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

The thing not to confuse is that the requirement to the insulation layers of the construction assembly is not the same as the total assembly U-value. Also, like Sean said, if you hand calculate the U-value of a construction as put forward in Appx. A, you will find that they closely match with those in the Tables (there are some rounding errors, especially from I-P to SI units. Just don't compare the R-values required of the insulation to the U-values of the entire assembly and expect to get the same values.

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

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).

Look to the 90.1 Users' Manual for more guidance.

Rahul

---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

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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