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Re: [EnergyPlus_Support] C-Factor for Underground Walls and F-Factor for Slab-on-Ground Floors (ASHRAE 90.1, Title 24 etc)
Sean,
1) you can use the f-factor & c-factor objects in e+ to create u-value appropriate equivalent constructions.
2) You are absolutely correct. There is no insulation requirement for slabongrade in CZ1. However, my spreadsheet method has a slight advantage that it respects as closely as possible the construction descriptions of Appx A including the amount of soil to be modeled under the slab, etc.
Actually, the standard works a bit backward. You have a building with a slab and the insulation under the slab has a certain layout and thickness. Based on this, you can lookup the F-factor (which is a kind of equivalent U-factor but considers perimeter "short-cicuit heat losses). So the goal was to come up with a thermally equivalent construction that meets the definition and uses the build up as closely as possible of Appx A.
At the end of the day the u-value (as if exposed to air) is what counts, and the internal mass is secondary (nice to have). I mean how much internal mass error do you incite for carpet and rubber?
At the time DesignBuilder had no F-factor object GUI so I had to do what e+ F-factor objects do automatically to create construction templates. I was taking to e+ developers at the time and some of my suggestion have been implemented in the latest v8.x versions regarding the rating of constructions.
2) f-factor c-factor objects create layered constructions internally.
Mit freundlichen Grü�en- Sent from my iPhone (excuse the brevity)
i. A.
Jean Marais
b.i.g. bechtold
Tel. +49 30 6706662-23
On 14.02.2014, at 17:25, <<mailto:seanking.1970@xxxxxxxxxxx>seanking.1970@xxxxxxxxxxx> wrote:
>
Hi Jean,
> Thanks again for your help.
While checking your spreadsheets, the sheet "Slab-on-grade" (the last one) especially, I have two questions.
(1) The table at the top of your sheet shows insulated value (R-value) against F-factor for a fully insulated unheated slab-on-grade floor (in order to work out how much insulation needed for F-factor 1.264), which must be extracted from ASHRAE 90.1-2007 Table A6.3 (SI units version page 97). I think you might not need to do regression (through trendline and get an equation) to calculate insulation level for F-factor = 1.264, as I found at the top of Table A6.3, F-factor of 1.264 (rounded to 1.26 in the Standard) requires no insulation (R-0.0).
(2) With EnergyPlus two objects CFactorUndergroundWall and FfactorGroundFloor available (they only require C-factor and F-factor values and some geometrical data), should we still need to define layered constructions for these two (we need to work out insulation thickness for a given/defined insulation material)? If yes, we might have nowhere to input these data via EnergyPlus idf editor.
Please do excuse me if I have more questions to ask after re-reading your reply and spreadsheets.
Best regards,
Sean
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