TM59 Overheating Assessment for Millbrook Independent Hospital

Project Overview

KJ Tait undertook a thermal comfort and overheating assessment for a proposed extension to Millbrook Independent Hospital, comprising seven single-occupancy patient rooms with ensuite facilities and kitchenettes. Dynamic thermal modelling was undertaken using IES Virtual Environment 2025 to assess compliance with CIBSE TM59 and to evaluate the building's resilience to future climate change. The assessment considered both current Manchester Design Summer Year (DSY1 2020) weather data and future climate projections for 2050.

The Challenge

Patient accommodation requires comfortable internal conditions during both daytime and sleeping hours. The proposed design relied primarily on natural ventilation through restricted-opening windows, while healthcare operational requirements limited opportunities for purge ventilation through external doors. As a result, it was necessary to understand whether the design could maintain acceptable temperatures under both current and future summer conditions.

Initial modelling identified overheating risk in several rooms under present-day conditions, with overheating driven primarily by elevated night-time temperatures. The future climate assessment demonstrated a significant increase in overheating risk, with all seven patient rooms failing the TM59 criteria without further design intervention.

Our Approach

KJ Tait developed a detailed dynamic thermal model from the architectural proposals and assessed occupancy, solar gains, building fabric performance, ventilation strategy and room orientation. Natural ventilation was modelled using MacroFlo to reflect the proposed window configurations, opening restrictions and occupant behaviour.

The assessment identified solar gains and limited night-time heat dissipation as the primary causes of overheating. A series of passive mitigation measures were therefore tested, including reducing the glazing g-value from 0.66 to 0.50 through improved solar control glazing and introducing secure night-time ventilation while maintaining the proposed 100 mm window restrictors.

The effectiveness of these measures was assessed against both current and future climate scenarios to establish a practical compliance strategy without introducing mechanical cooling.

Key Outcomes

• Assessed overheating risk for seven single-occupancy patient rooms using CIBSE TM59 methodology
• Developed a dynamic thermal model using IES VE 2025 and Manchester DSY weather data
• Identified night-time overheating as the principal thermal comfort risk
• Demonstrated that reducing glazing g-value from 0.66 to 0.50 achieved TM59 compliance under current climate conditions
• Reduced bedroom overheating hours from up to 48 hours to a maximum of 16 hours under the current climate scenario
• Demonstrated that secure night-time ventilation enabled all patient rooms to achieve TM59 compliance under the 2050 climate scenario
• Established a passive overheating mitigation strategy without the need for active cooling systems

Value to the Client

The assessment provided a clear understanding of overheating risk within the proposed development and identified a practical passive strategy to achieve compliance with CIBSE TM59. By combining improved solar control glazing with secure night-time ventilation, the design was shown to deliver acceptable thermal comfort for patients under both current and projected future climate conditions, supporting long-term resilience while avoiding the energy, maintenance and capital costs associated with mechanical cooling.