The Cold Surfaces Where a House Condenses First

Why this matters

Mold complaints and "water stain, no leak" calls cluster in the same few places in almost every house, and a tech who knows the map goes straight to them instead of chasing the plumbing. The places are the surfaces that run coldest in winter and, in summer, the surfaces the air conditioning makes cold. Most of them are made worse by the way people live: a closet stuffed to the back wall, a dresser pushed tight against an outside corner, a couch against the coldest wall in the room. The physics of why a surface below the dew point gets wet belongs to Dew Point and Condensation, the Reasoning Behind the Fault. This card is the field map of where it happens first and what living in the house does to it.

The map

 FLOOR PLAN, heating season (not to scale)
 north / exterior walls on top and left
 +--X--------------------+--------------W--+
 X CLOSET on            |  BEDROOM         |
 | exterior wall,       |  dresser tight   |
 | packed to the back   |  to outside      |
 |                      |  corner -> X     |
 +----------------------+                  |
 W window frame                   ceiling  |
 | and glass edge        thin insulation   |
 |                       at eaves above -> X
 | LIVING  couch tight                     |
 X against exterior wall                   |
 +--X--------------------------------------+
 X = cold surface, condenses first
 W = window frame, the usual first sign
 Summer adds: attic ducts, cold water pipes,
 basement walls and slab, supply registers

Winter: the envelope's cold spots

In the heating season the cold surfaces are on the inside face of the envelope wherever heat leaks out fastest or room air reaches it least.

Window frames and the glass edge. The edge of an insulated glass unit, where the spacer joins the panes, and the frame itself are usually the coldest interior surfaces in a room. Condensation on the bottom of the glass on a cold morning is the house's own dew point gauge: it tells you the indoor dew point is above that edge's temperature. See related: How to Work Out Where Condensation Will Form.

Outside corners. Where two exterior walls meet, there is more outside surface losing heat for each square foot of inside surface gaining it, and the corner gets less air circulation from the room. Corners run colder than the middle of the same wall, and the top corner where two walls meet the ceiling runs colder still.

The ceiling at the eaves. On a pitched roof, the insulation over the top-floor ceiling thins as the roof slope comes down toward the wall plate. The ceiling just inside the exterior wall is often the coldest stretch of the room. Wind washing at the eave can make it worse, which is the subject of Insulation Contact and Wind Washing.

Exterior-wall closets. A closet on an exterior wall is a small room with no heat supply of its own, a door that is mostly closed, and a back wall in contact with the outdoors. It runs colder than the bedroom it serves even when empty.

Framing lines and rim joists. Studs, headers and the rim at the floor line conduct heat around the insulation and show up as cold stripes. See related: Thermal Bridging in Ordinary Houses.

Behind anything pressed against an exterior wall. Furniture, stacked boxes, a mattress on its side, a bookcase. Covered next.

What stuffing a closet or pushing back the furniture does

A wall surface stays warm because room air washes heat onto it. The thin film of air at the surface is itself a small insulating layer, and the warm room air beyond it keeps that film supplied. Pack the closet with clothes or push a dresser flush to the wall and you add an insulating layer on the warm side and stop the air from reaching the wall. The wall loses heat to the outdoors at nearly the same rate but receives less from the room, so its surface temperature falls. Moisture still reaches it, because water vapor diffuses through clothing and slips around furniture, and the vapor that gets there meets a colder surface than before.

Worked example: an exterior closet through a cold week

The numbers for the wall's and the clothing's insulating value are illustrative, chosen to be plausible for an older partly insulated wall and a full closet. What the example teaches is the direction and size of the effect.

The house. Indoors at 70 F and 41 percent RH, which puts the indoor dew point at about 45 F. The closet's back wall is an older exterior wall with a total insulating value of about R-8 counting its surface films, of which the inside air film is about R-0.68.

Open wall, 5 F outdoors. The temperature difference across the wall is 70 minus 5, or 65 degrees. The inside surface sits below room temperature by the share of the wall's resistance that is the inside film: 65 times 0.68 divided by 8, about 5.5 degrees. So the open wall surface runs near 64.5 F, far above the 45 F dew point, at about 50 percent surface RH. Nothing happens.

