Likely causes
Failure modes and building-physics mechanisms connected to the clues that support or weaken them.
Combustion backdrafting
Flue gases spill into the building instead of rising safely outdoors.
Electrical overheating
Resistance, loose connections or failing equipment generate abnormal heat at an electrical component.
Loose electrical connection
A poor connection creates resistance, heat and possible arcing.
Overloaded electrical circuit
Connected demand exceeds the intended circuit capacity.
Refrigerant or chemical leak
A system or stored product releases volatile material.
Sewer backup
Wastewater reverses through a drain due to blockage, surcharge or system failure.
Sump-pump failure
Collected water rises because the pump, power, float or discharge has failed.
Air leakage into the attic
Warm humid indoor air escapes through ceiling penetrations and reaches cold attic materials.
Blocked condensate drain
Cooling or heating condensate cannot leave the appliance normally.
Boiler scale or flow restriction
Reduced flow or heat transfer causes local boiling and noise.
Chimney flashing leak
Water enters at the roof-to-chimney transition.
Damaged roof underlay
The secondary weather layer is torn, brittle, poorly lapped or penetrated.
Differential settlement
Parts of the foundation move by different amounts.
Disconnected bathroom exhaust duct
Humid exhaust air is released into an attic, ceiling or wall cavity.
Dishwasher hose or connection leak
Water escapes during a fill, wash or drain stage.
Failed roof flashing
A transition detail no longer reliably redirects water onto the drainage surface.
Failed toilet seal
The hidden connection at the toilet outlet allows wastewater or odor to escape.
Failed window flashing
Water is not reliably directed around and out of a wall opening.
Failing motor bearing
Wear creates friction, heat, vibration and noise in equipment.
Ground slopes toward the foundation
Surface water is directed toward rather than away from the building.
Hidden plumbing leak
Water escapes from a supply, drain, appliance connection or seal and appears away from the actual failure.
Hidden storm damage
Wind, hail or debris damages a weather layer without an obvious interior sign at first.
Ice-dam water backup
Meltwater is trapped behind roof-edge ice and moves beneath coverings.
Interior air leakage
Humid indoor air moves through cracks and penetrations into colder assemblies.
Loose plumbing compression fitting
A mechanical joint seeps under pressure or movement.
Missing or discontinuous wall drainage plane
Water entering behind cladding has no continuous path to drain outward.
Pinhole leak in a supply pipe
A small opening in a pressurized pipe releases water continuously or intermittently.
Restricted dryer exhaust
Lint, crushing or excessive duct length reduces moisture removal.
Roof water entry
Rain reaches the interior through a failed covering, flashing, penetration, joint or drainage detail.
Shower waterproofing defect
Water passes through a failed penetration, joint or membrane detail.
Solar-panel roof interface leak
A mounting or cable penetration allows water into the roof assembly.
Washing-machine hose leak
A pressurized or drain hose leaks during part of the appliance cycle.
Water entry through a foundation crack
Hydrostatic or wind-driven water follows a crack or joint through below-grade construction.
Wind-driven rain entry
Rain is pushed through a vulnerable joint or opening that remains dry during calm vertical rain.
Wood decay
Fungi break down persistently damp wood and reduce its strength.
Air trapped in a radiator
Air reduces circulation and can create gurgling or uneven heat.
Blocked gutter
Leaves, ice or debris restrict roof drainage and cause overflow.
Capillary moisture rise
Liquid water moves through fine pores from a wet source toward a drying surface.
Condensation on a cold pipe
Humid air reaches a pipe below dew point and deposits water.
Contaminated condensate pan or coil
Persistent moisture and debris in air-conditioning equipment create odor and deposits.
Downspout discharges too close to the house
Roof runoff is released where it can return toward the foundation.
Drain vent obstruction
Restricted venting changes pressure and drainage behavior.
Dry drain trap
Trap water evaporates or is siphoned away, removing the sewer-gas barrier.
Failed exterior sealant joint
A perimeter joint has split, detached or exceeded its movement capacity.
Floor-joist deflection
Floor framing bends under load or due to span, damage or support conditions.
Freeze-thaw spalling
Water in porous material freezes, expands and breaks the surface.
High indoor humidity
Moisture generation exceeds removal for the current temperature and air exchange.
Low bedroom air exchange
Overnight moisture and contaminants accumulate faster than they are removed.
Partial drain blockage
Accumulated material slows flow and changes sound or odor patterns.
Pest entry opening
An exterior gap permits insects, rodents or birds to enter a cavity.
Rapid valve closure
Fast water-flow interruption creates a pressure wave known as water hammer.
Soot deposition on a cold surface
Airborne particles collect preferentially at cold or high-airflow patterns.
Surface condensation
Moist indoor air reaches a surface below its dew point and deposits liquid water.
Surface microbial growth
Moisture and available nutrients allow growth on or within a surface layer.
Unbalanced house pressure
Mechanical systems create pressure differences that move air and odors through unintended paths.
Wind-driven snow through a vent
Fine snow enters an opening and later melts inside an attic or roof space.
Cold bridge at framing
Framing or a conductive element lowers the interior surface temperature in a repeated pattern.
Drywall joint movement
Seasonal or structural movement reopens a finish joint.
Duct pressure flexing
Thin sheet metal changes shape as system pressure changes.
Efflorescence
Water dissolves salts in masonry and leaves crystals as it evaporates at the surface.
Failed insulated-glass seal
The perimeter seal of a glazing unit no longer controls moisture between panes.
Failed insulated-glass seal
The sealed cavity between panes no longer remains dry.
Insulation void or compression
A gap or compressed area creates a repeatable cold surface.
Normal timber shrinkage
Wood dries and changes dimension, producing small gaps or checks.
Pipe thermal expansion
Pipe length changes with temperature and rubs or releases at supports.
Salt transport through masonry
Moisture carries dissolved salts to an evaporation surface.
Subfloor fastener movement
Floor layers move against fasteners and create squeaks.
Thermal bridge
A conductive or poorly insulated path produces a colder interior surface.