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NC-2009 EAp2:Minimum Energy Performance

slab-on-grade floor

In Table 5.3 point 5 of annex G of ASHRAE 90.1-2007 I find that for the baseline building performance “opaque assemblies used for new buildings (..) shall confirm with the following common lightweight assembly maximum U-factors in Tables 5.5-1 through 5.5-8: (..) - slab-on-grade floors shall match the F-factor for unheated slabs from the same tables.”. Given a building, how can I calculated the F-factor of its slab-on-grade floor? The technical standard EN ISO 13370:2008 considers the U-factor of the slab-on-grade floor as the thermal resistance between the internal surface of the floor and the external air, passing through the ground. “To allow for the three-dimensional nature of heat flow within the ground, the formulae in this International Standard are expressed in terms of the “characteristic dimension” of the floor, B', defined as the area of the floor divided by half the perimeter” (see. Par. 8 of EN ISO 13370:2008). Does ASHRAE use the same approach as EN ISO 13370:2008? How have I to calculate U-factor and F-factor for slab-on-grade floors, according to annex G of ASHRAE? Can I find documentation concerning this issue, please? With Regards

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Marcus Sheffer
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7group / Energy Opportunities
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Thu, 04/12/2012 - 19:12

I am not sure how the methodologies compare. There are more detailed explanations on F-factor in the ASHRAE Fundamentals Handbook and the 90.1 User's Manual. Here is a link to a paper we have used to help us model slabs (see page 6). http://simulationresearch.lbl.gov/dirun/1901.pdf The slab-on-grade F-factor accounts for the impact of the slab and the earth under the slab. If the energy model cannot reflect an F-factor, you can model the Baseline slab-on-grade floor as a six inch concrete slab over earth with a soil conductivity of 0.75 Btu/h-ft-°F in accordance with 90.1 Appendix A6.1.

Mon, 04/16/2012 - 08:46

Thank you for your answer. Thank the link http://simulationresearch.lbl.gov/dirun/1901.pdf (p.7) I understood that if I know shape and dimensions of the slab-on-grade floor and I know one between U-factor and F-factor I can easily get the other one. If ASHRAE accepts a method based on tables (like in http://simulationresearch.lbl.gov/dirun/1901.pdf ), I think the method of EN ISO 13370:2008 can be accepted as well, right? I didn’t find in ASHRAE Fundamentals Handbook and in the 90.1 User's Manual any similar method. The best solution would be a three-dimensional FEM model slab-on-grade floor + ground, but I would avoid it because of the much time which it requires. Best Regards

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