Un-Balanced Roof Intake vs Outlet Venting
Problems created with unbalanced roof venting systems
PROBLEMS with PARTIAL ROOF VENTILATION - CONTENTS: problems caused by un-balanced, incomplete, or just partial roof venting include ineffective ventilation, increased heating costs in cold climates, or incresed cooling costs in hot climates. The importance of a balance between venting air intake and outlet is explained.
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Problems with un-balanced 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 un-vented 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.
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.
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).
If you can't provide enough intake venting it is probably better to not vent at all in these conditions. Illustration at above left was provided courtesy of Carson Dunlop Associates, a Toronto education, engineering & home inspection firm.
Watch out for real world snafus, damage, leaks in roofs.
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.
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)" - 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).
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Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
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TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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