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[EnergyPlus_Support] RE: ASHRAE 90.1 building envelope compliance methods [1 Attachment]

Dear all,

Thank you very for replying my post.

I notice there is a rounded issue when ASHRAE generates SI edition for IP edition. To avoid this issue (sorry I do not have all documents needed in IP edition, so I still try to use SI to explain my question further), I hand calculate surface resistance figure and others: interior surface resistance 0.10743 (R-0.61 IP), exterior surface resistance 0.02994 (R-0.17 IP). Use roofs - insulation entirely above deck climate zone 4 as example: 

.                     Nonresidential                                Residential                             Semiheated

IP units             U-0.048           R-20.0ci             U-0.048          R-20.0ci              U-0.173         R-5.0ci

SI units (cal.)     U-0.27256       R-3.5220            U-0.27256       R-3.5220             U-0.98234     R-0.88055 

It would be fine to use both methods (will not see big difference between the two) when metal deck is used because on page 77 in Standard 90.1 2007 SI edition section A2.21, it states "R-0 for metal de  

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.


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d:\eudora\attach\comparison roofs insulation entirely above deck Assembly U calculation.PNG