Reducing Overheating Risk: Cavity vs Timber Frame

Understanding how construction methods impact thermal performance is critical for meeting TM59 overheating criteria and Part O of the Building Regulations. This technical study compares three construction approaches to help architects and clients make informed decisions about building fabric choices.

Why This Matters

With stricter overheating regulations under Part O and assessment requirements using TM59 methodology, choosing the right construction type can be the difference between compliance and costly retrofits. Our study reveals significant performance differences between cavity and timber frame construction.

The Study Parameters

Our analysis is based on a typical 100m² three-bedroom semi-detached house using the Birmingham DSY1 2020 High 50 weather file—a standard dataset for overheating assessments.

Key Terms & Definitions

TM59 – CIBSE guidance for assessing overheating risk in homes

Part O – Building Regulations addressing overheating mitigation

Option 1: Cavity Construction – 0.18 W/m²K walls with 150mm mineral wool cavity insulation and lightweight block (0.28 W/mK)

Option 2: Timber Frame Construction – 0.18 W/m²K timber frame walls

Option 3: Enhanced Cavity Construction – 0.15 W/m²K walls with 150mm PIR cavity insulation, improved roof to 0.08 W/m²K

Glazing ‘g’ value – Solar transmittance (0.40 = 40% solar energy passes through)

U Value – Heat loss through external elements (W/m²K)

Study 1: Cavity Construction vs Timber Frame

Key Finding: Thermal mass matters for overheating compliance.

Bedroom Performance:

• Timber frame showed 29% more overheating hours compared to cavity construction
• Cavity construction’s thermal mass helps regulate internal temperatures

Living Room Performance:

• Timber frame had 12% higher overheating levels
• Cavity construction demonstrated better temperature stability

Implication: For Part O compliance, cavity construction offers inherent advantages in managing overheating risk.

Study 2: Standard vs Enhanced Cavity Construction

Key Finding: Better insulation can reduce overheating when properly specified.

Bedroom Performance:

• Standard cavity (0.18 W/m²K walls, 0.10 W/m²K roof) showed 8% more overheating than enhanced specification
• Enhanced cavity (0.15 W/m²K walls, 0.08 W/m²K roof using PIR) performed better

Living Room Performance:

• Standard cavity had 2.5% higher overheating than enhanced specification

Implication: Investing in enhanced insulation can improve both energy efficiency and summer comfort.

Study 3: Solar Control Glazing

The Challenge: All living rooms initially failed TM59 Criterion A (3% threshold for hours above setpoint between May-September), achieving 4.03-4.31%.

The Solution:

• Reduced glazing ‘g’ value from 0.66 to 0.40 (solar control glazing) for only the Living Room.
• Result: All three construction options achieved compliance

Conclusion

The Cavity Wall construction performed better than Timber Frame as expected. The effect was less on Living spaces than on bedrooms. The higher thermal mass has assisted in reducing higher temperatures. But the surprise was that more insulation = less overheating. There is generally a notion that because we are building better insulated houses, the they are more likely to overheat. When in fact you are reducing heat flow in both directions. So when the su is beaming onto an exposed wall or the external temperature is high, then the ‘heat flow’ transferring into the dwelling is slowed down by the improved insulation. And just as a side study, where you have larger areas of glazing, then consider lower solar transmittance (‘g’ value) through the glass to help ground floor living spaces pass.