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SIA TeslaHouses
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Riga, Latvia
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Energy efficiency

All our projects are designed according to Passive House standard, which is the world‘s leading standard in energy efficient construction and is truly energy efficient, comfortable and affordable at the same time. The following criteria provide an introduction to the Passive House concept and an overview of the basic principles relating to the leading energy efficiency standard.

1 Insulation

In a Passive Home the whole building envelope has an excellent thermal insulation. The envelope consists of all parts of the construction, which separate the indoor climate from the outdoor climate. All construction methods can be used for Passive Houses and have been tested successfully: masonry construction, lightweight construction, prefabricated elements, insulating concrete formwork construction, steel construction, and all combinations of the methods above.

The thermal heat loss coefficients (U-Values) of external walls, slabs to the ground, and roofs are within 0.1 to 0.15 W/(m²K) (for Central European climate). These are upper level values for all contemporary constructions. As a consequence the transmission heat losses during the cold season are negligible. Another consequence is that the temperatures of the internal surfaces are almost the same as the indoor air temperature. This leads to a very comfortable indoor climate and avoids damages caused by the humidity of indoor air.

During hot periods in summer, a high thermal insulation is a protection against heat, too. To ensure high thermal comfort during summer, well designed shading and sufficient ventilation are important, too.

Good thermal insulation and an airtight construction are well proved in Passive Houses. Another basic principle used is "Construction Avoiding Thermal Bridging": The insulation is applied continuously around the envelope of the building without serious thermal bridging. By this method there will be no cold spots and no increased heat losses. This is a contribution to high quality, comfortable, and long lasting construction, too.

2 Thermal bridge free design

In a Passive home the heat losses of thermal bridges are significantly reduced, too. The reduction is made to a degree that the losses through thermal bridges become negligible. If the thermal insulation is not disturbed at any place of the envelope, the heat loss calculated with the U-values and the external surface areas of the building will be higher than the actual losses (including all thermal bridges).

Heat will flow the easiest path from the heated space to the outside - the path with the least resistance. And this will not necessarily be the path perpendicular to the surfaces. Very often heat will “short circuit” through an element which has a much higher conductivity than surrounding material. In such cases the experts call this a "thermal bridge".

Typical effects of thermal bridges are:

  • Decreased interior surface temperatures; in the worst cases this can result in high humidity in parts of the construction

  • Significantly increased heat losses.

Both can be avoided in Passive Houses: The internal surface temperatures are high enough that a critical humidity can not occur at any place, and the additional heat losses will be negligible. If the thermal bridge coefficient (which is an indicator of the extra heat losses of a thermal bridge) is lower than 0.01 W/(mK), the detail is said to be “Thermal Bridge Free”.

If this criterion of avoiding thermal bridges is fullfilled throughout the thermal envelope, neither the designer nor the builder has to worry about cold and humid parts in the construction - and it will be far much simpler to calculate the heat energy balance.

3 Airtight construction

The key to making a building airtight is the principle of a continuous, tight building envelope. Passing the blower door test is a necessary prerequisite for any new building.

If a construction is not sufficiently airtight, moist room air can penetrate into the construction, condense and cause damage. In hot humid climates infiltration of humid outside air can damage construction. The problem can be solved by thorough, air tight design. Air tightness is not a mere nicety of energy saving construction, it is essential to avoid construction damage. Gaps in the construction will lead to substantial humidity transport by convection.

Of course details are important. But an envelope can be air tight only if it consists of a uniform, in tact, airtight enclosure wrapping around the whole house volume. In first step, for each element of the envelope, that part which is to achieve air tightness must be specified (e.g. the particle board or OSB in a roof construction). In a second step how the airtight layers of elements will be connected has to be detailed, assuring lasting air tightness.

