[Bldg-sim] U Value

[Sheila]

Good morning Gentlemen,

 

Actually, the C-factor for below grade walls is the value of conductance for a given thickness of material and is basically the inverse of an R-value for a given thickness  i.e. R=1/C.  For the concrete in a below grade (BG)  wall, the overall C in the baseline BG wall should be 1.140 - see Table 4 in Chapter 26 of 2009 ASHRAE Fundamentals Handbook.

 

The wall assembly in the software should include the exterior soil, the concrete (with total C=1.140), and the interior airfilm.

 

I hope this helps.  Have a nice day.

 

Sheila Sagerer

Energy Engineer, EIT, LEED AP

Energy Opportunities, Inc, a 7group company

Phone: 717-880-9069

www.sevengroup.com

 

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Nick Caton
Sent: Thursday, April 15, 2010 10:02 AM
To: sambhav tiwari; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] U Value

 

Sambhav,

 

Don't have my copies of 90.1 handy.. but I think I can guide your questions - do ask again if I'm missing the mark.

 

1.       While your units look about right for a U-value (Btu/h·ft2·°F), baseline underground walls, in 90.1-2004, are defined with a C-factor, not a U-factor.  Appendix A section for underground walls explains how it's different from U-factor (I think C basically excludes air films), and has a table you can reference to find a C-value from chapter 5. Between that table and the relevant appendix A text section, you can build a custom construction to match the C-value specified.

2.       I don't think there's any specific guidance regarding wall/floor/partition construction absorptance values in 90.1.  It's one of those things that fall generally into "match the proposed."  90.1 does specify "solar reflectances" for roofs only, and we handle those (somewhat indirectly) through the absorptance inputs for roof constructions.

 

Hope this helps,

 

~Nick

 

 

 

NICK CATON, E.I.T.

PROJECT ENGINEER

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617

Check out our new web-site @ www.smithboucher.com 

 

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of sambhav tiwari
Sent: Thursday, April 15, 2010 6:55 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] U Value

 

Hi All,
             What should be  U value for below grade walls for basline is it 1.14 btu/hrsqft2F. and how much should i consider absoptamce fro base line below grade walls.

I look forward for your response.

Thanks
Sambhav Tiwari.

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