RE: Re: [EnergyPlus_Support] ASHRAE 90.1 building envelope compliance methods

If you use Appendix A in 90.1, you will find that the required R-values come very close to the required U-factor as long as you use the construction assemblies as described in the appendix. If your construction assemblies are significantly different than those specified in Appendix A, then the U-factor path should be used (for both baseline and proposed).

---In EnergyPlus_Support@xxxxxxxxxxxxxxx, <energyplus_support@xxxxxxxxxxxxxxx> wrote:

The consensus for modelling purposes at least is that the U-value of the assembly is to be held for the baseline case. The R-value of the insulation layer is not nessesary, although constructions can be made to apply to both. Note the R-values of insulation layers are "rated" values which sometimes need to be derated in accordance with appx A (eg. Steel-framing/insulation layer).

A note to some other recent posts regarding ASHRAE rated assembly U-Values: e+ calculates these according to the ASHRAE 90.1 standard for the summary reports.

Mit freundlichen Grüßen- Sent from my iPhone (excuse the brevity)

i. A.

Jean Marais

b.i.g. bechtold

Tel. +49 30 6706662-23

On 04.10.2013, at 12:58, <seanking.1970@...> wrote:

Dearl all,

In ASHRAE Standard 90.1 2007 SI Edition, Page.18, it states:

5.5.3 Opaque Areas. For all opaque surfaces except doors, compliance shall be demonstrated by one of the following two methods:
1. Minimum rated R-values of insulation for the thermal resistance of the added insulation in framing cavities and continuous insulation only. Specifications listed in Normative Appendix A for each class of construction shall be used to determine compliance.
2. Maximum U-factor, C-factor, or F-factor for the entire assembly. The values for typical construction assemblies listed in Normative Appendix A shall be used to determine compliance.

My question is simple but very confused to me:

While modelling baseline buildings, there must be different U-factors between using these two options (using certain R-value from column next to U-factor column will certainly not generate the same U-factor shown in that column). Therefore EnergyPlus will generate different outputs on thermal performance (I am quite sure E+ works out heat transfer through building element using its U-factor).

So the question is: How do we know which method we should use in the model? I suppose everyone might prefer a bigger U-factor (when creating a baseline building) for whichever method being used.

Is my understanding right? I look forward to any advisable comments.

Sean