Insulation Calculator
If your upstairs turns into an oven every summer and an icebox every winter, the attic is almost always the culprit. Heat rises straight through a thin layer of old insulation, and your heating and cooling bills pay for it all year. The good news: blowing extra insulation into an attic is one of the cheapest comfort fixes per dollar in all of home improvement — a weekend job with a rented blower, no special skills required. The part that trips people up is the shopping, not the work: how many bags do you actually need? This insulation calculator answers that. Enter your attic area, the R-value you are aiming for, what material you want, and what is already up there — it tells you how much R-value to add, how deep to blow it, and how many bags to buy. It works in metric too (RSI included), which most insulation calculators do not.
How to use this calculator
- Measure your attic floor area — length times width of the space you will insulate. Skip areas you cannot or should not cover, like the attic hatch opening and around recessed lights that are not rated for insulation contact.
- Pick a target R-value. R-49 suits most climates; go R-30 in hot climates and R-60 where winters are severe. The calculator shows the RSI equivalent automatically (R ÷ 5.678) if you think in metric.
- Choose your material. Blown cellulose gives about R-3.5 per inch, blown fiberglass about R-2.5 per inch — so cellulose reaches the same R-value in less depth. Fiberglass is the safer pick in damp attics because it does not absorb water the way cellulose can.
- Measure and enter your existing insulation. Poke a ruler down through it to the ceiling drywall in a few spots and average the readings. Old insulation still counts — R-values add when you stack materials, so the calculator only prices the top-up you actually need.
- Read the bag count and the depth to add. Depth is what you aim for while blowing; the bag count is what you buy. Both are rounded up, because running out mid-attic is worse than having a bag left over.
- Verify against your bag. Bag sizes differ by brand — one maker's 26-bag job is another's 84-bag job at the same R-value. Check the coverage chart printed on the bag you are actually buying and adjust if it disagrees.
- Plan the blow day. Most home centers lend you the blower free with a 10–20 bag purchase. Budget 2–4 hours for 1,000 sq ft, wear a dust mask and goggles, and keep soffit vents clear so the attic can still breathe.
Formula
Worked example
Take a 1,200 sq ft attic with nothing in it, target R-49, blown cellulose. Step one — depth: 49 ÷ 3.5 = 14.0 in of cellulose. Step two — bags: at R-49 the chart rate is 91 bags per 1,000 sq ft, so 1,200 × 91 ÷ 1,000 = 109.2, rounded up to 110 bags. Same attic in blown fiberglass: depth 49 ÷ 2.5 = 19.6 in, and 1,200 × 26 ÷ 1,000 = 31.2 → 32 bags. Notice the trade: fiberglass needs fewer bags but nearly 6 in more depth — which is exactly why the bag counts you see online disagree so wildly between brands.
Metric version: 100 m² attic (1,076 sq ft), target R-49 (RSI 8.6), blown cellulose. Depth: 14.0 in = 35.6 cm. Bags: 1,076 × 91 ÷ 1,000 = 97.9 → 98 bags.
Top-up version: the same 1,200 sq ft attic already has 6 in of old batts. Existing R = 6 × 3.2 = R-19.2. Added R needed = 49 − 19.2 = R-29.8. Depth to blow in cellulose = 29.8 ÷ 3.5 = 8.5 in, and the bag count drops to about 71 bags — the old insulation just saved you nearly 40 bags.
Measurement tips
- Measure existing depth in several spots and average them. Old insulation compresses unevenly — one reading near the hatch can lie to you by inches.
- Air-seal before you blow. Gaps around pipes, wires and the attic hatch leak more heat than thin insulation does; a few tubes of caulk and some foam pay off first.
- Do not bury soffit vents. Slide vent chutes (baffles) between the rafters at the eaves before blowing so air keeps flowing from soffit to ridge.
- Blow to the installed depth on the bag chart, not the settled depth. Cellulose settles 15–20% over time — the chart already accounts for it, so trust the chart, not your eyes a year later.
- Buy by the chart on YOUR bag. A 19 lb bag and a 27.5 lb bag cover very different areas; the bag count only makes sense for the brand you are holding.
- Keep insulation 3 in from recessed lights unless they are IC-rated (insulation contact). This is a fire-safety rule, not a suggestion.
Common mistakes
- Comparing bag counts between brands. One site says 26 bags, another says 91, for the same attic at the same R-value — because the bags are different sizes. Compare price per bag and coverage per bag instead.
- Ignoring what is already there. Blowing a full R-49 over 6 in of decent batts wastes roughly a third of your bags.
- Blocking the soffit vents. Buried vents trap moisture, and a damp attic rots wood and ruins insulation R-value alike.
- Walking on the drywall. Step only on joists or laid planks — a foot through the ceiling below turns a weekend project into a drywall repair.
