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Forum discussion

CS-2009 EAp2:Minimum Energy Performance

U-Value / F-Factor

Ciaran McCabe

Director

Passive Dynamics Sustainability Consultants

LEEDuser Basic Member

12 thumbs up

August 18, 2016

Hi, I have calculated the U-Value of a Slab on Grade Floor to be 0.12 (metric) using 100mm insulation. The Leed reviewer wants to see the F-Factor equivalent in agreement with ASHRAE 90.1 2007 Table A6.3 however the min horizontal insulation is 300mm (within this table) and it does not quote R values above R-2.6. Therefore there is no real direct match from this table that I can pick. What would be the best way to respond in this case. Will a calculated U-Value not suffice ?

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Guillaume Martel

Architect Partner

Provencher Roy Architectes

LEEDuser Basic Member

2 thumbs up

Thu, 08/18/2016 - 21:17

ASHRAE doesn't really allow other calculation methods for F-Factors. Since your 100mm thick insulation would adequate +/-Rsi-3.5, I suggest you use the F-Factor of Rsi 2.6 as there's not a major change in value for unheated slabs. What's the width of your insulation? Also, have a look at Figure 5-K in the 90.1 User's Manual to see if you have others options depending on your project specifics.

Marcus Sheffer

LEED Fellow

7group / Energy Opportunities

LEEDuser Expert

4454 thumbs up

Fri, 08/19/2016 - 15:56

You can extrapolate from the tables or provide a similar calculation to the method used in the table. LEED does allow other than F-factor calculations because some of the modeling software does not model F-factor.

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