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F-Factor for slabs with both vertical and horizontal insulation

asked 2016-02-11 09:13:35 -0600

RobbyH's avatar

updated 2017-05-08 15:58:22 -0600

ASHRAE 90.1 Table A6.3 has values for horizontal insulation and vertical insulation, but nothing for slabs that have both vertical and horizontal insulation. What f-factor should I use? I don't consider it to be a 'fully insulated slab' since the horizontal insulation only extends 24" inwards, and A6.1.3 specifices "continuous insulation...completely covers the entire area under the slab."

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Conservatively, I have used the vertical insulation F-factor. While it doesn't capture the benefit of both horizontal and vertical insulation, I haven't seen any source of better information.

@Neal Kruis did an excellent presentation related to Foundation Heat Transferat the ASHRAE modeling conference; and his development of a program called 'Kiva'. Hopefully he can add his knowledge to this and enlighten us all as the expert.

dradair's avatar dradair  ( 2016-02-12 05:54:49 -0600 )edit

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answered 2016-02-15 10:58:23 -0600

F-Factors are tricky because there is no referenced definition or methodology that others can use to generate them. I am considering putting together an ASHRAE Research Topic Acceptance Request (RTAR) to better define the ASHRAE 90.1 requirements for foundation insulation.

In the meantime, it might be possible to generate F-Factors using Kiva. You would have to simulate your foundation and fitting a line using the following equation:

$$F = \frac{\dot{Q}}{P \cdot (T_i - T_g)}$$


  • $P$ is the perimeter of your foundation,
  • $T_i$ is the interior temperature, and
  • $T_g$ is the ground temperature as defined by the ground temperatures 0.5 meters below the surface (included in EPW file headers)

Short of doing some kind of numerical calculation, it is not possible to generate new F-Factors.

Disclosure: I develop Kiva

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Asked: 2016-02-11 09:13:35 -0600

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Last updated: Feb 15 '16