Figure 2-57: Skylight roof venting details (C) J Wiley, S BlissRoof Ventilation Details for Skylights

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This article explains How to Correct Improper or Inadequate Attic or Under-Roof Ventilation at skylights.

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Roof Ventilation Details at Skylights to Provide an Air Path - Eaves to Ridge

Figure 2-57: Skylight roof venting details (C) J Wiley, S BlissThis article series describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems in buildings. It describes proper roof ventilation placement, amounts, and other details.

These recommendations are based on a survey of building science literature combined with 30 years of building inspections, on the observation of the locations of moisture, mold, ice dams, condensation stains, and other clues in buildings, and on the correlation of these clues with the roof venting conditions at those properties.

We have measured very large changes in airflow, temperature, and moisture before and after installing roof venting.

As described in Best Practices Guide to Residential Construction:

Localized hot spots such as skylights can also lead to ice dams below, due to blocked ventilation as well as melt water from skylight heat loss.

Notching the rafters on either side of the skylight will help maintain airflow above the skylight (Figure 2-59 at left).


If icing on your skylight window interior surface in winter is still a problem, add an interior storm window to reduce heat loss through the glass in cold weather.

Figure 2-5: Flashing at Skylights, details (C) J Wiley, S Bliss

As a backup to prevent leaks at skylights, during skylight installation and even though modern skylights are usually provided with a factory-built flashing and counterflashing, it is always a good idea to seal the skylight curb and surrounding roof area with a bituminous membrane (see Figure 2-5 at left).

Also see SKYLIGHT LEAK DIAGNOSIS & REPAIR and see Skylight Glass Breakage.

Also see Ice Dam Protection for Skylignts for cases where under-roof venting is not provided around a skylight.

Cathedral Ceiling Ventilation Alongside of Skylights

Figure 2-57: Cathedral ceiling venting details (C) J Wiley, S Bliss

Cathedral ceilings require the same continuous air barriers, and balanced soffit and ridge vents, as attics.

Both air sealing and ventilation are more critical, however, since any trapped moisture in the roof cavity will remain longer and potentially cause greater damage than in an open attic.

Also, since there is little or no communication from bay to bay, an effective ventilation system must reach every bay (Figure 2-57 at left).

Details about cathedral ceiling ventilation are at CATHEDRAL CEILING VENTILATION.


-- Adapted with permission from Best Practices Guide to Residential Construction.


Continue reading at ROOF VENTILATION SPECIFICATIONS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see ROOF FRAMING TIES & BEAMS for a discussion of proper framing of a cathedral ceiling

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SKYLIGHT VENTILATION DETAILS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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