Passive systems: barrier, ground store, earth tubes

How does a thermal barrier work in an external wall?

A low-energy building stays comfortable because three things work together: a thermal barrier that slows heat loss through the external wall, a ground store that holds heat or coolth below the building, and earth tubes that move air through the soil before it enters a room. None of them generates energy on its own; each one changes the path that heat takes, and the building is planned around that path.

The thermal barrier is the part of the wall that resists the flow of heat between inside and outside. In a well-insulated external wall it is usually a continuous layer of insulation, placed so that it is not interrupted by structural elements, and paired with an airtight layer that stops warm moist air from leaking through the construction. A useful plain description of this layer and its role in a whole-house concept appears in the Passive Climate Journal’s notes on the thermal barrier exterior wall, where the barrier is treated as one element of a passive climatisation scheme rather than as a product.

Heat moves through a wall by conduction, and the rate depends on the material and its thickness. A thermal barrier is a layer with low conductivity, so the temperature difference across the wall is taken up mostly inside that layer instead of at the inner surface. The inner surface then stays closer to room temperature, which reduces both the heat lost in winter and the radiant discomfort felt by someone sitting near the wall.

Two details decide whether the barrier performs as designed. The first is continuity: a gap, a metal fixing that bridges the insulation, or a concrete slab that runs through it creates a path of higher conductivity, and the heat finds that path. The second is airtightness. If indoor air can pass through the construction, it carries moisture with it, and that moisture can condense inside the insulation, where it lowers performance and can damage the structure. For this reason the barrier and the airtight layer are usually described together, with the vapour control placed on the warm side in a heating-dominated climate.

A barrier does not heat a building. It only slows the exchange, which means the smaller the remaining heat loss, the smaller the heat input needed to keep the rooms at temperature. That is why the barrier is normally the first measure in a low-energy project, before any active system is sized.

How do earth tubes and ground heat storage work?

A few metres below the surface, the ground temperature is close to the annual average air temperature of the place, and it changes very little between summer and winter. Earth tubes use that fact. Air is drawn through pipes buried at that depth before it reaches the building, so in summer the soil takes heat out of the air, and in winter it gives heat back. The result is a supply of air that is already tempered, which reduces the load on whatever heats or cools the building.

Ground heat storage works on the same principle at a larger scale. Instead of tempering air on its way in, the soil or a mass under and around the building is used as a store: heat collected in summer, from solar gains or from the building itself, is put into the ground and taken back in winter. The store is slow. It responds over weeks and months, not hours, so it is planned as a seasonal buffer rather than as a boiler. Its performance depends on the volume of ground involved, on how well that volume is insulated from the surrounding soil, and on whether the building can deliver heat into it and extract heat from it without large losses.

Both techniques share a practical condition: the ground has to be reachable, and the pipes have to be laid before the floor and the garden are finished. They are decisions taken at the design stage, not additions made afterwards.

Can a roof act as a solar collector?

Yes, and in a low-energy building it often does, in two different ways. The first is direct: a roof surface with a dark absorber, a transparent cover and an air or water circuit behind it collects solar heat and delivers it to a store or to the ventilation air. The second is indirect: a well-oriented roof with large glazing admits solar gains into the building, and the mass inside the rooms holds that heat long enough to be useful after sunset.

A roof collector is not a separate energy plant. It is a surface that is already there, given a second function, and its output depends on orientation, tilt, shading and the time of year. In summer it can produce more heat than the building needs, which is exactly the surplus that a ground store can absorb for winter. In winter, when the days are short, its output is modest, so it is sized as a contribution rather than as the only source.

The same logic applies to the whole set of passive elements. A thermal barrier reduces the demand, a ground store shifts heat from one season to another, earth tubes temper the incoming air, and a solar roof adds heat when the sun is available. Each one is described in the building physics literature on its own terms, and each one is planned with the others in view.

What a passive scheme asks of the design

The three elements above are often presented as separate technologies, but they interact. A very tight thermal barrier reduces the heat that a ground store has to supply, and it also reduces the losses that would otherwise make the store pointless. Earth tubes deliver air at a temperature close to the ground, which changes how much the solar roof has to contribute. Sizing one without the others usually leads to a system that is either too small in January or oversized in April.

This is why low-energy projects are usually documented as a whole: the wall build-up, the airtightness target, the volume of ground used for storage, the length and depth of the earth tubes, and the area of the solar roof. The numbers are not independent. A change in the barrier thickness changes the heat load, which changes the store volume that makes sense, which changes the collector area that is worth installing.

How such projects are measured

A passive scheme is judged on measurements, not on description. The usual set includes the indoor temperature through the year, the delivered heat or cooling, the electricity used by fans and pumps, and the airtightness of the envelope. These figures show whether the barrier is continuous in practice, whether the ground store is charging and discharging as expected, and whether the earth tubes are delivering the tempering that was predicted.

Measurement also reveals the limits. A ground store can be depleted if a winter is long and the summer recharge was poor. An earth tube can deliver less than expected if the soil around it is too dry or if the pipe is too short. A solar roof can be shaded by a neighbouring building in December. None of these findings invalidates the approach; they are the normal content of a monitoring report, and they are what makes the next project better.

For a reader planning a house or a retrofit, the practical order is clear. Start with the envelope, because it sets the demand. Then decide how the ground will be used, because that decision is fixed once the pipes are buried. Then size the solar contribution against what remains. The three parts are one system, and the documents that describe them are read together.

A greenhouse gas footprint is not a single measured quantity but a sum assembled from two kinds of input: activity data, such as litres of fuel or tonnes of material moved, and emission factors that convert each activity into a carbon dioxide equivalent. The boundary chosen for the exercise, whether a household, a site or a whole organisation, decides which activities enter the sum and which stay outside it. A calculator applies the same arithmetic but stops at the edge of its own scope. The footprint measurement note sets out this distinction and what a calculator leaves out.