"R" values for insulation products
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Michael Hurd
- WUFI User
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- Joined: Thu Oct 19, 2006 5:27 am -1100
"R" values for insulation products
Are "R" values for the insulation products in the WUFI program identified somewhere? Or, is there a way to convert data presented in WUFI to an "R" value? When we model walls I would like to have the thermal capabilities for the insulation products as accurate as possible, most of the time I know the "R" value or the "R" value per inch of the material, this is what we are most familiar with.
Respectfully,
Michael Hurd
Michael Hurd
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Manfred Kehrer
- WUFI International Support Team
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Dear Michael,
The R Value in SI [m²W/K] is defined as
R=S/L
S: Thickness [m]
L: Heat Conductivity [W/mK]
To have a reasonable R Value you should use a Heat Conductivity which belongs to an RH of 80%
In IP
S: Thickness [in]
L: Heat Conductivity [Btu/h ft °F]
So R will be in [ (h ft °F in)/Btu ]
I don´t know if this is a value you familiar with
The R Value in SI [m²W/K] is defined as
R=S/L
S: Thickness [m]
L: Heat Conductivity [W/mK]
To have a reasonable R Value you should use a Heat Conductivity which belongs to an RH of 80%
In IP
S: Thickness [in]
L: Heat Conductivity [Btu/h ft °F]
So R will be in [ (h ft °F in)/Btu ]
I don´t know if this is a value you familiar with
Last edited by Manfred Kehrer on Sun Oct 21, 2007 8:54 pm -1100, edited 1 time in total.
Manfred
Wiss, Janney, Elstner Associates, Inc.
Official WUFI® Collaboration Partner for USA/Canada
Enjoy WUFI®
.... It is easy and complex.
Wiss, Janney, Elstner Associates, Inc.
Official WUFI® Collaboration Partner for USA/Canada
Enjoy WUFI®
.... It is easy and complex.-
Michael Hurd
- WUFI User
- Posts: 103
- Joined: Thu Oct 19, 2006 5:27 am -1100
"R" values for materials
Manfred, is seems then that I could alter the heat conductivity in the database if I need to adjust the "R" value of a product to more nearly match the actual products we might use - does this sound reasonable? Thanks for your help by the way!
Respectfully,
Michael Hurd
Michael Hurd
-
Manfred Kehrer
- WUFI International Support Team
- Posts: 351
- Joined: Thu Mar 10, 2005 2:17 am -1100
- Location: Northbrook, IL; USA
- Contact:
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AUChE1997
- WUFI User
- Posts: 3
- Joined: Thu Apr 24, 2008 1:28 am -1100
- Location: Mount Airy, NC
- Contact:
Here's how I do it:
Let's take "Sprayed Polyurethane Foam; closed cell." In WUFI, it has a "Thermal Conductivity, Dry" = 0.014 Btu/h ft °F. For my simulation, I want to use the R-value of our InsulStar ccSPF which has an R-value of 6.4 per inch or:
6.4 h.ft².°F/Btu.in. I know these units look weird to some, but they work and for me as an engineer. It just needs to work and be simple.
You take the inverse of R-value to get Thermal Conductivity. So the inverse is:
0.156 Btu.in/ft².h.°F
Okay, but now the units don't match what you input into WUFI, so get rid of the inches and ft² by dividing that number by 12 in/ft.
0.156/12 = 0.013 Btu/ft.h.°F
Do it on paper and watch your units! The same idea works in SI units, and it's much easier! Just look at the units you start with and try to figure out how to get them to the units you want.
Make sure when you start with an R-value, it's the R-value at 1 inch. If not, divide it by however many inches thick the product data sheet states. If a data sheet does not report the R-value per inch, be careful. They could be trying to decieve you.
Let's take "Sprayed Polyurethane Foam; closed cell." In WUFI, it has a "Thermal Conductivity, Dry" = 0.014 Btu/h ft °F. For my simulation, I want to use the R-value of our InsulStar ccSPF which has an R-value of 6.4 per inch or:
6.4 h.ft².°F/Btu.in. I know these units look weird to some, but they work and for me as an engineer. It just needs to work and be simple.
You take the inverse of R-value to get Thermal Conductivity. So the inverse is:
0.156 Btu.in/ft².h.°F
Okay, but now the units don't match what you input into WUFI, so get rid of the inches and ft² by dividing that number by 12 in/ft.
0.156/12 = 0.013 Btu/ft.h.°F
Do it on paper and watch your units! The same idea works in SI units, and it's much easier! Just look at the units you start with and try to figure out how to get them to the units you want.
Make sure when you start with an R-value, it's the R-value at 1 inch. If not, divide it by however many inches thick the product data sheet states. If a data sheet does not report the R-value per inch, be careful. They could be trying to decieve you.