INTRODUCTION As computing power increases and numerical models for whole building heat@ air and moisture (HAM) transfer advance@ there remains a general need for more experimental data that quantify HAM transport in porous building materials. For example@ recent benchmark data for validating 1-D HAM simulation models produced in a large international project (Hagentoft et al. 2004) rely solely on numerical and analytical data because well-documented and accurate 1-D data are scarce. An important part of the research in IEA/ ECBCS Annex 41 has been on heat and moisture transfer between indoor air and hygroscopic materials during transient changes in indoor humidity because research has shown that moisture buffering may improve comfort@ air quality and energy consumption in buildings (Rode et al.@ 2004@ Holm et al@ 2004@ Simonson et al.@ 2002@ 2004a@ 2004b and Osanyintola and Simonson@ 2004). To validate models that simulate moisture buffering of hygroscopic materials@ new experimental data are needed that accurately quantify heat and moisture transfer between humid air and hygroscopic materials during transient changes in the air humidity. Experimental data are available in the literature@ but most data are not well suited to benchmark detailed numerical models because carefully planned laboratory experiments are best suited for model validation (Holm et al.@ 2004@ Simonson et al.@ 2004b@ Tariku and Kumaran@ 2006@ Svennberg et al.@ 2007@ Kalamees and Vinha@ 2006@ Jenssen et al.@ 2002@ Hens@ 2004@ Qin et al@ 2005 and Talukdar et al.@ 2007a and b). In addition many of the experiments in the literature are conducted on non-hygroscopic materials@ where a majority of the moisture accumulation is due to condensation and frosting near a cold surface. Furthermore@ in many cases the thermal transients dominate the problem. To benchmark models that intend to consider moisture buffering of hygroscopic materials in contact with indoor air@ experimental data are needed where the air humidity is changed in a transient manner as presented in this paper. The objective of this paper is to compare experimental and numerical data for 1-D heat and moisture transport in a bed of gypsum boards@ which was conducted as part of Subtask 2 of IEA/ECBCS Annex 41 (Roels@ 2008). Comparing the numerical and experimental data serves a dual purpose of validating the numerical models as well as confirming the experimental data. However@ as previous research (e.g. BCR 1992@ Time and Uvslokk 2003@ Roels et al. 2004) showed that apart from the lack of good benchmark models@ also attaining uniformity in measuring material properties remains difficult; therefore@ it was decided to perform first an interlaboratory comparison of the measurement of the hygric properties of porous materials by means of a round robin test. Not all material properties have been measured@ but the round robin focussed on those properties relevant for indoor moisture buffering: the water vapor transmission properties and the sorption isotherm. In total fourteen laboratories participated in the round robin test. The results of the round robin test are presented first. Then@ the experimental facility is explained and the different test cases and applied numerical models are presented. Finally@ a comparison is made between the experimental and numerical results. The large number of participants in the round robin tests@ also allowed a sensitivity study of the numerical simulations@ in which the influence of the uncertainty in material properties and boundary conditions were investigated.