How to choose the best passive solar house plan?
Large bay windows exposed to the south, sun visors to avoid overheating in the middle of summer, an inclined roof to follow the course of the sun season after season and a good seal: in a passive solar house, everything is thought to be able to do without heating or air conditioning from January to December. With the explosion of energy prices, this type of housing, the construction of which is more expensive than that of a conventional house, becomes very trendy. Construction principle, materials used, aids, advantages and disadvantages: find everything you need to know about the passive solar house.
What is a passive solar house ?
A passive solar house is a house whose structure, orientation and building materials, among others, allow in principle to do without winter heating and air conditioning in summer. How? ‘Or’ What ? Using solar radiation to store thermal energy in winter and occulting it in summer to keep as much freshness inside the home. So much for the theory.
These houses are said to be “passive” because they do not consume or little energy to maintain a comfortable room temperature all year round. This type of housing mainly uses electricity for lighting and the operation of electrical appliances. The ecological footprint of a passive house is therefore much lower than that of a conventional house. She is supposed to be able to get a hands-on at the Energy Performance Diagnosis (DPE).
As the environmental impact of domestic heating is one of the main culprits of global warming, passive solar houses have been experiencing renewed interest in recent decades, an interest recently reinforced by the rising cost of energy. However, their invention was not new. The Romans, for example, were already experimenting with this concept by building their villas around a central patio which made it possible to regulate the temperature summer and winter.
The Passivhaus label
The most recognized certification for determining whether a house is passive or not is the German label Passivhaus. For accommodation to benefit, it must have annual heating needs of less than 15 kWh ep / m². By way of comparison, a conventional house consumes on average each year for this item 110 kWh ep / m².
Note that the Passivhaus label is not only interested in the thermal performance of the frame. It is also very demanding with regard to the choice of construction materials, which must for example be biosourced, the size of glazing, etc.
What does the unit kWh ep / m² correspond to ?
Mainly used to measure the energy performance of a building, the unit kWh ep / m² corresponds to the primary energy consumption expressed in kilowatt hours per unit area. It makes it possible to express in m² the quantity of energy necessary, whatever it is (gas or electricity), to cover all the needs of the building and its occupants: heating, lighting, hot water production, etc.
The Low Consumption Building (BBC) standard
There is no specific label for passive houses in France which would be the equivalent of the German Pasivhaus. However, since RT 2005, the Bâtiment Basse Consumption (BBC) standard, which imposes on new constructions a maximum threshold of energy expenditure, includes passive solar houses. For lack of anything better, this is the closest standard to those expected from a passive solar home.
Find below the thresholds corresponding to each evolution of thermal regulations :
- RT 2005 (in force from September 2006 to December 2012): up to 150 kWh ep / m² / year;
- RT 2012 (in force from January 2013 to December 2021): up to 50 kWh ep / m² / year;
- RT 2020 (in force from January 2022): neutral or positive energy balance.
French regulations therefore include passive solar houses, low-consumption buildings and positive-energy buildings (BEPOS) in the same category and without distinction.
Find in the diagram below the different annual consumption thresholds in kWh ep / m² / year for each tranche of the real estate energy performance diagnosis and to what type of housing it corresponds:
Passive solar house, low house consumption with positive energy: what differences ?
These different names refer to relatively similar types of housing and generally offering excellent thermal performance but nevertheless having some nuances:
- one or BEPOS is a low-consumption house that produces more energy than it consumes;
- a passive solar house does not need energy, or very little, to heat itself. It does not necessarily have the vocation of being equipped with an energy production system;
- one must display a neutral relationship between the energy it produces (solar panels for example) and the energy it consumes, for heating but also for other energy consumption stations (lighting, production of hot water, etc.).
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What are the advantages and disadvantages of a passive solar house ?
Very popular in northern European countries and in Canada, a passive solar house has many advantages, particularly in terms of energy savings. Its drawbacks are rather to seek on the side of the architectural constraints linked to its construction and which do not make it a housing type adapted to all situations.
The benefits of a passive solar house
Thermal comfort, longevity, energy savings, etc. Find below the main positive points of a passive solar house:
- significant energy savings: this is the raison d’être of a passive solar house which, in the worst case, consumes 7 to 8 times less energy than a conventional house for heating, savings annual which can amount to several hundred euros each year
- a very reasonable carbon footprint: from an environmental point of view, a passive or bioclimatic solar house makes an excellent contribution to the fight against global warming by displaying very low carbon emissions, especially since it is often built in using biosourced materials
- a resistant and 100% reliable system: a passive solar house heats up simply by letting the sun’s rays enter through the windows. It does not need to be equipped with a boiler, radiators or pipes, installations always likely to fail or cause water leaks;
- mild and homogeneous heat: thanks to its great thermal inertia, a passive solar house rises and falls slowly in temperature, absorbing the sudden variations in the outside temperature. In addition, the indoor air being heated over the entire surface of the windows, it is easily distributed in the accommodation;
- an interesting resale value: with the context of rising energy prices and the application in the new of increasingly strict thermal performance regulations, the value and attractiveness of a passive solar home have the best chance of maintaining itself over time.
