The Ins and Outs of Roof Exhaust Vents

Winter rains have arrived and the summer’s warm, dry air has been soundly replaced by cold moisture. This can prevent a problem in your home if your roof isn’t properly ventilated. When you turn on the heat in your house in the winter that now warmed, moist air accumulates at the highest point in your home, and without an escape route through a roof exhaust vent or attic exhaust fan, that trapped air encourages mold and mildew growth and creates a stale odor. 



The Importance of Attic Ventilation

If you’ve ever spent any time in your attic—even just to poke your head in to take a quick look—you have no doubt noticed the difference in temperature from the rest of your home. Depending on the time of year, your attic space will be either warmer or cooler, which is just as it should be. The air in your attic should also feel dry, and with properly balanced attic ventilation, it will be.



Don’t Get Stuck in an Attic Vent Trap | Pacific West Roofing

attic insulation and ventilationAttics are both very useful and inconvenient at the same time. If you wonder how this could be, consider the pros and cons. Attics can provide indispensable storage space, but the things that can be stored in an attic are often limited to boxes and other small items that are light enough to be hoisted overhead. Another negative– yet entirely avoidable– characteristic of attics is extreme heat in the summer and trapped moisture in the winter. Proper ventilation can alleviate both of these environmental issues, but you need to make sure that your vents are installed properly and working together seamlessly for the best results.


Attic vents can prevent a handful of costly home repairs, but not if they are used improperly. It is extremely important to have these vents installed correctly, otherwise they can do more harm than good. There are several common mistakes people make when dealing with vents, but you can avoid them by keeping clear of these four problems:

  1. Unbalanced intake and exhaust installation – There are two types of vents that keep your attic cool and moisture free, they are intake and exhaust. By building a system that is overbalanced with one type of vent or the other, the ventilation system will not function properly.
  2. Concentrating only on summer heat – If you’ve climbed up into your unventilated attic during the summer, odds are high that you began sweating almost instantly. However, temperatures soaring well over 120 degrees are not the only attic issue you should worry about. The winter months can cause moisture buildup which can be damaging. Attic vents are designed to work all year long and should not be neglected during cooler times.
  3. Mixing and matching ventilation strategies – Attic vents are successful because of a delicate balance that keeps air flowing in and out, while also protecting the attic from outside weather. There are a few different combinations of vents you can use to draw air in and let it escape, but they are not interchangeable. For example, installing two or more different kinds of exhaust vents can sometimes cause the system to fail and allow wind and rain to enter into the attic.
  4. Not sealing off attic bypasses – In order for an attic ventilation system to work, the attic must be a contained air system, which means that openings like leaks or cracks that are not intended let in outside air should be sealed or repaired.


To guarantee that your attic is properly ventilated, call a professional roofer to have your vent system repaired or installed correctly. With the help of an experienced contractor, your attic will become much more efficient at dispelling hot or moist air, effectively lowering your energy bill and preventing water vapor from damaging your home.

Contact Pacific West Roofing about attic ventilation today. Call 503-635-8706 or click here to request an estimate!



Attic Ventilation: Critical to a healthy home | Pacific West Roofing

Houses built back in the day, could breathe both through the window frames, light fixtures, switch plates and not-so-tight siding with building paper that was very porous. Today is a totally different story. The building “wrap” installed under siding today is very tight and permeability is almost non-existent. Moisture that used to exit through the building now travels through the inside of the building, rising with heat and winding up in the attic.If not vented properly, this moist air in the attic will condensate at the lowest points of the roof deck during the colder months of the year.

Now here comes the true purpose for a balanced ventilation system in your attic. Once this moisture rises into the open attic, there needs to be adequate exhaust ventilation at the ridge as well as adequate intake ventilation at the soffit or bird blocks. Bird blocks are the blocks between the rafters where they rest on the plate of the exterior wall of the building. These bird blocks are usually 2×4’s of wood which space the rafters 2 feet apart. These blocks are either blank or have holes with screen covering the holes to keep birds and bugs out but allow air to flow in as heat rises out of the exhaust vents or ridge vent.  The formula is 18 inches of net free air (NFA) per running foot of ridge to exit at the ridge vent, or can vents, and 9 inches of NFA of intake air per side of the house times 2 sides equals 18 inches of intake air to match the 18 inches at the ridge exhaust. This is a balanced system.

The vented wood bird blocks are grossly undersized for intake and most have maybe 10 inches of NFA in the 2 foot block it sits in making it 8 inches short of the needed 18 inches of NFA needed in the 2 foot span it uses. There is a cure for the undersized bird blocks and that is to replace the bird block with a full screen replacement vent which is a corner-to-corner screen giving the vent a 48 inch NFA for that 2 foot span of a 2×4 bird block. That translates to 24 inches per running foot well over the 9 inches per foot needed for adequate intake vent. Now….how often should you space these intake vents? I say no more than two blanks between vents or you will end up with dead air pockets and then the condensation begins again. I recommend every other rafter bay to make the air move some of the air in the “dead” or blank rafter bay. Yes, it can be considered overkill and I love to be over than short of intake, which is exactly what Air Vent Inc. suggests we are to do. Visit for information on what I have discussed here.

Now that we have addressed the exhausting of heat and moisture in the attic, we need to look for moisture contributors in the same space. Bathroom exhaust fans are usually the culprit for shoving moisture into poorly vented attics causing the mold that eventually decays the roof deck to the point of needing total replacement. These vents are easy to fix by installing a baffled 4 inch vent with a goose neck or nipple sticking down off the bottom of the vent to slide the typical 4 inch ducting from the fan directly over the nipple then taping them together well with HVAC foil tape to stop air leaks…air now exits the building and not into the attic. This is step 1 in stopping the attic moisture source.  Step 2 is the kitchen vent where you will repeat step 1, taking the steam from your stove and putting it directly outside, not into the attic. With this solution, the only vaporized moisture that will get into the attic will be the moisture produced from inside the house from normal heating and living, which will easily be evacuated by the intake and exhaust vents we prescribed earlier in the article.

A happy attic is a moisture free, dry attic!

Portland Roofing Contractor Since 1980

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