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!


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