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
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,
Larry O. Degelman, P.E.
Janne et al,
Since IECC 2009 section 501.1 continues the happy tradition of
permitting one to dismiss IECC entirely and instead defer to 90.1-2007
for commercial buildings, here's my interpretation under 90.1-2007 (all
the figures should match regardless for this exercise)...
The prescriptive envelope option of 90.1-2007 has the same following 3
independent requirements for a steel framed wall in climate zone 5:
- R-13 batt
- R-7.5 c.i.
- Assembly Maximum U-0.064
The wall you're describing does not meet the prescriptive requirements
for a 'steel framed' assembly on two fronts: The U-value exceeds the
maximum (not minimum!) of 0.064, and from what your description there's
also no batt insulation...
Combining the first two steel framed requirements into an "effective
c.i." figure is an interpretation of compliance I haven't encountered...
But it would appear even by that metric that the proposed assembly falls
short from your description.
ON THE OTHER HAND, the envelope appears to fall neatly within the
requirements for a 'mass' wall (no batt requirement, min c.i. 11.4, max
U-0.09). A very important nuance (re: the 90.1 glossary and/or IECC
502.2.3) is that walls with steel framing can still be 'mass' walls. I
did the math once under 90.1's definitions, and came away with a rule of
thumb that any assembly including a 4" brick/masonry layer = a 'mass'
wall as far as these prescriptive tables are concerned.
Also to confirm Larry's hunch - The maximum assembly U-values correspond
closely to the assembly R-value you'd calculate if you added up all the
base assembly layers as detailed in 90.1 Appendix A section A3 (re:
archived discussion in the [equest-users] list if interested).
NICK CATON, E.I.T.
PS: Anyone in the position of designing around or enforcing IECC should
be keenly aware of that deferral to 90.1, and should probably have the
corresponding referenced version handy as a reference. I think you'll
find 90.1 is much more clearly organized and worded for those moments
when IECC causes raised eyebrows =).