Efficiency
Thermal mass changes when a house is warm, not how much heat it loses
A heavy building stores heat and releases it slowly, which shifts the timing of everything and does nothing at all to the rate at which heat escapes through the walls.
By Manish Trivedi4 min read
Storage and resistance are different properties
Two properties of a building element get confused constantly. One is how readily heat passes through it, which is what insulation addresses. The other is how much heat the material can hold for a given temperature rise, which is a function of its density and its specific heat capacity.
A solid masonry wall stores a great deal and resists heat flow poorly. A thick layer of mineral fibre stores almost nothing and resists heat flow very well. Neither substitutes for the other, and a claim that mass makes insulation unnecessary is confusing the two.
What mass does is change the timing. Everything else about the building decides the quantity.
Delay and damping, which are the useful effects
Heat arriving at the outer face of a heavy wall takes hours to make its way through, and it arrives at the inside diminished as well as delayed. A hot afternoon reaches the interior in the evening, softened. A cold night reaches it in the morning, softened.
This is genuinely valuable in a climate with a large difference between day and night temperatures. The mass absorbs the daytime heat, the interior stays comfortable through the afternoon, and cool night air is used to discharge the stored heat before the next day. That is why traditional buildings in hot dry regions are heavy.
In a climate where nights are not much cooler than days, there is nothing to discharge into and the mechanism has little to work with. The same wall in a humid tropical climate is a liability rather than an asset, which is why traditional buildings there are light and shaded instead.
Intermittent heating is where mass works against you
A heavy interior takes a long time to warm up, because the heating system has to raise the temperature of the structure and not merely the air. A house occupied for a few hours each evening and heated only then will spend much of that time bringing the fabric up, and it may never quite get there.
A lightweight interior responds quickly, reaches comfort fast and cools equally fast once the heating stops. For irregular occupancy that is exactly the behaviour you want, and it is why timber-framed rooms feel responsive in a way solid-walled ones do not.
Neither is better in general. They suit different patterns of use, and the mismatch between a heavy building and an intermittent schedule is a common source of complaints that get blamed on the heating system.
Which is why low-temperature heating and mass get on well
A system running steadily at a modest output pairs naturally with a building that stores heat, because the store smooths out the gap between a slow input and a variable demand. Underfloor heating in a screed is the clearest example: the floor holds a large quantity of heat and releases it gently for hours after the system stops calling.
The corollary is that such a system responds slowly to any change of setting, and trying to run it on a sharp schedule fights the physics. Long steady operation with gentle setback is the pattern that suits it, and treating it like a boiler with a timer produces disappointment.
This is a real trade rather than a drawback. Slow response is the price of the smoothing, and the smoothing is the point.
Internal mass is easier to add than people think
The mass that matters for these effects is the mass in contact with the internal air, which means floors, internal walls and exposed ceilings rather than the outer leaf of a wall. Insulating a solid wall on the inside disconnects the masonry from the room and removes most of its usefulness as a store; insulating externally keeps it.
That is one of the more interesting arguments for external insulation where it is feasible, and it is rarely the deciding one, since cost, appearance and planning constraints usually dominate. A solid floor left exposed rather than carpeted, or a masonry internal wall left unlined, contributes in a smaller way.
Adding mass deliberately to an existing house is seldom worth the structural trouble on its own. Knowing whether your house has it, and heating it accordingly, is worth a great deal more.
What to do with this
Work out which kind of building you are in, then match the heating pattern to it rather than the other way round. A heavy house rewards steady low-level heating and punishes sharp schedules. A light house rewards responsive control and loses heat quickly when the system is off.
And do not let anyone sell mass as a substitute for insulation, or insulation as a substitute for mass in a climate that needs both. They solve different problems, and the building needs whichever of those problems it actually has.
Common questions
Does thermal mass reduce my heating bill?
Not directly, because the rate at which heat escapes is set by insulation and air leakage rather than by how much the fabric stores. Mass can reduce consumption indirectly by smoothing demand, by allowing a heating system to run more steadily, and in summer by reducing or removing the need for cooling. In a poorly insulated building it does very little of any of that.
Is a stone house harder to heat than a modern one?
It is slower to heat rather than harder, and the two feel similar from inside. A solid masonry building takes hours to bring up to temperature and holds that temperature well afterwards, which suits continuous occupancy and frustrates intermittent use. The larger issue in such houses is usually the poor insulation of the walls themselves, which is a separate matter from their mass.
Should I insulate a solid wall internally or externally?
Both work and they have different consequences, so the answer depends on the building and on constraints that are not thermal. Internal insulation is cheaper and disruptive to the rooms, and it disconnects the wall’s mass from the interior. External insulation keeps the mass working and changes the appearance of the building, which may not be permitted. Moisture behaviour differs between the two as well, and that is a question for someone who can assess the specific construction.
Deputy editor, Power Your Roof
Manish has been reporting on solar basics, batteries, bills & tariffs since long before it was fashionable and would rather show the working than assert the conclusion.
