Insulation Calculator: R-Value, Depth & Cost

Measuring what you have

Area. For an attic floor, measure the footprint of the heated rooms below — length × width of each section, added together. For walls, multiply each wall’s length by its height and subtract large windows and doors. For a floor over a crawlspace or garage, use the floor area of the rooms above. The square footage calculator handles L-shapes and multiple rooms.

Existing R-value. In the attic, push a ruler down through the insulation to the ceiling drywall in a few spots and average the depth. Multiply by the material’s R per inch: loose fiberglass is about R-2.5, cellulose (gray, papery) about R-3.4 and fiberglass batts about R-3.2. Eight inches of loose fiberglass is roughly R-20. If the old insulation is wet, moldy or packed down, count it as less — or plan to remove it.

How much to add

The calculator uses DOE and ENERGY STAR attic recommendations for an uninsulated attic: R-30 in zone 1 (south Florida, Hawaii), R-49 in zones 2–3 and R-60 in zones 4–8. Walls default to R-13, which is what fits a standard 2x4 cavity with ordinary batts, and floors default to R-19. Those wall and floor targets are typical figures rather than official recommendations, so enter your local code requirement in the target field if it differs.

Worked example: a 1,000 sq ft attic in zone 5

The same R-49 with each material:

On an open attic floor, depth is rarely a problem, so blown-in insulation usually wins on cost. Before topping up, check that baffles keep the soffit vents clear and that the attic hatch is insulated and weatherstripped too.

The spray foam figures are national per-square-foot survey ranges, which reflect typical job depths. Foam is often priced by the board foot (1 sq ft, 1 in thick), so a layer as deep as the one in this example would cost considerably more than the table shows. Get a quote for the exact depth.

Walls and floors: the cavity limits you

In a wall or floor, insulation has to fit inside the framing. A 2x4 is actually 3.5 in deep, a 2x6 is 5.5 in and a 2x8 is 7.25 in. That caps the R-value:

The calculator warns when filling the whole cavity with your chosen material still can’t reach the target, even with a high-density version of it. Your options then are a higher-R material, deeper framing, or a layer of rigid foam over the studs. Walls that already hold insulation can’t simply take more on top — the old material comes out, or the added R-value goes outside the framing.

DIY or hire out?

Batts in an open attic or an unfinished floor, and blown-in insulation with a rented blowing machine, are common DIY jobs. The DIY figure is an editorial estimate that material is about half of the installed price. Spray foam is a professional job outside of small kit-sized areas: it requires protective equipment, ventilation and proper mixing.

Pros are also worth it when the attic needs air sealing first. Gaps around light fixtures, plumbing stacks and top plates leak conditioned air, and sealing them before adding insulation makes the new layer far more effective.

What’s not included

Air sealing, attic baffles, vapor retarders, removing old or damaged insulation, drywall repair after wall work (see the drywall calculator) and equipment rental. Better insulation can also let you buy a smaller air conditioner — see the HVAC replacement cost calculator.

How this calculator works

• R to add = target R − existing R (never below zero).
• Depth = R to add ÷ R-value per inch of the chosen material.
• Cost = area × installed cost per sq ft range × local cost level (× 0.5 for DIY material only).
• For walls and floors, a warning appears when filling the whole framing cavity with the chosen material cannot reach the target, even at the high end of its R-per-inch range (high-density products).

Assumptions and limitations

• Attic targets are DOE / ENERGY STAR recommendations for uninsulated attics: R-30 in zone 1, R-49 in zones 2–3 and R-60 in zones 4–8.
• Wall (R-13) and floor (R-19) targets are editorial defaults; R-13 to R-15 is typical for a 2x4 wall cavity. Enter your local code requirement to override.
• R-value per inch uses typical values: batts 3.2, loose fiberglass 2.5, cellulose 3.4, open-cell foam 3.6, closed-cell foam 6.0.
• Installed costs are national per-square-foot ranges from contractor surveys, adjusted for local cost level. DIY cost assumes material is about 50% of the installed price — an editorial estimate.
• Air sealing, baffles, vapor retarders, old insulation removal and equipment rental are not included.

These results are planning estimates, not quotes. Local prices, site conditions and building codes vary, so get written estimates from licensed contractors before you commit to a budget.

Frequently asked questions

How much attic insulation do I need?

DOE and ENERGY STAR recommend R-30 to R-49 in the hottest zone and R-49 to R-60 in most of the country for an empty attic. With 3–4 in already there, adding R-25 to R-49 depending on zone is the usual advice.

Can I put new insulation over old insulation?

In an attic, yes — loose fill or unfaced batts can go right on top, as long as the old material is dry and not moldy. Don’t use faced batts on top, because the facing can trap moisture.

Is spray foam worth the extra cost?

Closed-cell foam packs about twice the R-value per inch of fiberglass and also air-seals, which matters in shallow wall cavities and rim joists. In an open attic floor, blown-in insulation usually gives more R-value per dollar.

Sources and data

• HomeGuide (2026) — batt roll insulation cost
• HomeGuide (2026) — attic insulation cost
• HomeGuide (2026) — blown in insulation cost
• HomeGuide (2026) — spray foam insulation cost
• ENERGY STAR / DOE — insulation r values
• DOE Building Technologies Program insulation guide
• DOE Building America spray foam guide

Prices are US national ranges reviewed October 2026. A few values are editorial estimates where no single published source exists; theCost Index marks each one. Read our methodology and editorial policy.