How are insulation R-values calculated for wall cavities of inconsistent thickness?

Question

I'm designing a house which will have unconventional wall construction that varies in thickness. The wall cavities will be filled with dense-packed cellulose, rated at R-3.7 per inch. I want to understand how to be sure that I'll meet IECC/IRC code requirements for minimum R value for the wall insulation.

I know that it's typical, for example, for the ubiquitous 2x4 framed exterior wall with studs 16" OC to meet the minimum of R-13 if the stud bays are filled with fiberglass batts. Clearly, though, the wall's actual average R-value will be less than R-13 due to the studs themselves, which have a much lower R value than the fiberglass.

So, what exactly is the method used to evaluate a wall's insulation design against code? Is it:

• the R-value of the insulation material at its minimum thickness, ignoring studs?
• the R-value of the insulation material at its average thickness, ignoring studs?
• something else?

As a more concrete example, the illustration below depicts top-down views of wall cross sections, where light gray represents 2x4 studs (vertical) and sheathing (horizontal) and dark gray represents insulation fill. My understanding is that examples A and B would be rated equally at R-13. At what R-value would a wall like example C be rated?

Answer 1

The Building Code (Energy Code) stipulates the R - values allowed for various building components. (See ICC Chapter 11)

Likewise the Energy Code stipulates the maximum amount of windows, doors, skylights, etc. allowed. (See Table N1104.1(1) and Table N1104.1(2))

All calculations are based on the sum of the R - values in any particular component, (i.e.: walls, roof, etc.) To answer your question: when the component varies in thickness, then the AVERAGE of the R - value for any given AREA (given in square footage) is used.

The Energy Code gives allowable “Prescriptive Paths” that meet the Energy Code criteria for walls, slabs, skylights, HVAC, windows, roofs, etc. If for any reason your construction does NOT meet the “Prescriptive Path” for any component, then you can substitute a higher rating for another component...provided it meets or exceeds the rating and area. (It gets a little tricky substituting a higher rated HVAC unit for using more windows than what’s allowed by the prescriptive path, but there is a formula based on your heating / cooling degree days.)

There are energy calculators you can download, which are suitable for your area (zone). You can stipulate if the studs are 16” oc or 24” oc,I don’t think you can automatically do “no studs”, but you could add the R - values up longhand. I’d call your local Building Official to get the name of such a software program. (Even I can use the program and I have trouble adding 2 + 2)

Answer 2

Best thing to do in these kinds of situations is to go talk to the building inspection people for your local jurisdiction. Many areas require an insulation inspection (after framing, electrical, and plumbing rough ins are done, right before the wall is closed up). Get them on board with what you're doing to prevent or reduce the chance of having to go back and do something over. Some of the field inspectors may be reluctant to sign off on something if it's different from what they're used to dealing with.