What is a passive house?
The passive house is a building with a high level of comfort, comfort ventilation and a very low consumption of energy for heating purposes – less than 15 kWh per year per square meter of living space – compared to 150 – 200 kWh in a standard building.
The ultimate goal is to keep a combined total consumption of heating, hot water and electricity, at less than 120 kWh a year per square meter.
Components of the passive house
A passive house is different from other buildings because of a few essential components:
- good thermal insulation and compact form – all components of the exterior shell of a passive house are insulated to achieve a U-Value that does not exceed 0.15 W/m
- southern orientation and shade considerations: passive use of solar energy is a significant factor in passive house design.
- energy efficient window frames with triple low-e glazing – windows should have U-values not exceeding 0.80 W/m²K for both glazing and frames – this requires the window frame to incorporate insulation and the glazing to be triple. Solar Heat Gain Co-efficient through the glazing [g] should be at least 50%.
- thermal bridge free construction – even the most insignificant thermal bridges can cause important heat loss
- airtight building anvelope air leakage through unwanted gaps and cracks in the building fabric must be less than 0.6 times the house volume per hour under negative and positive pressurisation of 50 Pa.
- comfort ventilation with highly efficient heat recovery from exhaust air using an air-to-air heat exchanger – Fresh air may be brought into the house through underground ducts that exchange heat with the soil. This preheats fresh air to a temperature above 5°C, even on cold winter days. Most of the perceptible heat in the exhaust air is transferred to the incoming fresh air (heat recovery rate over 80%). The required air refreshment rate is 30m³/h per person.
Besides minimizing heat transmission through the building envelope and leaks, the passive house aims to optimize the heat input, be it a solar input or an internal one [the heat produced by lights, appliances, occupants, etc.].
Comfort in a passive house
The very well insulated shell reduces heat loss, increasing altogether the comfort index for the occupants. Temperature gradients inside the dwelling are avoided thus reducing the risk of mold or condensation. For optimal comfort, the inner surface temperature of the exterior walls should be close to that of the air (18 ° C – 20 ° C).
Very good levels of thermal insulation also translate into very good levels of acoustic insulation further increasing indoor climate comfort.
The continuous ventilation system does not require complicated technical solutions and brings extra comfort by heating or cooling uniformly all rooms. As such the interior climate is constant and the air quality is kept high around the clock, as opposed to traditional dwellings where one loses heat while ventilating through windows creating a decrease in indoor temperature. Filtering used in the ventilation system insures that no dust or pollen particles reach the inside of the house. A low rate ventilation system will also not create noise or draft. Foul air is absorbed in the house through the kitchen or the bathroom and fresh air is introduced in the living/ sleeping areas, which will create a flow through all the rooms in the house.
The leaks inherent of traditional housing technology usually lead to draft and other unpleasant consequences while the passive house stays free of these.
Designing a passive house
The design will be adapted to local climatic and environmental conditions. As such, judicious orientation of the windows – mostly towards the south – will bring a significant contribution towards passive use of solar energy: during wintertime the sun rises low in the sky, the windows allowing passive heating of the house, while in summertime the sun rises higher in the sky, the windows coupled with shading devices will not allow the sunlight in.
Windows are usually not oriented towards north and the east and west oriented ones will be heavily shaded in summertime in order to avoid overheating. East and west oriented windows do not contribute significantly to the energy balance of the house during the cold season. A large number of doors towards the outside translates into a negative influence on the energy balance due to their worse heat transmission coefficient.
The correct mounting of the windows in the passive house wall section is of great importance: the window will be positioned in the thermal insulation layer and its frame will also be covered in insulation in order to prevent thermal bridges.
A compact form with a better A/V (envelope area/ enclosed volume) ratio and a small footprint is desirable, in order to get the maximum [solar or internal] heat gain.
All these requirements can be obtained through rigorous planning and careful building.
Environmental protection
The passive house will help the environment through its reduced energy consumption, reduced CO2 emissions, judicious use of the natural resources, use of regenerative resources for heating/ cooling and domestic hot water preparation.
