Thermal Mass

  • Building Design Expert
  • 5 years ago

A church wedding in November not only leads to the requirement of a new outfit, and a hat for the ladies, but invariably one of those “very serviceable” coats just to keep warm. Assuming we are dealing with one of those nice old churches, you know, the ones with outlandishly thick stone walls; we can make reference to the internal environment being managed, to a large extent, by the ‘thermal mass’ provided by that very construction. But thermal mass or no, the heavy winter coat inside the church is needed to combat the drafts which will quite readily relieve us of our own body heat.

The actual construction is indicative – The illustration is intended to represent the thermal transmission process

Of course, today, most old churches have been fitted with central heating and had a lot of the drafts proofed. This tendency to minimise air movement, not only in churches, but all of our older buildings, has changed the way their internal environments operate. In many cases it is down to a lack of understanding of the building physics involved, often to the detriment of the human comfort of that environment, and, or the building fabric itself.

Depending on the height, our solid church walls may be 600 – 800mm thick, maybe even more, and known as ‘massive stone wall construction’. But closer examination reveals they are not solid at all. More of a hybrid cavity wall really, where in this case the cavity is infinitely variable, filled with much smaller stones and subject to countless direct cold bridges as a result of the irregular spaced bonding stones – linking outside to in. So we are a long way off ‘Passivhaus’ construction for churches, but of course the construction was ‘of its time’, and resolved the construction issues of the day.

So where are we going with this? Let’s bring it together and consider the unheated church in winter. The one caveat in this argument is a requirement for direct sunlight onto our stone wall face. The vertical walls provide the perfect incidence for the low angle of the sun, and the high thermal conductivity of the stone is perfect for absorbing the radiant heat it transmit, and transferring it by process of conduction throughout the wall thickness, ultimately to the cooler internal surface. But the sheer mass (thickness) of the construction dictates this process is incremental over the day, notwithstanding the obvious variables. Over time the conducted heat will reach the internal wall’s surface, such that the wall now acts as a giant radiator.

Now here is where we need to be careful in our use of terminology. Radiators, in our homes, are largely unsuccessful at heating a space through the emission of radiant heat alone. Radiant heat is composed of electro-magnetic waves within the infra-red spectrum and can only travel in straight lines. it will not heat a gas (air), but will heat surfaces and objects that are in the path of the EM waves. Hence, place a piece of furniture a short distance from the radiant source, and anything beyond it remains cool. A radiator will heat the air in direct contact with its surface. That air becomes less dense as a consequence, and rises. New cooler air moves into the space to replace it, gets heated, rises etc. etc. so the process perpetuates, in varying degrees, as long as the radiator remains hot / warm. So combination heating then via radiation, conduction and convection.

So back to our church wall ‘radiator’. The huge mass of the construction is absorbing a lot of heat from the sun. As long as it’s being fed from the outside, it will continue to perform a sync of that heat to the inside, as radiant heat. There is a crucial ‘however’, because remembering our drafty church; the movement of air  over the internal wall surface will relieve it of it’s stored heat, and thereafter distribute it via the process of convection.

Highly insulated internal face of an external wall can act to retain heat within its constructed form for release when ambient internal temperatures fall

Moving swiftly into the modern day, we utilise the principles of thermal mass described above, but in reverse. Because our prime aim is to prevent heat loss from building to outside world we  must provide an insulation to the building envelope internal lining. As long as that lining has a high thermal conductivity, we are on our way. As an obvious example, for internal lining read ‘inner’, or ‘structural leaf’ . Dense concrete block construction which is then highly insulated, with minimal cold bridge links to the outside world, we have a perfect heat sync for an internal environment of fluctuating temperature.

So why fluctuation temperatures? Consider this: In an office or domestic environment where the requirement is to maintain a uniform temperature throughout the day the thermal mass of the wall structure is constantly being fed with heat to become the ‘giant radiator’. The stored heat can only be released once the internal temperature drops below that of the walls.

However, the heat derived from thermal mass is ‘latent heat’, as the wall surface will never feel like a ‘radiator’ as we know it. Buildings designed specifically to take advantage of thermal mass, need to be designed and managed such that those internal wall areas are largely exposed, and not masked with furniture, fitted cupboards or fixtures. That will allow the radiant heat transfer. Further, there must also be a contrived air movement either through natural of mechanical ventilation. The air movement over the wall surface will release the stored heat by process of conduction / convection.

The process of liberating latent heat in this way is by no means a precise science, but if we can get the majority of our ducks in a row we know it works. The conditions that must prevail for optimal performance will not suit everyone, or every building, and may not even be achievable, but thermal mass can be a useful addition to an energy conscious building design.

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