Stop Condensation Under Metal Roof

Winter mornings in Nassau County reveal a strange kind of leak that nobody saw coming-you walk into your garage or bonus room under a metal roof and find water dripping from the underside, beading on screws, even pooling on the floor, yet last night’s sky was perfectly clear. Stopping condensation isn’t about patching holes or replacing flashing; it’s about balancing three dials: how much moisture your indoor air carries, how well you’ve insulated and sealed the space below the metal, and whether you’ve given warm air a clean escape route through proper ventilation.

Is That Really Condensation, Not a Leak?

On a 25° January morning in Seaford, here’s what’s actually happening on the underside of your metal roof: warm, humid air from your heated garage, workshop, or living space rises until it hits that cold metal deck, instantly losing its ability to hold moisture-the water vapor turns straight into liquid droplets, just like steam fogging up your bathroom mirror. That process is pure condensation, and it mimics a roof leak so well that I’ve spent seventeen years explaining to property owners that the water they’re seeing never came from outside.

If your “leaks” only happen on cold, clear nights, not during heavy rain, that’s your first big clue. Real roof leaks follow the weather-they show up during storms, wind-driven rain, or when snow melts. Condensation, by contrast, loves calm, frigid nights when your metal roof radiates heat to the sky and drops well below the dewpoint of the air trapped underneath. You might also notice the wetness spreads evenly across wide areas-sometimes an entire sheet of metal is covered in fine mist-rather than running in obvious tracks from a single breach point.

The good news: most condensation problems can be fixed without tearing off your roof.

Why Your Metal Roof Is “Sweating”

Think of warm indoor air like steam from a shower-it’s always looking for the first cold surface to grab onto. Metal is one of the most conductive materials in residential construction, meaning it instantly reflects whatever temperature the outside air or night sky delivers, creating a cold magnet for any moisture that drifts upward from below. In a Merrick cape conversion I worked on a few years back, a brand-new standing-seam metal roof over cathedral ceilings started “leaking” only on cold, clear nights; I cut a small access panel and found warm bathroom air escaping around an exhaust fan into the rafter bay, where it condensed on the underside of the metal and dripped down onto the drywall below. The fix wasn’t resealing the roof-it was air-sealing around that fan housing, adding a proper baffle channel so the rafter bay could breathe to the ridge vent, and keeping the moist air where it belonged.

Garages, workshops, and bonus rooms over the garage are the worst offenders in Nassau County. These spaces often have minimal insulation, no vapor control, and marginal ventilation, yet we heat them, park warm cars inside, run space heaters, or even do laundry-all activities that pump moisture into the air. The metal roof overhead stays ice-cold because there’s nothing between the heated space and the underside of the deck, so every cubic foot of humid air that rises turns into a droplet factory by midnight.

Where Condensation Shows Up Most Often

I’ve seen the same patterns emerge across hundreds of jobs in Nassau County. Uninsulated metal roofs over garage spaces, metal re-roofs installed directly over skip sheathing or open purlins with no thermal break, and low-slope or nearly flat metal roofs on additions where warm air pools under the deck with no exit path. In every case, the metal is doing its job-it’s the assembly underneath that’s out of balance.

Watch for these warning signs:

• Damp or compressed fiberglass insulation that never fully dries out, especially near eaves or valleys
• Rusty fastener heads, screw tips, or purlins showing surface corrosion even though the roof is only a few years old
• Musty or moldy smell in the attic or ceiling cavity, with no obvious rain intrusion
• Dark streaks or staining on rafters and sheathing that appear even during dry spells
• Frost buildup on the underside of metal panels during very cold snaps, melting into puddles as temperatures rise

The Three Dials You Need to Turn

Fixing condensation means adjusting one or more of three interconnected systems: ventilation (giving moist air a way out before it condenses), insulation and air sealing (keeping warm humid air from reaching cold metal), and vapor control (slowing or stopping moisture migration through walls and ceilings). Most jobs require tweaks to all three dials, not just cranking one to maximum and hoping for the best.

MOISTURE TRIANGLE
┌─────────────────┐
│ AIR (humidity) │ → Indoor moisture from breathing, cooking, heating, vehicles
└────────┬────────┘
         │
         │
┌────────┴────────┐        ┌────────────────────┐
│ TEMPERATURE │──────│ BARRIERS │
│ (surface) │ │ (insulation/films) │
└─────────────────┘      └────────────────────┘
All three sides must be managed to stop condensation effectively.

Improve Ventilation

Before you buy any fans or fancy vents, I want you to do one simple test: on a cold morning when condensation is visible, walk around outside and check whether you can see any airflow escaping from soffit vents, ridge vents, or gable louvers-if those vents are blocked by insulation, painted shut, or were never installed in the first place, you’ve found your first dial to turn. Proper ventilation creates a continuous path for air to enter low (typically at the eaves through soffit vents), travel upward along the underside of the roof deck, and exit high (ridge vent, gable vent, or powered fan), carrying moisture with it before it has a chance to condense. In a Massapequa detached garage I fixed one November, the owner had hung plastic sheeting under the rafters to “stop drafts,” which turned the space into a sealed moisture trap-every morning the plastic filled with beads of water and dumped onto his classic car below. I removed the plastic, added proper vented soffits and a continuous ridge vent, then installed a simple insulated ceiling layer below the rafters, creating a ventilated cold attic above and a conditioned space below, and the condensation stopped within a week.

