One attic, many questions: what R-value the zone demands, what the slope-adjusted area actually is, how many bags you need, and whether the project pays back. Every section reads the same inputs — so the numbers can never quietly disagree with each other the way figures copied from three different websites do.
This is the engine the whole site runs on. Quick Mode picks your climate zone, enters your attic geometry, and gives you the R-value gap, the bag count, and the slope-adjusted area instantly. Multi-Layer Mode stacks any combination of material layers — gypsum, batts, air gaps, blown cellulose, spray foam, rigid foam — and reports total R and U values. Cost & ROI Mode adds material price, dynamic labor at $1.5/sq ft, and fuel cost with unit matching to your fuel type, producing project cost, annual savings, payback period, and CO₂ reduction.
The presets do the tedious part. 30+ insulation materials arrive with R-values per inch from manufacturer product sheets; slope factors auto-compute from rise/run ratios; fuel cost changes its unit with your fuel type ($/therm for natural gas, $/gallon for fuel oil, $/kWh for electricity). Every result keeps both unit systems visible, and nothing hides behind an account.
Reach for it when…
Quick Mode for the 80% case, Multi-Layer Mode for existing assemblies, Cost & ROI Mode for budgets — all reading the same zone, geometry, and material data so numbers never drift apart.
Zone 1 → R-30, Zone 2–3 → R-30/R-38, Zone 4A/4B → R-49, Zone 5–7 → R-60. Local code amendments may push higher; the tool flags mismatches so you can confirm with your building department.
Blow-in cellulose, loose fiberglass, mineral wool; faced/unfaced batts; open-cell and closed-cell spray foam; XPS and EPS rigid boards; cotton; plus gypsum, plywood, and air gaps for multi-layer work. Each with manufacturer-sourced R-values.
Slope factor √(1 + (pitch/12)² auto-computed. Net area = gross × slope − cutouts. Attic Floor vs. Roof Slope insulation types supported. Joist cavity % for batts vs. blown-in.
Labor = $1.5/sq ft (not a fixed number). Fuel cost unit changes with fuel type: $/therm (natural gas), $/gallon (fuel oil, propane), $/kWh (electricity, heat pump). Input cost, get dollars.
Material cost + dynamic labor = total project cost. Annual savings modeled from ENERGY STAR's 15% baseline scaled by R-value added. Payback = total cost ÷ annual savings. CO₂ reduction estimate tied to fuel type.
Current R-value ≥ target R-value? The calculator says 'Your attic is already sufficiently insulated. No additional insulation needed.' Cost, savings, and payback show N/A — no false precision.
Six directions, ordered the way a real attic project unfolds — the zone first, then the material, then the geometry, then what it all costs. Each card links to the home-page section where that capability lives today, inside the main calculator and its reference sections.
Climate zone, gross attic area + roof pitch, material choice, and instant R-value gap calculation. Covers the vast majority of residential attic upgrades in one screen. The calculator auto-applies the slope factor and deducts cutouts.
Open Quick ModeStack any number of material layers — gypsum, batts, air gaps, plywood, blown cellulose, spray foam, rigid XPS/EPS — and see the total R-value and U-value. Works for existing assemblies and planned new builds alike.
Stack the layersMaterial price per bag or roll, labor dynamically calculated by square foot, fuel cost tied to fuel type ($/therm, $/gallon, $/kWh), annual HVAC spend — produces material + labor total, annual and 10-year savings, payback period, and CO₂ reduction.
Run the numbersDOE Zone 1 through Zone 7 mapped directly to ENERGY STAR recommended R-values. Zone 1 (hot FL) = R-30; Zone 4A/4B (cold) = R-49; Zone 5 (very cold) = R-60. Local code amendments may push higher — the tool flags zone code mismatches.
Pick your zoneSlope factor auto-computed from rise/run ratio: √(1 + (pitch/12)²). A 4/12 pitch adds 5.5% area; a 10/12 pitch adds 36%. Net area after cutouts for chimneys, skylights, and hatches — so your bag count is accurate, not aspirational.
Calculate slopeBlown cellulose R-3.2–3.8/inch, loose fiberglass R-2.2–3.2/inch, open-cell spray foam R-3.5–4.5/inch, closed-cell R-5.5–6.5/inch, XPS/EPS rigid foam, mineral wool batts, cotton batts. Each with coverage rates at target install depth — pick one and watch the bag count live-update.
Compare materialsNarrow, job-specific calculators that share the same engine — each with its own presets, warnings, and worked examples. They ship into the registry below as they are finished.
Calculate required Net Free Area (NFA) for attic and crawlspace ventilation. Follow IRC 1/150 & 1/300 rules with product matrix and cost estimates.
Open calculatorYour blown-in insulation settled 10-15%. See real R-value, R-value loss %, and extra bags needed for topping up.
Open calculatorQuantify heat loss through attic hatches, skylights, and chimneys. See BTU/hr loss, annual cost, and equivalent missing insulation.
Open calculatorIn the meantime, the single engine already answers these jobs: pick a scenario preset on the Quick Mode of the home page and the zone, material, labor rate, and payback all load together.
