Introduction Increasing energy costs and more energy efficient buildings have been driv- ing forces behind the recent emphasis on the construction of tighter building envelopes. Indoor relative humidity has in general increased in these build- ings leading to problems such as condensation on walls and window panes and visible damage to building materials, which often necessitates new re- medial measures.71229

Recent studies focus on the relationship between health problems and mois- ture in buildings (Sundell, 1994) (Spengler et al., 1993). Increased moisture levels in buildings have also been shown to increase the prevalence of both fungi (Nevalinen, Hyvärinen, Pasanen & Reponen, 1994) (Jaakkola, Jakkola & Ruotsalainen, 1993) and house dust mites (Korsgaard, 1999). Moulds and their spores may contain toxic substances and allergy-causing agents. Aller- gic reaction to house dust mites is a source of the growing incidence of al- lergy in the general population in Northern Europe. A comprehensive review (Bornehag et al., 2001) of more than a thousand studies has found a signifi- cant correlation between damp buildings and health-related problems such as asthma and respiratory symptoms.

The level of humidity in the indoor air depends on a number of factors includ- ing the humidity in the outdoor air and human activities like showering, cook- ing, washing and clothes drying. These sources of humidity are all of a dy- namic nature. Many building materials and interior furnishings absorb water and can therefore to some extent act as a buffer for the indoor air humidity. Increasing the ventilation can reduce indoor humidity and thereby eliminate the effects of surface fungi and dust mites, but it does not necessarily help prevent moisture problems in the building constructions. The strength of the moisture sources and their position in the building constructions are crucial for the capability of the ventilation to control the moisture content of indoor air.

Also, seasonal and daily rhythms influence humidity and temperature inside the buildings. Different materials have different capacities for absorbing and desorbing humidity. The sorption isotherms for common porous materials such as textile, stone, brick, concrete or wood depend on the nature of the material, the geometric and the total surface area, the pore size and the number of pores. This in turn plays an important role in the diffusion of hu- midity in and out of the materials. Certain materials require months or even years to achieve equilibrium with the average relative humidity indoors, while others such as textiles only need hours.

In most codes and standards the ventilation requirements are based on minimum ventilation rates. With demand-controlled ventilation real-time oc- cupancy and the activities of the occupants determine the ventilation rate. In office buildings CO2 is commonly used for controlling ventilation, and energy savings from 5 % to 80 % have been reported (Emmerich and Persily, 1997). In residential buildings, however, humidity is a more effective pa- rameter for demand-controlled ventilation.

In this study ventilation strategies are based on a general reduction of the continuous basic ventilation rate and possibilities for inpidual increase of

4 the ventilation rate adjusted to the actual needs within each dwelling and

room. The objective is to develop energy efficient ventilation strategies that will provide both a healthy and comfortable indoor climate and reduce en- ergy consumption when compared with present ventilation standards.

Danish Building Research Institute has initiated a 3-phased research pro- gramme. The main purpose of the programme is to develop energy efficient ventilation strategies that will provide both healthy and comfortable indoor climate and reduced energy consumption compared with present standards.

Results from Phase 1 of the research programme were presented at the AIVC conference 2000 in The Hague (Bergsøe, 2000). Based on an evalua- tion of ventilation needs in dwellings and a simulated moisture balance in the room air, the author concluded that the basic ventilation in a typical apart- ment under normal use could be reduced by 20-30 % without compromising indoor air quality. Phase 2 of the research programme is described in this report and covers development and testing of selected ventilation strategies in the laboratory. Phase 3 will cover tests in the field.