prescriptive and performance method

[mont1230]

I have a question regarding the 2009 IECC. In Table 405.5.2(1) page 35 it has a column for Standard and proposed design. In the Standard Reference design is for the prescriptive method and the proposed design for the performance method? When a house is done by Prescriptive method and when getting back the blower door results and calculating by the (standard reference design) specific leakage area it passes. The question is that most HERS and BPI raters are running a REMrate software and the natural ACH always comes in failing. Which results in more cfm's and costing more money and makes no since by doing a code by a prescriptive method. The raters do not need to run a REMrate software to do a prescriptive method? But why will it pass by the standard method, but not the Proposed design method? Can you give me an explanation between the two. For example if I took a 2500 square feet house and built it to the prescriptive method, should the house always pass if the blower test result is 7 ACH or less and that the specific leakage area passes too? But if I took the same house and did the performance method which usually requires less insulation requirements and passes the blower door test but always comes up failing in the minimum natural ACH of 0.35. How can this house that in most cases built in prescriptive method be built tighter than performance but passes. This is very hard to understand. We want to make sure that these houses are being design and built correctly.

 

[Wooly]

Mont 1230

I'm not sure I fully understand your question but I think I get the jist of it. First of all the reference home is not the same as the prescriptive home. If you were to enter the prescriptive values in REM as your model home in most cases it will fail. The home you are comparing to is designed to save the amount of energy that the new code is shooting for, 15% in this case. The reference model is set up for a net savings acquired from all energy saving aspects of a home. Second, a air change rate natural of 0.35 is very close in most cases to a ACH50 of 7. Your infiltration rates are the same. When using REM you must include mechanical ventilation if your inputs make the home tighter than the 0.35 ACHnat. For example, if you get a blower door of 3 ACH50 and input it into the program it will default to 0.35 ACH nat. Not the rate intended. You must add ventilation to get the actual ACH rate. I hope this helps.