Thermal Conductivity Showdown: Red Brick vs. Concrete Block vs. AAC

Key Takeaways

AAC Blocks Offer Superior Insulation:
Autoclaved Aerated Concrete (AAC) has the lowest thermal conductivity, so it blocks more heat and keeps buildings cooler than red brick or concrete blocks.

Lower U-Value Means Lower Electricity Bills:
AAC’s U-value is much lower than traditional materials, directly leading to big savings on air-conditioning—about $300/year per 100m² of wall.

AAC Combines Strength, Fire Resistance, and Practical Savings:
AAC blocks are strong enough for most buildings, fire-rated for up to 4 hours, and reduce both construction and long-term energy costs.

Proper Installation and Moisture Protection are Essential:
Using thin-bed adhesive and water-resistant plaster ensures AAC walls deliver maximum insulation and stay durable, even in humid climates.

Introduction

In the construction of the building envelope, the choice of wall material is often dictated by structural integrity and cost. However, with rising energy costs and increasing ambient temperatures, the thermal performance of the wall is now a critical third pillar of decision-making.

This technical breakdown analyzes three common walling materials: Fired Clay (Red Brick), Concrete Masonry Units (CMU/Hollow Block), and Autoclaved Aerated Concrete (AAC). We will derive their U-values from first principles and calculate the theoretical electricity savings for air conditioning.

What are the Core Metrics of Heat Transfer in Buildings?

Thermal transmittance (U-value) is the rate of heat transfer through a structure, where a lower value indicates better insulation and higher energy efficiency.

To compare these materials fairly, we must understand the three governing metrics of heat transfer in building physics:

1. Thermal Conductivity (k or λ):
   • Definition: The rate at which heat passes through a specific material.
   • Unit: Watts per meter-Kelvin (W/m·K).
   • Rule: Lower is better for insulation. It means the material fights heat flow.
2. Thermal Resistance (R-value):
   • Definition: The measure of how well a specific thickness of a material resists heat flow.
   • Formula: R = d/k (where d is thickness in meters).
   • Unit: Square meter-Kelvin per Watt (/m·K/W).
   • Rule: Higher is better.
3. Thermal Transmittance (U-value):
   • Definition: The rate of heat transfer through a structure (including surface air films), divided by the difference in temperature across that structure.
   • Formula: U = 1 / ΣR = 1 / (Rsi + Rwall + Rse)
     • Rsi = Internal surface resistance (typically 0.13)
     • Rse = External surface resistance (typically 0.04)
4. Unit: Watts per square meter-Kelvin (W/m²·K).
5. Rule: Lower is better.

How Do Material Properties Compare Across a Standard 200mm Wall?

For an “apples-to-apples” comparison, we assume a standard wall thickness of 200mm (0.2m) for all contenders.

AAC Block features a closed-cell porous structure with a thermal conductivity (k) of 0.16 W/m·K, offering superior insulation compared to high-density red brick and concrete.

*Note: Solid concrete has a k of ~1.6+. Hollow blocks are effectively ~1.1 depending on aggregate.

The Calculation: Which Material Has the Best U-Value?

The Winner: Autoclaved Aerated Concrete (AAC) reduces thermal transmittance by approximately 70% compared to Red Brick and 75% compared to Concrete Blocks.

Let us calculate the U-value for a 200mm uninsulated single-skin wall for each material.

• Formula: R_total = Rsi + (d / k) + Rse
• Surface Resistance (Combined): 0.13 + 0.04 = 0.17 m²·K/W

A. Red Brick Wall

• R-value (material): 0.2m / 0.8 = 0.25 m²·K/W
• Total Resistance: 0.25 + 0.17 = 0.42
• U-value: 1 / 0.42
• Result: U_brick ≈ 2.38 W/m²·K

B. Concrete Block Wall

• R-value (material): 0.2m / 1.1 = 0.18 m²·K/W
• Total Resistance: 0.18 + 0.17 = 0.35
• U-value: 1 / 0.35
• Result: U_concrete ≈ 2.85 W/m²·K

C. AAC Wall

• R-value (material): 0.2m / 0.16 = 1.25 m²·K/W
• Total Resistance: 1.25 + 0.17 = 1.42
• U-value: 1 / 1.42
• Result: U_AAC ≈ 0.70 W/m²·K

Economic Impact: How Much Electricity Can You Save with AAC?

Switching from Red Brick to AAC can save approximately $302.40 per year in electricity costs for a 100m² wall area under standard cooling conditions.

To translate these abstract U-values into financial metrics, we simulate a typical scenario:

• Wall Area: 100m².
• Temp Diff (ΔT): 10°C
• Cooling: 10~hrs/day
• AC Efficiency (COP): 3.0
• Tariff: $0.15/kWh

Savings Summary: AAC vs. Red Brick

Conclusion

The data confirms that while red brick and concrete blocks remain industry standards for structural mass, they are poor thermal performers. AAC Block stands out as the superior choice for modern, energy-efficient building envelopes.

By achieving a U-value of ~0.70 W/m²·K, AAC effectively reduces heat transmittance by over 70% compared to traditional materials. For developers and homeowners, this translates directly into significant long-term operational savings, reducing cooling costs and enhancing indoor thermal comfort in an era of rising global temperatures.

Frequently Asked Questions (FAQ)