Part I: Increase Attic Exhaust Ventilation to Drastically Cut Cooling Costs

Series: Attic VentilationPermalink

Reduce cooling costs by increasing attic intake and exhaust ventilation.

Is the top floor of your house a little too hot during the summer? Ours was, due to an acute lack of attic ventilation. This caused a significant heat buildup during the day that our air conditioner just couldn’t keep up with.

The first step to improving our attic ventilation was to install a ridge vent on our roof. This project was relatively straightforward, inexpensive, and I completed it in a day. I only had to return to the big box store once, too.

The results of the increased ventilation were immediately felt by human and cat alike in a cooler upstairs. We were also surprised by a 25% decrease in that month’s electric bill compared to the previous year, a savings that has continued over subsequent months of A/C usage.

Poor Little Unvented Hip Roof Permalink

Our roof had only one powered exhaust vent for ventilation, at least until I converted it into a passive vent by disconnecting the power and disassembling the fan. I did this because powered attic ventilators are a waste of money. The only reason they’re able to cool an attic is by pulling conditioned air from the floor below. Our ventilator will remain passive until we remove the dome, cover the hole with new decking, and replace the roof.

How Much Exhaust Ventilation Do I Need? Permalink

The ideal amount of attic ventilation is one square foot of vent space for every 150 square feet of attic area. This value is split evenly between exhaust and intake ventilation. In this post we cover the exhaust ventilation.

Attic length x Attic width ÷ 150 = total vent space or Net Free Area (NFA)

My attic has an area of 1,288 square feet, so my NFA is ~8.5 square feet. That is split into 4.25 square feet (or ~618 square inches) for both intake and exhaust. Ideally your exhaust ventilation equals your intake, but it’s likely your home is not balanced. Err on the side of having more intake than exhaust.

With a vapor barrier between the attic and conditioned space below you can halve the recommended Net Free Area. This results in using one square foot of vent space for every 300 square feet of attic area. Although my ceiling is air sealed, it’s not actual vapor barrier.

For a more thorough explanation, see How to Calculate Attic Vent Area Needed When Adding Soffit Vents.

Types of Roof Vents Permalink

There are many different types and styles of roof vents. I won’t bother to describe them here, but I will explain why I ultimately decided upon installing a ridge vent versus the other options available to me.

When considering exhaust vents carefully read the manufacturer’s specifications to ascertain the estimated Net Free Area per vent. Be aware that these metrics are often provided in square inches, seemingly to make your calculations just a little more difficult.

Box Vents (aka Low Profile, Louver, Flat, and Turtle) Permalink

Box vents were my second choice, but I decided against using them once I realized that I’d need about twelve to fourteen to have enough exhaust ventilation. It’s unlikely that half that number would fit near the ridge of my roof. I also wasn’t keen on cutting that many holes into my roof decking given the increased opportunity for me to mess something up during the installation.

Turbine Vents (aka Rotary and Whirlybirds) Permalink

Similar to the box vents, I would need six to eight turbine vents to have the recommended amount of exhaust ventilation. Again that’s too many points of failure during installation. I also read some horror stories about ball bearings failing, which prevents the turbines from spinning. That said, they do have their advocates.

Power Vents (aka Powered Attic Ventilators) Permalink

I did not consider keeping my existing power vent or supplementing it with another power vent. I shouldn’t need electricity to reduce the heat in my attic in the summer. Conversely I shouldn’t need electricity to reduce moisture in my attic in the winter, especially now that the plane between the attic and our living quarters is air sealed.

Ridge Vents Permalink

A ridge vent appealed to me because the required penetrations to the roof decking seemed easiest. Two long cuts with the circular saw along the ridge and no worries about the rafters, save for the imperative of not cutting into them.

It’s also the most efficient choice for my roof. My ridge is 16 feet long and with 18 square inches per linear foot of Net Free Area I get 288 square inches of NFA. That is a less than half of what’s recommended, but it’s the best that I can do short of peppering my roof with box vents (or installing hip vents, which I may have done when I pay someone to replace the roof).

The last benefit to ridge vents is that they are inconspicuous unless you’ve got venting on the mind. I don’t think most people notice a ridge vent, versus a turbine or power vent. It’s telling that those styles of vents are rarely seen on the street-facing side of the roof.

Not everything is rosy with ridge vents, although most of the downsides and risks are common to the other vent types. The foremost issue is moisture intrusion by rain or snow. To prevent this you need to do a good job installing the vent, per the instructions of the manufacturer. As a DIY-er this is risky since much of what we do is learned while actually doing it.

