Thermal Performance of Rammed Earth Walls

The thermal performance of rammed earth walls is often misunderstood. It works in an entirely different way than insulated timber frame walls and therefore comparing R-values, U-values or thermal conductivities doesn’t give the full picture and often makes rammed earth look like it will perform poorly. However, we know from thousands of years of history that rammed earth does perform well in almost all climates as long as the building is designed to utilise the strengths of rammed earth as a material.

Rammed earth is a very dense, solid material and has very little insulating ability. This is extremely important to remember. The two most important ideas to understand when talking about the thermal performance of rammed earth are:

  1. Thermal mass vs insulation
  2. Radiant heat vs air temperature

Thermal Mass of Rammed Earth

Rammed earth (and other solid earth building methods such as adobe or cob) does not insulate, but it does act as an extremely good thermal mass (thermal storage). This means that during the day the walls can absorb a large amount of energy from the sun, which is then slowly released at a later time when the temperature drops. The walls are always trying to find an equilibrium with the inside and outside air temperatures.

Increasing the thickness of the walls increases the total thermal mass and therefore the total amount of energy that can be absorbed. It also has another effect which is called thermal lag. In simple terms, it could be described as the time it takes for the heat to travel through the wall. For example, when the sun sets the outside temperature rapidly drops and often the inside of the house will be much warmer than outside, and therefore heat will be lost. With thick rammed earth walls, the thermal lag can be over 9 hours, slowing down the speed of heat loss and the inside of the house will only lose a small amount of heat overnight. The result is an extremely even and consistent inside temperature and in real rammed earth houses it has often been shown that it can take up to 2 days for inside temperature to react to the outside temperature.

In summer, rammed earth walls (and floors) help to absorb excess heat and prevent over-heating. Overnight as temperatures drop, the walls release heat to the outside. In winter, the same process occurs but the difference is the temperature range and amount of heat absorbed. The walls will continue to heat up (and gain heat from the surroundings) until they are hotter than the surrounding air, then the flow of heat reverses. If the internal air temperature is 26C, then the internal wall temperature can absorb heat up to 26C. In winter, however, the internal air temperature is much lower (e.g 18C) and therefore the walls will reach 18C quicker than 26C and start to release heat to the surroundings a lot quicker. This is why rammed earth can be both cool in summer and warm in winter.

Insulation doesn’t offer any thermal mass in most circumstances and relies entirely on the fact that it minimises the flow of heat from the inside to the outside. It is extremely effective at doing this, but there are some disadvantages. Insulation keeps the internal air temperature warm (or cool) through a physical barrier and all of the warmth (or energy/heat) is stored in the air. What this means is that an extremely airtight house is required for the insulation to work properly (this is why insulating old homes is not as effective as insulating new homes). If windows or doors are opened, or there are air leaks, then hot air from inside will naturally flow to the cold outside due to pressure differences. It is almost impossible to live without opening doors and windows at some point and every time you do that there are massive heat losses.

The fundamental difference between thermal mass and insulation is how they store their heat. Thermal mass stores the heat in its mass, whereas insulation relies on the internal air to store the heat. Heat can quickly be lost from air to the outside, but cannot be quickly lost from a dense thermal mass such as rammed earth.

Radiant Heat vs Air Temperature

Most people measure the warmth of their home in terms of air temperature. It is how we set our heat pumps and the way the weather forecast is given to us, but in reality, it’s probably not the best measure of warmth. It is only half the story. Think of the difference between sitting in the sun and then the shade on a winter’s and summer’s day. In winter, the air temperature is low and the shade is cold, but sitting in the sun instantly warms you. In summer, the shade is warm and out in the sun it is very hot.

Radiant heat is what the sun gives us. It is instantly warming and the air temperature does not need to be high for us to be warm. Nothing compares to the radiant heat from the sun or a bonfire. Air temperature, on the other hand, is generally either cold or hot, and we have to work very hard to keep it somewhere in between so we stay comfortable.

It may seem crazy that we have become so focused on air temperature as a measure of warmth when we can all relate to radiant heat, but the reason for this is that most traditional housing doesn’t have any form of radiant heating ability. Rammed earth is a good radiator, whereas timber and especially plasterboard are not. Rammed earth walls and floors take advantage of their thermal storage and radiating ability and make a living environment much more comfortable. Probably the biggest advantage of having radiant heat is that a lower inside air temperature feels warmer (the same way that sitting in the sun in winter feels warmer than the air temperature would make you think) and therefore there is less heating requirement for your house. Without the radiant heat, the air temperature would need to be higher to achieve a comfortable living environment.

Rammed Earth Thermal Performance FAQ’s

After reading about thermal mass and radiant heating you probably have a number of questions about how this best translates into designing and building with rammed earth, so here a few questions and answers that may be helpful.

How thick should a rammed earth wall be?

We don’t recommend anything less than 40cm, but really like 60cm as a thickness. This takes structural, thermal, construction and visual aspects into consideration. Thinner walls are not a great way to save construction costs.

Should you insulate rammed earth walls?

It depends, but in general the overall performance of thermal mass + insulation is a big improvement over thermal mass by itself. In regions where it gets cold in winter or not much sun, insulation is a good idea. In hot and/or sunny climates, such as Northland or Nelson it probably isn’t 100% neccessary.

Where is the best place to put the insulation?

In theory, the best location for insulation is on the exterior so the thermal mass is still open to the internal living environment and the insulation acts as a barrier to the outside. There is some suggestion and research that shows insulation in the middle of the rammed earth performs best. Insulation on the inside doesn’t make much sense. Each option has its pros and cons.

Will I still need to heat a rammed earth house?

Yes, most likely. We think a radiant heating option such as a masonry stove or hydronic water heating pipes are great options that can integrate with rammed earth. Good design is essential to reduce or potentially remove heating requirements.

Will I need an air conditioner or cooling?

We wouldn’t recommend or expect any rammed earth house to need cooling or AC.

What is the best climate for rammed earth?

Rammed earth is most common in hot, dry climates, but has also been historically used in wet, moist and cold climates. Rammed earth works amazingly in hot and dry, but a wet and cold climate is perfectly fine. Or course cold climates require more heating than warm climates, but this is true no matter the construction technique.