- Skipping the mask. Blown insulation dust is miserable to breathe; a proper dust mask and goggles are non-negotiable.
- Forgetting the attic hatch. An uninsulated hatch is a square hole in your R-49 — glue rigid foam to its back and weather-strip the edges.
Factors that affect accuracy
- Target R-value. Going from R-30 to R-60 roughly doubles the bags — the single biggest lever on the estimate.
- Material choice. Cellulose (R-3.5/in) needs less depth than fiberglass (R-2.5/in) for the same R, which changes both bag count and whether you have the headroom.
- Existing insulation. Every inch already up there subtracts directly from what you need to buy.
- Brand bag size. Coverage per bag varies widely between manufacturers — the chart on your bag is the final authority.
- Attic shape. Low-slope or cut-up attics with lots of obstructions waste more material than a clean rectangle.
- Settling. Cellulose loses 15–20% of installed thickness over time; installed depth (what the chart specifies) already accounts for this.
Sources & assumptions
- Needed depth = (target R − existing R) ÷ R-per-inch (cellulose 3.5, blown fiberglass 2.5, batts 3.2); existing R = depth × R-per-inch — R-values add when stacked.
- Bag rates (bags per 1,000 sq ft) from manufacturer coverage charts: cellulose 59/72/91/111 at R-30/38/49/60; blown fiberglass 20/26/32 at R-38/49/60, interpolated between points.
- Bag counts disagree across brands because bag sizes differ — compare price per bag, not bag count, per FreeFastCalculator's brand comparison.
- RSI = R ÷ 5.678 per the R-value definition; zone targets and method per CraftedCalcs' attic guide and DOE Energy Saver.
Safety note
What this calculator doesn't do
- This estimates blown-in attic insulation only — it doesn't cover wall cavities (dense-pack is a pro job), floors, or spray foam.
- It is a planning aid, not an energy-code approval. Required R-values are set by your local building or energy code — always check before you buy.
- Bag counts are estimates from typical manufacturer coverage charts. Bag sizes differ by brand, so always verify against the coverage chart printed on YOUR bag.
- It reports installed depth; blown insulation settles over time (cellulose 15–20%), so the depth you measure a year later will be less.
Frequently asked questions
How many bags of blown insulation do I need for 1,000 sq ft?
At R-49: about 91 bags of blown cellulose, or about 26 bags of blown fiberglass (AttiCat-type). At R-38: about 72 cellulose or 20 fiberglass. But brand bag sizes differ enormously — one maker's 26-bag job is another's 84-bag job — so always check the coverage chart on the bag you are buying.
Why do different websites give different bag counts?
Because they are quoting different brands' bags. A 19 lb bag and a 27.5 lb bag cover very different areas at the same R-value. The honest comparison is price per bag and square feet covered per bag, not the raw bag count.
Can I blow insulation over existing batts?
Yes — this is the most common attic upgrade there is. R-values add when you stack, so measure the existing depth, multiply by its R-per-inch (batts ≈ 3.2), and subtract from your target. The calculator above does exactly this: enter your existing depth and type, and it prices only the top-up.
What R-value do I actually need?
It depends on climate: R-30 for hot climates, R-49 for most of the temperate world, R-60 where winters are severe. R-49 is the widely used sweet spot. In metric terms that is RSI 5.3, 8.6 and 10.6 respectively (RSI = R ÷ 5.678). Your local energy code has the final word.
Cellulose or fiberglass — which is better?
Cellulose gives more R per inch (3.5 vs 2.5), seals air gaps slightly better, and is usually cheaper per R — but it settles 15–20% and absorbs moisture. Fiberglass resists moisture, settles less, but needs more depth for the same R. In a dry attic, cellulose usually wins on value; in a damp one, pick fiberglass.
How deep should insulation be for R-38?
About 11 in of blown cellulose (38 ÷ 3.5) or about 15 in of blown fiberglass (38 ÷ 2.5). Those are installed depths — cellulose will settle somewhat over the following months, which the manufacturer charts already account for.
Can I install blown insulation myself?
Yes. Most home centers lend you the blowing machine free with a 10–20 bag purchase. A 1,000 sq ft attic takes 2–4 hours with two people (one feeding the machine, one in the attic with the hose). Wear a dust mask, goggles, and a headlamp.
What is RSI, and how does it relate to R-value?
RSI is the metric version of R-value: RSI = R ÷ 5.678. So R-49 is RSI 8.6, R-38 is RSI 6.7, and R-30 is RSI 5.3. Same physics, different units — if your products are labelled in RSI, just multiply by 5.678 to get the R-value this calculator uses.
Next step
Take the bag count to the home center and check it against the coverage chart on the actual bag — then air-seal gaps around pipes, wires and the hatch before you blow, since air leaks waste more heat than thin insulation.
Save this estimate: use the Copy link button in the results box above to share these exact inputs, or Print results for a paper copy to take to the store.