Fig. 1 House retrofit nearing completion

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1 RIAI Sustainable Design Tools - Case Study 01: Passive House Domestic Retrofit of Rathdrum Residence using PHPP, Therm & RETScreen - Archie O Donnell of Integrated Energy Fig. 1 House retrofit nearing completion Introduction In April 2009, Integrated Energy was asked to advise on a specification for a house renovation, which would deliver a certified Passive House. The clients wanted to reflect a sustainable living philosophy in their choice of housing and felt a new build on a greenfield site would not be a sustainable choice. They purchased an existing house with a view to renovating it to become a house that produces more energy than it consumes (sometimes referred to as an Active House), delivered through low carbon technologies. Strategy In order to produce more energy than it consumes renewable strategies would need to be employed. However renewable energy equipment capital costs are high (this is reflected in a higher price per kwh delivered versus conventional fuel energy system costs) so reducing the peak demand on electrical and heat energy use would be key and mean that renewable plant could be used more cost effectively. To reduce the overall heat demand, heat losses though the building fabric and ventilation would need to be minimised. The passive house concept was seen as the best way of eliminating the wasteful use of energy while also providing an enhanced level of thermal comfort. In a new house, reducing heat demand is achieved by improved solar aspect and compact building form. As the clients did not meet the strict local needs criteria, there was a requirement that the design submitted for planning permission closely resembled the existing building. This restricted the ability to make significant modifications to the building s design. The house was built in the early 80s with various extensions added since. The original house required 190kWh/m 2 of energy for space heating to provide comfort, the renovated house would have to use less than 15kWh/m 2 to achieve Passive House requirements. The main glazed elevation faces west with insufficient south and east glazing and the existing front elevation faces north to address the roadway. With little scope to 1 of 5

2 change the building form and orientation and limitations on increasing the window to wall ratio, reducing the buildings heat load would have to be achieved in the building fabric. Fig. 2 House renovation in progress. PHPP The Passive House Planning Package (PHPP) was selected as the most appropriate tool for accurately determining the energy requirements of the house in question. It takes a whole systems approach to calculating the energy demand of a house, integrating the impact of fabric assemblies and components, local weather data and site topography to confirm the ability of the ventilation system to deliver a stable comfort level of 20 C year round. It achieves this by optimising solar and occupant gains while recovering heat through the ventilation system, resulting in a minimal requirement for heating. Budget Data outputs from the PHPP excel spreadsheet were used to inform decisions on how best to allocate the budget. Through various simulations of the model we were able to select the optimum choice of components. This validated the decision to prioritise fabric improvements over renewable technologies on economic grounds. Economically it is significantly cheaper to save energy through insulation than to generate energy using renewable technology. The initial focus in PHPP is to select wall, roof and floor assemblies and a glazing system. Thermal Bridges and Therm Assembly U values were calculated using PHPP and verified using Buildesk U. Repeating thermal bridges such as timber studs were included in the U-value calculation. Non-repeating linear thermal bridges at junctions between planer elements were calculated using Therm. The integrity of insulation is often compromised where walls meet floors and roofs, at window sills and reveals and at chimneys and gable walls. The cumulative effect of heat loss due to linear thermal bridging alone can equate to 10-15% of fabric heat loss. Heat loss through thermal bridges is calculated as the heat loss across the thermal bridge at a stated temperature difference between inside and outside, expressed as the combined heat loss across the 2 dimensional assembly minus the heat loss through the flanking planar elements. Therm software is available free on the internet and calculates heat flows across both the assembly and the flanking planer elements. The heat flow outputs are then inputted into the relevant 2 of 5