Insulation materials generally are NOT air tight (exception: foam glass). Therefor, the airtight envelope has to be designed and built separately. In timber constructions in most cases wooden composite boards are used (taped at the joints), in masonry construction a continuous inside plastering is sufficient. It is important, that the airtight envelope is continuous, without interruptions. This must be designed for, with particular attention to joints.

Architects of passive houses are familiar with the design of high quality and enduring joints. There are special products to facilitate airtight external construction. The exhibition held during Passive House Conferences is a valuable source of innovative and effective solutions to achieving air tight construction.

The external envelope of a building should be as airtight as possible - this is true for conventional as well as for passive houses. It is the only means to avoid damage caused by condensation of moist, room warm air penetrating the construction (see the figure on the left hand side). Such damage not only occur in cold climates; in hot and humid climates the problem can occur from airflows from the outside to the inside. The cause is the same in both cases: a leaky building envelope.

A key principle is maintaining "an undisturbed, airtight envelope", which can be recognised by the "rule of the red line" (see the section in the third figure on the left hand side).

4 Highly insulating windows

Compared with new standard windows Passive House windows reduce heat losses by more than 50 %. They allow for a new level of quality which translates into the advantages of a pleasant indoor climate and a positive energy balance even in the coldest climates and in the middle of winter.

To build Passive Houses, highly efficient windows have to be used. The type of glazing and frames will depend on climate, however. In the Central European climate there are three essentials:

  • Triple glazing with two low-e-coatings (or another combination of panes giving a comparable low heat loss)

  • "Warm Edge" - spacers,

  • Super-insulated frames.

These components harmonize in a way that the total heat loss of such a window is only half as high as compared to a conventional new window. But direct and indirect solar gains are collected through the glazing, too. Therefore, it has been demonstrated that by using these highly efficient windows, the result will be a positive energy balance even in the Central European winter period, as long as the orientation is suitable and the shading not excessive.

The thermal loss coefficients, Uw, of such Passive House windows are lower than 0.8 W/(m²K) according to the new European standard (EN 10077). One consequence of such a low heat loss is that the interior surface temperature of such a window, even in cold European winter nights, will exceed 17 °C. This results in excellent thermal comfort even near the window: There will be neither trouble with "cold radiation" from the window nor an unpleasant lake of cold air at the floor.

The 17 °C condition for minimum internal surface temperatures of windows in a Passive House is the defining requirement for Passive House windows in any given climate.

5 Heat recovery ventilation

The health and comfort of the inhabitants are the most important objectives of a Passive House design. Excellent indoor air quality is indispensable. But this can only be achieved if stale air is exchanged with fresh outdoor air at regular intervals. This can definitely not be done by just opening windows twice a day.

Ventilation will work accurately only if polluted air is removed constantly out of kitchen, bathrooms, and all other room with significant air pollution. Fresh air has to be supplied to the living room, children’s room, sleeping rooms, and workrooms to substitute the removed air.

The system will supply exactly as much fresh air as is needed for comfort and for good indoor air quality; only outdoor air will be supplied – no recirculated air. This will lead to a high level of indoor air quality.

What has been discussed so far could be satisfied by using a simple exhaust fan ventilation system, where the air is supplied through direct vents in external walls. These vents allow fresh (cold) air to enter the room at the required rate. However, for a Passive House, the heat losses caused by such a system are much to high.

In Central Europe Passive Houses will only work with highly efficient heat recovery. Heat from the exhaust air is recovered and applied to the supply air by a heat exchanger. The air flows are not mixed in the process. State of the art ventilation systems may have heat recovery rates of 75% to more than 95%. Of course this only works with counterflow heat exchangers and very energy efficient ventilators (using so called EC-motors with extraordinarily high efficiency). With this technology the recovered heat is 8- to 15-times higher than the electricity needed.

6 Solar heating system

Every household can produce a large part of their energy requirement themselves using modern and state of the art solar heating systems. They must be easy to assemble, low maintenance, easy to use and have a long useful life.

Read more about passive house

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