The design, construction and operation of low energy buildings should favour a simple ‘fabric first’ approach wherever possible writes, Tom De Saulles, building physicist at The Concrete Centre.
Avoid the green wash. For effective long-term sustainability you need to get the fundamentals of building design right. Increasingly, architects and their clients are returning to fundamental passive design principles that allow fabric performance to be fully optimised. This integrates the thermal mass of exposed structural elements with the design of glazing, ventilation, shading and mechanical systems. This helps ensures comfortable conditions can be maintained during spring and summer, whilst avoiding or minimising the need for mechanical cooling.
In practice, thermal mass is typically provided by heavy-weight floors synonymous with concrete frame buildings. Lightweight timber construction and typical steel frame buildings cannot match the performance provided by concrete floors, which can be constructed with an exposed soffit to fully access its inherent thermal mass. The large surface area of the soffit absorbs unwanted heat, helping regulate the internal temperature and peak cooling demand. Using concrete floor slabs in this way makes good sense, as they typically provide by far the greatest source of thermal mass in non-residential buildings and can readily absorb heat during the day and release it at night with the aid of night-time ventilation.
A question often asked by architects and designers is, ‘how much concrete do you need to provide thermal mass?’ The answer largely depends on the extent to which you want to optimise the building design. It is sometimes thought that a 100mm of concrete is sufficient, but this fails to take account of a range of factors including how buildings actually respond to real weather conditions. For example, a naturally ventilated office with exposed 100mm composite floors (steel decking/soffit with in-situ concrete topping) should have sufficient heat capacity to cope with a simple 24 hour heating and cooling cycle. However, in addition to a building’s daily cycle, there are also longer cycles related to a typical hot spell (usually three to five days) and also the five working days per week cycle, from which heat will reach different depths within the available thermal mass.
In the case of floors in a non-air conditioned building for example, the greater the slab depth, the longer the time period it responds to; the core of a 300mm thick concrete slab responds to the monthly average condition and draws heat in deeper over an extended period of hot weather. For longer time periods these factors are important because it is the longer-term average room temperatures that define the thermal storage core temperature and hence the temperature gradient that draws heat in. So, whilst a 100mm of concrete offers some an element of thermal mass, the thicker slabs used in concrete frame buildings provide greater temperature stability and increased cooling performance across a range of conditions including hot periods.
In terms of embodied CO2, research shows there is little difference between concrete and steel frame office buildings. Perhaps of more relevance, is the operational CO2 savings provided by thermal mass, through its ability to avoid or minimise the need for air conditioning. Over a 20 year period the savings achieved can account for around 75% of the initial embodied CO2 of the concrete, or in other terms, the whole life CO2 performance of a concrete frame office building is a tiny fraction of its initial embodied CO2 when the thermal mass is exploited.
When another factor known as carbonation (the absorption of CO2 by concrete) is factored in, along with a slightly longer life span, the initial embodied CO2 of the concrete can be fully offset. As this demonstrates, it is always more useful to view concrete buildings in whole life terms.
So there you have it, the simplest approach in office design, which utilises thermal mass can significantly reduce energy consumption, help maintain comfortable conditions and deliver impressive whole life CO2 performance.