How Much Attic Insulation Do I Need? R-Value and Depth Chart

Most homes in the US should have an attic insulated to somewhere between R-38 and R-60, depending on the climate. That is a lot more than many older houses have: R-11 to R-19 is common. The good news is that attic insulation is one of the cheapest, easiest and best-paying improvements a homeowner can make. Here is how to work out how much you need.

What R-value means

R-value is a material's resistance to heat flow: the higher the number, the better it insulates. R-values add up when you stack layers, so you can add R-19 to existing R-19 to get R-38. Each material gives a different R-value per inch, which is what decides the thickness you need.

What R-value should you aim for?

Energy Star's guidance for adding insulation to an existing home with an uninsulated attic floor:

Your local building code sets the minimum. Going above the minimum often pays off, especially where energy is expensive.

How many inches do you need?

Divide the target R-value by the R-value per inch of your material:

These are typical values. Loose-fill insulation also settles over time, so installers blow in a bit more than the final depth, and the bag shows the coverage needed for each R-value. The attic insulation calculator works out the depth and volume to add, for the R-value you have now and the one you want.

Worked example

A 40 × 30 ft attic (1,200 ft²) has R-11 now and the owner wants R-49. That is 38 more R-points. With blown fiberglass at R-2.5 per inch, they need to add 15.2 inches, or 1,520 ft³ of insulation. With blown cellulose (R-3.5 per inch), 10.9 inches would do it.

Batts or blown-in?

• Blown-in (loose-fill) fills around joists, wires and obstacles and covers the whole floor. It is the best choice for adding to existing insulation, and many stores lend the blowing machine if you buy enough bags.
• Batts (rolls) are easy to place in open, regular joist bays. Lay them across the joists on top of existing insulation, and use unfaced batts for the top layer.
• Spray foam gives the highest R-value per inch and seals air leaks, but costs much more, and is normally a professional job.

Do this before you insulate

1. Air-seal. Warm air leaking up into the attic carries more heat than the insulation can hold back. Seal gaps around pipes, wires, chimneys, recessed lights, the attic hatch and the tops of interior walls with caulk and foam.
2. Fix moisture and roof leaks. Wet insulation does not work, and may hide mold or rot.
3. Install baffles at the eaves so that insulation does not block the soffit vents. Ventilation and insulation work together: see the attic ventilation guide.
4. Check the fixtures. Keep insulation off recessed lights, unless they are rated for it (IC-rated), and 3 inches away from hot flues and chimneys.

Safety

• Wear a respirator or dust mask, goggles, gloves and long sleeves. Insulation fibers irritate skin, eyes and lungs.
• Walk only on the joists or on boards laid across them. The ceiling drywall will not carry your weight.
• Be careful of heat in summer attics, and take breaks.
• Vermiculite (loose, grey-gold pebbles) can contain asbestos. If you find it, do not disturb it: get it tested and removed by a professional.