I had a response from an energy model reviewer indicating my r-value calculations are not acceptable because we failed to derate the 3.5" R-11 batt insulation per Table A9.28B of ASHREA 90.1-2007. I am looking at the table and can't determine how to derate the insulation. I performed a weighted average calculation between the insulation and the metal stud per appendix a (78% through the insulation and 22% per the stud) which was acceptable with the reviewer; however, they indicated we didn't derate the insulation. Does anyone know how to derate the insulation per table A9.28B? This is a huge issue and I want to get it right so any help would be appreciated. Thanks, Todd

Post a reply

Marcus Sheffer

LEED Fellow

7group / Energy Opportunities

LEEDuser Expert

4454 thumbs up

Mon, 08/05/2013 - 17:12

You would derate the insulation in the cavity because you have steel studs in the assembly. You would not do a weighted average calculation and derate the insulation. This would derate the insulation value twice. So if the reviewer accepted your weighted average calculation they should not be asking you to also derate the insulation. Maybe I am missing something. Table A9.2B is pretty straight forward. Based on your R-11 batts the effective R-value is R5.5 with studs spaced 16 inches on center or R6.6 with 24 in OC. Any reason you don't just use Table A3.3? That is the simplest way to do it.

Todd Bundren

Associate Principal - Director of Sustainabilty

Lawrence Group

LEEDuser Premium Member

10 thumbs up

Mon, 08/05/2013 - 18:03

Marcus, thank you so much for the quick response. I wish I could send a pic; however, here goes. I have a 16.5" existing exterior concrete wall (R-1.3), then a 1/2" air gap (R-1), then a 3 5/8" stud wall @16" O.C.(R-0) with R-11 batt insulation(R-11) with 5/8" gyp on the interior face of the studs (R-0.56). We did a weighted average of the wall by drawing parallel lines through the stud portion of the wall and then one through the insulation portion of the wall, added up the R values and then used the following weighted average: 78% through the insulation portion and 22% through the stud portion of the wall, giving us an effective R-value of 11.44 for the total assembly. The reviewer indicated the following: "The weighted average between the insulation and the stud portions of the wall are acceptable. However, the insulation must still be derated in accordance with Table A9.2B which lists the effective R-value to be used in the weighted average calculations as R-5.5 if studs are 16" O.C." Does this mean I should use the R-5.5 for the insulation and metal stud portion, then add R-0.56 for gyp, R-1 for the air barrier and R-1.3 for the concrete giving me a wall assembly R value of 8.36? I am new to this type of R-value calculation, and my energy modeler gave no assistance, so I am a bit confused. I really appreciate the help and hope my explanation made sense. Thanks Todd...

Marcus Sheffer

LEED Fellow

7group / Energy Opportunities

LEEDuser Expert

4454 thumbs up

Mon, 08/05/2013 - 19:11

Your calculation already derates the assembly and there should be no need to further derate the insulation. In fact your calculation is somewhat conservative as it does not account for the inside and outside air films (R0.63 and R0.17). So you have three choices. 1. Explain the the reviewer that what they are asking you to do is redundant and explain to them the accuracy of your calculations. What they want you to do is model R8.36 for 78% of the assembly and R2.86 for the other 22% or R7.15 overall. This derates the insulation value twice. 2. Ignore your original calculations and use Table A9.2B. In this case just substitute the R5.5 for the R11 in your original overall R-value and use the R8.36 for the overall assembly. Do not do a weighted average of insulated wall to studs (the R5.5 already accounts for this derating) in this case. 3. Use Table A3.3 instead. Good luck.

Todd Bundren

Associate Principal - Director of Sustainabilty

Lawrence Group

LEEDuser Premium Member

10 thumbs up

Mon, 08/05/2013 - 21:23

Thanks Marcus.

Jean Marais

b.i.g. Bechtold DesignBuilder Expert

LEEDuser Basic Member

581 thumbs up

Tue, 08/06/2013 - 07:02

I second that. The table is the best derating tool and required to derate the R-rating of layers that are composite. The standard refers to other acceptable, but more complex methods to calculate the stud&insulation layer to include for the thermal bridging effects caused by the studs, which can be found in ASHRAE HOF -2009 27.4. So if you have really weird constructions, this may point the way.

Add new comment

To post a comment, you need to register for a LEEDuser Basic membership (free) or login to your existing profile.

You are currently logged in under an enterprise account

FAQs about EAp2 :

Derated Insulation

You rely on LEEDuser. Can we rely on you?

Add new comment