The disadvantages of a passive solar house
Construction cost, delicate design, difficulty in setting up in urban areas, etc. Find below the main negative points of a passive solar house:
- binding location: a solar house must occupy a clear location in order to be able to capture energy from the sun from morning to night, which is difficult to reconcile with certain urban or forest land. The shade created by a building, an abundant relief or vegetation can be very problematic when you count on the sun’s rays to heat all day;
- fairly rigid construction rules: a passive solar house can hardly afford certain architectural fantasies. Its compact shape, the absence of a balcony or a northern facade opening, can put some of them off. Building a solar house means dealing with many constraints that necessarily condition the interior and exterior layout of the accommodation;
- overheating problems: it can be difficult to calibrate the orientation of the house, the surface of the glazing and the position of the sun visors so that the interior temperature remains ideal in any season. Even if well designed, a passive solar house may tend to overheat in spring or autumn when the sun’s rays are no longer vertical enough to be stopped by sun visors;
- its construction cost: a passive solar house requires a more careful design, better quality materials, more skilled labor than for a conventional home and preliminary studies that result in additional cost, which will take many years to be eventually offset by the energy savings made on heating.
What main lines should follow the construction plan for a passive solar house ?
Building a home that can do without heating and keep a comfortable temperature even in the middle of winter seems to be an architectural challenge. However, this is quite possible, without spending exorbitant sums. However, care must be taken to respect some essential prerequisites.
The design and plan of a passive solar house must make it possible to respond to two major fundamental challenges:
- eliminate heat losses;
- maximize the thermal inputs of the sun.
To achieve this, all things considered: the orientation of the building, the layout of the openings, the choice of materials but also the interior or exterior layout. Planting a hedge of hardwoods, for example, which will protect the house from the sun in summer while letting its rays through in winter, can cause precious degrees to lose or gain.
While the techniques used are known and proven, the construction of a passive solar house nevertheless remains a matter of experienced professionals.
Orientation and location of the building
In a passive house, also called bioclimatic, the main (or even the only) source of heat comes from the sun’s rays. The orientation of construction is therefore essential to capture most of the radiation in winter without transforming housing into an oven in summer.
In general, water parts or service parts, with few windows, will be placed on the north side to avoid heat loss. Conversely, living rooms, such as living rooms or cooking, will be placed on the south side to enjoy the sun and store heat during the day. Orientation to the south is also the best way to protect yourself from the sun in summer by making it easier to install sun visors.
The location of the construction must also take into account the characteristics of the land. For example, gray areas caused by relief, a building or trees that could deprive the accommodation of the sun’s rays supposed to warm it. A clear location is therefore to be favored, while also taking care not to expose yourself too much to the prevailing winds.
The arrangement and size of doors and windows
The arrangement and size of the openings, doors and windows are very important in a passive solar house because it is the glazing that captures and concentrates the heat of the sun inside by serving as radiators in a way. But beware the openings are also responsible for most of the heat loss.
Large bay windows placed in the north are absolutely to be avoided. Only windows arranged on the south side can hope to maintain a positive energy balance (the ratio between heat gains and losses) in the middle of winter. It is generally estimated that the surface of glazing facing south must represent at least 50% of the total area of glazing so that they can properly play their role.
For glass, double or even triple glazing will be favored, possibly filled with a neutral gas such as argon to improve its insulating properties. However, care must be taken not to choose a glazing with too low emissivity on the south side to allow the windows to properly play their role as a thermal sensor. Indeed, the thicker a glass, the better it retains the heat, but the less it lets it in.
It’s all about balance.
Roof tilt, shutters and solar masks
A passive solar house must be designed so that it can adapt to the race of the sun. You have to succeed in both taking advantage of it or protecting yourself from it depending on the season and the energy needs of the household, and it is not always simple.
As such, the inclination of the roof is decisive in successfully regulating the thermal contributions of the sun as a function of the season. It will be tilted strongly on the south side so that it captures the horizontal rays of the winter sun and escapes the more vertical rays of the summer sun. The installation of architectural masks or sun visors on the south facade, which let the sun’s rays pass in winter and obstruct them in summer can also be very useful.
