Short answer: about 1,845 sq ft, which is 2 standard 1,000 sq ft rolls, for a 1,500 sq ft attic under a 6/12 roof installed staple-up. That is roughly $297.76 of foil. Here is the arithmetic, because the number changes with your roof pitch.
Why it is not 1,500 sq ft
The mistake almost everyone makes is buying foil equal to the attic floor area. If you are stapling to the underside of the rafters, you are covering the sloped roof plane, and a slope is longer than the floor beneath it. A 1,500 sq ft attic under a 6/12 roof has about 1,677 sq ft of rafter area.
| Roof pitch | Slope factor | Rafter area for a 1,500 sq ft attic |
|---|---|---|
| 4/12 | 1.054 | 1,581 sq ft |
| 5/12 | 1.083 | 1,625 sq ft |
| 6/12 | 1.118 | 1,677 sq ft |
| 8/12 | 1.202 | 1,803 sq ft |
| 10/12 | 1.302 | 1,953 sq ft |
| 12/12 | 1.414 | 2,121 sq ft |
If you do not know your pitch, a 6/12 is the most common residential slope in the US and a safe default. Measure it by holding a level 12 inches out from the rafter and measuring the drop.
The full calculation
| Attic floor area | 1,500 sq ft |
| × slope factor for 6/12 pitch | × 1.118 |
| = rafter area to cover | 1,677 sq ft |
| + 10% for overlap, offcuts and obstructions | 1,845 sq ft |
| ÷ 1,000 sq ft per roll, rounded up | 2 rolls |
| 2 × $148.88 (Reflectix radiant barrier, 4 ft x 250 ft perforated (1,000 sq ft)) | $297.76 |
| 3 × $17.85 foil tape (2 in x 30 ft) | $53.55 |
| Hammer tacker (Arrow HT55 hammer tacker) | $27.99 |
| Materials total | $379.30 |
The 10% allowance covers overlap at the seams, offcuts at hips and valleys, and the pieces you will waste working around vents and bracing. On a cut-up roof with dormers, use 15%.
If you are laying it over the attic floor instead
Deck-applied installations follow the floor, not the roof, so the slope factor does not apply. For a 1,500 sq ft attic you need about 1,575 sq ft including overlap, which is still 2 rolls. Use a perforated product, and read staple-up vs. deck-applied first because the dust problem is real.
| Product | Price | Rating |
|---|---|---|
| RadiantGUARD Xtreme, 1,000 sq ft reflective foil roll | $199 | 4.8★ (13) |
| Reflectix radiant barrier, 4 ft x 250 ft perforated (1,000 sq ft) | $148.88 | no ratings yet |
| Radiant barrier perforated aluminium, truss/rafter roll | $158.27 | no ratings yet |
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What else you need
Foil is most of the cost but not all of it. Budget $53.55 for foil tape (3 rolls at $17.85) and $27.99 for a hammer tacker if you do not own one. A hammer tacker, not a squeeze-handle staple gun: you will be driving several hundred staples overhead and the difference is your forearm.
Get the exact number for your attic → Free, no account for your first estimate.
Or read the complete radiant barrier guide for whether this project is worth doing in your climate at all.