What is Thermal Mass?
The best way to explain thermal mass is to think of the old bulky electric night storage heaters dating back to the 1970’s which were large metallic heavy heaters most common as retrofit background heating where a quick an inexpensive installation was required. Underneath the thin metal casing was a large lump of masonry, concrete or dense blocks, which were effectively plugged into the electric mains and used to store up heat overnight (when the electricity rate is cheap) and release it slowly throughout the day. There are much more sophisticated options today with centrifugal fans and refractory thermal storage blocks under the steel casing, but the core principle is the same; i.e. storage of heat and its subsequent slow release.
We’ve now got used to selecting building materials (and systems) based upon their conductive and thermal resistivity properties so that we balance strength and energy efficiency in complementary ways. But less readily understood are the physical properties for thermal mass which relate to a product’s density (how conductive it might be), its thermal capacity (the product of density and a heat source) and the speed with which it loses any stored heat through a thermal (time) lag. So, with a house which is bathed in sunlight, we would expect the glass in the windows (which has some density but little thermal lag) to quickly transfer heat as a quick heat flow. Conversely, any external masonry walls (which have density but also a long thermal lag) would store up heat as a slow heat flow, and release it slowly after temperatures have cooled over many hours. Good natural products for maximising the benefits from thermal mass are those which are dense, have a high heat storage capacity and a slow thermal lag.
How is it most efficiently used in dwellings?
Designers have always understood the potential value from solar gain but the real trick is working out how to control it. Really energy efficient buildings will focus on a very high performing building fabric (envelope) where heat loss through external walls is virtually eliminated and air leakage is kept to an absolute minimum. These are the design tenets of the German Passiv Haus philosophy where a building is not going to need much (energy) input if it doesn’t lose, consume or waste energy.
Add to this large amounts of glazing facing south, and the building will be able to soak up free heat energy magnified through the glass which, as we know, has a quick heat flow. However, this solar gain may not be wanted immediately during the day but instead later at night and the complementary design would include materials of a high thermal mass to soak up and store this heat, making it bearable during the day, and release it back into the internal atmosphere slowly overnight helping to keep temperatures stable and constant. Typical building elements to do this might be an exposed feature masonry/stone wall, a large brick built chimney breast, stone floors or the inside linings of the external cavity wall construction.
One French inventor in the 1950’s, Felix Trombe, designed large masonry walls positioned sometimes in suspension directly behind large section south-facing glazing, with a gap between wall and glass, the outside facing side of the wall painted black to maximise heat absorption and air vents or voids below and above the walls to allow a flow of air to transfer stored heat. As such, Trombe walls can be positioned as architectural features but with the physical purpose to store and transfer heat through natural convection.
However, midsummer heat may just be too powerful to use or control and here the designer might add a brise-soleil to the outside with permanently angled shutters to shield the glazing from overhead summer sun but to maximise the gains from lower angled autumnal, spring and winter sun. Anyone with a conservatory will know that they are difficult to use in full summer without some form of sun screen/shield strategy.
Thermal mass is also readily used with the popularity of underfloor heating when the heat source, water borne pipes of electric cables, are set in a floor screed. The whole screeded mass heats up over time and then radiates heat on a completely even distribution upwards into the room voids above. It is the thermal mass characteristics of the concrete floor and screed which make it one of the most efficient ways of doing this.
Less obvious benefits for cooling
Perhaps one of the greatest design challenges for designers of 21st century buildings relates to cooling rather heating and heat retention alone. More efficient, lighter weight building fabrics may improve speed of construction as well as overall life cycle costing but the bi-product of some of these improvements is the retention of unwanted heat at times of peak load when buildings are in use. Passive ventilation and thermal mass are useful tools here to help keep internal stability and comfort without resorting to air conditioning.
