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Construction:
The building is a hybrid of masonry and timber framed construction in order to achieve a balance between effective insulation and thermal mass.
The north walls onto the North Circular Road are heavily insulated cavity walls with a U-value of 0.19 W/m2K with a brick outerleaf painted with white Beeckosil mineral paint. Openings are kept to a minimum. The mineral paint is a non-film forming silicate coating with excellent durability. It is extremely weather and UV resistant, and the finished surface will not hold on to dirt to the same extent as normal masonry paints.
The wall onto the forecourt to the east and the garden court to the south contrasts with the solidity and weight of the masonry walls. These walls are heavily insulated timber framed and clad walls with a U-value of 0.17 W/m2K.
The sedum roof was used mainly for its environmental performance. The water retention capacity of the flat roof negated any need for a rainwater attenuation tank on the site. In addition to the reduction in drainage flow rates, the sedum roof will fulfil a number of roles. It will absorb nitrates and other harmful materials in the air/rainfall and bound them within the soil; protect the waterproofing from climatic extremes, UV exposure and mechanical damage, thereby increasing the life expectancy of the waterproofing; it will reduce reflective sound and improve sound insulation; and finally provide additional thermal insulation.
There were also a number of planning reasons for using a flat sedum roof. It was necessary to minimize the height of the building and retain the view of the original hospital building from the street. The sedum roof also provides an attractive view down from the upper floors of the northwest wing of the original hospital building.
Sustainable design:
The building aims to achieve a 50% energy reduction for heating and hot water with the most cost-effective specification.
Proposals concentrated on passive strategies and basic technologies such as insulation, airtightness, natural ventilation, Supply Air Windows, efficient boilers and low surface temperature radiators, and a sedum roof. Passive solar gain is maximised with south facing bedrooms.
Insulation:
The specification of insulation in the different building elements is as follows:
Ground floor slab: 100mm Kingspan Thermafloor TF73, U=0.1 W/m2K
(also insulated outside rising walls (rigid external insulation))
North Wall: 100mm Kingspan TW50 rigid insulation, U=0.19 W/m2K
South Wall: 150mm Rockwool; 40mm Kingspan K8, U=0.17 W/m2K
Roof: 200mm Kingspan insulation, U=0.12 W/m2K
Glass: Pilkington K glass with hard low emissivity coating.
Airtightness:
A high level of airtightness was required to maximize the effectiveness of the thermal insulation and reduce infiltration losses. The building is designed and specified to an airtightness specification of 1.5 air changes per hour at a pressure of 50 Pascals. This will reduce infiltration heat losses, control the internal environment and allow the passive ventilation system in the bedrooms to function effectively.
The outside wall lining membrane is the Solitex WA breather membrane, which will protect against wind and damp. The inside airtightness membrane is the ProClima Intello moisture variable vapour check, capped and sealed with Orcon glue. This will allow water vapour to pass across but not air. Depressurization tests were carried out in June and November 2005 by Ecological Building Systems.
Ventilation:
Each bedroom is passively ventilated to provide fresh, oxygen-rich air. This is essential in a well-insulated and airtight building. The passive ventilation system uses Supply Air Windows and a stack chimney with wind cowl.
Building Performance:
The community moved into the building in mid November. It is already evident that the building is very efficient at retaining heat – several bedrooms have the radiators turned off despite the low temperatures outside. The community has also reported a noticeable improvement in air quality from their former residence, and rarely open windows in the bedrooms.
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