Design Principles – Insulation and Thermal Mass

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These two systems are vital to the functioning of a building, particularly a passive building. The two work in concert to create a properly regulated internal environment in a home.

he insulation forms the outer shell of the home and it keeps the temperature inside the way you want it, rather than following the outside temperature. The thermal mass sits inside the insulation envelope and functions as a heat storage bank for the building.

What we often see in buildings in Australia, particularly in project homes is that these systems are being put completely backwards. You have an insulated layer inside (being the stud wall infill insulation, if they bother installing it) and the thermal mass installed on the outside of the building (being the brick veneer). This is essentially the worst possible way you can use these materials and is a waste of money and resources.

To look at this topic properly, we have to first talk about what thermal mass and installation are, how they work and how we use them in a building to achieve the results we want.

What Is Thermal Mass?

Thermal mass is a material which has high density, generally something like brick, concrete, rammed earth, cob, or mud brick. The most important feature of thermal mass is that it’s a heavy, dense material to soak up as much heat as possible.

How Does Thermal Mass Work?

The dense material in your thermal mass draws in heat and stores it within the mass itself. Thermal mass will always work best when it is thick, such as in a thick interior wall or a concrete slab.

The effect of thermal mass comes from the fact that heat will always move from a hotter material to a colder material. Being thick and dense, the thermal mass wall or floor will tend to take a long time to change temperature. So as the air in the building heats up, the thermal mass will draw that heat into itself and hold it for a long time.

Then as the air in the room cools down, the thermal mass will release its heat into the air.

How Is Thermal Mass Used?

When we install thermal mass internally and wrap it in a full skin of insulation, the thermal mass works as a heat battery.

In winter we use this effect to keep the building warm without using any active heating. The heat generated by sunshine entering the building or by a fireplace, air conditioner, or electric heater is stored in the thermal mass as the heat is generated. If you are using solar passive design or solar panels to run electric heaters or air conditioners then this heat is generated during the day and is a zero carbon form of heating.

Overnight you can switch off your heating (or the heat from the sun stops entering the home) and the heat which has been stored in the thermal mass is released into the air.

In summer we use the effect of thermal mass to keep a building cool. We protect the thermal mass from the sun as much as we possibly can to prevent it from heading up through sun exposure. During the day, the thermal mass will draw heat in from hot air which gets inside, or from heat generated by the exterior soaking in the solar radiation.

At night the building can be opened up to cool breezes, which will pick up the heat from the thermal mass creating a cool wall or floor the following day.

It’s worth noting that the amount of thermal mass you need if your building is well insulated is a lot less than if your building is poorly insulated.

What Is Insulation?

Insulation is a material which is low density and which slows or prevents heat flow from one side of itself to the other. These materials are things such as fiberglass or mineral wool batts, hempcrete, straw bales, cellulose, and various foam products such as EPS and XPS foam boards.

How Does Insulation Work?

Insulation is very low density and it traps a still layer of air within the fibers of the material in things such as mineral wool and cellulose, or within the air pockets inside hemp, straw, and foam.

The disconnect between the interior and the exterior air created by the insulation layer prevents the heat in the exterior air from directly reaching the interior, and vice versa. The thicker your insulation layer is the better it works at keeping the heat on the correct side as the heat does flow through the installation, it just does so slowly.

How Is Insulation Used?

We use installation on the exterior of a building, and we try to entirely wrap the building in a layer of insulating material. This is to prevent any gaps or leaks, which might allow hot air to enter or exit the building.

We use the installation as a blanket to make sure that the temperature we create inside the building is independent from the temperature outside the building. So if it’s very cold outside, we want it to stay warm inside, and if it’s very hot outside, we want it to stay cool inside. The installation layer means that we can create an air temperature inside the building and not have to expend excess energy to keep that temperature stable.

An Ideal Building System

If we were to look at the uses of insulation and thermal mass, we can come up with an ideal system for a building. This is one which we would want to put in place during our design process.

The exterior of the building should have a full layer of insulation within the structural layer. So that would be within the wall studs as cellulose infill, mineral wool, hempcrete, et cetera. This infill insulation would extend into the roof as ceiling installation or rafter infill installation in a cathedral ceiling.

If we have a building raised off the ground on piers, we would also insulate between the joists. However, if we have a slab it will generally sit on the ground. This is an area which we can improve on in Australia by installing foam or expanded clay insulation under a slab. This will mean that the thermal mass is inside the insulation layer and not leaking heat into the ground in winter and drawing it into the building in summer.

We can improve our main insulation by including a layer of rigid insulation outside the structural layer. This can be a material such as wood fiber insulation boards or foil backed foam panels. These are screwed entirely across the outside face of the walls and roof and taped to prevent air movement. The main purpose of this rigid layer though is to prevent thermal bridging, which is where heat travels through the structural members, circumventing the infill insulation. Windows in this ideal system would be made of an insulating materials such as timber or the lower cost option of uPVC and our glass should be double glazed. We also need to size our windows as small as possible while maintaining good views, light, and sun exposure where appropriate. Minimising this least insulated part of the outside walls will reduce our heating and cooling loads significantly. 

The inside of the building should contain a high but appropriate level of thermal mess as too much can be wasteful of resources and money, but not enough will mean poor thermal regulation. A concrete slab is a good start but concrete itself is not a very good material to use. So it would be better to have the internal floor made from brick or compressed earth, if possible, or at least concrete with a high percentage of Fly Ash.

Internal walls should be constructed from rammed earth, poured concrete, concrete block, brick, or mud brick. Obviously the lower energy materials on this list are mor desirable as again, concrete is a very energy intensive material. So something like recycled brick is by far the best option for our standard internal walls.

We of course want to maximize the use of our financial resources in the building and so this situation is the ideal, but not the expected outcome. Insulation and thermal mass are both quite expensive to purchase and to install, especially when you get into complicated systems and thick walls, but the effect is almost always worthwhile.

We’ll go into more detail on particular materials and how they’re used at another time. But for now, when you’re thinking about your future home, think about one which has a fully insulated exterior and thermal mass inside so that you have a comfortable, well regulated, low energy use home.

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