Retrofitting Passive House Components
Although the successful implementation of passive house standard in new buildings plays an important role in the overall strategy to reduce greenhouse gas emissions, the improvement of the energy efficiency of the existing building stock is of even bigger importance. In Austria, the yearly rate of new built apartments is about 1% of the existing building stock. Depending on age and building type, the specific space heat demand is 130–280 kWh/m2. As only about 1%–1.5% of the building stock is retrofitted per year and this rate cannot be increased to much more than 2.5%, the improvement of the energy efficiency quality of retrofits is essential in order to reach the national, European and international targets for the reduction of greenhouse gases.
However, since 2001, more and more renovations in Austria, Germany and Switzerland have been carried out using components that had previously been tested in new passive houses. Different names are used for these houses, sometimes called ‘factor 10-houses’ as the energy demand after renovation is only a tenth of the original demand.
In these projects, a specific heat demand after renovation of 23 kWh/m2 a, was achieved.
The main elements of the energy concept are typical passive house components:
Excellent insulation level of opaque building elements: u-values range from 0.10 W/m2K for walls and roof to 0.18 W/m2K for basement ceilings.
Triple glazed windows with adequate frames and an optimized installation.
Thermal bridges reduced to a minimum.
The airtightness was improved by a factor of 6–10, the limiting value for new passive houses was achieved.
A ventilation system with highly efficient heat recovery installed.
Thermal solar collectors installed covering up to 60% of the annual energy demand for domestic hot water.
Highly efficient condensing gas boilers were installed; where possible, ducts have been insulated to a very good level; in other projects biomass boilers have been successfully tested.
Benefits of the Retrofit Programme
Experience with the renovations up to passive house standard is so far very good in the Vorarlberg projects, as well as in projects in other regions of Austria, Germany and Switzerland.
Thermal comfort has been improved to a level superior to that of a conventional new house; due to the ventilation system the air quality is improved, and energy bills are reduced drastically. As thermal bridges are minimized, the airtightness is significantly improved and the air exchange is always up to hygienic standards due to the ventilation system; the main causes for structural damage and mould problems are also eliminated.
Furthermore, the measured energy consumption shows good comparison to the demand that was calculated in advance using the passive house planning package (PHPP).
Austrian and German research has shown that for bigger apartment buildings renovations to passive house standard or very close to it cost about €450–600/m2a. The extra cost compared to a renovation up to national building code is in the range €80–150/m2a.
For comparison a new apartment building costs about €1600/m2a in western Austria.
Nonetheless, detailed analyses show that most of the measures used in passive house retrofit are economically feasible, for example, the overall lifecycle cost for investment and energy is lower using the passive house insulation of 26 cm compared to the building code insulation of 12 cm. As for most renovations lifecycle costs are not calculated, home owners and housing companies tend to realise sub-optimal insulation thicknesses.
In some regions of Austria, the regional funding system therefore differentiates the rate of funding according to the energy efficiency quality of the house after renovation – the lower the energy demand, the higher the funding. The experience in Austria shows that this funding rate differentiation is an effective way of encouraging very high quality standards.