How to Measure Attic Insulation Depth and Work Out the R-Value

What you need

• A flashlight or headlamp, and a ruler or tape measure. A phone camera helps you record what you find.
• A dust mask or respirator, gloves, long sleeves and eye protection: insulation fibers irritate skin, eyes and lungs.
• A board to kneel on if the attic has no floor. Step only on the joists; the ceiling below will not hold your weight.

Step by step

1. Look across the attic floor first. If the insulation is at or below the tops of the floor joists, you can see the joists, and you probably need more. Energy Star gives this as the quick check.
2. Measure in several places. Push the ruler straight down until it stops on the drywall, without compressing the insulation, and read the depth at the top of the fluffy layer. Take readings in the middle of the attic and away from the eaves, where it is usually thinner.
3. Take the typical depth. If the readings vary a lot, use the lower ones, or level the old insulation out before adding more.
4. Identify the material (see below), because each type gives a different R-value per inch.
5. Multiply depth by R per inch to get the R-value you have now, using the chart below.
6. Work out what to add with the attic insulation calculator: enter the R-value you have and the one you want.

Identify what you have

• Fiberglass batts or blown fiberglass: pink, yellow or white, soft and fluffy, in rolls between the joists or as loose fill.
• Cellulose: gray, like shredded newspaper, always loose fill. Denser than fiberglass, so more R per inch.
• Mineral (rock) wool: gray, green or brown, denser than fiberglass. Use the label figure, or Energy Star's rule of thumb of about R-3 per inch.
• Vermiculite: loose, gray-brown or gold pebbles. Stop and do not disturb it: it can contain asbestos. Have it tested before any work.

Depth to R-value chart

R-value of the insulation already in the attic, using the R per inch in the attic insulation calculator and, in the last column, Energy Star's depth × 3 estimate:

How much to add on top

Inches of new loose fill to blow over existing fiberglass batts to reach R-49 or R-60:

Worked example: a 40 × 30 ft attic (1,200 ft²) with 6 inches of batts has about R-19. To reach R-49 with blown fiberglass, add 11.9 inches, which is 1,192 ft³ of insulation. The bag's coverage chart turns that into a number of bags.

Which R-value to aim for depends on your climate zone: see how much attic insulation you need for the Energy Star zone chart and depths for every material.

Before you add more

• Seal air leaks around pipes, wires, flues, light fixtures and the hatch first. Warm air leaking into the attic carries heat past the insulation.
• Fit baffles at the eaves so new insulation does not block the soffit vents. See how much attic ventilation you need.
• Keep insulation off recessed lights unless they are IC-rated, and clear of hot flues and chimneys.
• Wet, moldy or crushed insulation does not do its job: fix the roof leak or moisture problem first.

Source for the measuring method: Energy Star, How to check your home's attic insulation level. R-values per inch are typical; the label on the product you buy wins.

Frequently asked questions