This blog describes the construction of a small garden office for a client in Lewes designed to the Passivhaus standard. The building is a simple box and the footprint measures only 4m by 3.5m externally. One wall is formed of lift and slide doors orientated south-east, balancing the southerly solar gains with a pleasant aspect to the garden. There is a window to the south west to provide summertime secure nigh-time ventilation (in conjunction with the doors being lockable on trickle). A solar sail erected in the summer will control overheating. A pair of very quiet Lunos single room heat recovery extractors will provide high indoor air quality and reduce energy consumption. The floor is a 300mm Isoquick raft with integral upstands / kerb eliminating thermal bridges and providing a bowl into which the reinforced concrete slab is poured and tamped level with the top of the kerb. The slab will be polished as the final floor finish and provides thermal mass to regulate the internal temperature. Walls are timber I beams with pumped Warmcell clad in 100mm of woodfibre and timber rainscreen. The roof is PIR with a wild meadow green roof. Heating will be very minimal and is provided by an electric radiator. Although the Passivhaus logic has been applied, with U values around 0.10, PHPP modelling suggests a heat demand around 45 KWh/m2/year significantly above the 15 KWh/m2/year limit but the discrepancy is due to the high surface area to floor area inherent for a very small building and the minimal internal heat gains for this office (not having all the associated kit a house might contain).
