Insulating a
Quonset Hut
Without the Drip

Closed-cell spray foam sprayed directly on the steel is the most reliable option. It bonds to the corrugations, leaves no air gap and becomes a Class II vapor retarder at 1.5 in (Building Science Corporation); 2 in gives about R-13. A pinned fiberglass blanket with every seam taped costs less and suits storage buildings.

A 40 × 60 ft Q-model with both end walls has about 5,027 sq ft of steel. At Angi's $1.30–$3.10 per board foot (Sep. 2026), 2 in of closed-cell runs about $14,400–$34,300 with a 10% allowance. A faced fiberglass blanket runs about $4,000–$13,100. Many foam contractors charge a $1,000–$2,000 minimum.

Keep humid air off the cold steel and cut moisture at the source. Closed-cell foam bonded to the steel does the first, as does a blanket with every seam and pin taped. Vent heaters and welding outside, put a vapor barrier under new slabs and ventilate; Agribilt suggests 4–6 air changes an hour for a shop.

For closed-cell sprayed inside, 1.5 in is the practical minimum, because that is where it becomes a Class II vapor retarder. At R-6.5 per inch, 2 in gives R-13 and 3 in R-19.5 (ArmorThane). It goes on in lifts of about 1.5–2 in. Heating, climate and budget decide how far above the minimum you go.

Usually humid air is reaching the steel behind the insulation through open seams, torn facing or unsealed pins, and that steel sits near the outdoor temperature. Open-cell foam against steel lets vapor through. Bubble foil can sweat on its own face: in a 60°F, 50% humidity shop on a 10°F night, it sits only about 1°F above the dew point.

Not well against winter heat loss. Reflective insulation needs an air space; pressed against steel, Reflectix says its product gives R-1.1, and its FTC fact sheet rates a ceiling at R-1.0 in a northern winter. Foil helps more against summer sun. Treat vendor R-15 to R-22 figures as ratings for assemblies with air spaces.

Yes. Closed-cell foam sprayed over the arch under a UV-protective coating keeps the steel warm, so it cannot sweat. ArmorThane's roofing cost guide puts spray foam roofing at $4.00–$7.00 per sq ft installed, with 1.5 in (about R-9.8) common, and notes code requires the coating within 2–72 hours of spraying.

In an occupied building, generally yes. The residential code requires foam to be separated from the interior by 1/2 in gypsum wallboard or a material tested to NFPA 275, and the building code's section 2603.4 matches it. Alternatives on a curve include a framed interior or a fire-tested intumescent coating. Ask your building department what applies.

For a small hut or patches, possibly. Angi puts DIY kits at $350–$800 for 200–600 sq ft each, while a 40 × 60 Q-model has about 5,000 sq ft of steel. DOE notes most liquid foam requires special equipment and certification. Pinned blankets and foil are the realistic DIY route; SteelMaster says two people can install its kits.

For a semicircle (Q-model), the arch length is pi times half the width; multiply by the building length for the shell. Each end wall is pi × width squared ÷ 8. A 40 × 60 ft Q has a 62.8 ft arch, a 3,770 sq ft shell and two 628 sq ft end walls. S- and A-models add straight sidewalls.

Closed-cell for anything touching the steel. Open-cell costs less ($0.60–$1.60 vs $1.30–$3.10 per board foot, Angi) and stops air, but lets vapor through to the cold steel. ArmorThane's metal building guide says to use open-cell only as a fill layer over closed-cell, never against the metal of a heated building.

End walls are flat and often framed, so batts, rigid board or spray foam all fit. Seal the joints where the end wall meets the arch and the foundation. In a 40 × 60 Q-model the two end walls total about 1,257 sq ft, a quarter of the steel, so make sure any quote includes them.