Top 10 Attic Checks That Predict Condensation and Mould
Attic moisture problems don't start on the roof. They start with what's happening inside your home. Here are the 10 conditions I check first.
By Jonathan Gogan, RHI
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Last Updated: September, 2025
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5 min read
AT A GLANCE
Most attic moisture issues in Southern NB are caused by indoor air leaking into the attic, not a roof problem
Bathroom exhaust fans venting into the attic are the single most common cause of attic condensation and mould
Cold-weather inspections make condensation indicators like frost on nail tips and damp sheathing more visible
Staining on roof sheathing doesn't always mean a bad roof, it often means a ventilation or air sealing problem
QUICK NOTE
Every home is different. Attic conditions can be limited by insulation coverage, storage, tight access, and safety concerns. This checklist covers the most common conditions I encounter, but it's general guidance, not a substitute for a professional inspection.
One Roof, Two Views: What Attic Condensation Looks Like from Outside and In
Click image to see description (full screen for best viewing)
Attic moisture problems start quietly. A small air leak here, weak ventilation there, warm humid air finding its way into cold spaces. The result is condensation, staining, and mould-like growth on roof framing and sheathing, often developing for months or years before anyone notices.
After thousands of attic inspections across Southern New Brunswick, these are the 10 conditions that most reliably predict moisture problems. If I find more than a couple of these in the same attic, the conversation with the buyer usually shifts from "what's that stain?" to "here's what's actually driving it."
THE CHECKLIST
Bathroom Fan Duct Terminated in the Attic
This is the single most common cause of attic moisture problems I see. The flexible duct from the bathroom exhaust fan ends somewhere above the insulation, sometimes near the eaves, sometimes just hanging in the open attic space, dumping warm, humid air directly into a cold environment. The result is predictable: condensation forms on the cold roof sheathing and framing, and over time you get staining and mould like growth concentrated around the discharge point.
It's an easy fix, but one that gets missed for years because most homeowners never look up there.
Next step: Route bathroom exhaust to the exterior using a proper wall or roof cap termination.
Disconnected, Crushed, or Leaking Exhaust Ducts
Even when the duct is routed toward the exterior, it doesn't always make it there in one piece. I regularly find ducts that have separated at joints, been crushed by someone stepping on them, or are sagging badly enough to hold water in the low points. Sharp bends, loose tape, and under supported runs are all common, especially in older homes where someone has been in and out of the attic over the years.
Restricted or leaking ducts reduce airflow and can deposit moisture in the attic and inside the duct itself.
Next step: Restore proper connections, support, and slope. Replace flexible duct with rigid or semi-rigid where possible. If in a cold zone, insulated.
Frost or Condensation on Nails or Roof Sheathing
This is the one that tells me immediately what's happening. During cold weather inspections, I'll see frost forming on the tips of roofing nails that poke through the sheathing, or water droplets on the underside of the plywood or OSB. These are classic indicators that humid indoor air has found its way into the attic and condensed on cold surfaces.
On a mild day you might miss it entirely. On a cold January morning in Southern NB, the attic tells you everything.
Next step: Evaluate ventilation pathways and identify where air leakage is occurring.
Dark Staining on Roof Sheathing or Framing
Dark patches on the sheathing, rafters, or truss chords. Spotty growth patterns near the eaves, in valleys, or around penetrations. The tricky part is figuring out whether you're looking at condensation damage or a roof leak, because the staining can look the same from the attic side.
Condensation staining tends to be more widespread, especially near the eaves where cold air meets warm. Roof leak staining tends to concentrate around specific penetrations and flashing points. The distinction matters because it changes the repair plan entirely.
Next step: Determine the source, condensation vs. roof leakage, before planning any remediation.
Blocked Soffit Vents at the Eaves
Soffit vents are the intake side of your attic's ventilation system, fresh air enters at the soffits, rises along the underside of the sheathing, and exits at the ridge or roof vents. When insulation is packed tightly into the eaves with no baffles in place, that intake gets choked off. The attic can't ventilate as intended, and moisture retention goes up.
I see this in a lot of homes where insulation has been blown in over the years without anyone installing proper baffles to maintain the air channel. It also increases ice dam risk in winter.
Next step: Install or restore baffles (chutes) at each rafter bay to maintain a clear air path from soffit to attic.
Inadequate Insulation Coverage and Voids
Thin, uneven insulation. Exposed drywall or ceiling areas. Insulation disturbed or displaced at the eaves by wind washing. All of these increase heat loss from the conditioned space below, warming attic surfaces that should stay cold. When you combine heat loss with air leakage, condensation potential goes up significantly.
I use thermal imaging to map exactly where the gaps are. It's often eye opening for buyers to see how much heat is escaping through areas that look fine from the living space below.
Next step: Evaluate insulation depth and coverage. Top up to current code levels and address any voids or displaced areas.
