How Much Attic Insulation Do I Need?

There is no single right answer, because the correct amount of attic insulation depends on how hard your climate works the house. What follows is how the target is actually set, and why the insulation you already have is probably performing below its rating.

Targets by climate zone

The US Department of Energy publishes recommended attic R-values by climate zone, and those recommendations are what most contractors, inspectors and rebate programs work from.

Two things are worth noticing. First, cold climates need more, because the temperature difference across the ceiling is larger for longer. Second, the hot-climate numbers are not small, and plenty of Las Vegas and Phoenix homes built in the 1990s are sitting well below them.

Why code minimum is not the target

Every house was built to the energy code in force the year it went up, and those codes have tightened repeatedly. A home built to code in 1996 is not a home built to code today. Code minimum is also, by definition, the least the builder was allowed to install, which is a floor rather than a recommendation. In a severe climate, the gap between the floor and the sensible target is money every single season.

Labeled depth and installed depth are different numbers

This is the part that surprises people. Loose fill insulation settles. Fiberglass batts slump and compress. Anything that has been walked on, had storage stacked on it, or had a technician crawl through it to service an air handler is thinner than it was on day one. It is completely normal to find an attic that was installed at R-38 measuring closer to R-25 twenty years later.

Compression matters more than people expect, because R-value comes largely from trapped air. Squash the material and you remove the air, so an inch of compressed batt is worth considerably less than an inch of uncompressed batt.

Depth is only half the question

Insulation slows heat conducting through a surface. It does nothing about air moving through a hole. If your attic floor has open top plates, unsealed plumbing and wiring penetrations, recessed light housings and a bare access hatch, air is moving straight past the insulation regardless of how deep it is.

This is why we recommend air sealing before adding depth on almost every job. Adding more material over an unsealed ceiling buries the problem instead of fixing it, and the homeowner concludes that insulation does not work.

How to check what you have

You can get a rough idea yourself without going into the attic: open the hatch and look. If you can see the tops of the ceiling joists, you have somewhere around six inches or less, which is well short of target in every climate we work in. If the material is level with or below the joists, it needs attention.

What you should not do is start moving around up there. Attic floors are usually not floored, the joists are the only safe footing, and stepping between them puts you through the ceiling. There are also live knob-and-tube circuits in some older homes, and recessed light housings that have clearance requirements for a reason. A visual inspection is free from us, and we do it with the right footing and the right lighting.