3 formula from ISO 10211:2007 and BRE document BR 497 Conventions for Calculating Linear Thermal Transmittance and Temperature Factors. When calculating thermal bridging in Ireland and the UK, the thermal bridge heat loss is calculated from the internal dimensions. In PHPP, the heat loss is calculated from the external surface. The rising wall cold bridge, at the existing walls of the house renovation was the most significant cold bridge. The transmission per linear meter for this cold bridge was calculated at Ψ= Psi. This figure is then multiplied by the length of cold bridge measured with exterior dimensions. This cold bridge was responsible for heat loss equivalent to 0.9kWh/m 2 /yr. The example below illustrates the method for calculating thermal bridging coefficient Ψ or Psi for the Irish or British Building Regulations, this uses internal dimensions and should result in roughly the same heat loss. Fig 3 Therm software showing infrared results Fig 4 Therm simulation software showing isotherm results 3 of 5

4 Referring to BR 497 Conventions for Calculating Linear Thermal Transmittance and Temperature Factors: U-values for planer flanking elements calculated with Therm Ux = U-value for floor = W/m 2 K Uy = U-value for wall = W/m 2 K Ireland & UK Method (INTERNAL) Thermal coupling coefficient for composite assembly: L 2D = Therm U-value * length = Projected X - L 2D = * 1.0 = W/mK = Projected Y - L 2D = * 1.4 = W/mK Ψ PSI = L 2D (Lx * Ux + Ly * Uy) = (1 * * 0.115) = ( ) = Ψ = W/mK Once the walls, roof and floor details and their junctions are designed to minimise heat losses, various window specifications can be compared in PHPP. In Ireland, due to our mild winters, certified Passive House can be achieved using good quality glazing without strictly requiring a Passive House certified window. However in the renovation, the aspect, overshadowing by an adjoining outbuilding and the high ratio of exposed surface area meant that in order to reduce the heating load below the Passive House threshold, a window with a combined frame and glass U-value less than 0.08 W/m 2 K had to be used, installed in a fitting free from thermal bridges. Performance The graph below indicates the projected performance of the renovated house design. The free heat gains for the months of the year are indicated as the yellow blocks. From May to September there is virtually no requirement for heating. In the coldest months of December and January, a heating demand of 3.6 kwh/m 2 is required. Presently, for a 200m 2 house, this equates to approx. 100 of oil for the two coldest months and less than 200 for heating for the entire year. In addition, hot water would also cost approx However, with the introduction of solar panels, 50-70% of the hot water requirements could be met, saving 120 to 150 per annum. Fig 5 PHPP monthly specific heat demand 4 of 5

5 Renewables and RETScreen Heating is delivered in the Rathdrum renovation by underfloor heating on the ground floor, distributed to the upper floor using the heat recovery ventilation system. A combination of 15m 2 of solar thermal and a 7kW electric heat pump fed by photo voltaic and 2.5 kw of micro wind, provide heat to a 4000 litre heat bank. This is a stratified water storage tank, which can have a temperature of 25 degrees at the base and up to 98 degrees at the top of the tank. Excess energy produced by the PV and wind is sold back to the grid. Equally when the house s electricity demand exceeds the power that can be drawn down from the micro renewables, this power is bought from the grid. The EMMA power management system performs the task of feeding into the grid. The feasibility of both wind and solar renewable was assessed as a desktop analysis using the RETScreen software. This determined the potential output from the systems and the potential return on Investment. In the case of our site, the results were inconclusive as to the financial feasibility of solar PV and micro wind, but the sensitivity analysis tools were able to give assurance that the installation would at least be cost neutral over the installation lifetime. This eliminated the risk to the homeowner whose reasons for installing the technologies went beyond financial considerations. Conclusion The building was completed in October 2009 and is being monitored for performance and tested to assess if it does in fact meet the strict Passive House criteria. The air tightness was measured at an impressive 0.7 air changes per hour and some modifications are being made to ensure that air tightness achieves the onerous 0.6 ach required by the Passive House Institute (PHI). Should this benchmark be achieved and pending verification by the PHI that design calculations match the completed building, then the build would be one of only 3 renovations worldwide to meet the Passive House standard set for new build construction. 5 of 5