For my case study of an international, certified Passive House, I have chosen ‘the Smith House’ in Urbana, Illinois. It was the first house to be certified Passive in the United States and subsequently Urbana is now home to the Passive House Institute of the United States or PHIUS. This house faced particular difficulties with weather conditions, as Illinois is subject to wide variations in temperature, with very hot summers and very cold winters. The mean summer temperature is 28°C and the winter temperature is in the region of -7°C. This temperature difference would have limited the choice of material for certain building elements.
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| Large, South Facing Windows ( barrier-busting.com) |
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| With Grape Trellis (zigersnead.com) |
The house has a Concrete Block Frost Wall foundation with 6 inch Expanded Polystyrene (EPS) Foundation Perimeter, which has an R-Value of 24. The R-Value is the level of thermal resistance of an element, the higher it is, the better the insulating properties of that element. There is a 4in. concrete slab on top of 14in. of EPS (R-Value = 56), and the total floor area is 1200 ft2. The walls are timber framed with 12in. I-Joists and 12in. blown-in fibreglass insulation, along with 4in. exterior EPS; this has a total R-Value of 60. The windows are argon-filled and triple-glazed, with EPS insulated fibreglass frames. The South facing windows in the Smith House are exposed to sunlight for more hours in the day so they have a Solar Heating Gain Coefficient (SHGC) value of .61, while the other windows in the house receive less sunlight so have a SHGC value of .37. The windows and doors were sourced from Thermotech Ltd. The roof is standard 16in. I-Joists with 16in. blown-in fibreglass insulation (R-60), with venting channels installed above the sheathing. The airtightness of the building is 0.6 ACH50, which means essentially that there are 0.6 air changes per hour in the house. Airtightness was maintained in the building by gluing and taping all panel joints. The ventilation system in the house incorporates a 1kW electric resistance element in a Westaflex WAC 250 Heat Recovery Ventilator (HRV). The element is the only supplementary heating source for the house. The finished house has a heating load of 11kWh/m2, which is safely below the Passivhaus standard of 15kWh/m2.
The Smith House is very well insulated and the large South-facing windows, shown in the top figure, allow for significant Solar gain. This means that the house was more that capable of staying comfortably warm in the very cold winters that Urbana experiences. For the same reasons, however, overheating in the summer proved to be a problem. The incorporation of shrubbery to provide shade in the summer time helped in solving this problem. The grape trellis used can also be seen in the 2nd picture. The choice of a deciduous shrub meant that the shade is only provided in the summer time when it is needed. The fact that the electricity bill for January 2004 for the house was $35 speaks for itself and is all the financial incentive that should be needed to encourage the incorporation of Passive House construction in the future.