Unsealed Attic Hatch or Pull Down Stairs
This is one of the biggest and most overlooked air leakage points in any home. The attic hatch, or pull down folding stairs, sits right in the ceiling between your heated living space and the cold attic. If there's no weatherstripping, no latch pulling it tight, and no insulation on the cover, it's essentially an open hole pumping warm, moist air straight into the attic every hour of every day.
I see this in almost every home I inspect. It's a cheap and easy fix that makes a measurable difference.
Next step: Add weatherstripping, a latching mechanism, and an insulated cover. For pull down stairs, consider an insulated tent box.
Unsealed Ceiling Penetrations
Open gaps around bathroom fan housings, plumbing vents, electrical wiring, pot lights, top plates, any break in the ceiling plane is a potential air leakage pathway. Even small gaps can move a surprising amount of warm, moist air into the attic under stack effect pressure.
Stack effect is the natural upward movement of warm air through the building. The higher the temperature difference between inside and outside, the stronger the pull.
In a cold New Brunswick winter, stack effect is significant. Every unsealed penetration is working against you.
Next step: Seal all ceiling penetrations with appropriate materials (fire rated caulk, foam, or gaskets depending on location).
Plumbing Stacks, Chimneys, and Roof Penetrations
Stains or dampness around vent pipes, chimneys, skylights, or valley areas. Rusted fasteners, wet insulation, or localized discolouration. These signs can indicate roof leaks, and roof leaks can look very similar to condensation staining from the attic side.
The key difference is location. Roof leaks tend to concentrate around specific penetrations where flashing has failed or sealant has deteriorated. If staining is isolated to one spot and you can trace it to a penetration point, it's more likely a flashing issue than a ventilation problem.
Next step: Evaluate flashings and seals at all roof penetrations. Repair or replace as needed.
Vermiculite or Other Suspect Insulation
That granular, pebble like insulation, often grey brown that sits in the attics of many older NB homes. As I've written about in my older homes guide, not all vermiculite contains asbestos, but a significant percentage was contaminated at the source. Until it's tested, it should be treated as suspect and left undisturbed.
From a moisture perspective, the real issue is access. When I can't safely enter or move through an attic because of suspect insulation, conditions underneath it, air leaks, staining, ducting problems go unreported. It's a blind spot that can hide exactly the kind of moisture issues this entire checklist is about.
Next step: Have the material tested. If it contains asbestos, professional abatement is required before any attic work can proceed.
What This Checklist Doesn't Cover
Lab confirmation of mould, a visual inspection identifies indicators, not species.
Hidden conditions under deep insulation or finished ceilings.
Exact ventilation sizing requirements, which vary by roof design and assembly.
Indoor lifestyle moisture sources, high humidity from cooking, showers, and drying laundry may require separate humidity control strategies.
When to Bring In a Home Inspector
If you're buying, an attic review can identify moisture indicators early, before they become expensive repairs. If you're selling, resolving the big drivers (exhaust routing, air leakage, and ventilation pathways) before listing can prevent surprises during negotiations.
Most of the items on this list are fixable. The expensive part is not knowing they're there.
Common Questions
How can I tell if attic staining is from condensation or a roof leak? Condensation indicators tend to appear more widespread, nail frost, broad sheathing staining near the eaves, while roof leaks tend to cluster around penetrations, valleys, and flashing points. A visual check can't always confirm the source, which is why identifying the pattern matters.
Is attic mould always a roof problem? No. Many attic moisture issues are driven by indoor air leakage and poor exhaust routing rather than failed shingles. The roof may be fine, the problem is often what's happening below it.
Does adding insulation fix attic condensation? Sometimes, but not by itself. If air leakage and exhaust issues remain, more insulation can still leave moisture pathways active. Insulation slows heat loss, but it doesn't stop air movement. You need both.
Should I run a dehumidifier to solve attic moisture? A dehumidifier may reduce indoor humidity levels, but attic moisture problems usually need targeted corrections: exhaust routed to the exterior, ceiling air leaks sealed, and ventilation pathways restored. Treating the symptoms without fixing the source rarely solves the problem long term.
What time of year is best to check an attic for moisture issues? Cold weather makes condensation indicators more visible, frost on nails, damp sheathing, active condensation. But staining patterns from past moisture events can be seen year-round. A winter inspection tends to reveal more, but any time of year has value.
Attic moisture problems don't announce themselves. They build quietly until someone finally looks up. The earlier you look, the less it costs to fix.
About the Author
Jonathan Gogan, RHI, WETT, CNRPP
Jonathan Gogan is the owner and operator of East Coast Home Inspection Ltd, based in Hampton, New Brunswick. With over 10 years of experience and thousands of completed inspections across Southern NB, Jonathan uses thermal imaging on every inspection to identify heat loss, moisture, and air leakage conditions that aren't visible to the naked eye.