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Attic Ventilation 101

attic ventilation 101

A vented attic is one of the most efficient ways to keep a house cool in the summer and free of high humidity and condensation in the winter. Although builders and homeowners didn’t always recognize this fact, great attic ventilation is a must in any residence. It extends the life of your roof and helps maintain more consistent indoor temperatures in both warmer and colder months.

Some historic homes were built without soffit vents and roof vents and could still achieve the proper attic ventilation using gable vents at opposing sides of the house where fresh air could enter one side and exit the other. Even on a calm day the flow of cool air in through one gable out through the other side can occur at a regular rate.

The key part to proper attic ventilation is that there is adequate surface area for fresh air to be taken in and hot humid air to escape. This can be achieved in several ways and in this post I’ll talk about many of them. I’ll also help you to check your own home to see if you have adequate ventilation.

You may discover as I did on my “new” old house that I was woefully undersized on one or more pieces in your attic ventilation puzzle. Read on and see how your house stacks up.

Soffit Vents

The first place to start is the soffit vent. Soffit vents are installed on the underside of your home’s eaves and allow fresh outdoor air to circulate into the attic. This operation may sound simple in premise, but attic ventilation is in fact crucial. It helps cool your home in the summer – and cut air conditioning costs – and in the winter reduces moisture buildup that leads to the “ugly three” mold, mildew, and wood rot.

The word soffit derives from the Latin word suffixus, meaning fixed underneath. This aptly describes where a soffit is located in your home. You might think of it as the underbelly of your roof; it’s that material between the roof’s eaves where gutters and the fascia are placed to the wall. Only roofs with overhangs have soffits, and the vents are installed directly into the soffit material.

soffit vent
Soffit vents on a modern home

Soffit vents are not made to be flashy. They are designed to blend in and be unnoticeable and come in a variety of shapes, sizes and materials. Below are just a few types you may commonly find.

  • Continuous Vents – These are typically found in homes built after the 1980s and are still used extensively today. Typically a perforated aluminum or vinyl sheet that is cut to size and installed like a drop ceiling throughout the soffit area of the whole house. If you see this style soffit vent retrofit onto a historic home it may not actually be a soffit vent but simply a cosmetic covering over a sealed eave so you may have to do a little more digging.
  • Circular Vents – Available in both plastic and aluminum the vents can be installed in frieze boards between rafter tails when there isn’t a dropped soffit installed. These are useful for Bungalow style homes with exposed rafter tails in the eaves.
  • Rectangular Vents – Lots of variety and sizes can be found here to fit almost any application. These can be retrofit very easily in older homes with a dropped wood soffit. Often these are installed in the place of a single 1×4 board or a piece of beadboard soffit up to 8 feet long.

Roof Vents

The soffit vents are the intake piece of the attic ventilation puzzles and roof vents are the exhaust piece. With soffit vents you give your house a way to bring cooler air into the attic space, but in order for The Stack Effect to really work you need somewhere for the hot or humid attic air to escape so it can replaced by the cooler or drier soffit air and that is the job of the roof vent.

Roof vents are typically installed in one of two designs.

  • Ridge Vent – Installed at the peak of a gable roof the ridge vent allows hot air to escape evenly across the entire peak of the roof and is virtually invisible.
  • Off-Ridge Vent – On some styles of house, like a hip roof shown below, there may not be enough or even any space at the peak of the roof to allow proper ventilation. In that case an off-ridge vent can be installed to attain the proper amount of exhaust ventilation.

How Many Vents Should My Attic Have?

The 2018 edition of the International Residential Code (IRC) specifies 1 square foot of venting for every 150 square feet of attic space but makes the exception that it can be 300 square feet in certain circumstances and certain areas. To be safe I would stay within the 1:150 range to ensure you have the proper ventilation.

The code also specifies, for good reason, that the proportion of soffit vents to exhaust vents be balanced at 50:50. Your intake area (soffit vents) should always be equal to your exhaust area (roof vents). If not balanced then air may be pulled from the interior of your home through openings in unsealed ceilings like recessed lights, electrical boxes, attic access doors, and anything else that isn’t completely sealed on your ceiling.

