Center for the Built Environment

This paper presents a discussion of first cost differences (premiums and savings) between a prototype commercial office building with underfloor air distribution (UFAD) and the same building with a conventional overhead (OH) system, based on a series of sensitivity studies using a detailed spreadsheetbased cost model. The model focuses on the first cost differences between four UFAD system alternatives and a baseline conventional OH design, and is designed to investigate tradeoffs based on nine categories of affected building elements: raised floor, HVAC system, electrical system, façade, ceiling treatment, voice and data cable, raised core, carpeting, and furniture. The first cost model was developed during the first phase of an ongoing research project whose overall objective is to develop a UFAD cost model covering both first and life-cycle cost differences between UFAD and OH buildings. In its current form, the model is intended to be used as a research tool to gain a better understanding of the tradeoffs inherent in investing in an UFAD system. Future versions of the cost model could be used to provide assistance early in the conceptual design process. This report describes the results of sensitivity studies covering a wide range of parameters using the first-cost part of the model.

Our experience in using this model for the studies included in this paper leads us to generally conclude that UFAD buildings cost more than OH buildings on a first cost basis when following the baseline assumptions of our model. These assumptions incorporate our best estimates of typical design and construction practices for UFAD and overhead systems, and were developed in collaboration with several CBE industry partners during the early stages of this project. Our baseline assumptions yield a cost premium of approximately $3.50/gross square foot (gsf) between the median UFAD building and the baseline OH building, although the HVAC system alone is slightly cheaper for the UFAD building. No single cost saving measure was sufficient to reduce the UFAD first costs enough to make up for the greater than $6/gsf premium incurred by the raised floor itself. However, the multi-parameter cost model provides an opportunity to investigate different combinations of cost-saving strategies from a variety of factors. Results for these integrated scenarios involving more aggressive strategies to maximize cost savings indicate that UFAD can be cost competitive with even the baseline OH building. Furthermore, when we changed the baseline OH assumptions to represent a higher quality overhead HVAC installation (more expensive, but still representative of typical high-end HVAC practice), the impact on cost differentials was dramatic. In this case, all UFAD buildings exhibited a significant cost savings compared to the OH building, demonstrating the important influence of the assumptions about the quality of the OH baseline building.

In this study we found that UFAD total building costs (independent of comparison to the baseline overhead building) are most sensitive to differences in material and labor markets, furniture and electrical configurations, perimeter HVAC parameters and the cost of the raised floor. Changing the parameters in any of these categories heavily influences the total cost of buildings using UFAD. However, of most concern to us in this study are those design parameters that create first cost premiums or savings for UFAD buildings with respect to the overhead convention. Generally, the areas that yielded the greatest sensitivity (cost differences >$1/gsf over the range of conditions tested) were interior zoning configurations, wall height differences, UFAD return ducting in the perimeter, airflow rate, and naturally, the quality of the baseline OH system against which all UFAD costs were compared. Those parameters found to have the least effect on cost differences between UFAD and OH buildings include workstation size, private to open office ratios (independent of zoning), floorplate size, building orientation and climate.

The process of developing and using this model leads us to believe that comparison studies done without such a tool must be scrutinized heavily for their ability to provide a true apples-to-apples comparison given the complexity of the cost tradeoffs. Comparisons of constructed systems are more daunting yet. First cost differences as described in this report, in combination with results from CBE’s upcoming UFAD life-cycle cost model (currently nearing completion), will provide a more complete basis for evaluating the cost advantages and disadvantages of UFAD buildings in comparison to OH buildings.