Insulation Calculator
Your details
Advanced options
Result
Depth to add
- R-value to add
- R-19 (R-49 target less R-30 existing)
- Bags needed
- 23 bags
- Area
- 800 sq ft
- Coverage per bag at this depth
- 35.9 sq ft
- Total depth when finished
- 14 in total
- Resulting R-value
- R-49
- R-values add, so reaching R-49 from R-30 needs only R-19 more — not R-49 of new material. At R-3.5 per inch that is 5.43 inches of blown cellulose on top of what is there.
- A bag covering 195 sq ft at one inch covers 35.9 sq ft at 5.43 inches, so 800 sq ft needs 23 bags. Coverage charts are printed on the bag and vary by product — use the real figure.
- Air sealing comes first. Insulation slows conduction but does nothing about air moving through gaps around wiring, plumbing, light fittings and the attic hatch. Sealing those leaks before insulating is consistently the better value of the two, and insulation laid over unsealed penetrations underperforms its rating.
- Do not block soffit vents or bury recessed lights that are not IC-rated. Baffles keep the ventilation path open at the eaves, and covering a non-IC fixture is a genuine fire risk.
Depth needed by material
All reaching R-19. A higher R per inch means less depth, which matters where headroom or cavity depth is limited.
| Material | R per inch | Depth needed |
|---|---|---|
| Blown cellulose (yours) | R-3.5 | 5.43 in |
| Blown fiberglass | R-2.5 | 7.6 in |
| Fiberglass batt | R-3.2 | 5.94 in |
| Mineral wool batt | R-4 | 4.75 in |
| Rigid foam board (XPS) | R-5 | 3.8 in |
Common attic targets
DOE recommendations run from R-30 in the warmest zones to R-60 in the coldest.
| Target | R to add | Depth of your material |
|---|---|---|
| R-30 | already met | — |
| R-38 | R-8 | 2.29 in |
| R-49 (yours) | R-19 | 5.43 in |
| R-60 | R-30 | 8.57 in |
How much insulation do I need to reach R-49?
Subtract your existing R-value from the target, then divide by the material's R-value per inch. R-values add, so going from R-30 to R-49 needs only R-19 more — which is 5.43 inches of blown cellulose at R-3.5 per inch, not a full replacement.
R to add = target R − existing R depth = R to add ÷ R per inch- target R
- What you are aiming for; DOE suggests R-30 to R-60 for attics by climate zone
- existing R
- What is already in place — it counts in full
- R per inch
- 3.5 cellulose, 2.5 blown fiberglass, 5.0 rigid foam
R-values of layered insulation add, which is the single most useful fact here: if you already have R-30 in the attic, reaching R-49 needs only R-19 of new material, not R-49. That turns a large job into a modest one, and it is why topping up existing insulation is usually the cheapest efficiency measure available on a house. Depth then follows from the material, since R per inch varies from about 2.5 for blown fiberglass to 5.0 for rigid foam — a factor of two, which matters whenever headroom or cavity depth is limited. Two things the arithmetic does not capture are more important than the number it produces. The first is air sealing: insulation slows conduction but does nothing about air moving through gaps around wiring, plumbing and light fittings, and sealing those first is consistently better value than adding depth. The second is diminishing returns. Going from R-11 to R-30 cuts heat loss through that surface by roughly two thirds; going from R-49 to R-60 saves a few percent more. The first inches do nearly all the work.
- Measure the area being insulated, not the building footprint.
- Determine the R-value already in place from its depth and type.
- Subtract the existing R-value from your target R-value.
- Divide the difference by the material's R-value per inch.
- Read the bag's coverage chart at that depth for bag count.