The test of a shared engine is simple: run the same attic through every lens and the story has to stay consistent. Here is a 1200 sq ft attic in Zone 4B, at a 4/12 pitch, with no existing insulation — through the figures it meets on the way to a verdict. Every number traces back to the same inputs, and you can reproduce each one in the tool that names it.
Zone 4B → R-49
Zone 4B (cold-humid: Detroit, Omaha) recommends R-49 for attics per ENERGY STAR. Your local code may push higher — always confirm with the building department.
1266 sq ft · 1.055× slope
1200 sq ft gross × √(1 + 0.33²) ≈ 1.055 slope factor = 1266 sq ft. No major cutouts to deduct. 4/12 pitch adds 66 sq ft you might otherwise forget.
R-49 (no existing insulation)
Empty attic, no insulation = R-0 existing. Gap = target R-49 − R-0 = R-49 needed. Enter 12 inches of settled cellulose (R-3.5/inch × 12 = R-42) + 2 inches of air gap and you're still short.
14 in · ≈15 bags cellulose
Cellulose at R-3.5/inch needs R-49 / R-3.5 = 14 inches of settled depth. Coverage per bag at 14-inch depth ≈ 85 sq ft. 1266 ÷ 85 ≈ 15 bags — that's what you buy.
≈$6,490 (professional)
15 bags × $35/bag = $525 material. Labor: 1200 sq ft × $1.50/sq ft = $1,800. Plus blower rental for DIY; we used professional here. Compare DIY (no labor) to decide.
≈24 yrs · 1,500 lb CO₂/yr
Annual savings ≈ $270 on $1,800 HVAC spend (15% baseline × R-49/R-49). Payback = $6,490 ÷ $270 ≈ 24 years, which fits the 25–30 year life of blown cellulose.
Same attic, every lens, every timePhoto: UnsplashThe classic failure is not one bad number — it is two good numbers that were never talking. A zone target taken from a ZIP code guess, a material coverage rate entered from the wrong bag, a slope factor forgotten because it "only adds a little." Each is an honest estimate made in isolation; stacked together they produce an attic that's short R-value, short bags, or short on the bill compared to the actual job.
Sharing one engine is the structural fix. The zone that set your R-value target also validates it against your local code; the material preset that set your R-value per inch also gives you coverage per bag; the slope factor that expanded your area also warns you when you skip it. When the definitions are shared, the mistakes get caught by arithmetic instead of by the blower-truck driver halfway through day two.
See more worked examplesFind the question you are holding, follow the row, open the section. Every destination is one click away — and back to the main calculator when the next question starts.
Every tool has a guide behind itPhoto: UnsplashEvery capability on this page has a matching set of guides that explain the ideas behind its inputs — why Zone 4 needs R-49 and Zone 5 needs R-60, why cellulose settles 10–15% in the first year, why the slope factor applies to the ceiling surface, and what a payback period really means for your HVAC budget.
The guides and the tools are written as a pair: the reading explains why a number moves, the tool shows how far it moves. If a result ever surprises you, the guide library is the fastest way to find out whether the surprise is the math or the measurement.
Browse the guide libraryYes — every tool on this page is free, requires no account, and has no usage caps or paywalled fields. The site is a reference toolset, not a lead-generation funnel: nothing gates a result, and no result is emailed to anyone.
One engine, three modes. The zone target, slope area, material coverage, and cost math all read the same zone, area, and material data you enter, so a figure from Quick Mode is always consistent with Multi-Layer Mode and Cost & ROI Mode. There is no second copy of the math to drift out of date — and no re-typing your zone when you switch modes.
Climate zone R-value targets (DOE Zones 1–7), R-value gap analysis — current vs. target — slope-adjusted net area with cutout deductions, coverage conversion to bag/roll counts, dynamic labor cost by area, fuel cost with unit matching to fuel type, cost & ROI with payback period, and CO₂ reduction estimate. The Multi-Layer builder handles any stack of materials.
The 30+ presets carry R-values per inch from recognized industry sources: NAIMA for mineral wool, FPSC for cellulose, ACC for spray foam, DOE and manufacturer product data sheets for fiberglass and rigid foam. Where a material's R-value is a range (loose fiberglass R-2.2–3.2/inch), the calculator shows the full range and lets you pick the low, mid, or high end — because real installed R-value depends on coverage quality and settling.
Yes. Switch the unit system and the inputs and outputs all move together — square meters, centimeters, and Joules per kilogram Kelvin in, or GPM and SCFM if the specification you were handed is imperial. Both unit sets stay visible on results, so a metric drawing can still be checked against an imperial nameplate.
Please do. Send the calculation you keep doing by hand — what inputs you start from, what result you need out — through the contact page. Requests that describe a real, repeated attic problem (e.g., 'calculator that accounts for attic hatch heat loss separately') go to the top of the queue, and the toolset grows the same way the guide library does: from questions that were worth writing down.
Pick your climate zone, enter your attic size and pitch, choose a material — the engine returns the R-value gap, the bag count, the project cost, the annual savings, and the payback period before your coffee cools. Free, private, and instant.