Whole wall R-value simplifies wall thermal analysis and creates a level playing field for energy savings comparisons. With whole wall analysis, five separate components of the wall assembly are considered: wall and roof interfaces, corner interfaces, clear-wall assemblies, window and door interfaces, and foundation and wall interfaces.
To provide some perspective the scale of differences in thermal performance is demonstrated below:
With a consistent and accurate metric for thermal performance in use to allow comparison, that being whole wall R-value, the benefits of thermal mass, air tightness and moisture tolerance can be evaluated and solid conclusions can be drawn about which wall assembly is right for the circumstance.
A note on insulation installation quality; the HERS index.
A home energy rating includes modeling to see how energy efficient abuilding is or will be once it is constructed. It is usually done for contractors that want to qualify for an energy efficiency certification. It used to be that even for such certification an R-value was an R-value no matter the quality of installation. Today, however, a HERS energy rater will inspect the workmanship on a construction site with regard to the installation of insulation in all of the wall assemblies in the building. They grade the installation as a Grade I, II, or III.
- Grade I – is the best possible rating and it means the insulation is installed according to the manufacturer’s instructions. It completely fills the cavities and is encapsulated on six sides. It is cut around electrical junction boxes, split around wires and pipes and is generally not compressed anywhere.
- Grade II – is the second best installation. Some allowance is given for imperfections in installation as in moderate to frequent gaps around wiring, electrical outlets, plumbing and other intrusions, rounded edges or incomplete fill.
- Grade III – is the lowest grade given and will reflect installation that has substantial gaps and voids.
In a grade II scenario, the R-value in the energy efficiency model is reduced 2% from the stated R-value and with a grade III, 5% is deducted.
The reason for the deductions to the R-value for poor insulation installation is due to the fact that the movement of heat through and around insulation decreases the material’s ability to be an effective thermal barrier. Creating as much as a 5% gap in insulation coverage can reduce the effective R-value by 50%