Night Sky Radiational Cooling:
Why Clear Winter Nights Wet Portland Attics
Some attic moisture has no leak, no bad vent, and no indoor source behind it. In the maritime Northwest, the clear night sky can do it on its own. Here is the physics and what it looks like in the attic.
On a clear October night in the Pacific Northwest, the air temperature reads 43 degrees F, the overnight low will be 38, and the relative humidity is above 90 percent. On rooftops across the Portland metro and Southwest Washington, dew is forming on the shingles. More important, moisture can be condensing on the underside of the roof sheathing inside attics that are vented to code.
No interior humidity problem. No roof leak. No blocked vents. Just physics.
The name for it is night sky radiational cooling, and it is one of the most important and least understood factors in roof and attic health in the maritime Pacific Northwest.
What Is Night Sky Radiational Cooling?
Every object above absolute zero radiates heat as infrared energy, and a roof is no exception. On a clear night, with no cloud cover to send that energy back, the roof radiates heat toward the clear sky, which behaves like a surface far colder than the air around the house.
The roof surface loses heat faster than the surrounding air can replace it. On clear, humid nights with temperatures in the 35 to 45 degree F range, conditions common in a Pacific Northwest fall or spring, the roof surface can drop several degrees below the air temperature, and the sheathing underneath follows it down. Those numbers sound small, but near the dew point even a degree or two is enough to trigger condensation.
Field research by RDH Building Science on test roofs in Vancouver, British Columbia, found plywood sheathing moisture content between 20 and 30 percent through the rainy months, with no interior moisture source. Mold grew on the underside of the sheathing within the first year, and it appeared first on the north-facing slopes. Moisture collects on cold sheathing at night and does not fully dry before the next cycle begins.
You have likely felt this effect without a name for it. It is one reason clear nights feel colder than cloudy nights at the same temperature. It is part of why frost can form on a roof or windshield when the air is still above freezing. And it is why you feel cold sitting near a large single-pane window: your body is radiating heat toward the cold glass.
Why It Shows Up in Newer Homes
Night sky radiational cooling has always existed. So why is it causing problems in attics now?
The answer is energy efficiency, specifically a side effect of building homes that are well insulated and tightly sealed at the ceiling.
In homes built before the 1970s, the ceiling was leaky and insulation was minimal or missing. Heat from the living space escaped freely into the attic and kept the sheathing warm enough that condensation from radiational cooling was either prevented or minor enough to dry out. The attic was, in effect, partly heated by the house below.
Homes built to current energy codes have the opposite situation. High R-value insulation and air-sealed ceilings do their job, and from the attic’s point of view, they do it too well. With little heat reaching it from below, the attic runs cold and the sheathing sits at or below the outdoor air temperature. That makes it a ready condensation surface on clear nights.
The tighter and better insulated the home, the more exposed the attic sheathing is to this effect. It is an irony of modern building practice: the more efficient the home, the more vulnerable the roof structure.
What It Looks Like in the Attic
When home inspectors find night sky radiational cooling damage, it presents in a recognizable pattern, one that sets it apart from moisture problems caused by interior air leaks or roof leaks.
The discoloration is broad and fairly uniform across large areas of sheathing, rather than concentrated around specific penetrations. It follows the roof plane rather than framing members. And it is consistently heavier on north-facing slopes, which get the least sun during the day and have the least chance to dry out between condensation cycles.
South-facing slopes on the same house often show little or no staining. On sunny days, south-facing shingles get far warmer than the air, which drives moisture out of the sheathing and resets the drying cycle. North-facing slopes get much less of that solar warming, so the moisture has nowhere to go.
The sheathing material matters too. OSB (oriented strand board), which has largely replaced plywood for roof sheathing since the 1990s, tends to dry more slowly than plywood once it gets wet, and both behave differently from the solid sawn boards used in pre-war homes. Holding moisture longer gives mold more time to take hold. That is part of why the problem shows up so often in homes built from the mid-1990s on: tight enough to starve the attic of heat, and built with OSB sheathing.
Real Inspection Photos
What Sheathing Moisture Looks Like in Real Attics
Attic photos from Russ's reports in Oregon and Southwest Washington showing the kinds of sheathing staining described above.
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Staining above the soffit vents. The report notes this is common locally, often from damp outdoor air drawn in and cooled overnight. -
Discoloration spread across the sheathing, not clustered at a leak or penetration, fits the pattern above. A photo alone can't confirm the cause. -
Stained OSB with nail tips poking through. Nail tips are often the coldest spots on the deck, so moisture tends to show up there first.