This can result in roof vents pulling your air conditioning or heat right out through the roof causing higher heating and cooling bills than even an unvented attic would create.

Calculating Attic Vent Size

I know, I know, you don’t want to do math right now, but I’ll try to make this as simple as I possibly can. That’s just the way we do things here at The Craftsman Blog!

First you need to find the square footage of your attic (not the cubic footage!). This is a fairly simple calculation. Let’s assume a simple square house that is 30’x50′. Here’s how the math works:

Step #1: 30 x 50 = 1500 SF

Now you need a 1:150 SF ratio for attic ventilation in total so you divide your attic square footage by 150.

Step #2: 1500 / 150 = 10 SF

That tells you that you will need a total of 10 SF of attic ventilation. Since the building code recommends splitting that 50/50 between your soffit vents (the intake) and roof vents (the exhaust) you need 5 SF of roof vents and 5 SF of soffit vents spaced evenly around the perimeter of your house. Pencils down!

Where to Position Soffit and Attic Vents

You now know how much ventilation you need, but you don’t know where to put those vents. There are a few simple rules to follow that I’ll outline below.

Roof Vent Position

Placing your exhaust vents as close to the top of the roof is preferred. This usually means a ridge vent which works great for a gable roof. It sits at the very top of the roof and allows maximum and even exhaust of the attic air since it is at the peak.

Depending on the shape and design of your roof this may not be possible. Some hip roofs come to a point and don’t allow for a ridge vent so in situations like this or more complex roof shapes an off-ridge vent or multiple off-ridge vents should be installed. The code requires that these vents be within three feet of the highest point of the roof to be effective. Hot air rises so the higher the vent is placed the more hot air it can vent.

Soffit Vent Position

Soffit vents should be spread out evenly along the soffit and should not be grouped in one area or another. In a simple gable roof scenario that will mean that you need them placed along the eaves, but they are not necessary on the gables end eaves since those areas are not open the attic.

On a hip roof the soffit vents can be spread evenly across all 4 sides of the building.

Are Powered Attic Fans Worth It?

When the powered attic fans on my house all died this summer I figured it was time for me to get them replaced. After getting several quotes and doing more research than I ever did for any of my college reports I realized that replacing my powered attic fans would be a huge mistake.

Study after study has shown that powered attic fans do not save on energy costs. Passive ventilation is by far the best way to ventilate an attic. The main issues with powered attic fans are below.

  • Use Too Much Power – The savings you gain in less strain on your AC are usually eaten up by the increased power consumption of a wired attic fan. Solar models don’t have this problem, but they can cost four to five times the cost of a wired unit.
  • Pressurizes Attic – We just did all that math concerning vent size and with a powered attic fan that can seriously change the equations. Often when a powered attic fan is installed it will pull so much air that you end up sucking the air from the conditioned rooms and driving up utility costs higher than they would have been without the fan.

Joe Lstiburek of Building Science Corp. puts it this way:

“In order for the fan to work the air needs to come from the outside and not be pulled from the house so this means that the attic ceiling needs to be airtight. If the attic ceiling is airtight you don’t need the fan. Your money is better spent on something else.”

Matt Rising on the build show has a great video on attic ventilation and attic fans you should check out below.

Attic Ventilation For Older Homes

Early American homes featured roofs made of metal, tile, wood shake, or slate. The chosen material was directly attached to wood lath, meaning the roof was porous enough to allow air movement that provided some natural ventilation which made stand alone vents unnecessary.

It wasn’t until the late 1800s that bituminous roofing became more common. Continuous roof sheathing likewise became more standard, allowing roofers to use felt to improve the home’s moisture barrier. But without proper airflow, these homes were susceptible to:

  • Ice damming in cooler temperatures
  • Heat damage to shingles
  • Moisture in the attics

Homes Built Before and During the 1950s

In other words, tighter roof assemblies gave way to the need for better ventilation. Building standards for natural ventilation, however, didn’t appear in writing until the 1940s. Still, homes built before and during the 1950s contained little weatherstripping and plentiful air leaks. So much air freely flowed into these homes that indoor humidity issues were rare.

