Types of thermal mass

From ;types of Thermal mass
Concrete slabs can be either constructed on-ground or suspended above ground.
Suspended slabs should always be insulated.
In some climates it is useful to connect the thermal mass in floors to the earth.
The most common example is slab-on-ground construction.
Less common examples are brick or earthen floors or earth-covered homes.
This is known as earth coupling.
The earth acts as an insulator to reduce heat loss from the slab and connects the house to deeper ground temperatures, which are more stable.
Walls
Masonry walls provide good thermal mass if they are located internally or protected by insulation.
Avoid finishing masonry walls with plasterboard because this insulates the thermal mass from the interior and reduces its capacity to absorb and release heat.
Reverse brick veneer construction is an example of good thermal mass practice for external walls because the mass is on the inside and externally insulated.
Water
Water-filled containers can be used as a mass substitute.
Water has double the thermal storage capacity of concrete and heat absorption is substantially higher because of convection within the container.
Water can supply similar storage capacity to masonry with significantly less mass and bulk, making water a cost-effective mass option for upper storeys.
Phase-change materials
There is growing interest in the use of phase-change materials (PCMs) as a lightweight thermal mass substitute in construction.
- All materials require energy input to change state (that is, from a solid to a liquid or a liquid to a gas).
-  This energy does not change their temperature, only their state.
- All materials change state at different temperatures (for example, water changes state to ice at 0°C).

I think a typical (non pcm) thermal mass only absorbs heat when it increases in temperature, and it only releases heat when it decreases in temperature.  I think thermal mass has no effect when at constant temperature. I think there could be times or situations when it could be a benefit to be able to change whether a thermal mass is insulated or not, both from the inside temperature and outside temperature.  

Also one of the most common applications of thermal mass is as the base of a solar collector in passive solar heating. The most extreme example is in a greenhouse. This is in order to absorb heat from the sun while minimizing the increase in indoor air temperature, and to release it at night while minimizing the decrease in indoor air temperature.

This is fascinating, John. I always thought that plasterboard was a decent, if not stellar, thermal mass. One of my permie colleagues in the region talks it up and even put a second layer on all the walls in his seaside home. So it's not all that great, or is it just not as good as masonry?

Mike, your comment is not clear.
I think you are saying that the mass will only heat up when its temperature increases.
I think the actual process is similar but different.
Thermal mass will increase in temperature, and store that heat, when it is subject to heating by the sun, warmer air or even electric or wood heat.

Phil, I believe the issue with plaster is its thinness.
I have sometimes removed plaster, filled the wall with bricks and replastered the wall.
That has helped.

I think John meant that if you already have a masonry wall it should be just plastered (or not if someone want the masonry material to be exposed). Putting some wood and then plasterboard is counterproductive - it separates the interior from the masonry mass. It is also waste of material and it increases flammability and last but not least - creates cavities for vermin.
I always wonder when watching rare videos of masonry (usually concrete block)  construction in US - why they always put drywall instead of chasing the walls for electricity and water and applying some natural plaster - earth or lime based.  It would improve thermal mass performance, eliminate vermin cavities and add some quality/natural touch to the interior that would not look the same like tens of millions of other houses with cheap drywall feel.

That was my instinct, Cristobal. And I totally agree on the missed opportunity with masonry exterior walls...they should have a Larsen truss on the exterior, not furring on the interior to create a cavity that then gets insulated.

The house I lived in while growing up was cinder block with brick veneer and had plaster applied directly to the inside of the block walls. You really felt the difference in winter if you leaned against one of the outer walls. During my early teens my bed was right up against the wall in the NE corner of the house, and I rolled up a blanket and jammed it against the wall just to keep the cold away from me as I slept.

I did not see foundations mentioned.

A concrete slab foundation is a huge thermal mass.

The forum on RMH is a great place to learn more about thermal mass:

https://permies.com/f/260/rocket-mass-heaters

>Putting some wood and then plasterboard is counterproductive - it separates the interior from the masonry mass….

I can see why that might be said if it was an *interior* wall , where someone might want to decrease the thermal lag. However on an exterior wall often times the goal is to *increase* the thermal lag. I think adding wood and/or plasterboard would do this.