Fiberglass Blown Insulation Calculator
Estimate attic area, added R-value, settled depth, bag count, coverage, density check, and framing or waste allowance for blown fiberglass insulation.
🏠Named Attic Presets
📏Attic Inputs
Many blown fiberglass products list settled values near R-2.5 to R-2.8 per inch.
Use the coverage printed on the exact bag chart whenever available. This calculator cross-checks bag coverage against density and bag weight so unusually light or heavy estimates are easier to catch.
📊Fiberglass Material Grid
📋Settled Depth Reference
| Target R | Existing R | Added R | Depth at R-2.6/in | Common Attic Use |
|---|---|---|---|---|
| R-30 | R-11 | R-19 | 7.3 in | Garage or mild-zone top-up |
| R-38 | R-13 | R-25 | 9.6 in | Moderate climate attic |
| R-49 | R-19 | R-30 | 11.5 in | Typical cold-zone upgrade |
| R-60 | R-19 | R-41 | 15.8 in | Very cold attic upgrade |
🧱Bag Coverage Planning Table
| Added R | Settled Depth Range | Coverage Trend | Field Check | Planning Note |
|---|---|---|---|---|
| R-19 | 7-8 in | Higher sq ft per bag | Depth rulers visible | Good for light top-ups |
| R-30 | 11-12 in | Medium sq ft per bag | Check bay-to-bay depth | Common R-49 upgrade from R-19 |
| R-38 | 14-15 in | Lower sq ft per bag | Watch baffles and dams | Heavy top-up for open attics |
| R-49+ | 18 in and above | Lowest sq ft per bag | Confirm access depth | Use manufacturer chart closely |
📝Preset Assumptions Table
| Preset | Area | R Goal | Existing R | Waste |
|---|---|---|---|---|
| Small Ranch R-49 Top-Up | 1200 sq ft | R-49 | R-19 | 10% |
| Cape Cod Low Pitch | 780 sq ft | R-38 | R-13 | 20% |
| Cold Climate R-60 | 1600 sq ft | R-60 | R-19 | 15% |
| Bonus Room Over Garage | 420 sq ft | R-49 | R-15 | 15% |
💡Calculator Tips
⚠Safety Notes
Results are planning estimates for loose-fill fiberglass. Final bag counts should be checked against the exact manufacturer coverage chart, local code requirements, and any attic ventilation or ignition-barrier requirements.
To calculate the amount of insulation that is needed for the attic, one must have a good understanding of both how insulation settles and how insulation work within the attic itself. Insulation itself isnt static in its properties; however, insulation is a form of air that is trapped within the glass fibers of the fiberglass insulation that is commonly used within attics. The fiberglass insulation appears light and airy when it is first loaded into the house insulation bag, but will settle into a lower depth within the attic space over the course of several week.
Should the amount of insulation that is calculated be based upon the initial, “fluffed” amount of the fiberglass insulation, the layer that is placed into the attic will be too thin to provide the necessary level of insulation to allow the attic to reach the target R-value. A layer of insulation that do not reach the target R-value will not effectively prevent the loss of heat from the attic. Therefore, one must calculate the amount of insulation needed according to the settled depth of the fiberglass insulation.
How Much Insulation Does Your Attic Need
In order to determine the amount of insulation required for the attic, the difference between the existing R-value of the attic and the goal R-value of the attic must first be determined. Most homes has existing insulation within the attic. Thus, it is not necessary to calculate how much insulation would be needed if the attic contained no insulation at all.
The existing insulation will not have its original R-value; the fiberglass insulation within the attic will have settled over time. One will need to measure the depth of the existing insulation to determine how much additional insulation is required to reach the goal R-value of the attic. In addition to the depth of the attic floor, which is not likely to be a series of perfect rectangle, allowances for waste must also be accounted for.
Allowances for the wooden chords that make up the attic floor will need to be accounted for within the calculation of the amount of insulation required. If such allowances are not made for the attic floor, the amount of insulation may be insufficiently. In addition to calculating the amount of insulation that will be required, it is also important to ensure that the calculations and measurements of the attic are accurate.
Every bag of fiberglass insulation has a specific weight and a target density. The target density is the amount of glass fibers that are contained within each cubic foot of the insulation. The weight of the bag of insulation and the area that it will cover can be referenced against the manufacturer’s chart to determine whether the calculations is accurate.
Should the weight of the bag of insulation and the area that it will cover not correspond to the chart that the manufacturer of the insulation publishes, it is possible that either the calculations or the manufacturer’s chart are incorrect. Thus, performing such a check is an essential step that will ensure that the purchase of fiberglass insulation will be sufficient for the attic. In addition to the calculations of the amount of insulation that is required for the attic, it is important to also ensure that the installation of the insulation does not interfere with the attic’s ability to naturaly ventilate.
Specifically, the installation of the fiberglass insulation must not block the soffit vents within the attic. The soffit vents allow for cool air to entering the attic. If the fiberglass insulation is packed too close to the roof deck, it will block these vents and prevent air from circulating within the attic.
To prevent this from occurring, one should install plastic baffles in the attic floor near the eaves of the roof to ensure that the soffit vents remain open and functional. In addition to ensuring that the attic has adequate ventilation, it is also necessary to perform air sealing prior to the installation of the fiberglass insulation. While fiberglass insulation is good at stopping conductive heat loss, fiberglass is not effective at stopping the movement of air.
If warm air is allowed to enter the attic through gaps in the structure, that air will move through those gaps and through the fiberglass insulation. Thus, before installing the fiberglass insulation, any gap should be sealed with canned foam. Finally, the total amount of fiberglass insulation that is purchased should always be rounded up to the next highest bag.
Purchasing the calculated amount may be convenient, but purchasing too little will result in insufficient coverage within the attic. Thus, rounding up the total amount of bags to be purchased is a simple step that will prevent the attic from losing heat due to insufficient insulation.
