U Value for a wall

Jack Kram · March 2006

RMC has been asked to design a radiant floor heating system for a home built in the 1940's. The structure is concrete and brick, interior and exterior. The floors 3 of them are 6" re-enforced concrete. The exterior walls are as follows; 2 courses thick red brick, 2" of Pittsburgh Corning Foamglas R-value @ 3, 1 course brick, rock lathe and plaster. I am having trouble determining the U-value so I can calculate the heating btuh's with Wrightsoft radiant.
Open for suggestions.

Thanks, Jack Kram

[Deleted User] · March 2006

1 divided by R = U....

just as 1 divided by U = R. All factors must be of the same value. No mixing R's with U's

From the formula department, heat loss in a conductive mode can be gotten by taking U times the area times delta T. Conversely, Area divided by R value and that sum times delta T will get you the same answer. Just a different means of getting the same answer.

HTH

ME

Doug_7 · March 2006

R-value table

ColoradoENERGY.org provides a very useful R-value table to help you determine the total R-value of your wall, floor or ceiling assemblies.

Go to:http://coloradoenergy.org/procorner/stuff/r-values.htm

Add up the R-values, then 1 divided by R = U

Brad White_9 · March 2006

U Value, Me Value, R Value...

Brick, if common brick not face brick. has an R value of 0.20 per inch. Face brick which is hard glazed and denser is 0.11 per inch. So you would have to give a thickness in inches. If 8 inches, a fair assumption, let me give you the breakdown with assumptions that I would use, indoor to outdoor (direction of heat flow):

1. Indoor Air Film: 0.68

2. Rock Lath and Plaster, 1" total: 0.50 est.

2A: Assume strapping and furring here? Add 0.97 for air space and/or wood as a layer.

3. 4" common brick (3.625"): 0.725

4. 2" Foam Glass @R3: 6.0

5. 8" Common Brick see above: 1.45

6. Exterior Air Film 15 MPH Wind: 0.17

Total R WITH air space at furring: 10.342 u=0.0967

Total R WITHOUT air space at furring: 9.372 u=0.1067

Your wall is thicker than most. As a point of reference and ball park guesstimate method/rule of thumb, I usually take all non-insulation R-values as about 2.5 to 3.0 and add that to the insulation. Your extra thicknesses and possible air space make that a tad higher.

And by the way, when between studs as in frame construction, another rule of thumb I have found is that the R value of the insulation is about the aggregate R value of the wall as a whole. Not a calculation basis of course, just shorthand for ballpark figuring.

Hope this helps.

Brad

U Value for a wall

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