Financial economy
A comfortable indoor climate can be maintained without an active system [and therefore an expensive one] for heating/ cooling. A passive house translates in low energy use – very significant in the long term. Compared to a traditional building, even a newly built one, a passive house will consume ca. 90% less energy. Following careful planning and good building techniques, a passive house will exhibit following characteristics:
- yearly heating energy consumption ≤ 15kWh / m² of living space;
- yearly heating + domestic hot water (DHW) energy consumption ≤ 42kWh / m² of living space;
- yearly primary energy demand (heating, DHW, appliances) ≤ 120kWh / m² of living space.
Of course, all domestic appliances will be A or A+ energy class, and the lighting will be based on efficient bulbs.
How expensive is a passive house?
The passive house is 5-15% more expensive than a standard one. The premium is paid off during the first 10 years of life of the building, through the significantly lower energy consumption. The superior indoor comfort level, the independence from traditional energy sources and the reduced running costs further raise the market value of such an estate.
In order to encourage the spread of passive house buildings, certain financial institutions offer more affordable credit options than for standard buildings – for passive house newbuilds and for reahabilitations of traditional buildings to passive house standard.
The inovation
The materials and technologies on the market allow reasonably easy for a passive house to be built, even having a reduced budget. If we stay by the aforementioned principles and carefully adapt construction details, any type of structure can be used in order to obtain comfortable and sustainable buildings – concrete, wood, brick and mortar – we only need to maintain a very well insulated envelope.
Heating a passive house
Most of the passive house heating is obtained via passive use of solar energy. This is why the building only requires a peak heating load of 10W/m2 which can easily be delivered through the low rate ventilation system. All of this is possible only because of the judicious orientation of the windows, the well insulated shell and the heat recovering ventilation unit.
Experimental approach
In our country the passive house approach has so far been mostly theoretical. The building we propose here will have a network of sensors monitoring indoor/ outdoor climate conditions as well as energy consumption. In order to further improve energy consumption a logic device will control shading devices and lighting intensity.
The aim is to thoroughly investigate the behavior of such a building in local climate conditions which pose a great challenge having rather large extremes (quite cold in wintertime but rather warm during summertime). While wintertime heating is easily attainable in a passive house, summertime cooling still poses a great challenge.
The approach allows a direct comparison of the energy consumption and indoor climate of a passive house and a well insulated standard house.
Structure
Foundations: The structure discharges the loads on the ground via isolated concrete blocks linked together by concrete beams supported on isolated bearings. The reinforced concrete beams are 30 x 80cm in section. The concrete blocks sizes are different, according to the necessary load to be transmitted to the earth: 1,40 x 1,40 x 1,00m on the building corners; 2,00 x 2,00 x 2,00m in intermediary positions; 2,40 x 2,40 x 1,00m at the intersections of structural walls.
The foundation depth is -1,40 m.
Materials used: reinforced concrete beams C16/20, concrete blocks C12/15.
Superstructure:Load bearing walls will be built from Porotherm brick 25cm thick. The 1st story floor will consist of wood beams 15 x 25cm sections laid at a distance of 50cm between axes. The beams are simple beams resting on the reinforced concrete block at the top of the load bearing walls.
Access at the first floor will occur via a wooden staircase.
Roof: The roof structure is similar to the 1st story floor, insulated with a waterproof membrane and corresponding thermal insulation.
Concept
The building will consist of two adjacent housing units built using the same structure concept but used in different ways: one will be insulated and fitted mechanically as a passive house, while the other will be just as well insulated but will use traditional fixtures and appliances. This will allow a better understanding of the contributions the mechanical and ventilation fixtures have in a passive house and will provide a basis of comparison in order to educate the population towards energy efficiency.
Unit no. 1will have a heat recovery ventilation unit to provide fresh air while preserving the heat. Additional heat necessary to maintain the indoor climate will be sourced by an air/air or a brine/air heat pump. Solar panels on the roof will prepare domestic hot water in the summer and contribute to space heating during winter.
Unit no. 2 will ventilate in the traditional way, by opening windows. Heating and hot water will be prepared with a heat pump and an electric boiler.