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The external wall of a house built in the 1970s is constructed using a single leaf 215 mm hollow concrete block - Leaving Cert Construction Studies - Question 5 - 2011
Question 5
The external wall of a house built in the 1970s is constructed using a single leaf 215 mm hollow concrete block. The wall is rendered externally and plasterboard is ... show full transcript
Worked Solution & Example Answer:The external wall of a house built in the 1970s is constructed using a single leaf 215 mm hollow concrete block - Leaving Cert Construction Studies - Question 5 - 2011
Step 1
Calculate the U-value of the external hollow block wall
Answer
To calculate the U-value, we first need to find the total resistance of the wall components:
- Resistance of External Surface (R₁): 0.048 m² °C/W
- Resistance of External Render (R₂): R_2 = rac{0.015}{2.170} = 0.00691 \ m^2 \ °C/W
- Resistance of Hollow Block (R₃): 0.210 m² °C/W
- Resistance of Air Space (R₄): 0.170 m² °C/W
- Resistance of Plasterboard (R₅): R_5 = rac{0.012}{0.160} = 0.075 m² °C/W
- Resistance of Internal Surface (R₆): 0.104 m² °C/W
Total Resistance (R_total):
The U-value is then calculated as: .
Therefore, the U-value of the external hollow block wall is approximately 1.63 W/m² °C.
Step 2
Calculate the thickness of expanded polystyrene insulation required
Answer
To determine the thickness of expanded polystyrene needed to achieve a U-value of 0.27 W/m² °C:
- Using the U-value formula:
- Calculate the required total resistance for the new U-value:
- Calculate the difference in resistance needed:
- Use the formula to find the thickness T of EPS insulation: Where, λ (thermal conductivity of EPS) = 0.037 W/m °C. Thus: Thus, the required thickness of expanded polystyrene insulation is approximately 113 mm.
Step 3
Discuss two disadvantages of hollow concrete block construction
Answer
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Thermal Bridging: Hollow concrete blocks can create thermal bridges where heat flows through the material at higher rates than through insulating materials, resulting in significant heat loss. This can lead to higher heating costs and an uncomfortable living environment.
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Moisture Penetration: The porous nature of concrete can allow moisture to penetrate the walls, leading to damp issues. This is particularly problematic in cold weather, as it can lead to the formation of mold and mildew, affecting the health of occupants. This dampness can also compromise the structural integrity of the building over time.
Recommendation for External Wall Type: A preferred external wall type could be a timber frame with external insulation. This construction method allows for better thermal performance and flexibility in design, while minimizing thermal bridging compared to standard block construction.