In my own timber framed three storey property we have two relevant summer-cooling examples; one a design flaw and the other a major design benefit. The design flaw relates to the 2nd floor attic bedroom where summer radiated heat is transferred and trapped in the bedroom with solar conduction occurring through the roof covering, the warm bridging of the roof structure and the imperfect insulation. There is no masonry and no other thermal mass properties to any of the building elements in the zone and, at times, the room is in my opinion too warm. Conversely, the ground floor is a haven in the summer with stone floors and two large chimney breasts soaking up excess ambient heat and helping to keep the zones cool.
Given that the roof structure to most houses is always going to be timber, the opportunity exists to line these walls and sloping ceilings with denser materials to ordinary plasterboard, like cement board or clay boards. In addition, any chimneys structures could be left plastered as a feature rather than clad and hidden with drylining so as to expose the brickwork, and stone floors used in the bathroom. These inclusions should make a very real contribution to summer temperature stability and comfort.
Practical considerations and some misunderstandings
Thermal mass can make the biggest contribution when there are likely to be big swings in temperature through any daily cycle and equally where seasonal weather patterns will affect the building’s dynamics; i.e. from a heating requirement to cooling. But, to maximise gains, the external side of the thermal mass elements must be securely insulated which is vital to stop the heat conductivity continuing right through the element. If you are using the inside skin of your external masonry walls for thermal mass, then you don’t want any imperfections in the cavity insulation as that would lead to heat loss. Equally, your concrete screeded floor will only collect, store and dissipate underfloor heat evenly into the room if you have a continuous and robust insulation layer beneath it.
Insulated concrete formwork (ICF) would therefore be a poor product to choose if thermal mass is part of your design requirements. By definition, an ICF insulated block is insulated on all four sides and although it may have a thicker external side, the internal side is nonetheless a layer of insulation which will separate the internal room space from the dense concrete poured into the ICF cavity. The concrete won’t be able to temporarily store excess heat (for later release) or act as a thermal buffer to soak up heat and cool the building in the summer.
Steel, timber frame and wood based SIPS panels are also poor choices for thermal mass as these products don’t have the physical properties to buffer heat. Exchanging the wall linings for denser materials would assist and there is a growing awareness of choosing other material to plasterboard for this very reason.
There is also a general lack of understanding amongst many builders and developers about the benefits of thermal mass. So even those builders who choose masonry construction are then sometimes voluntarily removing any thermal mass benefit by drylining the internal surfaces with either plasterboard or, worse still, insulated laminated plasterboard. If at the outset they put all of their thermal layering on the cavity side of the structure all of their external walls could be directly plastered thereby using plaster and concrete(block) for maximum buffering in winter and summer. But, this makes the installation of services of the external wall slightly more complex and one should probably expect more shrinkage cracks to repair after the first 12 months, whereas plasterboard drylining disguises most of this. The distinction therefore is the on-going building performance thinking from the occupiers perspective on the one hand, versus ease of construction for the builder and developer on the other.
Cost savings are difficult to measure but, if you can harness, store and use solar energy to help heat your house you will consume less paid-for fuels and, in the summer if you can keep buildings cooler without the need for air conditioning or fans, then that saves on other power requirements and installations. The other benefit, of course, is enhanced comfort!
Phase change materials
In response to the growing requirement to find ways of cooling buildings more efficiently, industry has responded with the development of some very creative solutions. Phase change materials, (PCM) works on the basis of temporarily storing heat in building products whilst they change from one state to another. Consider the energy required to change water from a liquid state to a gas (steam) or from a liquid to a solid (ice). Both require energy to do that but, once that energy is removed, so it returns back to its original normal state, i.e. water. Manufacturers are working with other materials whereby the change in phase occurs at very low temperature ranges, i.e. certain waxes can undergo phase changes at between 23 and 26°C. Within this range they can change from liquid to solid with the liquid to solid transformation known as solidification and the solid to liquid transformation known as fusion soaking up heat or releasing it accordingly. The challenge for industry is how to incorporate tiny proportions of these PCM’s, fully encapsulated, into ordinary building products so that they cannot be seen to the naked eye but where they can still react to temperature shifts. Various manufactures have successfully brought products to market as alternative building board linings to plasterboard although currently they are still very expensive options.
Written and published in October 2017
