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Depth Markers

What the rule usually says

Codes based on the International Residential Code (IRC) commonly require that when loose-fill insulation is blown into an attic, depth markers be installed showing the minimum initial installed thickness, marked in inches, attached to the trusses or joists and facing the attic access. A frequently cited density is at least one marker per 300 square feet. Codes also commonly require an installer certificate near the attic access listing the product, R-value, installed thickness, settled thickness, coverage and number of bags. Requirements vary by jurisdiction and code edition, and Canadian provincial codes have their own rules, so confirm with your building department before relying on any figure here. We are not giving code advice for your project.

Why they matter

Loose-fill depth is easy to get wrong: thin spots near the eaves, high piles near the hatch, and settling over time. Markers let you check coverage as you blow and confirm the job afterward. For hired work, they are an easy way to verify you got what you paid for. For incentive programs, inspectors may look for them.

How many markers?

Markers ≈ attic area ÷ 300, rounded up. Spread them evenly, including some away from the hatch, so they represent the whole attic rather than just the easy areas.

Placement

• Fasten each marker vertically to a joist or truss, with zero at the ceiling drywall surface (the top of the ceiling, not the top of the joist).
• Mark the target installed depth from the bag chart with tape or a marker.
• Face the numbers toward the access opening so they can be read with a flashlight.
• Place a few near eaves and corners where depth is often thin.

Making your own

Commercial rulers are inexpensive and labelled for the purpose. You can also cut strips of cardboard or wood and mark inches clearly. Whatever you use, it should stand above the final depth, stay put, and be readable from the hatch.

Installed vs settled thickness

Bag charts list the installed thickness you blow to, and the settled thickness the product will reach over time, especially with cellulose. Blow to the installed thickness at every marker. Our blown-in calculator estimates bags; the bag chart gives thickness.

Checking your existing insulation

Even without new work, you can measure existing depth at several spots with a ruler to estimate current R. Average your readings and use the attic insulation calculator. Do not dig into material that might be vermiculite; if you see granular pebble-like insulation, stop and do not disturb it.

Worked example

Figures below are illustrative examples (2026-10-29), not quotes or guarantees.

Assumptions: a 1,350 sq ft attic, one marker per 300 sq ft (an assumed local rule), a target installed thickness of 17 in from an assumed bag chart. Markers = 1,350 ÷ 300 = 4.5, round up to 5. Mark each at 17 in and place two near the eaves, two in the middle and one opposite the hatch.

The insulation certificate

Many codes require a signed certificate posted near the attic access. A typical one lists the insulation type and manufacturer, the R-value, the installed and settled thickness, the coverage area, and the number of bags. Keep a copy with your house records. If you install it yourself, recording the same information is good practice.

Common mistakes

• Placing all markers near the hatch, where depth is usually highest.
• Measuring from the top of joists instead of the ceiling surface.
• Using markers too short to show the target depth.
• Marking settled depth instead of installed depth.
• Forgetting to record bag count and product on a certificate.

Inspecting a contractor's job

After a contractor blows insulation, look from the hatch with a flashlight. Read markers in several directions. If depth looks short in areas you can see, ask the contractor to check and top up. Count empty bags if possible and compare with the certificate and bag chart.

Over time

Check depth every few years or after any attic work, such as electrical or roofing repairs. Workers can compress or push aside insulation. Top up thin areas to keep the target R.

Batt attics

Even with batts, measure thickness after installation to make sure they are not compressed and that gaps are filled.

Records for the future

A clear certificate and markers help future buyers, inspectors and incentive programs understand what you installed and when.

Making your own markers

If markers are not supplied, you can make them: cut stiff cardboard or thin wood strips, mark inches from the bottom with a permanent marker, and highlight the target installed depth. Staple or tape them to the side of joists so the zero mark sits on the ceiling surface. Number them and note their positions on a simple sketch of the attic, so you can find them later. This costs almost nothing and gives you the same check a contractor's markers provide. Space them roughly every 300 sq ft, and always put one at the far corners and near the eaves on each side, where depth is most often short.

Bottom line

Markers turn a promise of R-value into something you can see. Place them before insulating, check several, and keep the certificate with your house records.

Quick tip

Photograph each marker with a date.

FAQ

Are depth markers required for batts?

Usually the requirement is for blown loose-fill; batt thickness is labelled.

Where does the 300 sq ft figure come from?

It is common in IRC-based codes; your local code governs.

Should zero start at the top of the joist?

No, measure from the ceiling surface.

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