How Does a Passive House Work?

We admit it, we are passionate about passive houses, and that is why we never tire of explaining how a passive house works, whether via blog, email, phone, meetings at the studio… we love telling you what building an eco-friendly, zero-consumption house—what is known as a passive house—entails.

At Slow Studio, we have set out to ensure this is not the case—that by using key strategies from traditional passive architecture, such as opening windows in the right orientation, achieving natural ventilation to cool the house, and working with natural materials that accumulate heat, it is possible to build a passive house economically.

To do this, it is necessary to understand how a passive house works, what the key strategies are for a house to take advantage of the natural resources available in its immediate surroundings, and to achieve indoor comfort by reducing energy needs to a minimum so they can be met through renewable, clean, and affordable energy sources.

This is the topic we are going to address in today’s article: helping you understand what a passive house is and how it works, as well as the key factors that will make the difference compared to a conventional house.

Let’s see how a passive house works.

What is a passive house

Well, before going into detail about how a passive house works, it is worth recalling what passive houses are all about, because the reality is that they are houses in which common sense and traditional knowledge of materials, land, and the environment in which we are building are applied to make them work in our favor and achieve indoor comfort at minimal cost.

Thus, a passive house is a house that uses passive architecture strategies to function efficiently, reducing consumption by approximately 60 to 90% compared to a conventional house.

Passive architecture strategies use design elements or architectural features to achieve energy gains—for example, a window placed facing south or a porch to protect us from the sun in summer are simple strategies that help us understand how a passive house works.

1- Reduce Energy Demand

Reducing demand is the key factor in understanding how a passive house works.

To reduce energy demand, we must apply a series of strategies during the design of the dwelling. The fundamental objective is to take advantage of the characteristics of the environment and optimize construction systems in relation to it.

Harness Solar Radiation

Harnessing light and solar radiation is fundamental in any type of architecture, but when it comes to a passive house, harnessing the sun is essential.

The sun is an inexhaustible source of energy that provides light and heat to a dwelling. The location and size of windows, therefore, are of vital importance.

In fact, the general recommendation is that whenever we have a south-facing façade through which we can capture sunlight, we will install a glazing surface corresponding to approximately 70% of the total façade.

This percentage is high and may seem excessive, but the truth is that the sun’s capacity to heat a dwelling is very high if windows are designed appropriately.

Another aspect to consider is the use of the so-called greenhouse effect, which can be very beneficial for heating a house. The greenhouse effect is achieved by placing glazed galleries in front of the sunny façades of the house. A gallery means having a glazed roof that multiplies solar capture.

Of course, in summer we must be able to protect all solar capture systems, either with solar filters such as porches or pergolas, or by ventilating the spaces that receive direct radiation.

Understanding how a passive house works involves knowing the local climate and knowing how to harness radiation to heat the house, but also knowing how to protect ourselves in summer without losing interior light quality.

Harness Natural Ventilation

Heating the house in winter is all well and good, but in summer we must also ensure an adequate indoor temperature. For this, it is important to guarantee cross ventilation in all rooms of the house—or at least in bedrooms and common areas.

To achieve cross ventilation, it is necessary to place operable windows on opposite façades.

In large dwellings or those with extensive compartmentalization, it is often difficult to generate this cross ventilation. For this reason, courtyards are usually a good strategy to achieve cross ventilation, as well as a healthy indoor environment through vegetation that helps regulate humidity and retain dust particles.

In the Mediterranean climate, interior courtyards work excellently well, as there are no extreme cold or heat peaks throughout the year, and they allow us to effectively regulate indoor temperature.

Insulate the House Well

When we achieve an adequate temperature inside the dwelling, we must maintain it for as long as possible. This is achieved by preventing heat loss.
Heat exchange in a house occurs mainly through the façade (wall and glazing), through the roof, and, to a lesser extent, through the floor in contact with the ground.

In a passive house, the thickness of insulation is considerably greater than in a conventional house. The less insulation, the greater the thermal transmittance and, therefore, the greater the heat exchange.

For this reason, while a conventional house typically has between 5 and 12 centimeters of insulation, a passive house has a minimum of 15 to 20 cm of insulation thickness.

Windows

Now, as we have mentioned, it is advisable that on the south façade approximately 70% of the surface be allocated to windows. If these are not well insulated or do not have a proper chamber that prevents heat loss through the glass, installing 20 cm of thermal insulation will be of little use.

