Wonder exactly what are attic fans?
Attic fans can be a great way to ventilate your attic for a few reasons, but they also have some disadvantages.
In this guide, I will go over...
- What are attic fans and how do they work
- The number one reason for attic fans may be to protect asphalt roof shingles
- A couple reasons why standard “passive vents” don't always work
- And more...
Let's get started with this guide!
“Who Cares How Hot Your Attic Gets?”
This seems to be the recurring recommendation that I hear from many people. Here is a screenshot of one popular website that says it doesn’t matter if your attic gets too hot…
In my viewpoint as a home inspector, they miss a very important benefit. Heat stress — according to Berkeley Lawrence Labs — is the #1 reason for premature aging of asphalt roof shingles. For most homeowners, one of the biggest investments that we make is on the outside; the asphalt shingle roof covering. Right?
Asphalt shingle roofs are costly!
Architectural shingles (the most popular roofing shingle in the USA) are approximately $5 per square foot, so for the average home that is ten to twenty grand. I would do everything possible to protect that investment. Wouldn’t you?
I understand where the naysayers are coming from however — I used to discourage attic fans for a long time during my home inspections. I urged homeowners to install additional passive venting or to fix the venting problems that I discovered.
The best passive venting for homes is known as the “soffit and ridge” vent system. Soffit vents are the air vents under the roof overhangs, and the ridge vent is at the very top (ridge) of the roof. To see if you have a ridge vent installed, just take a look at the ridge or peak of your roof. If it looks like there is a raised portion about 2-3 inches, then you have ridge vent.
The Best “Passive” Vent System For Most Homes
The soffit vents and ridge vents work together as a single system to push out hot air from the attic, and to keep the air moving. Hot outside air rises up into the soffit venting, and it then moves up to the ridge vent where it exits from the attic. It creates a natural suction that feeds on itself, and this system “should” keep the attic from getting too hot — at least with most homes.
One obvious benefit of the passive vent system is that it doesn’t use electricity, and it requires very little maintenance.
Common Problems With Ridge & Soffit Venting
During my home inspections, I realized that quite a number of ridge vents are not working properly. Primarily, ridge vents were either obstructed by debris (organic matter such as leaves, dust, etc.) or the roofer failed to cut away the roofing felt. On rare occasions, the actual wood sheathing wasn’t even cut away!
And I can’t even tell you how many times I have seen soffit vents covered over with fiberglass insulation. To prevent insulation from covering the soffit vents, there are inserts called “baffles” which should be installed to prevent insulation obstructions.
Baffles are usually made out of cardboard or plastic.
Another common problem with soffit venting is that an unknowing painters covers the soffit vent with paint. Sometimes the vent holes are just partially blocked with paint, and other times it is completely covered.
The only way to fix this problem is to take a small pointed tool like an awl and painstakingly removing all of the paint. Using a metal brush may also work depending on the vent and type of paint.
If the paint is too difficult to remove, the soffit venting may have to be completely replaced.
Lack Of Continuous Passive Venting
Especially with older homes, I frequently come across soffit vents that are discontinuous. Rather than an unbroken vent under the whole roof overhang or eave, it is broken up in maybe 2-5 isolated vents. Obviously, this greatly reduces the amount of air flow into the attic, and consequently reduces air flow out of the ridge vent.
Unless you have a qualified home inspector or contractor point out all of these deficiencies in your passive venting, it is unlikely that the average homeowner will become aware of these issues.
When The Attic Is Still Extremely Hot With Passive Venting…
I realized something was wrong when I started measuring the attic temperatures with my infrared thermometer during home inspections — something weird was going on. I noticed that even homes with seemingly perfect ventilation (unobstructed AND continuous soffit/ridge venting)…the attic still got extremely hot!
I vividly remember these times because going into the attic felt like walking into a tropical rain forest. The heat and humidity was off the charts. And what is especially odd is that this happened even on “cool” spring days; not even during the heat of summer!
The excessive heat in the attic may due to an unusual layout of the attic, or some other unknown reason, but all I know is that my senses don’t lie. And I don’t mean just like 20 degrees warmer than the outside. I am talking about more in the range of 50-70 degrees warmer than the outdoor air.
I remember recently going into an attic on a cool spring day (picture below) when it was around 70 degrees outside, but the attic measured 127 degrees. On a sweltering summer day, these attics were going to be pushing over 160+.