Roof vents were similarly uncommon, and if they were incorporated into the home’s design, they were few in number. In other words, they failed to work effectively. If you own an older home, take a walk outside and look under the eves. Regularly placed vents mean you’re in good shape. Individual vents closely resemble furnace registers, while continuous venting appears as a long, narrow aluminum strip that runs the length of the soffit.

gable vent
A traditional gable vent with unvented soffits

A Little Caution When Installing Vents

If you don’t see the vents we just described, or only a few scattered about, you may need to install some. Many older homes have since undergone major renovations that include caulked air leaks and new, modern weatherstripping. You may have a house that was built for a wood shingle roof, but has been converted to asphalt shingles. All these things serve to change the level of air change within the attic and your house as a whole.

This means the air inside is likely to be much more humid than it was 100 years ago, and your attic and roof could be at risk for damage caused by ice damming, mold, or excessive heat which will dramatically shorten the lifespan of shingles.

When embarking on your installation project, keep vents evenly spaced and place them on both sides of the roof. Also keep them free of obstructions, including attic insulation; this should never be closer than three inches to the vent. Simply pull back any insulation if it’s made of fiberglass batts or, if you have blown-in insulation, rake it away from the vents with a long garden rake or hoe.

If you’re not sure what type of insulation you have in an old house do be careful and have it tested for asbestos. Moving or otherwise disturbing this type of insulation can release asbestos fibers that may enter your lungs and cause serious health issues down the line. Caution is best.

Ensuring your attic ventilation is adequate and properly placed will go a long way to reducing your energy bills and extend the life of your roof covering. It’s not always a need for more insulation or a higher SEER HVAC unit. With something as simple as improving your attic ventilation you could lower the temperature in your attic as much as 30-40 degrees for a minimum investment.

The right attic ventilation will not only keep those utility bills down but add a lot of extra comfort in your house, and while we all want to save money, it really comes down to being comfortable in our homes.

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6 thoughts on “Attic Ventilation 101

  1. I’m confused. If I’m reading this right you’re saying lowered attic fans are not worth the expense. But Scott did a video on fixing his in his house when his house was having a hard time cooling and once fixed solved the issue. This made me realize I should fix my roof fan. So are they worth it or not?

  2. Soffit vents are not allowed for new construction in areas of high fire danger in California, particularly those areas labeled ‘urban wildlife interface’ (which means there are lots of trees around) as they pull air into the attic during a fire and feed the combustion. Instead, you will see a verticle strip of roof vent about half way up the shingles. I don’t’ know if other states require this or not.

  3. Hi Scott,
    thanks for the great work you do with old houses. We are restoring an 1879 folk/italianate? victorian in central Massachusetts. It’s in a “T” shape. Our attic has two gable vents at either end of the top of the “T” – which is a gable roof. The vertical part of the “T” is basically a mansard roof. We can’t do soffits because the 2nd floor ceiling/walls is the underside of the roof and is currently filled (sorta) with blown in insulation. We don’t have AC. The house originally had a metal roof and no insulation. The previous owners put black asphalt shingles on – and did the bad insulation. I’ve been wondering if the metal roof helped keep the attic cool in the summer and this was how they dealt with heat. Any thoughts are appreciated.

  4. Hi
    My home was not built with gable vents. We do have soffit and ridge vents but my attic temp is above 110 degrees.
    My wife does not want turbines or any vents that would cause holes in the roof.
    What do I do?

  5. if I have A traditional gable vent with unvented soffits and in my case a powered attic fan at the other end of the attic in front of the other gable vent what should I do with it to make it better?

    1. This is my solution as well. I’d be interested in what Scott says. I think the answer is that you won’t “save” on electricity usage, but properly size fan may give you adequate/appropriate ventilation without having to reconstruct any vents, if the existing vents are undersized.

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