About this calculator
This calculator estimates how much insulation to buy for an attic floor laid between (and, optionally, crossing over) the ceiling joists — batts/rolls or loose-fill, plus a vapour barrier. Enter the attic floor area, the joist spacing and width, and the total thickness you want to reach.
Formula
layer 1 area (between joists) = floor area x (1 - joist width / joist spacing)
layer 2 area (cross layer, if used) = full floor area, no joist deduction
R-value = total thickness / lambda [ISO 6946]
batts: packs = ceil((layer 1 area + layer 2 area) / pack coverage)
loose-fill: volume = layer 1 area x layer 1 thickness + layer 2 area x layer 2 thickness
volume (with settling) = volume x (1 + settling allowance%)
mass = volume x installed density
bags = ceil(mass / bag mass)
vapour barrier area = floor area x (1 + overlap%)
vapour barrier rolls = ceil(area / roll area)
tape length = floor perimeter; tape rolls = ceil(tape length / tape roll length)
A single layer takes the full target thickness; two layers split it evenly between the between-joist layer and the cross layer, so the cross layer's own thickness can differ from the first once installed.
Worked example
Defaults: an 8 x 6 m attic floor, 600 mm joist spacing, 45 mm joists, 250 mm total thickness in two layers, batts at 6 m² per pack, λ = 0.04 W/m·K.
floor area = 48 m²
layer 1 area = 48 x (1 - 45/600) = 44.4 m²; layer 2 area = 48 m² (full area)
packs = ceil((44.4 + 48) / 6) = ceil(15.4) = 16
R = 0.25 / 0.04 = 6.25 m²·K/W
FAQ
Why does the second layer cover the full area, with no joist deduction? The second layer is laid crossways over the joists (the standard way to break the thermal bridge they create), so it covers the whole floor rather than fitting between anything.
Why is the joist deduction a simple percentage, not an actual joist count? This calculator doesn't know which way the joists run relative to the entered floor shape, so it can't place real joist lines on the outline. Instead it removes the same fraction of the area that one joist bay's width/spacing ratio implies — accurate for a rectangular attic with joists running one way, less exact for an irregular shape.
Does loose-fill settle over time? Yes — blown or poured loose-fill compacts under its own weight and with air movement, which reduces the installed R-value if not accounted for. The settling allowance adds extra material up front so the settled thickness still reaches the target; it is a practical estimate, not a manufacturer-specific figure.
Which standard does this follow? ISO 6946 for the R = thickness / lambda formula. Pack coverage, loose-fill density, settling allowance, and vapour barrier roll size are all product-specific figures with no single governing standard — check the product's own data sheet.
Assumptions and limits
Pack coverage, loose-fill bag mass and installed density, the settling allowance, and the vapour barrier roll size and overlap are all user inputs with example defaults, not data for any specific product. The joist deduction assumes joists run consistently across the whole floor at the entered spacing; an irregular attic with cut-in obstructions (chimneys, hatches, water tanks) needs a manual adjustment. Vapour barrier tape only covers sealing the floor perimeter — seams between rolls depend on the actual roll layout, which this calculator does not model. The section drawing is schematic, not to a real joist profile.