How Much Attic Insulation Do I Need?
For a vented attic in Western Washington, the target is R-49, the level current Washington energy code calls for above ceilings in new homes. In blown cellulose that is about 14 inches; in blown fiberglass, 17 to 20 inches. Most homes in our area built before the 1990s have R-11 to R-19, so even a partial top-up makes a noticeable difference.
Depth needed for R-49 by material
| Material | Approx. R per inch | Inches for R-38 | Inches for R-49 |
|---|---|---|---|
| Blown cellulose | 3.5 | 11 | 14 |
| Blown fiberglass | 2.5 – 2.9 | 13 – 15 | 17 – 20 |
| Fiberglass batts | 3.0 – 3.5 | 11 – 13 | 14 – 16 |
| Rock wool (loose) | 2.8 – 3.0 | 13 | 17 |
Manufacturer coverage charts give the exact installed and settled depth for each product. That is why we staple depth rulers in the attic: the installed depth should match the chart.
How to measure what you have
- Open the hatch and look with a flashlight, without climbing onto the insulation.
- Push a ruler or a yardstick straight down through the insulation until it touches the ceiling drywall or plaster.
- Measure in at least three spots away from the hatch, which is often disturbed.
- Identify the material: fluffy pink, yellow or white fibers are fiberglass; gray, shredded paper is cellulose; gray-brown or silvery pebbles are probably vermiculite — stop there and read our vermiculite guide.
- Multiply inches by the R per inch from the table.
If the tops of the ceiling joists are visible, you can stop measuring: you are at roughly R-19 or less.
What the numbers mean for your house
| Current level | Recommendation |
|---|---|
| R-0 to R-19 | Air seal and insulate to R-49; high value |
| R-19 to R-30 | Air seal and top up; good value, especially with electric heat |
| R-30 to R-38 | Check for gaps, missing baffles and air leaks first; top-up is optional |
| R-38 and up, evenly distributed | Leave the depth alone; focus on air sealing if the house is drafty |
Coverage matters as much as depth. A 10-inch average with bare spots at the eaves and a trench where someone crawled to the furnace performs worse than the average suggests.
Why air sealing comes first
Air carries far more heat and moisture through a ceiling than conduction through the insulation does. A heavily insulated attic floor with open top plates and unsealed lights still leaks warm, humid air into the attic. That is also how attic condensation starts. See air sealing and our guide on winter attic condensation.
Diminishing returns, honestly
Each added inch saves less than the one before. Going from R-11 to R-30 does most of the work; R-30 to R-49 adds a smaller saving. We still usually recommend finishing at R-49 because the setup and labor are the same, the material cost difference is small, and the attic will not need revisiting.
Situations where R-49 is hard to reach
- Low-slope roofs where the eaves leave only a few inches above the top plate — baffles keep the airway open, and insulation is as deep as space allows
- Sloped Craftsman ceilings where rafter bays are 2×4 or 2×6 — dense-packed cellulose fills the bay, but the R-value is limited by its depth
- Knee walls and side attics — these are wall assemblies, not floors, and need their own approach
Verdict: R-49 if you are below eight inches
Measure first. If you see joist tops or have less than about eight inches of material, sealing and insulating the attic to R-49 is one of the most reliable improvements you can make to an older Western Washington house. If you already have R-38 or more, look for gaps and leaks rather than more depth.
Attic insulation FAQ
Does more insulation help in summer?
Yes. It slows heat coming down from a hot roof, which matters for upstairs bedrooms in July and August.
Can too much attic insulation cause problems?
Not by depth alone. Problems come from blocking soffit vents, covering non-IC-rated lights or burying live knob-and-tube wiring. Proper baffles and clearances prevent those.
Is cellulose safe?
Cellulose for building insulation is treated with borate compounds for fire and pest resistance and is widely used. It does generate dust during installation, which stays in the attic.