Attic Blown Insulation Calculator
Estimate added insulation depth, bags, material weight, covered area, and blower time from attic size, target R-value, existing insulation, and chosen loose-fill material.
★Attic presets
⚙Insulation inputs
Calculation breakdown
▣Selected material spec grid
📋Material R-value reference
| Loose-fill material | Typical settled R per inch | Installed density used | Common attic use |
|---|---|---|---|
| Cellulose loose-fill | R-3.7 per inch | 1.6 lb/ft³ | Retrofit attics, dense coverage around framing |
| Fiberglass loose-fill | R-2.5 per inch | 0.55 lb/ft³ | Open attics where lower weight is useful |
| Mineral wool loose-fill | R-3.1 per inch | 1.8 lb/ft³ | High-density fills and fire-resistance assemblies |
| Stabilized cellulose | R-3.8 per inch | 1.7 lb/ft³ | Attics with slopes, wind wash, or better cling needed |
🌡Common attic target R-values
| Target | Typical climate use | Cellulose depth | Fiberglass depth |
|---|---|---|---|
| R-30 | Mild climate or garage attic | About 8.1 in | About 12.0 in |
| R-38 | Moderate retrofit target | About 10.3 in | About 15.2 in |
| R-49 | Cold climate common target | About 13.2 in | About 19.6 in |
| R-60 | Very cold attic upgrade | About 16.2 in | About 24.0 in |
📦Bag and blower planning reference
| Planning item | Low-clearance attic | Typical attic | Open new attic |
|---|---|---|---|
| Waste and touch-up factor | 15% to 20% | 8% to 12% | 5% to 8% |
| Framing adjustment | 10% to 14% | 3% to 6% | 0% to 3% |
| Depth marker spacing | Every 200 ft² | Every 300 ft² | Every 400 ft² |
| Blower rate planning | 250 to 350 ft³/hr | 350 to 500 ft³/hr | 500 to 700 ft³/hr |
📏Depth ruler and coverage checks
| Field check | What to measure | Why it matters | Calculator input |
|---|---|---|---|
| Existing depth | Average depth across several bays | Prevents buying for R-value already present | Existing insulation depth |
| Vent clearance | Soffit and baffle openings | Keeps air path from being buried | Obstruction factor |
| Settled depth | Depth after light leveling | Confirms the final R target is reached | Added depth result |
| Bag count | Empty bags used versus planned | Cross-checks density and coverage card | Rounded bags result |
💡Blown insulation calculation tips
When your heat comes on in January, sometimes that’s just heat leaking out and up through your attic, not doing anything but making you shiver. Attic insulation works based off its density over time, rather than by forming a solid wall. Materials such as fiberglass or cellulose is loose-fill, so they settle. That’s why, unless you have contractors hired to do this, it’s difficult for homeowners to be accurate in their calculations.
With the calculator above, you can get the real-life r-value instead of a guess based on eyeballing how much you put in. It’ll handle math behind adding thickness to what was already there. People typicaly get square footage of their attic right, but overlook what’s already there.
How to Use the Insulation Calculator
It may appear to be a deep 4-inches of old fiberglass insulation, but it will compress and lose effectiveness. If you assume it has no R-value at all, you’ll end up purchasing more than necessary. If you assume it retains full value, you’ll purchase less then needed, leaving gaps in your thermal envelope. To avoid this, the tool prompts for old insulation type so it can guesstimate how much is left, preventing you from over-filling low clearance areas where you can barely stand between joists.
In general, fiberglass versus cellulose is not an apples-to-apples comparison by actual R-value (per inch), but rather by weight and/or density. Generally speaking, cellulose have better thermal performance because it is denser. This density help it hold up better against air leaks. It is also heavier, so it stays in place better on a slope. However, fiberglass is light enough to be easy to install, but to get the desired amount of thermal resistance take more depth.
This is where comparison table is useful. It details those ratings and explains why a typical recommendation might be R-49 in a cold climate. This would require almost 20” of fiberglass but just under 13” of cellulose. Consider that this added dead load of insulation may or may not be supported by your existing floor framing. Especially important when insulating attics in older houses with weak foundation support.
Obstacles such as attic trusses and wall plates also reduce cost efficiency; while blowing in, those usual 2x6s or 2×10 wall plates and truss webs simply gobble up insulation space. To do this, the calculator find the actual coverage area based on how dense or sparse your framing is. So if you’re working in a moddern open-attic style house without many interior walls, the blower can go straight across it. But if you’re doing a retro-fit of an old ranch with plumbing stacks and chimney chases all over, there’ll be spaces the hose doesn’t reach.
You also don’t want to run out of bags partway through process. That would leave you with thin spots right where the heat escapes most badly. That’s why it adds in a waste factor for such areas.
Before you rent a blower, plug all air leaks in fan boots and other areas where wires penetrate. Even the thickest layer of blown-in cellulose has no chance against blowing air through it, much like how a wind chill effect can defeat thermal resistance. After plugging the leaks with caulk or foam, concentrate on getting an even fill. Lay a long ruler across multiple joists to make a guide for your target depth, then keep blowing till every inch looks like this, insulation up to the ruler’s level, everywhere. Tedious work, but it definitely tops the thermal escape hatch. You should of checked your insulation levels before starting.
