[EnergyPlus_Support] Re: ASHRAE 90.1 U-factor with or without surface film

["Xing" <xing.shi.2006@xxxxxxxxx>]

Hi! Dr. Li.  I understand the differences between U and R and what the surface film is for.  I guess what I am not sure about is this 0.272 W/m2-K is for wall assembly with or without surface film.

Let's say we design a wall with a U-value (not including surface film) of 0.275 W/m2-K.  Apparent, it is larger than 0.272, which means that it does not meet the ASHRAE minimum requirement.  However, with exterior and interior surface films added, the U-value may be down to 0.270, now meeting the requirement.

I know this is a little hair-splitting.  But if you would kindly answer, I very much appreciate it.



--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, YuanLu Li <yli006@...> wrote:
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> Hi, Xing 0.272 W/m2-K  U-value is only a guide for minimum requirement. The film is required to prevent moisture condensation on the insulating material. I think, this U value is equivalent to R4.  The current minimum for Toronto is R8.  The usual value used is R12,  The ceiling is R20 and above.  A two storey residential house with a basement, and with these insulation hardly need any heating in Winter. When you use the R-value, the resultant is simply added together. When you use U-value, it will be 1/(1/U1 + 1/U2).  Therefore, the one with large U-value can be ignored. You need to consider the thickness of the material as well  to make the calculation for the resultant U-value, using the conductance value. In EPlus material, I think the value of conductance is given.  The material with a low conductance is a better heat insulator  Dr. Li   
>  To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> From: xing.shi.2006@...
> Date: Mon, 15 Aug 2011 11:44:33 +0000
> Subject: [EnergyPlus_Support] ASHRAE 90.1 U-factor with or without surface film
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>       ASHRAE 90.1-2007 Table 5.5-3 require that U-value of exterior wall shall be 0.272 W/m2-K.  E+ provides U-values with and without film.  Which one should match 0.272?  We think it should be the U-value without film.  Are we right?
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> Thanks!
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