[Bldg-sim] Mass. Energy Code

[Larry Degelman]

Janne,

R-values in the energy code tables are just for insulation materials.  The intent of the code when limiting U-factors is that you include all layers of the entire assembly, and not just the insulating materials.  This is why the U-value requirement is very seldom equal to 1/R.  What you should do is add all R-values for the insulation, skin materials on the wall, and the indoor and outdoor air film coefficients.  Then compute the U-value and see if it meets the 0.064 limit.

I believe that most of the values presented in the tables have already assumed certain bridging conditions, and that is why (repeating what I said above) the R-values for insulation materials do not usually match the inverse of the overall U-factors.  You can see this more clearly when you look at below grade walls.  Since no air film would be present, the insulation requirement is expressed as C.  The value C-0.119 inverted is R-8.4, whereas the R-value requirement is just R-7.5.  In this case, the remaining 0.9 R-value is assumed to be available from the masonry wall. 

Regards,

LOD   
===========================
Larry O. Degelman, P.E.
Professor Emeritus of Architecture
Texas A&M University
ldegelman at suddenlink.net
===========================


From: Janne Kairento 
Sent: Wednesday, December 15, 2010 2:16 PM
To: bldg-sim at lists.onebuilding.org 
Subject: [Bldg-sim] Mass. Energy Code


Hello!

 

For Climate Zone 5, IECC requires a minimum insulation value of R-13 + R-7.5 c.i. for a steel framed building. If the studs are 16" on center, the thermal bridging degrades the batt R-value of 13 down to an effective R-6 c.i. which bring the total insulation value to R-13.5 c.i. Currently, I am reviewing a building with 2.5" of R-5 extruded polystyrene total an insulation value of R-12.5 c.i. However, the building wall assembly exceeds the 'Assembly Minimum' of U-value 0.064. Since IECC 2009 is the effective energy code of Massachusetts since June 30, 2010, is the insulation assembly in agreement with the Minimum Envelope Requirements if the 'Assembly Maximum' is exceeded? Any input will be much appreciated!

 

Regards,

 

Janne Kairento

 

 



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