Problems with unbalanced roof ventilation intake versus outlet:

This article explains why an incorrect ratio or balance of intake venting and outlet venting in a roof ventilation system can cause more problems than any possible benefit of the roof existing ventilation system.

This article series describes various solutions for unvented cathedral ceilings and similar under-roof spaces, offering advice on how to avoid condensation, leaks, attic mold, & structural damage when roof venting is not possible.

Problems with Partial Roof Venting

Some buildings, by their shape or design, simply don't make it easy to install continuous intake venting at the eaves or lower roof edge, or continuous outlet venting along a ridge.

Illustration of soffit intake venting courtesy of Carson Dunlop Associates, a Toronto home inspection, education, & report writing tool firm.

[Click to enlarge any image]

For example, a house which has no roof overhang at all makes intake venting at the eaves difficult.

A house with a pyramid roof shape or complex roof shapes makes outlet venting at a ridge difficult.

And a home with cathedral ceilings following the roof line may be difficult to vent - a common case where the hot roof design is appealing.

On these roofs, partial venting can be worse than no venting.

For example, adding a ridge vent, or several roof "spot vents" or roof turbine vents on a few roof slopes, typically mid-slope or in the upper third of the slope on roof surfaces not visible from the front of the building, may please the installer, but they are worse than ineffective.

Placing an outlet vent on a roof without adequate inlet venting works against the interests of the building and its occupants.

As convection currents and heat loss into the roof space or attic vent out through these vents, the intake air needed to satisfy the exhausted air leaving the building will be drawn from the building interior - increasing building heating costs and possibly increasing particle movement from basements or crawl spaces (if there is a mold concern in the building).

See ROOF VENTIILATION INTAKE-OUTLET RATIO

If you can't provide enough intake venting it is probably better to not vent at all in these conditions. The roof ventilation illustration above was provided courtesy of Carson Dunlop Associates, a Toronto education, engineering & home inspection firm.

Watch out for real world snafus, damage, leaks in roofs.

• Attic or roof cavity condensation 
  
  due to indoor moisture sources vary widely as individual building moisture conditions vary (we start our roof inspection by looking for evidence of basement or crawl space flooding) as well as by quality of construction and the number of penetrations in the building ceilings and walls, not to mention the effects of air pressure differences within the building.
  
  Samuelson (1995) pointed out that "... to guarantee no indoor air movement into the attic, the ceiling has to be airtight and the pressure of the attic needs to be higher than that of the indoor air
  
  (i.e. pressurized attic or depressurized living space)"[7] - not normal conditions in most climates.
• An under-roof cathedral ceiling or attic ventilation system
  
  to help the building in any way, needs to be properly designed and installed with adequate intake venting at the eaves, outlet venting at the ridge, and with careful sealing of air leaks from the occupied space into the attic or roof cavity (to avoid heat losses or increases in both moisture movement into the roof cavity and increased heating or cooling costs).
• Moisture problems originating outside the building 
  
  such as due to damage or poor workmanship involve an element of luck and appear to have been excluded by experts and studies in the field, notwithstanding the experience of repair-roofers and home inspectors that those leaks appear on a great many homes, can cause severe rot damage, costly mold damage, and that leaks eventually happen on a great many if not most homes that remain standing for 20 years or more.
• Unbalanced roof ventilation:
  
  outlet venting high on the roof (at the ridge preferably) should be about 1/2 the amount of inlet opening at the building eaves.
  
  If the outlet venting is too large relative to the inlet venting for outdoor air, warm air escaping the attic or roof cavity will tend to draw conditioned air out of the occupied space of the building (through leaks into the attic or roof space), thereby increasing the building's heating or cooling costs or both.
  
  Details are at ROOF VENTIILATION INTAKE-OUTLET RATIOS

...

Continue reading at HARD to VENT ROOF SOLUTIONS for HIPS & CATHEDRAL CEILINGS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• CATHEDRAL CEILING VENTILATION
• CAPE COD ROOF & KNEE WALL INSULATION & VENTILATION
• CATHEDRAL CEILING INSULATION
• HOT ROOF DESIGN PROBLEMS
• HOT UN-VENTED ROOF DESIGN SOLUTIONS
• HARD to VENT ROOF SOLUTIONS for HIPS & CATHEDRAL CEILINGS
• INSULATION LOCATION & QUANTITY for ATTICS
• PROBLEMS with PARTIAL ROOF VENTILATION
• ROOF NOISE TRANSMISSION
• ROOF VENTILATION SPECIFICATIONS pros and cons of hot roof designs
• ROOF VENTILATION IMPROVEMENTS
• ROOF VENTIILATION INTAKE-OUTLET RATIO

Suggested citation for this web page

PROBLEMS with PARTIAL ROOF VENTILATION at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING VENTILATION

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