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.
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.
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.
If it's a below grade wall absorptance should not be a factor since the term
absorptance refers to the ability of a body to absorb light. Unless light is
somehow striking your underground wall absorptance should not be an issue.
eQUEST will know this, I'm sure.
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 CATON, E.I.T.
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
Hi Sambav,
If it's a below grade wall absorptance should not be a factor since the term
absorptance refers to the ability of a body to absorb light. Unless light is
somehow striking your underground wall absorptance should not be an issue.
eQUEST will know this, I'm sure.
Carol