HowMuchIt / Insulation
How much insulation do I need?
Insulation is measured in R-value, and each material gives a certain R per inch. Pick the R-value for your attic or walls, subtract what is already there, and the calculator gives the inches to add.
Work out your insulation depth
| Material | R per inch |
|---|---|
| Fiberglass batts | 3.1 to 4.2 |
| Blown fiberglass | 2.2 to 2.7 |
| Blown cellulose | 3.2 to 3.8 |
| Open-cell spray foam | 3.5 to 3.8 |
| Closed-cell spray foam | 6 to 7 |
Open-cell foam is about R-3.6 per inch (sources span 3.5 to 3.8). Layers add up; squashed insulation loses part of its R-value. Bags per 1,000 sq ft for each R-value are on the bag's coverage chart.
How much R
For an attic with no insulation, ENERGY STAR recommends R-30 in the warmest zone, R-49 in zones 2 and 3, and R-60 in zones 4 to 8; R-30 to R-49 is the range most often quoted for attics. Walls run from about R-13 to R-21. Layers add up, so new insulation on top of old counts both. Insulation that is squashed down does not give its full rating: let it loft.
In the attic
For blown-in insulation, the bag has a coverage chart: for each R-value, the bags per 1,000 square feet and the minimum depth. Set attic rulers around the space so you can see when you reach it. Keep the soffit vents clear, and keep loose-fill at least 3 inches from recessed lights and other heat sources.
Questions people ask
How many inches of cellulose for R-49?
At R-3.2 to 3.8 per inch, about 13 to 15 inches.
What is R-value?
How well a material resists heat flow: the higher, the better it insulates.
How many bags of blown-in insulation?
The bag's coverage chart gives bags per 1,000 sq ft for each R-value.
Good to know
- For an uninsulated attic the recommended level rises with climate zone: ENERGY STAR gives R-30 in zone 1, R-49 in zones 2-3 and R-60 in zones 4-8; CertainTeed's DOE table gives ranges that contain these (R-30 to R-49, R-38 to R-60, R-49 to R-60).1,2
- R-30 to R-49 is the range most often quoted for attics and ceilings.3,4
- Typical wall recommendations run from about R-13 to R-21 (one maker goes up to R-23).3,5
- Fiberglass batts give about R-3.1 to R-4.2 per inch.4,6
- Blown (loose-fill) fiberglass gives about R-2.2 to R-2.7 per inch, depending on density.4,6
- Blown cellulose gives about R-3.2 to R-3.8 per inch.4,6
- Open-cell spray foam gives about R-3.6 per inch (sources span R-3.5 to R-3.8).4,6
- Closed-cell spray foam gives about R-6 to R-7 per inch (around R-6.5).4,6
- Layers add up: the total R-value is the sum of the R-values of each layer.5,7
- Compressed insulation does not deliver its full rated R-value.4,7
- For blown-in insulation, the coverage chart on the bag gives, for each R-value, the number of bags per 1,000 sq ft and the minimum thickness.8,9
- Set attic rulers (depth markers) around the attic so you can see when the target depth is reached.8,9
- Do not cover or block soffit (eave) vents.8,9
- Keep loose-fill insulation at least 3 in away from recessed light fixtures and other heat sources.8,9
How we checked this page
Checked on October 27, 2026. We kept a fact only when two independent publishers agree, and each one is numbered to its sources below. Where sources give different numbers, we keep only what they share. We never write a price. This page was written with an AI assistant and checked against the sources below.
Sources
- Recommended Home Insulation R-Values, ENERGY STAR
- Insulating Spaces: Attics, CertainTeed
- What R-value Do I Need?, Johns Manville
- Understanding Insulation and R-Value, This Old House
- How to Increase Insulation R-Values in Attics, Ceilings & More, Lowe's
- What to Consider When Choosing Insulation, Fine Homebuilding
- Insulation, U.S. Department of Energy (Energy Saver)
- Do-it Yourself Attic Installation Instructions (DM-6.3-38), Greenfiber
- Installation Instructions for Loose-Fill Blown-In Fiber Glass, Johns Manville