Thermal mass – flipping marvellous

April 25th, 2014 by

Used in combination with insulation and controlled ventilation, thermal mass is one of the keys to achieving really low energy buildings.

When it comes to energy efficiency the average Victorian house leaves a lot to be desired. Solid walls, and more holes than a string vest. But the one thing Victorian homes generally have in spades is mass, thermal mass. Inch thick, dense, lime plasters, and internal walls made of bricks on end mean lots of mass just where it is needed, inside the building.

Thermal Mass - Flipping Marvellous

Plenty of thermal mass, but outside the insulated envelope

Thermal mass and the benefit it brings in keeping buildings warm in the winter and cool in the summer is probably the least understood of the three elements of really low energy buildings with the other two being insulation, and ventilation.

Used in the right way thermal mass is like a battery storing and releasing energy in the form of heat and smoothing out the peaks and troughs in daily (and nightly) temperature variation. It’s the reason why the 1893, Victorian mid-terraced house I live in has been holding an internal temperature of around 17 deg C since the beginning of April without any heating beyond the four occupants, solar gains and heat from our lights and appliances (and yes, we are quite content living at 17 deg C).

At a much larger scale it also helps to explain why the converted warehouse, also Victorian, in which we have our offices, with its thick concrete floors, beams and columns requires no heating in the autumn for at least a month after the lightweight offices in town have been running theirs.

The beauty of thermal mass is that if you use it in the right way it just will just sit there doing its stuff for as long as the building is standing. Part of the trick is to make sure it’s in the right place, inside the insulated envelope. Traditional house construction in the UK has put the heavy, thermally massive materials outside the insulated envelope. This is either to provide the structure and load bearing or as the case of much timber framed construction, to provide protection from the elements, and a ‘traditional’ appearance.

The problem is that standing outside the insulation its benefits as a thermal energy store are lost. Flipping the construction of walls so that dense heavy materials are inside the insulation and the lightweight materials are on the outside would transform the ability of buildings to regulate their temperature passively, keeping warm in the winter and cool in the summer.

A criticism often levelled at high mass buildings is that all that heavy material means high embodied energy. This is true, but when used in combination with insulation and controlled ventilation it is not difficult to knock out the requirement for whole house heating, meaning the lifetime energy running costs will a fraction of the a lightweight building with an active heating system. What’s more, traditional construction methods often produce the worst of both worlds, high embodied energy, and none of the thermal benefits of the heavy materials to offset that embodied energy.

Early versions of SAP and the Code for Sustainable Homes (RIP) tended to penalize ‘heavy and tight’ dwellings when compared to ‘light and tight’  equivalents of the same form and size. Both underestimated the energy benefits of high mass in reducing heating and cooling loads. More recent versions have sought to rectify this and factor in the heating and cooling benefits of thermal mass. It’s no accident that the use and careful placement of heavy materials is integral to the design of many of the really low energy buildings built over the last 30 years and under construction today.

Thermal mass is unlikely to woo the crowds at Ecobuild, but when combined with insulation and controlled ventilation it’s crucial to unlocking self-heating and self-cooling buildings and to cutting space heating and space cooling loads.

Written by

Environmental consultant, facilitator, founder & Director of Climate Works Ltd.