How long does attic insulation last?

No expiry date, plenty of ways to wear out

You won't find a lifespan printed on a bag of insulation, and the official sources don't give one either. That's because the common attic materials don't break down on their own. Glass fibers and rock fibers don't rot. Dry paper fiber, kept that way, doesn't either.

What does happen is damage. Insulation's job depends on trapped air, so anything that crushes it, soaks it, scatters it or fills it with nests takes R-value away. In a typical Bay Area attic, those things are more likely to end its working life than the calendar.

By material

• Fiberglass, loose fill or batts. Spun glass. It doesn't decay, but it only insulates while it keeps its loft. Crushed under boxes or trampled into a path, it does less. Kraft facings on batts are paper and age like paper.
• Mineral wool. Rock or slag spun into fibers. Like fiberglass, it doesn't decay, and the DOE guide notes it loses R-value when compressed.
• Cellulose. Recycled paper treated with fire retardant. Dry, it holds up. The DOE guide warns that accidental wetting and high humidity can make it absorb large quantities of water, which reduces its insulating value, drains the fire retardant and raises the risk of mold.
• Rigid foam board. The DOE guide explains that foam with closed cells holds a low-conductivity gas, and that over time some of that gas escapes and is replaced by air, so the R-value drops. Foil and plastic facings help slow that.

Settling is built into the label

Loose fill does settle after it's blown, and that's expected. Federal labeling rules for home insulation (16 CFR 460.12) require every loose-fill coverage chart to show "the minimum settled thickness, initial installed thickness, maximum net coverage area," along with bags per 1,000 square feet and minimum weight per square foot at each R-value. In other words, the label tells the installer how deep to blow it so it still reaches the target after settling.

The problem is fill that wasn't blown to the label. The DOE guide puts it bluntly: if the density is too low, which the trade calls "fluffing", the insulation "will settle excessively and have a lower R-value." Its advice is for the contractor to staple rulers in several places across the attic, which also lets you measure settling over the years.

What shortens it in Bay Area attics

These are the things we'd look for first in an attic around here:

• Rats and mice. Roof rats in particular nest in attic insulation, tunnel through it and soil it. Soiled insulation can't be cleaned; it has to come out. If you hear them, start with rats in the attic.
• Roof leaks. One wet winter storm through a failed flashing or a cracked tile can leave a patch of matted, stained insulation that never fully recovers, especially cellulose.
• Other trades. HVAC techs replacing a furnace or ducts, cable and internet installers, electricians and solar crews running conduit all crawl across the attic. Every path they make is a strip of crushed insulation, and fill pushed aside to reach a junction box rarely gets raked back.
• Storage. Plywood laid over the joists and boxes stacked on top compress whatever's underneath.
• Thin original fill. Many houses here got their first attic insulation decades ago, when far less was expected. What remains is often a few inches, uneven and packed down, which is less a lifespan issue than a starting-point issue.

How to tell if yours is still working

Depth and evenness tell you most of it. Our guide on how to check attic insulation walks through measuring depth and reading what you see, and ENERGY STAR's attic guidance suggests checking your levels before planning any project.

If what you find is dry, clean and simply thin, the usual answer is to top it up with blown-in insulation rather than start over. If it's soiled, wet or crushed, our insulation removal page lists the signs it's time to replace it.

What a manufacturer warranty covers

New insulation comes with whatever manufacturer warranty its maker offers, and those terms usually depend on the product being installed according to its label. We pass that manufacturer warranty on to you, and we don't offer a separate workmanship promise. Read the manufacturer's terms for the product named on any quote, ours included.

Making the next layer last

Most of what shortens insulation's life can be prevented: close rodent entry points before new insulation goes in, fix roof leaks first, and ask anyone who works in the attic later to put the insulation back the way they found it. A clear walkway or marked path to equipment helps too.

If you'd like a second pair of eyes on what's up there now, book a free inspection or send us photos.