[Equest-users] Semiheated envelope constructions

[Keith Swartz]

Ryan,

Yes, the layered approach is better. It takes into account the thermal lag while the straight U-value method does not. If your energy model is for LEED, then you have to use the layered approach.

To hit a target assembly U-value I include an insulation layer and find the thickness that gives the target U-value. eQUEST displays the overall R-value, which is the reciprocal of the U-value. The overall R-value that eQUEST calculates does not include the exterior film coefficient. The film coefficient is calculated for each hour based on the wind speed from the weather file and the surface roughness. So start with your target U-value, take the reciprocal to get the target R-value, subtract the film coefficient R-value (See 2013 ASHRAE Fundamentals, Chapter 26, Table 10.), then play with the thickness of the insulation layer in eQUEST to get that adjusted R-value.

Keith Swartz, PE, BEMP, LEED AP
Senior Energy Engineer | Energy Center of Wisconsin | Madison.Chicago.Minneapolis
608.210.7123 | www.ecw.org<http://www.ecw.org/>

From: Ryan Black [mailto:ryanblack17 at gmail.com]
Sent: Thursday, September 19, 2013 1:31 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Semiheated envelope constructions

I am working on building my first model in eQuest. I have a warehouse that falls into the semiheated category and I'm wondering how to build the layers up to reach the given construction U-value. I know that you can simply hard enter that value, but I've been told it is better to build the layers. Is this true? And if so, any pointers on how to get the layer assembly value reasonably close to the table U-values would be greatly appreciated!

Thanks,

Ryan
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