Still a bit confused here:
The following is from Users' Manual 2007 SI edition:
For above-grade opaque constructions, the design criteria are expressed in terms of a maximum U-factor or a minimum R-value. If insulation is installed that has the prescribed R-value, then there is no need to demonstrate compliance with the thermal performance (U-factor) of the construction assembly. When using the maximum U-factor criteria, Appendix A of the Standard has defaulted U-factors for most constructions so that you rarely have to calculate a U-factor to show compliance.
If we follow the lines in red shown above, the U-factor used (calculated based on R-value of insulation)) for modelling might still be different with the U-factor read from Table 5.5-1 through 5.5-8.
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.
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.
On 04.10.2013, at 12:58, <seanking.1970@...> wrote:
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