If construction is subjected to a continental climate with large temperature differences between summer and winter, the installation of shutters on the windows can also be of great help in order to avoid significant heat loss in the middle of winter or on the contrary protect yourself from the sun’s rays in summer and thus keep a maximum of freshness inside housing during a scorching episode.
The more compact a house, the more effectively it isolates itself, the more it avoids excessive heat loss. Most passive solar houses thus form a large rectangle within which the different rooms are organized on a single level of preference. This cubic structure, which will avoid any balcony or prominence likely to create a thermal bridge, has the immense advantage of being able to be easily isolated both from the inside and from the outside.
To benefit from good inertia, a passive house must integrate a thermal mass into its construction. Composed of heavy materials (brick or concrete for example) capable of “storing” heat, it plays the role of thermal regulator by accumulating heat during the day and restoring it at night. In summer, the reverse process allows you to slowly restore the freshness accumulated during the night until the evening.
The walls or floor of the house often act as a thermal mass. Their ability to retain heat depends on the specific volumetric heat of the material used, measured in kJ / m³. The more important it is, the more heat the material is capable of accumulating heat. Find in the table below the specific volumetric heat of the main materials used for the construction of a passive house:
Note that if a wall is used as thermal mass, its exterior coating will preferably be dark to allow it to better absorb solar radiation.
To manage to conserve enough heat to dispense with a heating system, a solar house must have the most waterproof insulating envelope possible. This insulation, which can be done from the inside, from the outside or from both sides at the same time, must be lined with an external airwall and an internal vapor barrier to avoid air renewal and heat loss.
The insulation must be carried out using a material having good thermal resistance and avoiding the circulation of air such as rock wool, cellulose wadding or other insulation of plant origin (wooden wool, hemp wool , etc.) or animal (sheep wool). It must apply to floors, walls but also to roofs, which represent up to more than 30% of the heat loss of a building.
It is generally believed that the insulation of a passive house should be almost doubled on all surfaces compared to a conventional house.
Air arrester, vapor barrier: what is their role ?
An airwall sealing insulation by preventing outside air from entering the housing through an air leak or insulation defect.
Conversely, a vapor barrier prevents water vapor present in the indoor air from diffusing into the walls where it would promote heat transfers with the outside.
The ventilation of a passive house
A passive solar house is supposed to be as waterproof as possible to avoid heat loss and prevent air exchange with the outside. It must therefore be equipped with an efficient ventilation system in order to be able to renew the indoor air without cooling it. Ventilation outlets or single flow VMC are therefore to be avoided because they promote heat exchanges with the outside.
Double flux VMC is arguably one of the best systems for ventilating a passive solar house as it avoids the exchange of direct air with the outside. Equipped with a heat exchanger, it ejects the stale air outwards which warms the healthy air coming from outside. Another advantage is that a double flow VMC is reversible and allows fresh and healthy indoor air to be kept in summer.
The Canadian well: a complement to the VMC in cold regions ?
A Canadian well uses the principles of geothermal energy and thermal inertia of the basement, which in France stays all year round at a temperature between 8 and 18 ° C, to cool or preheat the outside air used to ventilate housing. Concretely, we bury a long underground pipeline, about two meters deep, which will play the role of heat exchanger. The outside air uses this pipeline to be preheated, or refreshed, before being blown into the home.
How to renovate an existing house in a passive solar house ?
Given the requirements of a passive solar house, it is very difficult, if not impossible, to transform an existing building into a passive building if the latter does not already meet certain basic criteria, such as the existence of a main facade oriented full south or the absence of a thermal bridge (balcon, dog-seated, etc.).
If you don’t make your home really passive, you can do work to improve its thermal performance and reduce your heating needs. To meet the constraints of a renovation project, the EnerPHit certification, which labels renovation projects as liabilities, offers a more flexible framework than the Passivhaus label or the thresholds provided for in RT 2020, by authorizing heating consumption up to 25 kWh / m² / year.
Before embarking on renovation works to transform your accommodation into a passive solar house in order to obtain this certification, you will first have to carry out a thermal study and an energy balance which will give you valuable indications of their feasibility. You will then most certainly have to implement many costly improvements, namely:
- reinforcement of the insulation, preferably from the inside and from the outside, including the installation of airwalls and vapor bars;
- replacement of simple glazing with double or even triple glazing;
- installation of a double flow VMC or a VMI;
- replacement of the heating system by a low-energy backup system;
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How much does it cost to build a passive solar house ?
The cost of a passive solar house depends on the specifics of each project and the constraints linked to climatic conditions. The insulation of a passive house built in Menton will obviously not have the same thickness or the same cost as that of a passive house built in Roubaix. Where a VMC will suffice to ventilate the house, elsewhere the installation of a Canadian well will be essential, etc. It is therefore impossible to set a basic construction price per m².
