Condensation and venting: the part foam guides skip
Insulation guides love R-value because it’s easy to sell. But the thing that actually decides whether a foamed pole barn or house thrives or quietly rots is where the moisture goes — and that part rarely makes the brochure. What follows is the moisture playbook of a man who sprayed urethane foam for 30 years across Idaho and Wyoming, in freezing climates where getting the dew point wrong isn’t theoretical.
The dew point rule: make it land inside the closed-cell
Warm air holds moisture; the dew point is the temperature where that moisture condenses into water. Somewhere between your heated interior and the frozen outdoors, that temperature line exists inside your wall or roof assembly — the only question is what material it lands in.
His rule from three decades of cold-climate work: put enough closed-cell foam on that the dew point lands inside the foam itself — two to three inches. Closed-cell foam is the one common material where condensation landing inside it is harmless: the foam is closed to water, so there’s nothing to soak, wet, or rot. Let the dew point land anywhere else — in soft insulation, on the metal skin, on wood sheathing — and you’ve built a condensation collector.
That’s the quiet second reason his standard pole barn job was two inches of 2-lb closed-cell (the first reason is cost — the first two inches do most of the insulating work). Two to three inches isn’t just the economic sweet spot; in his climate it’s where the dew point math works out too.
The cold roof: how roofs were done right
For roofs, his flat answer: the number one way to do a roof properly is a cold roof. The principle is simple — keep a ventilated air gap between the insulation and the roof skin, so the roof stays cold and any moisture that gets up there is carried out by moving air.
How he built them: insulation goes on or above the ceiling plane, then two to three inches of open gap under the roof deck, with vents low (eaves) and high (ridge). Cold outside air enters low, warms slightly, rises, and exits high — pulling moisture out of the assembly continuously, for free, forever.
Most ordinary houses are already cold roofs without anyone calling them that: insulation on the attic floor, vented soffits, ridge or gable vents, cold attic above. That’s why the era of blowing insulation onto attic floors produced so few moisture disasters — the venting was doing silent work. It’s also why sealing a vented roof up with foam changes the physics entirely: you’ve removed the escape path, and the moisture that used to ride the airflow out now has to go somewhere else. Foaming a roof deck isn’t adding insulation to a cold roof — it’s converting the building to a different system, and the rest of the system (below) has to come along.
Open-cell belongs on walls, not roof decks
The distinction he wanted shouted from the rooftops, so to speak:
- Walls: open-cell is fine. He sprayed soft foam in walls without trouble — walls dry differently, gravity helps, and the exposure is gentler.
- Roof decks: no. Open-cell foam is vapor-open. Spray it under a cold roof deck and moisture migrates into the foam; when the dew point lands inside that soft foam, it gathers water — in the insulation itself, against your sheathing. This was his #1 chew-the-installer-out practice when he saw it in videos.
Building code agrees with him in cold climates: for unvented roof assemblies, the IRC (R806.5) requires air-impermeable insulation in climate zones 5–8 to be, or be coated with, a Class II vapor retarder — the code’s way of stopping exactly this moisture path (see the US DOE Building America code brief). If a contractor proposes open-cell on your roof deck, the words you want to hear from them are “vapor retarder” and “your climate zone” — and if they’ve never heard of either, that’s your answer about the contractor.
Sealed buildings need a breathing plan
The flip side of foam’s greatest strength. Foam stops air infiltration completely — which means it also stops the accidental ventilation that used to carry moisture (cooking, showers, breathing, animals) out of leaky buildings.
His rules for encapsulated buildings:
- Foam the ceiling and walls of a house and you must bring in fresh air mechanically — a heat-exchanging ventilation system tied to the HVAC. His words: “if you don’t put an exchanger in, you’re begging for problems.” The moisture your household produces has to leave somehow; you removed the leaks that used to do it.
- He talked people out of foaming ceilings when there was no such plan. A fiberglass or blown-in ceiling under a vented attic lets house moisture escape upward; encapsulate that ceiling with foam and the escape route is gone. Foam the walls, leave the vented ceiling alone — that was often his actual recommendation, and it’s cheaper too.
- High-humidity buildings need dehumidification on top of everything. The Idaho potato cellars he knew are the extreme case — a potato cellar is a humidity box, and humidity is moisture waiting for a cold surface. Any building with wet processes, livestock, or stored produce needs its humidity managed, not just its walls insulated. Humidity causes moisture; moisture causes every problem in this guide.
The one-paragraph version
Closed-cell, thick enough that the dew point lands inside it (2–3 inches in cold country). Roofs: keep a ventilated cold roof if you can; if you seal the roof deck instead, use closed-cell and understand you’ve changed the system. Open-cell on walls only. Fully-foamed buildings get mechanical fresh air, no exceptions. High humidity gets dehumidified. That’s the playbook that kept his jobs dry for 30 years — and everything it protects against is catalogued on our problems page.
Choosing materials first? The metal building guide and retrofit guide put this moisture logic in context — or describe your building and climate below and we’ll give you a straight answer on the venting plan.