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How To Know If Your Attic Is Venting Properly

Bad attic ventilation can destroy a roof prematurely.  It used to be believed that the purpose of a roof is to keep water from penetrating from the outside, but the industry has learned that water can come from the inside too by way of condensation.

On pitched roofs with composition shingles, condensation typically happens on the plywood that the shingles are nailed to.  With enough condensation, the plywood can rot out, leading to malfunctioning shingles.  This is avoided by proper attic ventilation.

In this article, we’ll discuss how to check if your attic is ventilating correctly, and what to do about it if it’s not.

Method 1: check attic and outdoor temperature


Condensation can occur when surfaces are colder than the ambient air.  So the goal is to keep the roof plywood and ambient air in your attic the same temperature, because then there won’t be any rot-causing condensation.

Take a thermometer and compare the air outside your house near the intake vents to the air inside your attic.  If there is a sizeable difference, then the attic needs more ventilation and is at risk of having the roof plywood rotting out.

Not every house is the same, and there are multiple factors that go into rotting plywood, so the temperatures don’t need to be exactly the same.  But the closer the better.

Method 2: the smoke candle test


If you’re unsure about the temperature difference between outside and inside the attic, you can do the smoke test for air travel through the attic.  Incense or sometimes a match can work.  Grab a ladder and blow smoke into a soffit vent under the low edge of your roof.  Then stand back and watch to see if any smoke comes out the exhaust vents at the top of the roof.

If the smoke escapes at the top vents, you’re good to go.  If it doesn’t or if only a small amount does, it means that air in the attic isn’t venting properly.

It’s best to conduct both of these tests because different parts of the attic can have different temperatures and airflow.

What to do if the attic isn’t venting well


If the two tests above fail, your roof is at risk of premature failure due to plywood rot.  It’s better to address attic ventilation when installing a new roof, but if you don’t yet need a new roof, ventilation can often still be fixed for the existing roof.  And you will want to fix the ventilation in order to get the full life out of the existing roof.

Check gable end vents


Go look at the side of your house where the two roof decks come together at a peak.  If there is a vent in the siding going into the attic, it’s a gable vent.  These should be blocked off with wood from the inside.  The attic should vent from the bottom outside edge to the top peak, which will get all the air flowing.  But gable vents can act as intake or exhaust and disrupt air flow, leaving dead spots of air in the attic.

Check internal attic vents from inside house to roof


Oftentimes vents that intake from bathrooms or kitchens end up not being properly attached in the attic.  They’re meant to pipe out into exhaust vents on the outside of the roof, but if they’re not connected in the attic correctly, they leak conditioned air from inside the house, changing the attic temperature and bringing moisture.

These vents need to be reattached so that no air escapes through them.

Check for attic insulation blocking intake vents


Insulation blocking air flow is a common cause of poor ventilation.  Homeowners and contractors alike sometimes push insulation around when working in the attic.  If the intakes aren’t baffled properly, this can lead to stuffing up the vents with insulation, and will almost certainly cause condensation issues.

If insulation is blocking the vents, it needs to be pulled back and baffles should be installed or seated properly so that intake air flows along the underside of the plywood up into the attic.

Call a roofing contractor


Unless you’re DIY, the best course of action is to have a roofing contractor inspect and provide an estimate for work that needs to be done.  We can fix any of the above problems, as well as have other tools to get attics venting smoothly.

If you think you may have attic ventilation issues and live in the South Puget Sound of Washington State, we would love to give you a free estimate for fixing them.  Please call us at 253.445.8950 or fill out our quick estimate form.

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