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F-Factor Calculation - ASHRAE 90.1-2007 Table A6.3

asked 2015-03-12 15:30:43 -0500

updated 2017-05-08 15:59:02 -0500

Does anyone know what the two different columns for unheated and heated slabs represent in Table A6.3 ? Why are there different F-Factors for the same horizontal insulation distance and the same R-value? The example below that is highlighted is that if I have a slab with 12 inches of R-5 horizontal insulation is the F-Factor 0.72 or 0.61?

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Does anyone know the basis of this table?

Joe Huang gravatar imageJoe Huang ( 2015-03-13 09:27:18 -0500 )edit

These values come from work David Baylon and Mike Kennedy did for the state of Washington. See this publication for more background.

Neal Kruis gravatar imageNeal Kruis ( 2015-03-13 15:07:22 -0500 )edit

Hhhmmm. That's interesting. These numbers are somewhat lower than those in my 1988 paper. I guess I'll have to read the Baylon paper to find out how he and Mike got the F-Factors. I just did a regression of the fluxes to the Inside-Outside Air Temp Difference, so the F-Factors can vary depending on location.

Joe Huang gravatar imageJoe Huang ( 2015-03-13 18:49:39 -0500 )edit

I fixed the broken link for Neal's comment above.

Anna Osborne Brannon gravatar imageAnna Osborne Brannon ( 2017-09-05 20:25:19 -0500 )edit

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answered 2015-03-12 16:46:23 -0500

Eric Ringold gravatar image

updated 2015-03-12 16:47:59 -0500

It's an error in the table. The second set of values for unheated and heated slabs should be for vertical insulation, not horizontal. See the errata sheet for ASHRAE 90.1-2007 from August 15, 2011. Your 12 inches of R-5 horizontal insulation has an F-Factor of 0.72.

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Asked: 2015-03-12 15:30:43 -0500

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Last updated: Mar 12 '15