In general, the construction of a passive solar house is more expensive than that of a classic house. There are several fairly obvious reasons for this, namely:
- the high cost of construction materials which must display excellent thermal properties;
- the additional cost linked to the installation of double or triple glazing;
- the essential installation of an efficient ventilation system;
- the employment of a trained and qualified workforce;
If, on average, the construction price of a new house according to thermal regulations 2012 (RT2012) fluctuates between 1,200 and 1,600 € according to analysts, it is estimated that sometimes it takes up to double for a solar house passive. As a general rule, there is an additional cost of 20% to 25% compared to a conventional house. This type of housing, even if it allows significant energy savings, therefore remains relatively expensive and requires to be well supported in its project.
As an indication, count between 300,000 and 450,000 euros for the construction of a passive solar house of 150 square meters.
Permit to build passive solar house: watch out for PLU !
Take care, certain architectural requirements linked to the construction of a passive solar house, such as the inclination of the roof, the installation of large bay windows to the south or that of sun visors can be in contradiction with the recommendations of the Local Plan of town planning (PLU). If it does not strictly fall within the framework defined by your municipality in terms of town planning, your project may not obtain a building permit.
What aid can be obtained from the construction of a passive solar house ?
To compensate for its relatively high cost, aid devices put in place by the public authorities can be used for the construction of passive solar houses or low-consumption houses, namely:
- zero rate loan, or PTZ: this interest-free loan is granted by your bank and guaranteed by the State to finance the construction of a principal residence. It must complete a first classic loan. To benefit from it, your resources must not exceed a maximum amount and your land must be located in an eligible area
- exemption from property tax: any new house is exempt from property tax for 2 years following the end of its construction. Note that if you build a passive solar house that meets BBC standards, this land exemption can be extended by a few years if you request it from your municipality or the EPCI (Public Cooperation Establishment) concerned;
- aid from local authorities: certain local authorities (municipalities, inter-municipalities, departments or regions) grant specific aid or subsidies to encourage the construction of passive houses on their territory. Do not hesitate to inquire in town hall to find out if such aid may relate to the implementation of your project.
Despite the existence of these devices which can be applied to the construction of a passive house, it is regrettable that no aid has been specifically provided to encourage the use of this type of housing, which nevertheless offers a particularly suitable response to the challenges of the energy transition, heating representing on average more than 70% of CO emissions2 housing.
Aid for energy improvement or renovation works
If you want to improve the thermal performance of an existing passive solar house or undertake work to transform your accommodation into a passive house, other aids are available than those already mentioned. Note that they are only accessible for renovation works:
- : this premium, the amount of which is fixed and depends mainly on the level of your resources, replaces ISCED and aid from Anah. It is only accessible for dwellings over 15 years of age and makes it possible in particular to finance insulation or ventilation works;
- reduced VAT rate: the VAT rate for renovations to housing that dates back more than 2 years to construction, such as the installation of double glazing or the insulation of roofs, may benefit from a reduced rate of 10% or 5.50% granted under certain means test.
What savings does a passive solar house make ?
From a strictly accounting point of view, it is difficult to determine after how long the savings made by a passive solar house are greater than the additional cost represented by its construction compared to that of a traditional house. However, let’s try a little calculation to get an idea of the question.
- If it is estimated that a passive solar house consumes a maximum of 15 kWh per m² per year to heat up, at the current price of electricity, for a house of 100 m², a maximum annual heating budget of 233.70 euros is obtained. TTC
- If we do the same calculation for a traditional house by taking the average value which is in France of 110 kWh per m² and per year, we obtain for the same surface an annual heating budget of 1,713.80 euros including tax
For an area of 100 m², a passive solar house therefore saves around 1,500 euros in heating per year, or even more, a figure that must of course be weighted according to the geographic location of the accommodation. In any event, it therefore takes 20 to 30 years to amortize the additional construction cost.
Note, however, that certain benefits of a passive solar house, such as the absence of air flow or condensation and overall everything related to interior comfort and provides well-being to the occupants, are difficult to quantify in hard and hard currencies. . They are however very real !
What an electricity supply for a passive solar house ?
If it consumes little energy, a passive solar house must nevertheless be connected to the electricity network for lighting or to supply additional heating when the sun is struggling to maintain a comfortable temperature in the middle of winter.
Total consumption in kWh, which corresponds to the variable part of the electricity bill, not very important in a passive solar house, rather, an offer with the lowest possible subscription price will be favored for its electricity supply to reduce the total amount of invoices.
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