Low energy architecture for a severe US climate: Design and evaluation of a hybrid ventilation strategy Kevin J. Lomas, Malcolm J. Cook * , Dusan Fiala Institute of Energy and Sustainable Development, De Montfort University, Queens Building, The Gateway, Leicester LE1 9BH, UK Received 14 September 2005; received in revised form 14 March 2006; accepted 15 March 2006 Abstract Natural ventilation, relying on openings in the fac ¸ade, is applicable to a limited range of climates, sites and building types. Advanced naturally ventilated buildings, such as those using stacks to encourage buoyancy driven airflow, or hybrid buildings, which integrate both natural and mechanical systems, can extend the range of buildings and climate within which natural ventilation might be used. This paper describes the design of a new library building for a college, located near Chicago, which uses a new hybrid ventilation concept despite the severe continental climate. The likely operation of the building is illustrated using dynamic thermal modelling and computational fluid dynamics analyses. The new building challenges ingrained preconceptions about building designs for severe climates and exposes barriers to low energy buildings posed by national standards and guidelines. # 2006 Elsevier B.V. All rights reserved. Keywords: Hybrid ventilation; Low energy buildings; Climate; Dynamic thermal model; Computational fluid dynamics 1. Introduction Natural ventilation (NV) is widely recognised as a low energy approach to conditioning the interiors of buildings. The term ‘natural ventilation’ usually conjures up an image of a small scale building, with a shallow plan depth, operable perimeter windows and variable and unpredictable internal temperatures, air quality and air speeds. The manual operation of windows can be inconvenient, but on the other hand it has been shown that the provision of personal environmental control can, in part, enhance satisfaction with the building’s internal conditions through adaptive opportunity [1] and the ‘connectivity’ with the outside world can be welcome. The simple NV strategy is therefore only well suited to temperate climates, sites with a benign micro-environment, buildings with modest internal heat gain and when occupant activities will tolerate variations of internal temperature. Where such conditions do not prevail, or where deep plan buildings and sealed fac ¸ades are required, designers generally presume that a mechanical ventilation strategy is necessary— and this usually means air conditioning. This presumption is, in fact, enshrined in some standards. For example, ASHRAE Standard 62.1 [2] states that NV systems are permitted ‘in lieu of or in conjunction with mechanical ventilation systems’ but goes on to list a number of pre-requisites which should be met. As the work of Bordass et al. [3] has shown, air-conditioned buildings invariably consume more energy to offer the same level of service to occupants than NV alternatives. The fans, pumps and control equipment being responsible for much of the additional electrical energy consumption (rather than the chillers). Hybrid buildings can reduce energy consumption whilst offering the potential to combat tough climatic and site conditions and the prospect of meeting stringent internal environmental conditions. Such buildings are therefore becoming increasingly popular in Europe, however they tend to consume more energy than simple NV buildings, but less than those with air conditioning [3]. They also contain all the paraphernalia associated with mechanical air delivery systems (as well as the passive building elements) and can therefore be costly to build. If energy use is to be reduced significantly, and at an acceptable building cost, then more innovative thinking is needed. www.elsevier.com/locate/enbuild Energy and Buildings 39 (2007) 32–44 * Corresponding author. Tel.: +44 116 257 7969; fax: +44 116 257 7981. E-mail address: mcook@dmu.ac.uk (M.J. Cook). 0378-7788/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2006.03.032