R-Value problem

4 posts / 0 new
Last post

Hi all eQUEST user!
I'm a bit confused about my input values inside the Wizard mode VS Detail
mode.
In "Building Envelope Constructions" window (3 of 25) I put an Overall
R-Value of 20 h-ft2-?F/Btu...
But, when I go into the spreadsheet view (in Detail mode) and take a look at
the Constructions details, I see a U Value of 20 Btu/h-ft2-?F!?
It's not realy the same thing!

Did you know why the software does not make the convertion R to U value
correctly? Or is it my mistake? Should I put a U value when the software
asks me to put an R value in the Wizard window?

Thanks to all and sorry for my poor English (it's not my first language) !
:-)

Martin

marmen007's picture
Offline
Joined: 2011-09-30
Reputation: 0

Martin,

when you are in the wizard, it is actually asking you to build-up your wall
construction for eQuest to calculate the overall Assembly u-value which
includes the thermal effects of the exterior surface/cladding + the
insulation + the frame conductance (for frame walls, etc.) The R-value to
which you refer is specific only to the thermal properties of the
insulation--nothing else in the assembly.

when you are in DDedit mode in the spreadsheet the input approch is
different and more flexible. In this mode you can build-up your
constructions based on the types of layers in the construction from the
outside to inside surfaces. OR, under the column labled "specification
method" you can use the drop down box to select either 'layers' or
'u-value'. When you select u-value with this approach this is the input
U-value for the overall assembly construction, not just the u-value for the
insulation. The u-value approach is the simplified, quick way, but when you
have requirements as in ASHRAE 90.1 Appedix G, you have to use the layers
method. If you do use the u-value in DDedit mode, make sure you know what
your overall assembly u-value will be. The insulation R-value will always
be higher than your overall construction assembly R-value (u-value
equivalent), becuse of the additional thermal losses conducted through the
other surfaces--especially dramatic when you evaluate a steel-frame
construction. There is a software program that will calculate the overal
construction thermal performance as a built up building (outside of
eQuest.) I think this program is called Therm, but not sure.

Pasha

Pasha Korber-Gonzalez's picture
Joined: 2011-09-30
Reputation: 600

The conversion is accurate. If your calculated values don?t agree, it?s usually due to one or both of two things:

1) The conversion accounts for the inside air film (default of R-0.68), so you cannot simply take 1/R of the physical assembly to get the expected U-value

2) If you are specifying steel framed construction in the wizard, the insulation will be de-rated according to the stud depth and spacing. Refer to Table A9.2B in 90.1 for details.

Regards,

DAKOTA KELLEY

Dakota Kelley's picture
Offline
Joined: 2011-09-30
Reputation: 1

All,

I don?t think air films or steel studs are going to account for a U-value of 20, but it?s hard to say what?s causing the problem without more information. We would probably need screenshots at a minimum, but the project .pd2 and .inp files would be the most helpful.

Eric

Eric O'Neill's picture
Joined: 2011-09-30
Reputation: 0