Same wall, closet packed, 5 F outdoors. Say the packed clothes and trapped air add about R-3 on the room side. The total is now 8 plus 3, or R-11, and the layers between the room and the wall surface are 0.68 plus 3, or R-3.68. The surface sits 65 times 3.68 divided by 11 below room temperature, about 21.7 degrees, so near 48.3 F.

• 48.3 F is still above the 45 F dew point, so no visible water.
• But the air at that surface sits near 89 percent RH. Sustained surface RH around 80 percent and above is the range commonly associated with mold growth, and in this house that threshold falls at a surface temperature of about 51 F. This wall is below it.

So the closet grows mold on the back wall with no condensation anyone ever sees, and the homeowner calls about a smell, not about water.

A colder week, minus 5 F outdoors. The difference is now 75 degrees. The packed-closet surface drops 75 times 3.68 divided by 11, about 25.1 degrees, to near 44.9 F. That is just below the 45 F dew point. Now the wall is wet at the surface, frost is possible in the corners, and the stain the homeowner eventually finds follows the coldest part of the wall, usually the bottom corner and the stud lines.

What changes the answer. Lower the indoor dew point and the whole map moves down: at 30 percent RH and 70 F the dew point is about 37 F, and the packed closet at minus 5 F stays above it, though still near the mold range at its surface. Empty the back 6 inches of the closet and leave the door ajar and the surface warms most of the way back to the open-wall figure. Insulate the wall properly and both the open and packed surface temperatures rise. And a closet on an interior wall does not enter the map at all, because its back wall is warm.

Summer: the surfaces the air conditioning makes cold

In the cooling season the map flips. The envelope is warm, and the cold surfaces are the ones chilled by the cooling system or by the ground, sitting in contact with warm humid air.

• Ducts in an attic or crawlspace. A supply duct carries air in the 50s F through a space full of hot air, which in humid climates can have a dew point near or above the duct's surface temperature. Torn jackets, uninsulated boots and gaps at the ceiling let that air reach the cold metal, and the water runs down onto the drywall below as a ceiling stain shaped like the duct.
• Supply registers in a humid house, especially where attic air leaks around the boot.
• Cold water pipes and toilet tanks in a humid basement or bathroom, dripping where they hang.
• Basement walls and the slab, cooled by the ground, wet whenever humid outdoor air gets in. See related: Relative Humidity Versus the Moisture Actually in the Air.
• The underside of a floor over a vented crawlspace, cooled from above by an air-conditioned house and bathed below in humid outdoor air.

Where each trade meets the map

Cleaning and restoration crews see the stains and the smell first. A mold spot that returns in the same place after cleaning is a map location, not a cleaning failure.

Painters and drywall crews see dark lines along studs and in corners. Painting over them treats the symptom.

HVAC techs see the summer half: duct sweat, register drip, a coil closet that stays damp.

Plumbers get called to wet ceilings and walls with no leak on the pressure test, and the map is how they explain it to the customer rather than leaving with "could not find anything".

The failure mode

The common mistake is treating the location as the cause. A closet back wall gets bleached and repainted, the furniture goes back where it was, the indoor humidity stays where it was, and the spot comes back next January in the same place. The cause is always a pair: a surface that runs cold and air that carries enough water to reach it. The US EPA's guidance on mold in homes makes the same point: control the moisture or the growth comes back. A fix has to move one of them, by warming the surface, lowering the indoor dew point, or letting room air reach the wall again. Finding the specific cold spot behind a complaint is the procedure in How to Find the Cold Spot Behind a Closet Mold Complaint.

How to know you have the right spot

You have the right location if the stain or growth sits at or below the coldest surface temperature you can measure in the room, if its edges follow a cold feature (a corner, a stud line, the eave, a duct run), and if the indoor dew point you measure sits within a few degrees of that surface temperature on a cold or humid day. A stain that sits on a warm surface far above the dew point is not this problem. Look for a leak.

References

• See related: Dew Point and Condensation, the Reasoning Behind the Fault; How to Work Out Where Condensation Will Form
• See related: Thermal Bridging in Ordinary Houses; Insulation Contact and Wind Washing
• See related: How to Find the Cold Spot Behind a Closet Mold Complaint; The Tightened House That Grew Mold in the Closets
• US EPA guidance on moisture and mold in homes