This report describes a closed-chamber method for laboratory measurements of the rate at which radon-222 degasses (exhales) from small building material samples. The chamber is 55 L in volume and the main sample geometry is a slab of dimensions 5x30x30cm"3 . Numerical modelling is used to assess (and partly remove) the bias of the method relative to an ideal measurement of the free exhalation rate. Experimental results obtained with the method are found to be in agreement with the results of anopen-chamber method (which is subject to different sources of error). Results of radon-222 exhalation rate measurements for 10 samples of Danish building materials are reported. Samples include ordinary concrete, lightweight aggregate concrete,autoclaved aerated concrete, bricks, and gypsum board. The maximum mass-specific exhalation rate is about 20 m Bq h"-"1 kg "-"1. Under consideration of the specific applications of the investigated building materials, the contribution to the indoor radon-222concentration in a single-family reference house is calculated. Numerical modelling is used to help extrapolate the laboratory measurements on small samples to full scale walls. Application of typical materials will increase the indoor concentration byless than 10 Bq m"-"3.