Insulation Calculator
Rolls or bags of insulation needed for a wall, floor or ceiling cavity of a given area.
Insulation is priced per square foot of R-value, and the coverage printed on the bag shrinks as the R-value climbs. Two rolls of R-13 will not cover the same wall as two rolls of R-19, even though the packaging looks identical from across the aisle. Always calculate from the coverage figure on the specific product, not from a rule of thumb.
The calculation itself is trivial; the failures are all in the details. Compressing a batt into a thinner cavity lowers its R-value rather than raising it. Leaving a gap at the top plate or around outlets creates a convection loop that can halve the effective performance of an otherwise perfect wall.
How this calculator works
Every field is editable and the result updates as you type — nothing is submitted anywhere, all the maths runs in your browser. That matters for a number you are about to spend money on: you can see exactly which input moved the answer, and by how much.
What this calculation assumes
Assumes standard stud bays with no obstructions and a roll coverage taken from the product label. Compressing a batt into a shallower cavity reduces its R-value, and gaps around outlets, top plates and window frames are not accounted for here.
Treat the result as an order-of-magnitude check rather than a final quantity. Product packaging changes, regional labour rates vary by a factor of two, and no calculator has seen your actual site.
Frequently asked questions
What R-value do I need?
It depends on climate zone: roughly R-13 to R-15 for 2x4 walls, R-19 to R-21 for 2x6 walls, and R-30 to R-60 in attics depending on how far north you are. The US Department of Energy publishes a zone map — check it rather than guessing, because under-insulating is expensive to correct later.
Rolls or blown-in insulation?
Rolls for open stud walls and simple joist bays where you can reach everything. Blown-in wins for attics with awkward framing, existing finished walls, and anywhere you want a seamless blanket. Blown-in also reaches places a batt cannot, which matters more than the per-bag price difference.
Should I leave a gap between insulation and the roof deck?
Yes, in a vented roof assembly. Insulation pressed against the underside of the deck blocks airflow from the soffit vents and traps moisture, which leads to rot. Use rafter vents (baffles) to hold a clear channel, and never stuff a batt into the eave.
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