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Hristo Stankushev Dipl. Arch, Svetoslav Michev Dipl. Arch. Rumen Yordanov Dipl. Eng. MEP and Sustainability Consultant - AEE Asian European Engineering Ltd.

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PHPP2007_Hristo_Stankushev_and _AEE_Ltd.pdf

For the design of the Lozen Passive House we have studied different options for situation and form of the building. Starting from a narrow single-storey house with all rooms facing south for maximum solar gains, through an open plan narrow house with long east and west facades for best natural ventilation, we have come to the nearly cubical shape of a two-and-a-half storey semi-sunken into the ground building. During the design process, a dynamic computer modeling and simulation has been undertaken involving Ecotech for sight insolation modeling, Radiance for interior daylighting and illuminance modeling and IES VE for energy modeling and computational fluid dynamics.

Our Lozen Passive House is fully compliant with the Passive House standard for energy efficient building design. All exterior above grade walls have a U-value of 0.096 W/(m2K), roofs have a U-value of 0.094 W/(m2K) and below grade walls and ground-contact floors have a U-value of 0.121 W/(m2K). The construction is designed to be entirely thermal bridge free. All glazing is Passive House certified with U-value of 0.7 (glazing only) and g-value of 0.5. As a result the envelope is airtight with infiltration rate of 0.3 ACH. All building services equipment use variable speed drives and has the highest class of energy efficiency as do all home appliances. Energy recovery with the highest recovery efficiency is utilized for ventilation and DHW. Our Lozen Passive House has a Specific Space Heat Demand of 12 kWh/(m2a) and Specific Useful Cooling Energy Demand of 2kWh/(m2a) with only 6% frequency of overheating. The total Specific Primary Energy Demand is 73 kWh(m2a). Furthermore, there are 25 m2 of PV panels providing 2,380 kWh electricity per year. Additionally Lozen Passive House contains a number of energy efficient and environmental features described below exceeding the requirements of Passive House Standard.

During the design development international sustainability standards as LEED, ASHRAE Standards 62.1 and 90.1 and CIBSE Guidelines have been considered. Numerous aspects of sustainability have been explored and included in the design adding to and exceeding energy efficiency concepts.

For better winter solar gains we have incorporated into the design a large picture window on the south façade and up to 30% glazing on the west and east facades, while having minimum openings on the north one. The whole second floor is open plan in order to optimize natural ventilation with predominant winds in the area from the west and the east. For better insulation of the building skin we have buried the mechanical, storage and sleeping areas as much as possible.

The house is situated in the north part of the plot, next to the street, providing an optimal view to Vitosha Mountain to the west and Sofia city to the north-west. This location also contributes to enhanced indoor and site ventilation.

The shape of the building is almost cubic with a roof slightly inclined to the south, thus, achieving an optimum façade-to-volume ratio (0.69) and facade-to-area ratio (2.57). The inclined roof accommodates solar panels, skylights and a roof garden. The building skin is airtight and super insulated with 30cm of extruded polystyrene and it features cladding made of certified wood from local producers. The south façade is equipped with a ventilated façade wall for better solar gains in the winter and stack effect ventilation plus shading to reduce overheating in summers.

The house entrance and the garage are on terrain level and are approached from the north. The entrance lobby leads through a storage space to the kitchen, to the midlevel living and dining areas and to the subterranean level of the bedrooms. The fulcrum of the building is the sky lit central stairwell which goes through all the levels of the house and ends in a garden on the subterranean floor, providing natural light at the core of the house.

All the main rooms (i.e. living, dining, bedrooms) have south facing windows while, the library has north-west facing window with a view to the city of Sofia. The guest bedroom and the kitchen have eastern windows, while all the rest (i.e. mechanical, storage and garage) have no windows. The room height varies according to the function– from 300cm in the living and dining areas, to 240cm in the kitchen, bedrooms and library, and 210cm in the storage and mechanical rooms.

Our Lozen Passive House utilizes a number of passive house features. Most importantly, it has been designed with a fully Passive House compliant construction featuring Airtight passive house certified windows with insulated spacers. We have combined less than 30% overall windows-to-wall ratio with an optimum indoor environment. We have paid special attention to thermal envelope avoiding thermal bridges. For that reason we have places foundations on aerated concrete blocks and insulated foundation slab to eliminate below ground thermal bridges while providing a radon barrier.

Moreover, full consideration has been given to optimization of the energy efficient design. Energy saving features implemented within our Lozen passive house are summarized in the table below:

Energy Efficient Features



Ventilated Facade

Provides stack effect ventilation and shading during summers and additional insulation during winters

Snow storage for cooling in summers

Utilizes freely available snow in winters to offset summer cooling load

Geothermal heat pump with horizontal ground loops

Achieves high efficiency compared to conventional heat pumps

Energy recovery ventilation

Pre-treatment of air utilizing exhaust air energy

Earth-to-air preconditioning

Pre-treatment of air utilizing year round stable ground temperature

All motors equipped with variable speed drives

VSD controls achieve energy savings under part load conditions

Energy efficient labeled appliances

Efficient appliances minimize electricity used by the household

Low energy CFL and LED lighting fixtures

Efficient light bulbs achieve the same lumen output using less energy

Optimized Illuminance for interior lighting

8 W/m2 for indoor lighting and task lighting addressing the work areas

Enhanced lighting controls – occupancy sensors and daylight controls

Eliminate energy consumption for lighting unoccupied or daylit areas

Light tube providing natural light on the top floor.

It introduces daylight over the working area in the study room saving energy for artificial lights

Domestic hot water heat recovery

Pre-treatment of domestic hot water by utilizing process water and shower water’s heat

Solar hot water collectors

Converts solar energy into heat source for domestic hot water

Renewable Energy - photovoltaic panels

Converts solar energy into electrical energy


Water saving features implemented within our Lozen passive house are summarized in the table below:

Water Efficient Features



Rainwater, grey water and HVAC condensing water collection for irrigation purposes

Recycles water from these sources to offset irrigation demand

Underground water toilets flushing and irrigation purposes

Utilizes underground water for flushing and irrigation, thus, reducing utility demand

Low-flow water fittings

Optimizes water use for household needs

Drip irrigation system

Reduces the water demand for irrigation


Other Sustainable Features have also been incorporated into the design such as a vegetated roof which provides an enhanced thermal insulation and increased wildlife environment while alleviated the heat island effect. Also all exterior lights face the ground in order to minimize light pollution.

Moreover, we have given a full consideration to materials and waste management during construction and occupancy stages. All wooden products specified for interior and exterior uses will be delivered from certified and controlled sources. Furthermore, a material budget and a target of 50% of the materials are from local sources – extracted or harvested, as well as manufactured, within 800km from the project site and also the recycled content is targeted at 30% of the materials used.

Also a construction management plan has been drafted, identifying construction wastes to be diverted from disposal and reused or recycled. During the occupancy period a waste management system has been incorporated in the design, including on-site waste sorting and separated disposal in labeled bins.


Design by:

Architecture - dontdiy – arch.Hristo Stankushev and arch.Svetoslav Michev

MEP and Sustainability Consultant - AEE Asian European Engineering Ltd. - eng.Rumen Yordanov