safety design for special scenarios
Explosion-proof freezer
: In freezers used in flammable and explosive environments (such as chemical warehouses), the glass needs to pass explosion-proof certification, and the double-layer insulating glass can be filled with inert gas and add an explosion-proof film to prevent glass fragments from splashing during internal explosions.
UV protection design
: Some foods (such as medicines, cosmetics) are sensitive to ultraviolet rays, and the freezer glass can be made of anti-ultraviolet tempered glass, with a transmittance of ≤1% to protect the quality of the contents.
9. Future trends: technological innovation and application expansion
With the improvement of material technology and energy-saving requirements, the design of refrigerator glass is developing in the direction of higher performance and lower cost, and the following trends are worth paying attention to:

(1) Commercial exploration of vacuum glass
Vacuum glass draws the air layer below 10^-3Pa, almost completely eliminates gas conduction and convection, and the thermal conductivity coefficient can be as low as 0.5W/(㎡・K) or less, making it the most ideal thermal insulation glass at present. However, due to the complex manufacturing process (support is required to prevent glass from being compressed and deformed) and high cost, it is currently mainly used in high-end buildings, and if the cost decreases in the future, it may replace double-layer insulating glass in high-end freezers.

(2) Research and development of intelligent temperature-controlled glass
Electrochromic glass automatically adjusts the thermal insulation performance according to the ambient temperature by applying voltage to change the light transmittance and thermal radiation transmittance. For example, it improves thermal insulation during high temperatures in summer and reduces thermal insulation during low temperatures in winter, achieving dynamic energy savings. This technology is currently in the laboratory stage and may be applied in the future to mobile freezers that need to adapt to different climates, such as refrigerated trucks.

(3) Application of environmental protection materials
Lead-free tempered glass
: Respond to the EU RoHS directive to reduce the use of harmful substances such as lead and meet environmental protection requirements.
Water-based sealant
: Replace solvent-based sealants to reduce VOC emissions and improve the production environment.
Recyclable glass
: Improve the recycling rate of cullet and reduce raw material consumption.
(4) Integrated design trend
The integrated design of freezer glass and cabinet body is becoming more and more important, such as:


Borderless glass door
: Through the hidden sealing design, the thermal bridge effect of the metal frame is eliminated, and the thermal insulation performance is further improved.
Glass and display integration
: Embedded LCD display on the glass surface to realize the integration of product information display and temperature display to improve user experience.
Conclusion: The essence of technology selection is demand matching
The difference between single-layer and double-layer hollow design of refrigerated glass is essentially a different balance of the "performance-cost-scenario" triangle. Single-layer glass meets the basic needs of household and low-end commercial scenarios with simple and economical characteristics, while double-layer insulating glass achieves efficient thermal insulation, condensation control and energy-saving benefits in commercial scenarios through complex structural design and higher cost investment. With the improvement of energy efficiency standards and the decline of manufacturing costs, double-layer insulating glass is penetrating from the high-end market to the mid-market, and the emergence of new technologies (such as vacuum glass, smart glass) will further reshape the technical landscape of refrigerated glass. For consumers and businesses, understanding the logic behind these designs can help them make choices that better meet their needs in procurement and applications, while for the industry, continuous technological innovation will promote the development of freezer products in a more energy-saving, smarter and more environmentally friendly direction.