Soffit Vents Permalink

A soffit vent is actually an intake vent, but it merits mention because all the exhaust vent styles need an equal or greater volume of intake air to work as advertised. I have more to say about this in the next post in this series, Increase Attic Intake Ventilation to Drastically Cut Cooling Costs.

Ridge Vent, I Pick You! Permalink

There are lots of different types of ridge vents, but I just used the brand that was in-stock at my local big box store. My architectural roofing shingles are twelve years old, so the best case scenario is that we replace the roof eighteen years from now. Hopefully this ridge vent will last until that time.

Remove the Ridge Cap Shingles Permalink

The first step to installing the ridge vent was to remove the existing cap shingles. This is most easily done with a small pry bar. Start on the last installed shingle, which should betray itself by its visible roofing nail heads.

Foam Kneeling Pad, Would Recommend Permalink

I was hesitant to complete this project due to the risk of doing something horribly wrong and then having a very long slot in my roof. Who knows how long it would take to get a professional on the roof to fix my mistakes. The last thing I wanted was an unexpected downpour to fill my attic with water during the interim.

I procrastinated all spring and finally worked up the courage to install the ridge vent during the middle of the summer. It was very warm and I didn’t wake up particularly early to start the job. The shingles were hot to the touch and without my foam kneeling pad I would have surely burned my knees. Not even all my sweat would have prevented that and I got real sweaty.

The State of the Ridge Permalink

Once I pried all the shingles off the ridge I noticed that in some parts I could see into the attic below. Was there really that large a gap between the roof decking and the ridge beam? I investigated by entering the attic from the hatch below.

It seems that my roof once had a ridge vent. The last roofer must have removed the vent and replaced the decking on one side, but not the other, hence the gap.

Remove the Roof Decking Permalink

Along the existing gap I used a utility knife to cut away the shingles so they were flush with the roof deck. That as the straightforward side of the ridge vent slot.

For the other side I used my circular saw to cut through the shingles and decking. I was very careful to make sure the depth of the cut was no deeper than the thickness of the roof decking. Thankfully I could use the existing gap to gauge the correct depth. You’ll want to read the instructions that come with your ridge vent to determine the correct width of the slot.

Secure Metal Wire Mesh Along the Length of the Vent Permalink

Some folks on the internet recommend installing wire mesh under the ridge vent to discourage vermin from entering the attic. I had some lying around so I cut it to fit as best I could. I am unsure how much it will benefit the installation, but I figured it couldn’t hurt. I used a hand stapler to secure it to the top of the ridge beam.

Some Tools I Used to Install the Ridge Vent Permalink

The only thing to note about the tools and supplies I used is that 3” ring shank nails should have come packaged with the ridge vents. Unfortunately they did not, presumably due to the spartan packaging. My guess is that they fell out somewhere along the distribution chain.

I could not find 3” ring shank nails in the right quantity at any of the big box stores near me. I even went to three different stores: Menard’s, Lowe’s, and Home Depot. They were in-stock in very large quantities and collated for nail guns, but didn’t need 1,000. This was a weekend, so the roofing supply stores near me weren’t open.

Ring shank nails are ideal for roofing because they better resist removal compared to smooth shank nails. Unfortunately I had to settle for the smooth shank nails, which increases the likelihood that a windstorm could separate the ridge vent from my roof. I’ll update this post if that ever happens, which I hope is never.

Discourage Water Damage with Roofing Cement Permalink

With my tube of roofing cement I filled the nail holes left from the ridge cap shingle removal. I also covered the exposed nail heads that secure the now topmost row of shingles. Better to be dry than have water seep into the roof decking.

Install the Ridge Vents Permalink

Install the ridge vents one length at a time by pounding the 3” roofing nail into the designated holes to reach the underlying roof decking. The brand I used was designed to under/over lap the neighboring length of vent. Be sure to have at least six inches of overlap at the ends of the ridge.

Cover the Ridge Vent with Shingles Permalink

The previous owner of our house saved a whole stack of shingles, so I was able to cut them into appropriately sized cap shingles. The old shingles weren’t reusable, unfortunately. I used a pair of metal snips to cut through the shingles and then secured them with 3” nails and an excessive amount of roofing cement. It doesn’t always look so good, but who’s going to see it, save for you dear blog reader.

Performance Thus Far Permalink

It’s been nine months since the installation and the ridge vent hasn’t blown away and there’s been no moisture infiltration (so far as I know). With the increased attic intake ventilation this project has paid for itself in savings on our electric bill, which we’ll enjoy every year for so long as we live in the house. I’d definitely recommend that homeowners investigate their attic ventilation to evaluate whether it would be worthwhile to undertake such a project as this.

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