A Prematurely Aged Asphalt Shingle Roof (There Goes $15,000)
Nowadays, 95% of all residential roofing is made of asphalt such as the lowly 3-tab shingle and the very popular “architectural shingle” — which has taken hold of almost all new homes.
I remember I did a home inspection in McLean, Virginia and the 3-tab asphalt shingles were badly faded — almost a white color — with some of the edges already curling.
To me, it looked like the roof was over 15 years old. And when the Realtor told me that the Seller gave a receipt of a roof replacement only 7 years ago, my jaw dropped. The buyer, Realtor, and me just kept looking at the roof — wondering how a seven year old roof can look so old.
Well, after going into the attic, it wasn’t difficult to put two and two together. The attic was like raging inferno of heat. In fact, it was difficult to inspect the attic because it was like working in a sauna. I told the buyer that in short, “your roof has probably aged twice it’s years due to the poor attic ventilation”.
How Exactly Do Asphalt Shingles Age?
According to Wikipedia.org, asphalt shingles are mainly made of hydrocarbons, which is basically a substance derived from crude oil. These oil-based hydrocarbons gives the asphalt shingles the water repelling or “hydrophobic” qualities but which is also sensitive to heat. And these oil-based molecules will turn to liquid at a high enough temperature.
What happens is that when the hot sun bakes the roof shingles, and when the temperature cycles between hot and cool — it softens the hydrocarbons. Hydrocarbons are an organic substance, and all organic materials are susceptible to ultraviolet radiation as explained by this paper from the Office of Scientific And Technical Information.
Unfortunately, these softened hydrocarbons get washed away from the shingles when it rains. As these “oil molecules” get washed to the ground, the shingles will begin to shrink. And as the shingles get smaller, it breaks the surface coating of the shingles, causing the underlying paper to disintegrate. The nail heads then get exposed to the weather, allowing water to seep through, wetting the wood sheathing and rafters.
What Are the Downsides To Powered Attic Fans?
One of the biggest reasons cited against attic fans is that they are not energy efficient. You have to understand that when people think of attic fans, they usually aren’t thinking of roof protection. And purely in terms of energy efficiency, it probably isn’t a smart investment. According to Home Energy Magazine, a research study was conducted on 8 homes over a period of three months. The researchers detected air leakage from the home’s interior flowing into the attic.
Obviously, if the attic is pulling in conditioned air into the attic, that isn’t exactly energy efficient.
The most important step to stop conditioned air from getting pulled into the attic is to have enough passive venting in the attic so that the attic fan will work properly and pull in outside air. A good amount of insulation may also help because it helps to retard the flow of air from the home’s interior and keep the hot attic air away from the home interior.
Natural-Gas Combustion Appliances
In addition to the questionable energy efficiency of powered attic fans, there is also the problem of combustion appliances. If a powered attic ventilator creates a negative air pressure in the home, it may cause gas appliances to “backdraft” which means the exhaust gases flow into the home (rather than being pushed to the outside). Of course, if you don’t have gas appliances, then this isn’t an issue.
And yet another reason why having enough intake/passive venting in the attic for the fan to work properly is so crucial. If the attic isn’t pulling enough air from the outside, it will pull in air from the interior.
Safety Tip: Always have a carbon monoxide alarm installed on each level when you have a home with natural gas or a garage (with or without attic fans)!
Should I Install An Attic Fan?
I think homeowners should seriously consider installing an attic fan if you have a very hot attic (in excess of 30F degrees above the outdoor temperature) and you have the conventional soffit/ridge vent system already installed. And to repeat — the biggest reason isn’t for energy efficiency — but it is to protect your asphalt roof shingles from premature aging.
Since asphalt shingles are an oil-derived product, it is susceptible to high temperatures (and the swings between hot & cold).
Even if your cooling bill actually increases somewhat, I would still want to keep my attic cool — whatever it takes. With solar powered attic fans, the only cost you have is for the installation and the fan itself.
One or two of these fans just may do the trick.
However, there are many anecdotes of homeowners lowering their A/C bill by installing a standard electrically-powered attic fan.
But seriously, if you have the continuous soffit and ridge vent system, but your attic still gets hot — and there are no obvious defects — it may be time to consider doing something different to protect one of your largest home investments.
Read Also: How To Install a Solar Attic Fan