The largest-ever exercise to validate dynamic thermal simulation progr
ams (DSPs) of buildings has recently been completed, It involved 25 pr
ogram/user combinations from Europe, the USA and Australia, and includ
ed both commercial and public domain programs, Predictions were produc
ed for three single zone test rooms in the UK. These had either a sing
le-glazed or double-glazed south-facing window, or no window at all. I
n one 10-day period the rooms were intermittently heated and in anothe
r 10-day period they were unheated. The predictions of heating energy
demands and air temperatures were compared. The observed interprogram
variability was highly likely to be due to inherent differences betwee
n the DSPs, rather than the way they were used. Predictions of the dif
ference in performance of two rooms were no more consistent than predi
ctions of the absolute performance of a single room, By comparing the
predictions with the measurements and taking due account of experiment
al uncertainty, the DSPs that are likely to contain significant intern
al errors are distinguished from those which, in these tests, performe
d much better. The likely sources of internal error are discussed. It
is recommended that empirical validation exercises should consist of a
n initial blind phase in which program users are unaware of the actual
measured performance of the building, and then an open phase in which
the measurements are made available. The work has produced five empir
ical validation benchmarks, which have significant practical benefits
for program users, vendors and potential purchasers. There is consider
able scope for improving the predictive ability of DSPs and so suggest
ions for further work are made. (C) 1997 Building Research Establishme
nt. Published by Elsevier Science S.A.