Updated: Sep 25, 2020
Imagine a perfectly insulated building, as if it had been covered with a coat that protects it in winter as well as in summer. It's nice inside, since you occupy it and radiate heat back into it. If the weather is sunny, it's even better, and you don't have to heat it. That's what it feels like in a passive house.
TLDR: Passive houses, compared to traditional houses, are particularly well insulated and energy efficient, especially in terms of heating. As such, they are much cheaper to maintain, despite the fact that they cost about 10% more to build. They represent a real asset for the future of the ecological and energy transition, and allow you to be easily independent from the energy inputs of your local distribution network, as a few solar panels are enough to power you. In addition, they can be built with environmentally friendly materials to further minimise the environmental impact.
The concept of the passive building is that the heat released inside the building (living beings, electrical appliances) and the heat brought in from outside (sunshine) is sufficient to meet the heating needs. An occupied building that does not loose internal heat does not need heating to remain pleasant to live in.
Compared to a building that complies with current thermal standards, a passive construction saves a lot of energy. The energy expenditure for heating must be ten times less than a conventional building. As we have seen above, this energy saving considerably reduces the impact of the house on the environment.
However, a passive building can be built with many materials, from the most artificial to the most natural. So it is quite possible to have both: ecological design/build and passive performance. The studies are formal: the biggest impact of a building is not during its construction, but its energy expenditure during its years of occupation.
Heating less homes
The big advantage of these houses is that there is almost no need to heat them, and this is good news because heating is never ideal:
Heating creates pollution
Radiators are rarely pretty and create layout and maintenance inconvenience
You always have hot and cold areas
It can costs you quite a lot overall
Characteristics of the passive house
It is quite obvious that in order to keep the heat in a building (or leave it outside when it is hot), the walls must be well insulated. The main losses occur when heat passes through these walls: first the roof (hot air rises), then the walls and finally the floor. Doors and windows are considered as special walls (transparent and/or opening).
In addition to efficient insulation of the walls, particular attention must be paid to the suppression of heat passage through particular points of the structure, called "thermal bridges" because these are passages (bridges) that promote heat loss. In practice, insulation from the outside must be favoured because it eliminates these passage points.
Doors and windows, which are less insulating than fixed opaque walls, must also achieve a higher level of insulation. The use of triple glazing is advisable or even necessary to achieve sufficient insulation performance in a passive building.
2. Ventilation and airtightness
To avoid heat loss, a passive building must avoid any air passage. You know those unpleasant cold air streams in poorly finished buildings? Before the advent of controlled ventilation, these passages allowed the renewal of air, essential to the well-being of the inhabitants. They are now to be eradicated because they jeopardise thermal performance and can cause damage to the envelope.
Moreover, the ventilation of a passive building is the only "forced" passage of air, and no longer by "leaks". Like insulation, airtightness is therefore an essential criterion in passive buildings. It should be noted that the air passing through a double flow ventilation system is filtered, which allows passive buildings to benefit from excellent Indoor Air Quality (IAQ).
3. Outgoing heat recovery
A passive house, like any modern and comfortable house, is well ventilated. But since ventilation draws in outside air and then expels it outside after passing through the warm zone, there is no question of heating this incoming air and then finally throwing this heat outside. When insulation is satisfactory, ventilation becomes a major channel for heat loss. So the idea is simple: heat is recovered from the outgoing air (not the air itself, just its heat) to warm the incoming air. In cold periods, there is no more question of bringing in frozen air!
For this purpose, passive constructions are most often equipped with a so-called "double-flow" ventilation (incoming and outgoing flows pass through the ventilation system) with a heat exchanger. To have its place in a passive building, this system must be able to recover more than 75% of the heat from the outgoing air and communicate it to the incoming air (efficiency calculated on the extracted air).
It is now possible, to save even more money, to recover heat from outgoing "grey" water (dishwasher, washing machine, shower, sinks) to preheat incoming water from the network (or incoming air).
The price of a passive house
Passive houses are more expensive than traditional buildings, but only if the initial investment is taken into account. The thermal study, the careful construction, the quantity and quality of the insulation and other materials, the use of specific joinery increase the cost of building a passive building. It is estimated at 5-10%.
But when taking all the costs into account, it is in fact much cheaper. Thanks to the energy savings made possible by passive construction, the operating cost will be lower than that of a "regulatory" building for decades to come. If you are interested in passive construction, it is probably because your horizon is not limited to next year. So one of the advantages of passive construction is to be ahead of current thermal regulations.
By building (or renovating) according to passive criteria, you know that the resale of your property will be easier than the resale of the vast majority of properties. The asset value of a passive building is unquestionably higher than that of an equivalent regulatory building, with the difference likely to increase steadily as energy costs rise.
What you need to know
A true passive construction must meet 3 specific criteria:
1. The heating requirement must be less than 15 kWh/m²/year. This is the result of economic optimisation (no independent heating system). For a 100 m² house, this means a maximum of 1,500 kWh for a full year.
2. Primary energy consumption must be less than 60 kWh/m²/year of renewable energy for all uses, from heating to household appliances. Depending on the energy source used, the conversion of primary energy into final energy may or may not be penalising. Renewable energies are much more favoured by this conversion.
3. Air tightness n50 < 0.6 /h: this criterion is difficult for a novice to understand, it is enough to know that this air tightness is tested using the "BlowerDoor" test and that it means the absence of leaks and air passages.