Telling Night Sky Cooling From Other Moisture Sources
Not every case of attic staining comes from night sky radiational cooling, and it is a mistake to jump to that conclusion before ruling out more common causes. Interior air leaks, such as exhaust fans venting into the attic, unsealed can lights, poorly sealed attic hatches, and gaps in the ceiling, cause most attic moisture problems and get investigated first.
The inspector compares the staining pattern with the home’s construction and looks for interior moisture sources before drawing any conclusion.
- Home is newer, airtight, and well insulated at the attic floor
- No exhaust fans end in the attic and no major air leaks at the ceiling
- Staining is uniform and follows roof orientation rather than framing or penetrations
- North-facing slopes are clearly more affected than south-facing slopes
- Areas shielded from the night sky, such as under an overlapping roof, are noticeably cleaner
- Active mold growth or damp, soft sheathing, which warrants evaluation by a qualified specialist
That shielded-area sign is especially telling. A porch roof or addition roof that overlaps a section of the main roof creates an area of sheathing that is blocked from the night sky. That area is often noticeably cleaner than the exposed sheathing on either side. When an inspector sees this pattern, it works like a controlled demonstration of the effect: the main difference between the two areas is exposure to the night sky.
Why More Ventilation Can Make It Worse
The instinctive response to attic moisture, adding more ventilation, can backfire when night sky radiational cooling is the cause. The moisture is not coming from interior air leaks in these cases. It is coming from the outdoor air itself, which is cold and saturated with humidity through much of the Pacific Northwest’s long wet season.
Adding vent area brings more cold, humid outdoor air into contact with even colder sheathing. More air means more moisture available to condense. The RDH Building Science researchers concluded that in the coastal climate, the usual approach of more attic ventilation can make the problem worse, not better.
In tight, well-insulated Pacific Northwest homes, adding attic ventilation is not a reliable fix for sheathing condensation and can make it worse. Interior moisture sources still get ruled out first, but more vents is not the default answer.
For more on where common ventilation advice goes wrong, see attic ventilation myths for Portland homeowners.
What Can Be Done?
The building science community is candid about the limits of current knowledge here. For night sky radiational cooling in tight modern homes, there is no simple fix that works everywhere. Several approaches have been proposed and studied, and none has emerged as the definitive answer.
Reduced ventilation. Cutting ventilation below code minimums is counterintuitive and runs into code requirements, but some field evidence suggests it can help by reducing the cold, humid air delivered to the sheathing. This is not something inspectors recommend. It is part of ongoing research.
More vapor-permeable roof assemblies. Moving away from vapor-tight underlayments toward more breathable materials can let moisture that collects in the sheathing dry upward through the roof instead of staying trapped.
Fungal-resistant treatments. Treating the underside of the sheathing is another area of research. The RDH test roofs compared bleach, a water sealer, a primer paint, a borate product, and copper and zinc naphthenates. Bleach and the water sealer did poorly in the early results, while products with mold-fighting ingredients held up better. These treatments do not stop condensation. They make the sheathing more resistant to its effects.
Unvented, conditioned attics. Some building scientists point to unvented attics, with insulation at the roof line, as the most reliable way to avoid this problem in new construction. That is a design decision for the builder and designer, not something added after the fact.
When interior moisture sources have been ruled out, attic ventilation is correctly sized, and staining still develops on new or recently cleaned sheathing, night sky radiational cooling belongs on the list of likely causes, and adding more vents is not the answer.
What This Means for Your Home Inspection
At Trusted Home Inspections, thermal imaging is included at no additional charge on every inspection. Cool sheathing, damp areas, and frost can show up on an infrared camera at the time of the inspection in ways a visual check can miss. That lets the report document the conditions present that day, not just the visible staining.
If a report noted attic staining, moisture, or mold in a newer, well-insulated home, night sky radiational cooling may be part of the explanation. Understanding the mechanism does not make the problem go away, but it helps explain why some common fixes do not work. For what the inspector looks at in the attic generally, see the guide to attic mold in Portland homes.
Questions about your inspection findings? Email Trusted Home Inspections at office@trustedhome.org. Russ is happy to walk through what was observed and what it means for the specific home. Still shopping? Schedule an inspection online.
Night Sky Radiational Cooling FAQ
