The successful Computational Fluid Dynamics (CFD) benchmarking activity,originally started within the EC-funded Network of Excellence HySafe (2004-2009),continueswithin the research topicsof the recently established "International Association of Hydrogen Safety" (IA-HySafe). The present contributionreports the results of the standard benchmark problem SBEP-V21.Focus is given to hydrogen dispersionand accumulation within a non-ventilated ambient pressure garage both during the release and post-release periods but for very low release rates as compared to earlier work (SBEP-V3). The current experiments were performed by CEA at the GARAGE facility under highly controlled conditions. Helium was vertically released from the centre of the 5.76 m (length) x 2.96 m (width) x 2.42 m (height)facility, 22 cm from the floor, from a 29.7 mm diameter opening at a volumetric rate of 18 L/min (0.027g/s equivalent hydrogen release rate compared to 1 g/s for SBEP-V3) and for a period of 3740seconds. Helium concentrations were measured with 57 catharometric sensors at various locations for a period up to 1.1 days. The simulations were performed using a variety of CFD codes and turbulence models. The paper compares the results predicted by the participating partners and attempts to identify the reasons for any observed disagreements.