Manage Insulation and Vapor Barriers

Foil-faced insulation stapled straight to the rafters looks tidy, but it can trap moisture in all the wrong places. If you place a vapor barrier (including foil facings, vinyl-backed batts, or polyethylene sheeting) on the warm side of your insulation without providing ventilation above it, any moisture that sneaks past-through gaps, tears, or simple diffusion-gets trapped between the barrier and the cold metal, creating a perfect condensation zone with no escape. The better approach in most retrofit scenarios is to use unfaced or kraft-faced insulation in rafter bays, maintain at least a one-inch air gap between the top of the insulation and the underside of the metal deck using baffles or spacers, and let that gap vent continuously from soffit to ridge. For conditioned spaces like finished bonus rooms, closed-cell spray foam applied directly to the underside of the metal deck can work because it acts as both insulation and air barrier, eliminating the temperature difference and blocking moisture movement in one step-but that’s a professional install with code and moisture considerations that need careful planning, especially in our humid coastal climate.

In a Farmingdale workshop, the homeowner had installed a bare metal roof over open purlins with no insulation or underlayment at all; summer was fine, but in February every screw was dripping like a showerhead because indoor humidity hit freezing metal with nothing in between. We added a layer of synthetic underlayment with slight permeability to the underside of the existing panels (a condensation-control membrane designed for retrofit), then furred out a new insulated ceiling below with ventilation channels built in, and installed a small gable exhaust fan on a humidistat to pull excess moisture out whenever levels spiked-the combination turned a rain cave into a dry, usable space within one cold season.

Practical Steps to Get Condensation Under Control

Once you’ve figured out where the warm air is getting in, the fix usually follows this order:

1. Document the pattern. Take photos of where water appears, note the outside temperature and weather conditions, and check whether the problem happens year-round or only during certain months. This helps you (or your contractor) pinpoint whether the issue is ventilation, insulation, or an indoor moisture source like an unvented dryer or propane heater.
2. Check existing ventilation. Verify that soffit vents are open and unblocked, that ridge or gable vents are functional, and that there’s a clear path for air to move from low to high. If your metal roof was installed over solid sheathing with no venting provision, adding ventilation will require cutting channels or installing a new vent system-not a DIY afternoon project, but very doable for an experienced roofer.
3. Inspect and upgrade insulation. Pull back accessible insulation and look for compression, wetness, or gaps. Add baffles between rafters to maintain airflow, increase insulation thickness if there’s room, and make sure any vapor retarder is on the warm (interior) side and properly sealed at edges and penetrations.
4. Air-seal the interior. Caulk around ceiling penetrations (lights, fans, pipes), seal gaps at top plates where walls meet ceilings, and make sure bath fans duct all the way outside rather than venting into attic or rafter spaces. This single step often cuts condensation more than any vent upgrade because it stops the moisture from ever reaching the cold zone.
5. Consider vapor control carefully. In retrofit situations, adding plastic sheeting or foil barriers can do more harm than good if the assembly isn’t designed for it. If you’re unsure, bring in a contractor or building scientist who understands moisture movement in your climate-coastal Nassau County is not the same as inland or northern climates, and the wrong fix can trap problems instead of solving them.
6. Call a professional if the issue persists. If you’ve improved ventilation and sealed obvious air leaks but still see heavy condensation, there may be hidden moisture sources (slab moisture, foundation leaks, or unusually high indoor humidity from equipment or activities) or the roof assembly may need a more comprehensive redesign involving spray foam, new sheathing, or mechanical dehumidification.

Visual checks, small vent upgrades, and basic caulking are reasonably DIY-friendly if you’re comfortable on a ladder and can safely access attic or ceiling spaces. Cutting new vents into existing roofs, installing baffles in tight rafter bays, applying spray foam, or diagnosing complex moisture patterns should involve a roofing or insulation contractor who’s dealt with metal-roof condensation before-this isn’t the same skill set as patching shingles, and the wrong move can make things worse.

Never seal up a ceiling cavity or add plastic sheeting without understanding where the moisture will go.

Why Nassau County Homes See Condensation More Often

Coastal Humidity and Temperature Swings

On homes near the water-Freeport, Island Park, Long Beach-humidity loads are higher to begin with, thanks to marine air, frequent fog, and Nor’easters that deliver soaking rains followed by cold, clear nights. That combination is ideal for condensation: the storm raises indoor humidity (especially if your house isn’t perfectly tight), then the temperature drops and the metal roof radiates heat to a clear sky, falling well below the dewpoint while warm, moist air is still trapped underneath. Inland properties see the same physics, but the swings are sharper and the baseline humidity slightly lower, so the problem often shows up most dramatically in unheated outbuildings rather than attached living spaces.

Plan your fixes during shoulder seasons-late spring or early fall-when temperatures are moderate and you can open up ceilings or attics without dealing with extreme cold or summer heat. Pay extra attention to properties with heavy tree cover or those close to tidal water, where morning dew, ground moisture, and shaded roof surfaces all contribute to higher condensation risk even when the weather looks mild.

When to Call TWI Roofing for a Condensation Assessment

Condensation under a metal roof is almost always fixable with the right combination of improved airflow, thoughtful insulation strategy, and careful air sealing-no tear-off required. If you’re seeing drips, rust, or damp insulation in your Nassau County garage, workshop, or addition, TWI Roofing can assess your specific assembly, identify which of those three dials needs adjustment, and recommend practical fixes that fit your building and budget.