How much insulation is too much? There must be a threshold of cost and efficiency that keeps a house from becoming an overstuffed pillow. That’s what’s keeping “Bill 12901” (we will call him Bill) awake at night, and what he asks about in this Q&A post.

Bill is building a new house in Climate Zone 6 with R-30 walls and an airtightness goal of 0.5 ACH50. He’s trying to figure out how much blown-in cellulose he should put in his attic. His local code specifies R-60 in the roof assembly, and he’d like to build better than code, but wonders if there’s a point where going beyond code doesn’t make sense.

When does the return on investment of extra insulation stop working? That’s the topic for today’s Spotlight.

Most of the readers who commented agree that R-60 is the sweet spot for the money–energy efficiency balance. Member tkzz points to a reader thread about the diminishing returns of added insulation. Others recommend using the National Renewable Energy Laboratory’s BEopt software to calculate energy usage and return on investment. Walta100 also advises watching a series of training videos  to offset the steep learning curve of the program.

Some point out that increasing the amount of cellulose to get to R-80 doesn’t cost that much (about $600).  As gstan puts it, “Putting it in deeper is cheap insurance.” He notes, too, that cellulose will settle over time, so why not go overboard? Malcolm Taylor asks whether the extra money would be better spent for upgrades somewhere else in the assembly, such as windows.

Our expert’s opinion

We’ve asked building science consultant and regular GBA contributor Jon Harrod to weigh in. Here are his thoughts:

When I’m deciding whether to go with…