"INTRODUCTION The Radiant Time Series Method (RTSM) is a simplified cooling load calculation procedure@ originally developed (Spitler et al. 1997) to provide a rigorously-derived approximation to the Heat Balance Method (HBM) (Pedersen et al. 1997). It effectively replaced all other simplified (non-heat-balance) methods such as the Cooling Load Temperature Difference/Solar Cooling Load/Cooling Load Factor Method (CLTD/SCL/CLF)@ the Total Equivalent Temperature Difference/Time Averaging Method (TETD/TAM)@ and the Transfer Function Method (TFM.) Compared to the methods previously available@ the RTSM is quite similar to the TFM. (Spitler and Fisher 1999) Like the TFM@ the RTSM may be classified as a two-step method ?C heat gains are computed first@ then cooling loads. Both methods compute loads for a 24-hour design day. In the TFM@ conduction heat gains were computed with conduction transfer functions (CTF) and cooling loads were computed with weighting factors (WF); in both cases@ iteration was needed to arrive at the solution for a single day. The most important difference between the two methods is that the RTSM eliminated the need for iterative solutions of conduction heat gains and cooling loads by assuming steady periodic boundary conditions and then deriving 24-term response factor series. For conduction heat gains@ a 24-term series of periodic response factors (PRF) related the conduction heat gain to the 24 hourly sol-air temperatures and a constant room air temperature. The periodic response factors were later replaced with conduction time series factors (CTSF)@ which non-dimensionalize the PRF by dividing by the U-factor. For determination of cooling loads from heat gains@ a 24-term series of radiant time factors (RTF) were derived. Elimination of the need for iteration made the RTSM well-suited for spreadsheet application. For the TFM@ CTF and WF were available from electronic databases (Falconer et al. 1993) and printed tables. For the RTSM@ the Cooling and Heating Load Calculation Principles book (Pedersen et al. 1998) (Pedersen@ et al. 1998) was accompanied by an HBM computer program that could calculate PRF and RTF. Printed tables of select PRF and RTF were later developed (Spitler and Fisher 1999; ASHRAE 2001). For both the TFM and RTSM@ pre-tabulated factors for conduction heat gain and cooling load calculations require the user to select wall types or zone types that most closely match the actual wall type or zone type. What ""most closely matches"" may not be clear in all cases@ even to experienced designers. Use of a separate computer program to determine PRF@ CTSF@ and RTF@ which must then be input to a spreadsheet is also less than desirable. Therefore@ one improvement to the method has been development of compact procedures for computing CTSF and RTF. Since the original development of the RTSM@ solar heat gain coefficients (SHGC) have replaced shading coefficients (SC) as the key figure of merit characterizing window performance. Therefore@ the fenestration model of the RTSM was updated to utilize SHGC. Shading coefficients gave transmitted solar radiation and inward-flowing absorbed solar radiation as separate quantities; each of which in the RTSM had different recommended values of radiant fraction. Since SHGC give the total window heat gain due to solar radiation@ including both transmitted solar radiation and inward-flowing absorbed solar radiation in one quantity@ a new recommendation for radiant fractions was developed based on a large parametric study. As shown by Rees@ et al. (1998)@ the RTSM@ like the TFM@ generally showed a small amount of overprediction of peak cooling load compared to the HBM. As both the TFM and the RTSM are approximations to the HBM@ it is generally desirable that any inaccuracies result in overprediction rather than underprediction. However@ for rooms with high percentages of the exterior fa?ade covered with single-pane glass@ the overprediction can be significant@ exceeding 40% in extreme cases@ in cooler climates where the room geometry approaches that of a vertical solar collector. Rees@ et al. (1998) demonstrated for one extreme case with 37% peak cooling load overprediction that about 80% of the overprediction was due to heat gains that entered the room but that were then lost through the single-pane glass. Therefore@ one refinement to the RTSM described here is a correction factor that can be applied for rooms where significant overprediction is possibility. Finally@ performance of the RTSM performance is verified against the HBM for a large number of cases with a parametric study. The study and results are described briefly in this paper."