Thermal Bridging in Ordinary Houses

Why this matters

A painter rolls a ceiling white and in two winters it has faint gray stripes every 16 inches. A cleaner scrubs dark lines off a bathroom wall and they come back on the same lines. A customer asks why the bedroom wall with "R-13 in it" feels cold to the hand. All three are thermal bridging: the framing that holds the house up conducts heat around the insulation, so every stud, header, rim joist and slab edge is a cold stripe on the inside face in winter. It is not a defect in the usual sense, and it is not a cleaning or painting problem. It is a property of how ordinary houses are built, and an interior trade who recognizes it can tell the customer what they are looking at instead of selling them the same job again next year.

What the bridge is

Insulation works by trapping still air in fiber or foam. Wood framing is solid. A softwood stud conducts heat roughly three times as readily as the fiberglass batt beside it, inch for inch, so a 2x4 stud resists heat flow at only about R-4.4 across its 3-1/2 inches, against R-13 for the batt in the cavity next to it. Heat takes the easy path: through the stud, the double top plate, the header over every window and door, the rim joist at every floor line, and the edge of a slab where it meets the outdoors. Each of those is a thermal bridge, a conductive path that bypasses the insulation layer.

Steel is far worse. Steel studs, steel lintels over brick openings and steel angles conduct heat many times more readily than wood, and a steel-framed wall can lose a large share of its cavity insulation's value through the studs alone. Masonry ties, balcony slabs and concrete that runs from inside to outside behave the same way.

The section, with the bridges traced

 WALL AND CEILING SECTION (winter, inside at right)
            ATTIC  insulation thins at eave
   roof  \  ............................
          \ ceiling joist -> cold line on ceiling
   -------+-------------------------------
   top    | <- double top plate, cold stripe
   plates |
   STEEL  | <- lintel over brick window, cold
   LINTEL | header -> solid wood, cold band
   window |
          | stud every 16 in -> cold stripe
          |   batt between studs, warm field
   RIM    | <- rim joist at floor line, cold
   -------+-------------------------------
   SLAB   | <- slab edge meets outdoors,
   EDGE   |    cold strip along the floor
   OUTSIDE               INSIDE

The pattern on the inside face is the framing drawn in temperature: vertical stripes at studs, a horizontal band at the top plates and another at the rim, a wider block over each opening, and a cold strip of floor along an uninsulated slab edge.

Worked example: how much colder a stud line runs

The layer values below are common published figures for a wood-framed 2x4 wall, of the kind tabulated in the ASHRAE Handbook, Fundamentals, carried as approximate.

Through the cavity: inside air film 0.68, half-inch drywall 0.45, R-13 batt, sheathing about 0.6, siding about 0.6, outside air film 0.17. Total: 0.68 plus 0.45 plus 13 plus 0.6 plus 0.6 plus 0.17, which is R-15.5.

Through the stud: the same layers with the stud's R-4.4 in place of the batt. Total: 0.68 plus 0.45 plus 4.4 plus 0.6 plus 0.6 plus 0.17, about R-6.9.

Surface temperature on a cold night, inside at 68 F, outside at 0 F, a 68 degree difference. The inside surface sits below room temperature by the share of the total resistance that is the inside film:

• Over the cavity: 68 times 0.68 divided by 15.5, about 3.0 degrees, so the surface is near 65.0 F.
• Over the stud: 68 times 0.68 divided by 6.9, about 6.7 degrees, so the surface is near 61.3 F.

The stud line runs nearly 4 degrees colder than the field of the wall beside it. This is a one-dimensional estimate; heat spreading sideways through the drywall warms the stud line a little in practice, so read it as an upper bound on the difference rather than an exact figure.

What that does to the whole wall. Framing, counting studs, plates and headers, is commonly estimated at around a quarter of a 16 inch on-center wall's area. Treating the wall as parallel paths, 75 percent cavity and 25 percent framing, the overall conductance is 0.75 divided by 15.5 plus 0.25 divided by 6.9, about 0.0846, which is an overall R of about 11.8. The "R-13 wall" performs, counting films and sheathing, at about R-11.8 rather than the R-15.5 its clear-cavity section suggests. That is why the customer feels the wall as cooler than the label promised.

