Note the air path provided over the insulation and under the roof deck at the house eaves. Where to Locate Roof Vents to Prevent Attic Condensation, Leaks, Mold, & Structural Damage

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Here we explain how Proper location of roof intake and outlet venting prevents Attic Condensation, Ice Dam Leaks, Attic Mold, & Roof Structure D

This article 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.Intake venting needs to be at the eaves or lower roof edges. Otherwise these roof areas will not be dry and cool, and we'll find water damage, condensation damage, ice dams, and often mold in these areas.

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Proper Roof Vent Location for Air Intake and Outlet

Attic ventilation sketch

Some people install the ridge vent (the easy part) and leave older gable end vents as "intake." This does not work very well. Simply pulling in the needed makeup air from typical gable end wall vents will not move air under the lower and center areas of roof sheathing, thus inadvertently encouraging moisture and mold to form in these areas.

In buildings where this vent design has been used we see pretty clean roof sheathing near the ridge and near the building gable walls, but at the lower roof center and looking down to the eaves, the sheathing will often be wet, moldy, damaged.

The sketch at page top is not quite right. It shows the soffit opening along the house wall where wind-blown rain may enter the soffit and wall. A better location for this opening would be at the outer edge of the soffit, just behind the fascia!

Attic knee wall insulationOutlet Venting needs to be along the length of the ridge. On some complex roof designs and on pyramid roofs there is no ridge.

The sketch at left is an example of the insulation placement and air flow pathway for homes that combine an attic knee wall space with a partial cathedral ceiling space. We recommend closing the gable-end vent on older homes that have one, installing a continuous ridge vent to assure air flow between every rafter pair.

On some hip roof designs the length of ridge is very short compared with the total roof length. In these cases there may be no option but to add multiple individual exit vent openings across the roof field. In this case they should be placed near the ridge so that the upper attic will be vented.

Putting them at mid roof or lower is ineffective. Usually such vents are placed on just one slope of the roof so that they are not visible from the front of the building. This is ok, provided the vents are near the ridge. Otherwise the un-vented roof slope simply won't be treated.

On lower single-plane shed roofs that abut an upper story building wall, venting can be provided by installing a half-ridge at the upper roof slope. Spot vents and gable end vents are a distant second choice for these designs for the reasons I outlined above.

For cathedral ceilings that have minimal vent space see our suggestions at CATHEDRAL CEILING INSULATION

For roofs that have no soffit or eaves overhang to provide an intake opening, see ROOF VENTILATION INTAKE if NO OVERHANG.

For roofs that cannot be vented, roofs whose structure, shape, or choice of insulation (such as sprayed foam insulation under the roof deck, or the "hot roof" design, see Roof Venting: Un-Vented Hot Roof Solutions.

For irregular or hard-to-vent roof shapes, condensation (winter) or summer heat build-up can be relieved by a thermostatically operated attic fan - with a cutout switch to turn off the fan in case of a building fire. (Otherwise the fan spreads and speeds the fire.) But such fans do not run in cold winter weather, so you're only solving the heat problem not the winter condensation and ice dam problem.

Optimum roof ventilation design: provides continuous ridge venting, continuous soffit venting, and then, close off those old gable-end vents to force the intake air to come where you want it to flow. Otherwise the exiting air at the ridge will usually pull its makeup air from the closer gable end vents and not from the soffit or eaves vents.


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