Considering that we lose up to 8 times more heat through glass than through walls, in a passive house we must install double or triple glazing, with an air chamber or low-emissivity heat filters. In this way, we achieve transmittances or heat losses similar to those we have in the façades.

In any case, when choosing glazing, we must consider not only insulation regarding heat loss but also the loss of light capture. In this regard, our architect specialized in eco-friendly homes should guide us on the best option according to the climatic characteristics of the area as well as our needs.

Harness the Thermal Mass of Materials

Thermal mass is a characteristic of materials that, while not insulating as such, does control heat exchange between the interior and exterior.

Thermal mass is the capacity of a material to store heat, so it usually corresponds to materials with high mass. Concrete or stone, for example, are materials that, while slow to heat up, accumulate heat for long hours.

In addition, materials with thermal mass tend to accumulate heat through the façades and release it to the interior with a certain time delay—generally, it is recommended that the house be designed so that the delay is about 8 hours. In this way, the heat accumulated during the day will be released to the interior of the house during the night.

This is very beneficial in summer, as having the heat indoors at night, when it cools outside, allows us to open windows and quickly ventilate the environment.

Heating systems such as radiant floor heating work by harnessing thermal mass, which is why their installation under wooden floors is not usually recommended—not because it is not possible, but because wood does not accumulate heat and therefore will not be as efficient as with a ceramic floor.

Harness the Ground

When discussing how a passive house works and having clarified the concept of thermal mass, we must consider a natural element with infinite thermal mass: the ground itself.

If there is a slope on our land, there is the possibility of semi-burying part of the dwelling and thus harnessing the thermal mass of the ground. Ideally, the north façade should be covered, as it does not receive direct sunlight and cannot capture heat.

2- Generate the remaining energy through renewable sources

Through solar capture, ventilation, and the thermal mass of materials, we can achieve a pleasant ambient temperature. However, depending on the area and its local climate, an extra energy input may be needed to achieve a certain level of climate comfort in the dwelling.

In addition, we must consider that we will need to generate energy to heat shower water or to operate appliances, for example.
Let’s see how a passive house works when it comes to generating energy through clean and renewable sources.

Heat Water Using Solar Panels

Although there are other methods for heating water, the most common and widespread is through solar panels. Solar panels should not be confused with photovoltaic panels, intended to generate electrical energy, as a solar panel functions solely to heat water.

A solar panel works through a liquid that heats up and passes through a closed circuit that runs from the panels to a water tank that is heated.

This tank can also be heated using biomass, for example, on days when the sun is not sufficient to meet our hot water needs.

Generate Electricity Using Photovoltaic Panels

Electricity can be obtained from photovoltaic panels. A photovoltaic panel works through chemical processes that transform the sun’s heat into electricity.

Storing electricity is more difficult than storing water—in fact, today the storage problem has not yet been solved, and most installations must feed the surplus into the conventional electrical company’s grid and consume from it during nighttime periods when there is no sun.

This will change in a few years, with the arrival of systems such as Tesla batteries that allow us to store electricity in our own home to consume it when necessary.

Generate Heat Using Biomass

Although there are also other methods or non-renewable sources, we usually recommend installing biomass boilers to obtain the remaining heat needed.

Biomass boilers are fueled either with pellets or with organic waste, whether wood or olive pits, nut shells, or forestry residues. Generally, for convenience, a biomass installation usually includes an automatic pellet tank that we only need to refill from time to time and that automatically supplies fuel to the boiler according to energy demand.

Another option is thermo-fireplaces, which, just as they heat the area where they are located, can be adapted to generate domestic hot water or for the heating system installed in the dwelling.

Generate Heat Using a Canadian Well

A Canadian well is a duct that is passed underground or behind a retaining wall through which air captured from the outside circulates.

Due to the thermal mass of the ground, as we dig deeper we will see that the temperature is more stable. Thus, if we circulate an air duct through the ground, we stabilize its temperature between 18 and 20 degrees.

Once we have this air at an adequate temperature, we only need to conduct it indoors using a simple pump.

We must remember that Spanish building regulations require constant ventilation in the home. Through a Canadian well system, the air for ventilating the house enters at an adequate temperature that neither heats nor cools the environment.

Generate Heat Using Geothermal Energy

Geothermal energy applied to a house works by circulating ducts through which water passes and is heated. These ducts can be located superficially underground or at great depth.

While the temperature to which the water is heated is usually not sufficient to heat the environment, only a small extra energy input will be necessary to reach the comfort temperature, and as we have seen, this input can be provided through affordable and renewable energy systems such as biomass.