Where the stripe becomes mold rather than dust. In the living room at 68 F and 35 percent RH, the indoor dew point is about 39 F. The stud line at 61 F sits at about 44 percent surface RH, comfortably dry. Now take a bathroom on the same exterior wall, at 70 F, with a weak exhaust fan, so it sits around 70 percent RH for hours after showers. Recomputed at 70 F inside and 0 F outside:

• Cavity surface near 66.9 F, at about 78 percent surface RH.
• Stud surface near 63.1 F, at about 89 percent surface RH.

Sustained surface RH around 80 percent and above is the range commonly associated with mold growth. The field of the wall sits just below it and the studs sit well above it, which is why bathroom mold so often grows in neat vertical lines. The fix is the bathroom's humidity, not the paint, which is the same point US EPA guidance on moisture and mold in homes makes: control the moisture. See related: The Cold Surfaces Where a House Condenses First.

Ghosting: the stripes that are dust, not mold

Ghosting is the gray or dark pattern of framing showing through drywall and paint. It forms because airborne particles, from candles, cooking, a fireplace, a gas range or ordinary household dust, deposit preferentially on cooler surfaces as room air moves across them, and they cling better to a surface that is slightly damper. Over a winter or two, the cold stud and joist lines collect a visible film. Screw and nail heads show as dots for the same reason, since the fastener is a tiny steel bridge.

The tells that separate ghosting from mold:

• It is evenly spaced and straight, following framing exactly, often on ceilings below an attic and on exterior walls.
• It wipes or washes off as a gray film without the fuzzy texture or musty smell of growth.
• It returns on the same lines after cleaning, because the cause is temperature, not contamination.
• It is worst near the source of particles, above a frequently used candle, near a wood stove, on the ceiling over a gas range.

Ceiling ghosting is often heaviest near the exterior walls, where the attic insulation thins at the eave and the joist ends sit in the coldest part of the attic. That overlaps with wind washing, covered in Insulation Contact and Wind Washing.

What each interior trade is actually looking at

Painters. Stripes that bleed back through fresh paint are ghosting. A stain-blocking primer hides them for a while; reducing indoor particle sources and fixing insulation gaps at the eave are what stop them. Say so in the estimate so the next callback is not yours.

Cleaners and restoration crews. Lines that come back on the same studs in the same room are a temperature pattern. Note the location and the spacing in the job record and tell the customer it is a building issue; repeatedly selling cleaning for a pattern that returns is how a shop loses the customer's trust.

Drywall crews. A ceiling with dark lines at the joists only near the exterior wall points to thin insulation at the eave, not bad taping.

Flooring installers. A cold, sometimes damp strip of floor along the exterior wall of a slab-on-grade house is the slab edge. Moisture-sensitive flooring laid right to that edge can cup or stain along it.

HVAC and energy trades. Thermal bridging explains why a house underperforms its nominal R-values and why stud lines show clearly on an infrared camera on a cold night.

What changes the answer

Continuous insulation outside the framing covers the studs, plates and rim with an uninterrupted layer, so the bridges stop reaching the outside and the stripes largely disappear. That is a wall design decision, and the options belong to Building Science and Envelope Reference.

Advanced framing, with studs at 24 inches on center, single top plates where allowed, insulated headers and two-stud corners, reduces the framing share and the number of stripes.

Steel framing turns a minor effect into a major one; expect strong ghosting and condensation risk at studs in a humid room unless the wall has continuous insulation.

Climate and season. The effect is a heating-season issue in cold climates. In a hot humid climate the bridges run the other way in summer, warm on the inside face, and the condensation risk moves to the cold surfaces the air conditioning creates.

The failure mode

The failure is treating the pattern as a surface problem and repeating the surface treatment. A homeowner who gets the ceiling repainted three winters in a row has paid three times for an eave insulation gap, a candle habit and a fan that does not move enough air. The useful job is to name the pattern, measure the surface temperatures if you carry an infrared thermometer, and route the cause to the trade that owns it: insulation at the eave, bathroom exhaust, or indoor humidity.

References

• ASHRAE Handbook, Fundamentals, for material thermal resistance values and the parallel-path method
• See related: Building Science and Envelope Reference; The Cold Surfaces Where a House Condenses First
• See related: Insulation Contact and Wind Washing; Dew Point and Condensation, the Reasoning Behind the Fault
• US EPA guidance on moisture and mold in homes