A metal roof over a heated space in a St. Louis winter is a condensing surface, and the water that forms on it drips. Every barndominium insulation decision is really a condensation decision: keep warm, humid indoor air from reaching cold steel, and keep the steel warm enough that what reaches it does not condense. The climate makes both sides of that hard — about 4,400 heating degree days a year, a January normal low near 24°F, and a July dew point normal near 67°F — so the assembly has to work in both directions.
Why metal buildings drip
Steel has no thermal mass and no absorbency. In January the underside of a metal roof is within a degree or two of outdoor temperature; indoor air at 70°F and 40% humidity has a dew point around 45°F; wherever that air touches the steel, it condenses. Fiberglass batts pressed against the steel do not stop it — they let the air through and hold the water. In July the reverse happens on an air-conditioned shop slab without a vapor retarder: humid outdoor air meets a cool floor and sweats.
The three assemblies that work
| Assembly | How it works | Best for | Cost (2026) |
|---|---|---|---|
| Closed-cell spray foam on the steel | 1.5–3 in. of closed-cell foam sprayed directly to the underside of roof and wall steel; air-seals and insulates in one layer; the steel never sees indoor air; batts or more foam can go inside it | Living areas; the simplest reliable assembly in a post-frame building | $1.44–$4.70 per sq ft per HomeAdvisor (June 2026); typically 2 in. roof, 1.5 in. walls |
| Vented assembly with a vapor retarder | Interior ceiling with blown or batt insulation above it, a continuous vapor retarder on the warm side, and a ventilated attic between insulation and roof steel (eave and ridge vents); walls get batts in the girt cavities with a warm-side vapor retarder and a housewrap/condensation blanket under the steel | Owners finishing a shell on a budget; works if the air barrier is continuous | Lower material cost; depends on detailing |
| Liner / insulated panel system | Faced insulation rolls or rigid board with a liner panel on the interior face, sealed at seams; or insulated metal panels | Shops, where a washable interior and a modest R-value are enough | Between the two |
What does not work: unfaced fiberglass against the steel; a "radiant barrier" alone; a vapor retarder on the cold side of the insulation.
R-values for this climate
Missouri is generally mapped IECC climate zone 4 (St. Charles County's code references the zone-4 energy provisions; confirm your county's). Prescriptive targets for a new home in zone 4 are roughly R-49 ceiling, R-20 wall (or R-13 cavity plus R-5 continuous), R-10 slab edge; in the counties with no code there is no requirement, but the heating bill and the lender's appraiser both notice. A 2-inch closed-cell layer on the roof (about R-13) plus R-38 batts below it, or 3 inches of foam alone on a cathedral assembly, is a common living-side spec; shop bays usually get 1.5–2 inches of foam and nothing more.
The slab and the shop
- Vapor retarder under the slab, living and shop, every time. A shop slab poured without one sweats in July and lifts any coating.
- Slab-edge insulation on the living side; under-slab insulation if the living floor is heated.
- Shop condensation happens at the overhead doors and on uninsulated steel when a cold shop is opened to humid air. Insulated doors and the foam layer on the steel handle most of it; a dehumidifier handles the rest.
Air sealing and ventilation
A foamed post-frame building is tight. Tight houses need mechanical ventilation — bath and kitchen exhaust at minimum, a balanced ERV in a well-sealed living side — and combustion appliances need sealed combustion or outside air. In the code counties the mechanical code covers it; in Warren and Lincoln it is on the builder.
Frequently asked questions
Can I use fiberglass in a barndominium? Yes, inside a continuous air and vapor barrier with a vented space between the insulation and the roof steel. Not against the steel.
Open-cell or closed-cell foam? Closed-cell on the steel — it is a vapor retarder and adds rigidity; open-cell lets vapor through and should not touch the roof skin in this climate.
Do I need to insulate the shop? If you condition it at all, yes, at least on the steel to stop drips. An unconditioned shop with a vapor retarder under the slab and good ventilation can go bare.
Is the condensation blanket under the steel enough? It catches drips in an unconditioned building. It is not insulation for a living space.
Next step
Shell finish-out starts with this assembly. Related: slab, soils and frost · post-frame vs. steel vs. stick · the complete guide.