Every three years, national building codes change. Oklahoma is a little behind, since we still use 2009 International Energy Conservation Code. But oklahoma will eventually adopt the new code which introduces greater insulation measures. We encourage to meet or exceed the 2012/2015 IECC, and below are some details on what the code states for residential homes in our climate zone.
Walls either have to be R-20 or R-13+R5 Continuous Insulation. This is easily accomplished with Cellulose or Foam in the wall cavity. Cellulose 2x4 wall is R-14 and 2x6 wall is R-21. Cellulose totally fills the stud cavity and it does a good job of air sealing inside that stud bay. Spray Foam is comparable to Cellulose in R-Factor. Although it falls extremely short in other considerations, like fire, moisture, and environment. With Fiberglass we have never been convinced that it's advertised R-4 per inch in the wall was ever real world, after all the same material is advertised at R-2.2 per inch in the attic, doesn't make sense does it? Continuous Insulation has been a recommendation of ours for years, especially with a good radiant barrier attached, and we're very excited that code is enforcing this now (for 2x4 at least.) We recommend XPS on the exterior, although EPS works fine too. As far as our final recommendation we feel if it were our home we would have 2x6 walls with R-21 of Cellulose and 1 inch of XPS Continuous Insulation on the exterior of the wall to prevent any thermal bridging through the framing members.
Attic's have to be a minimum of R-38. Fiberglass is not our recommendation because of its low R-Value per inch and poor ability to stop air currents. It takes approximately 17 inches to meet the minimum R-Value. With Stabilized Cellulose you can easily achieve the minimum R-38 at only 10 inches in the attic. But we recommend having a R-49, which is 13 inches. This is all considering our recommendation of keeping the HVAC and its ductwork inside of the thermal envelope. This creates a far more efficient home for numerous reasons. A "cathedralized" attic can be achieved with Spray Foam or Cellulose applied to the roofline of a home, but we feel it is a great compromise on the amount of insulation for a lot more dollars. Also, Spray Foam is bad news in our books with it's fire, moisture, and legal problems.
Ductwork sealing is mandatory and in the attic R-8 duct insulation is required. Also, ductwork tightness has to be measured at less than 4 CFM/per 100 sq ft of conditioned floor area. The exception to this is if the HVAC Unit and ductwork are inside the thermal envelope of course.
Home Air Tightness:
Air leakage rate for a home in our zone has to be equal to or less than 3 ACH at 50 Pascals. A tight home is accomplished with strategic air sealing in the attic, around windows & doors, and the frame including the sill plate. Blower door testing has now become mandatory, it's nice to know we have never failed a blower either.
I hope this information helps a little and gives a good picture of what the future of weatherization and codes holds.
- Jesse L. Cox