How much insulation does a walk-in cooler need?
R-25 on the walls and ceiling is the usual target for a cooler held in the mid-30s. That's about 5 inches of XPS or polyiso, or 6½ inches of EPS. Freezers need more, around R-32. Floors on a slab often get by with less, but a trailer floor with air underneath needs real insulation.
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Rigid foam, not fiberglass
Classified between the tips. Live ads would name a product; this is the hole they fill.
Build tips that matter more than R-value
- Stagger the seams between layers so no gap runs straight through.
- Tape or foam every seam. Air leaks carry moisture that turns into ice on the AC coil.
- Keep the vapor barrier on the warm side, which is the outside of the box.
- Avoid fiberglass. It soaks up condensation and loses most of its value in a cold room.
Assumptions
Wall area is the perimeter times the height, minus each door. The ceiling and optional floor are length times width. Sheets per layer are the area plus your waste allowance, divided by 32 square feet and rounded up. Polyiso is rated near R-6 per inch, but it loses some performance when cold, so it's figured at R-5 here. Framing, liner, door, and caulk aren't included.