Worked example: A 32 × 25 ft attic going from R-30 to R-49 with blown cellulose
A 32 × 25 ft attic going from R-30 to R-49 with blown cellulose
The attic floor is 32 × 25 = 800 square feet. Because R-values add, reaching R-49 from an existing R-30 requires only R-19 of new material rather than R-49 — the existing insulation counts in full, which is what makes a top-up so much cheaper than a replacement. Blown cellulose is about R-3.5 per inch, so R-19 needs 19 ÷ 3.5 = 5.43 inches added on top of the roughly 8.6 inches already there, finishing at about 14 inches. For bags, a product covering 195 square feet at one inch depth covers 195 ÷ 5.43 = 35.9 square feet at 5.43 inches, so 800 ÷ 35.9 = 22.3 bags, meaning 23. Choosing blown fiberglass instead at R-2.5 per inch would need 7.6 inches for the same R-19, which is 40% more depth for the same thermal result — the trade-off that matters when headroom above the joists is limited.
What each insulation input means
- Area length
- Length of the attic floor or wall being insulated.
- Area width
- The other dimension. Use the insulated area, not the building footprint.
- Target R-value
- What you are aiming for. The Department of Energy recommends R-30 to R-60 for attics depending on climate zone, with R-49 to R-60 across most of the northern US.
- Existing R-value
- What is already in place. R-values add, so you only need to make up the difference — which is why topping up is far cheaper than starting over.
- Material
- R-value per inch varies by material, so the depth needed for a given R-value does too. Product labels govern over these typical figures.
- Bag coverage at 1 inch
- Blown insulation bags print a coverage chart by depth. This is the square footage one bag covers at one inch, which the calculator divides by your depth. Use the figure on the bag.
What this insulation does not account for
- R-values add, so existing insulation counts in full. Reaching R-49 from R-30 needs only R-19 of new material, which is why topping up is far cheaper than replacing.
- Air sealing first is better value than extra depth. Insulation slows conduction but does nothing about air leaking around wiring, plumbing, fittings and the attic hatch.
- Returns diminish sharply. R-11 to R-30 cuts heat loss through that surface by about two thirds; R-49 to R-60 saves only a few percent more.
- Do not block soffit vents. Baffles must keep the ventilation path open at the eaves, or trapped moisture will damage the roof deck.
- Never bury recessed lights that are not IC-rated, and keep insulation clear of flues and chimneys. Both are genuine fire risks rather than code technicalities.
- Compressing batts reduces their R-value. An R-19 batt squeezed into a 2×4 cavity does not deliver R-19, which is why the cavity depth has to suit the product.
- R-value per inch figures here are typical. Product labels govern, and blown material settles over time, so installed depth should allow for settling.
- Wall and floor assemblies have thermal bridging through the framing that reduces the whole-assembly R-value below the insulation's own rating.
Frequently asked questions
How many inches of insulation is R-49?
About 14 inches of blown cellulose at R-3.5 per inch, or close to 20 inches of blown fiberglass at R-2.5. If you already have R-30, you need only the 5.43 inches that makes up the difference.
Do R-values add together?
Yes. Layered insulation adds, so R-30 already in place plus R-19 added gives R-49. This is what makes topping up existing attic insulation the cheapest efficiency measure on most houses.
What R-value do I need in my attic?
The Department of Energy recommends R-30 to R-60 depending on climate zone, with R-49 to R-60 across most of the northern US and R-30 to R-49 in warmer zones.
Is air sealing more important than insulation?
Usually, yes, and it should come first. Insulation slows conduction but does not stop air movement through gaps, and insulation laid over unsealed penetrations underperforms its rating.
Can you have too much insulation?
Not in a harmful sense, but returns diminish steeply. Beyond roughly R-49 in an attic the additional saving is a few percent, and the money is usually better spent on air sealing or windows.
Which insulation has the highest R-value per inch?
Among common options, rigid foam board at about R-5 per inch, then mineral wool at R-4 and cellulose at R-3.5. Blown fiberglass is lowest at around R-2.5.
Sources and review
- Insulation — recommended R-values by climate zone — ENERGY STAR, U.S. Environmental Protection Agency
Last reviewed .