How Much Attic Insulation is Enough?

This seems like a simple question. It deserves a simple answer.

We recommend R-49 in the St. Louis area. That’s about 17 inches of blown insulation. 

The ins and outs of attic insulation can get complicated. Blown insulation has advantages over roll or batt insulation.  When it comes to blown insulation, cellulose and fiberglass have their own comparative advantages and disadvantages. Spray foam is a superior option that costs much more. In addition, each home is different, and the one-size fits all prescription don’t always fit.

That said, the “R” in R-49 stands for “resistance” to the flow of heat by conduction. When it comes to the R-value of building materials, those R-values are determined in the laboratory. Normal lab testing under very specific and set conditions take into account only resistance to the flow of heat by conduction. Heat transfer through convection and radiation are largely or completely ignored.

Regardless of its shortcomings, the R-value has been widely adopted. The US Department of Energy, through the Home Performance with Energy Star program, recommends between R-38 and R-60 for the St. Louis area. The 2012 International Residential Code (IRC) requires R-49 for new construction.

Illinois adopted a state version of the IRC on January 1, 2013 and requires R-49 as a minimum. Although Missouri does not have statewide building or energy codes, local authorities typically set their own standards. For example, St. Louis County requires a minimum of R-30 (based on the 2003 International Energy Conservation Code) and some municipalities within the county set higher minimums.

Of course, all these code minimums are for new construction. Existing homes are not required to measure up. For homeowners who are interested in improving their homes, code minimums are generally viewed as just that:  minimums. In the case of insulation, the minimum depends on which code you read. For some municipalities R-49 meets the minimum, for others it exceeds the minimum.  

So how much R-value should I have in my home?

For the energy-conscious homeowner, a good case could be made for R-60, the maximum recommended by the Home Performance with Energy Star standard. R-60 is the point beyond which very little additional benefit would come from adding more.

Is R-30 enough? How about R-38? R-60? We have insulated many attics in the St. Louis area to R-49 with outstanding results in terms of energy efficiency and comfort. That level of insulation represents an effective and sensible choice for most homes in the St. Louis area.