Table C402.1.2 vs Table C402.2

[Codegeek]

I'm looking at the 2012 IECC. Can someone help me understand when I would use Table C402.1.2 rather than Table C402.2? I know Table C402.1.2 is allowed to be used in lieu of Table C402.2 per Section C402.1, but I'm not sure I understand when a situation would warrant using the U-factor alternative.

Thanks.

 

[knightj]

It is my understsnding the U-factor alternative is used when using the"trade off approach and the R-factor chart in used for a prescriptive approach.

 

[Codegeek]

It is my understsnding the U-factor alternative is used when using the"trade off approach and the R-factor chart in used for a prescriptive approach.

I'm not sure that is correct. Section C401.2 of the 2012 IECC states that there are three options for compliance - ASHRAE 90.1 OR the requirements of C402, C403, C404 and C405 (Prescriptive) OR C407 and some various other mandatory requirements (Performance). The various other requirements for the performance option do not include Table C402.1.2 as it is part of the prescriptive section. The only reference to Section 402 at all in the performance method is Section 402.4, which is air leakage, which is mandatory.

Anyone else have any idea when Table C402.1.2 would be used rather than Table 402.2?