Attic Vents Work…Right?!?

by Erik North on July 20, 2011

Attic Vents Work



Attic vents (and basement, crawlspace and kneewall vents) are often treated as moisture cure-alls in residential building.

Too much moisture? Ventilate.

Not enough moisture? Ventilate.

Having car problems? Ventilate.

Do properly installed attic vents work? Yes…but notice the caveat…’properly installed’.


Moisture? Attic Vents!

The concept goes that poking enough holes in your house allows it to breathe and dry out. This sounds REALLY reasonable and straightforward. You mean all I have to do is drill holes in my soffit and Skilsaw 4 inches off the attic peak? And the roof will last longer and I won’t have moisture problems? In the Paleolithic era before electric and gas dryers, people hung their clothes out to dry. Why wouldn’t you air your house out to dry?

Because building science is as straightforward as the tax code, that’s why.

The facts of climate, building science and the evolving nature of building materials tell us that it just ain’t so.

Attic vents are a common approach where I live, in New England. While the weather can be a bit, *ahem*, erratic (Mark Twain reportedly once said, “If you don’t like the weather in New England, wait a minute”), the winters are cold and generally dry. If you want to dry out your house, attic vents that move cold dry air through the roof couldn’t hurt and may even work. It also cools the roof deck, slowing the formation of ice dams. In hot climates, attic vents move hot air out of the house, easing the air conditioner’s cooling load.

(A quick aside on what a vented roof system is: Attic vents are where perforated soffits allow air to flow up into the attic space. Channel vents (or attic vents or proper vents or attic baffles or whatever you call them) direct air flow up the underside of the roof deck toward the peak. At the peak, air exits from either a ridge vent or gable end vents).

What if you lived in Louisiana or Georgia? What if you were attempting to control house moisture while sweltering in the exact opposite climate? We can safely call Louisiana a warm, wet climate. A well-meaning contractor would perform their residential tracheotomy, letting in the swampy Southern air. A vented attic can still control temperature in this climate, IF installed right. But the odds are against the interior getting drier.

Just Tack The Attic Vents On, Right?

Which gets us to installation…improper installation (or misunderstanding the dynamics) often undermine the intended venting. Here is a cross section of a cape knee wall:

Cape Knee Wall Attic Venting

Oh! That

Theoretically, the air should flow into the soffits, along the roof deck, up and out the venting in the peak. In theory. In reality, air goes wherever. What if (like I saw in a recent audit), the homeowner has installed an oven vent that would not be out of place on an aircraft carrier (it moved nearly 1000 CFM)? This created a negative pressure in the house, sucking the cold soffit air into the house. The air wasn’t moving up the roof deck and certainly not out the peak.

Unless the attic vents are installed correctly, sealed against the soffits, run sufficiently far over the insulation, spaced far enough from the insulation, the attic floor is COMPLETELY sealed (we’ll get to this in a bit) and provide sufficient vent volume…DEEP BREATH…it won’t work right.

(A technical aside…building code calls for 1 sq ft of net free vent area (NFVA) per 300 sq ft of attic space. Code calls for this to be balanced but Joe Lstiburek of Building Science recommends more eave venting than ridge vents. This creates a slight positive air pressure in the attic space, preventing conditioned air from leaking out. Check out his recent article on attic venting here).

But My Grandparents House Never Vented!

The changes in construction materials over the last 60 years created another problem. Yes, they ‘don’t make ‘em like they used to’ and in part because we chopped down all the first growth lumber. Older homes used whole timbers and planed boards, were uninsulated, leaked like an open barn (getting literal, some were actual old barns) and (in the post-wood stoves era) were heated with massively oversized coal and later oil burning heating systems. The construction was FAR more tolerant of moisture combined with massive energy transfer continually baking moisture out.

Mold and rot weren’t issues…heating the house WAS but at $0.25 a gallon (or 4 farthings per bushel of coal), it was an affordable therefore ignorable one.

First, planed board roof decking are both more permeable to air and much more resistant to mold growth than plywood or oriented strand board (OSB). The explosive growth of OSB use as an exterior shell (growing 30x from 1980 to 2005) has entirely supplanted planed boards and largely replaced plywood. The good news for homeowners is that its use has reduced residential construction costs. The bad news (for our conversation) is that continuous sheets of OSB/plywood create, relative to older construction techniques, an airtight building shell.

During audits, we blower door test for air leakage. Air flow and leakage are measured in Air Changes per Hour – Natural (ACHn). This is simply the number of times under natural conditions that the air volume in your house changes per hour. Lower is better…it means less heat is leaking from your house. Older homes may test from 1.0 to 1.5 ACH where newer houses may range from 0.40 to 0.75 ACH (but can get much lower). Older homes can be 2 to 3 times leakier than a new one.

Second, insulation wasn’t exactly a priority or really used in pre-1950s homes. The insulation options boiled down to straw bales, newspaper, REALLY thick plaster, double wythe plaster with a dead air space or wishful thinking. Modern homes are much more well insulated (uh, in theory) and more tightly built.

Heating systems have also become more refined along with homes. Everyone has seen pictures like this:

Giant asbestos wrapped boiler

Yes, that is asbestos

Ahh, the 50s! It was a big time for big cars, big houses and water boilers large enough to merit their own congressman.

Yes, this was once a coal boiler (now converted to natural gas) and, yes, that boiler wrap is asbestos.

These monstrosities (which can only be removed by blowtorching them into manageable chunks) put out monstrous amounts of heat. Or an entirely appropriate amount if your house is uninsulated. Modern homes require fewer BTUs to achieve the same comfort but less heat means less drying. Water vapor that was once sunblasted out now lingers much longer.

Bringing It All Back Home

How does this tie together?

In older buildings, no insulation meant enormous amounts of heat flowed into the attic space. The gigantic boiler is chewing through hundreds of gallons of oil, keeping this sieve of a house warm. The warm air rises, carrying with it a heavy moisture load (the warmer the air, the more water vapor it can hold). This rising heat keeps the attic space relatively warm, ensuring that solid surfaces stay over the dew point (the temperature at which moisture condenses on a solid surface – think a glass of ice water).

When properly insulated the interior heat is cut off, chilling the attic space. The attic is colder and often falls below the dew point. Unless you’ve hire a bang up contractor, odds are good the attic floor is not airsealed well. This well insulated modern house is much tighter than an older home, so the air lingers 2 to 3 times longer and the modern boiler’s lower output does much less to dry out the house.

Moisture loaded warm air sloughs into the attic, dumping its water vapor onto the first cold hard surface. Usually your roof. After time (sometimes not much time), the roof system decays and possibly fails. So you call a roofer…who recommends more attic ventilation. Unless the attic ceiling is completely airsealed, poking a hole into your attic only causes the warm moist air to updraft faster. And more air movement means more moisture and probably more damage.

If done correctly, a vented roof assembly is one of the most effective approaches in building science. But attic vents can fail and are not a cure all.

Further Reading:
I linked this earlier but a great (recently published) primer on attic venting by Joe Lstiburek of Building Science, Inc. can be found here: A Crash Course In Roof Venting                             attic vents attic vents attic vents attic vents

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