Soffits and Fascia
Soffit and Attic Venting In Your Home
If you are considering the purchase of a solar powered attic fan, we encourage you to read this page. Get the facts before you buy any energy conservation product.
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The importance of venting your attic cannot be overlooked. In cold climates, improper venting can lead to moisture buildup which causes mold to grow, wood rot in extreme cases, or even ice in the attic insulation. During the summer, stagnant, superheated attic air can dry out your trusses, increase your energy costs for air conditioning, and make your house uncomfortable.
How did we become experts on attic venting? Our patented reflective insulation products reject the infrared heat emitted by hot roofs which, in turn, makes attics even hotter. Getting rid of this waste heat then forced us to look at not only how homes are vented, but the U.S. home building codes that dictate what is needed for adequate ventilation. What we found was amazing. Codes for attic venting are the same all over the U.S., regardless of the climate, and are designed for moisture egress, not the removal of hot air. Furthermore, most homes that do have venting have vents that are plugged with insulation.
Let’s start with an understanding of basic physics: Hot air rises. But why? Heated air rises due to a differential in density (mass), which causes an upward movement of the fluid (air is a fluid). Like a lava lamp, the colored liquid is heated, rises to the top, cools off (density is increased), then falls back to the bottom.
Think of the force required to raise a hot air balloon off the ground with 10 passengers, the tanks of propane, the weight of the basket, and the balloon. Pretty impressive force if you think about it. It should be easy for hot air to rise out of your attic, right? Driven by the density differential, the exhausting of the hot air should bring in fresh air where vents are located around the base of the roof (soffit).
There is no “pressure” that drives hot air out of an attic. The pressure (atmospheric) inside your attic is the same as it is outside (based on a no-wind condition). Wind blown across a roof can create a positive pressure on the windward side and a lower pressure on the lee side. Only the differential buoyancy of heated air over the temperature of ambient air is different.
Using a tracer gas and smoke generators, we have documented all types of attic venting, the appropriate sizes that work in hot climates, and have posted our findings below. This is not a scientific study, but observations made over the study of hundreds of homes in all types of climates.
Our field-proven results and recommendations: (if using a radiant barrier and passive vents)
- 10 square feet of venting per 1000 square feet of attic area minimum. 50% at the peak, 50% at the soffits (with no radiant barrier, use 8 square feet per 1000 square feet of attic area in hot sunny climates).
- Ridge venting, placed at the peak of the roof — while attractive and easy to install — do not allow sufficient air to exhaust as this defies gravity, physics, and logic. Hot air rises — this is a law of thermodynamics — yet a ridge vent consists of an inverted “V”. This creates a thermal check valve which does not allow the hot air to escape properly during the summer and, once covered with 2″ of snow, is rendered useless. (See ice dam page for more information.)
- Stay away from solar attic fans. The claims of CFM movement are overrated and not tested by any third-party laboratory. Our testing has shown a solar fan with an 11-watt panel, facing south, moves 315CFM. That’s less than half of what is advertised. Good idea but not enough power. Even if 22 watts moved twice the CFM, it’s still less than half of what a 1400 square foot home needs for active powered venting.
- Whirlybirds (the aluminum spinning vents) have less than one square foot of roof opening and do not induce air movement while spinning. Larger, less conspicuous static vents are more efficient at moving air.
- Aspirate your attic naturally (no power fans) wherever and whenever possible. A well-ventilated attic does not need power venting. Coming soon: E-book on do-it-yourself attic venting.
- In cold climates it is imperative that each rafter bay be open to the soffit area and free air allowed to move from the soffit area into the attic. This will keep mold from forming, ice dams at bay, and the attic cooler during the summer. (See ice dam page.) It is also imperative in cold climates to keep any attic insulation that is in direct contact with the ceiling from touching the roof as the slope nears the outside walls. This is a very common scenario. Direct contact between the insulation and roof decking creates a “thermal wick” and provides a path of heat that melts the snow on the roof which then travels to the edge of the roof, freezes, and can create ice dams and other problems.
- If you have to use a power fan (tile roofs, inaccessible areas, flat roofs), don’t look for one at a home store — they are not what they used to be. The older fans (of 10 years ago) had better motors, better bearings, and a thermostat that could be relied upon to control the fan. Having installed hundreds in our radiant barrier applications and replacing most of them (within a year), we were forced to find an alternative to this liability. Our radiant barrier installations rely heavily on the removal of the waste heat generated by our products which has made us venting experts over the last 20 years. We have found a fan made in Germany, used as a poultry house exhauster, that is awesome — quiet, vibration-free, has permanently lubricated bearings (never needs oiling) — which we mate with a quality high-voltage adjustable thermostat. (Scroll down for more information.) If you have to use a fan, this is the one to use.
What Works, What Doesn’t
CONTINUOUS SOFFIT VENTING
Continuous soffit venting is typically a 2″ or 3″ slot cut into the soffit into which is placed a manufactured venting system made from plastic or metal. The plastic is typically molded with thousands of small holes (1/32″ in diameter) to keep bugs out. It’s a great idea and easy to install, but falls short of offering sufficient net-free area to allow air to move.
Another popular type is an aluminum strip with small louvers pressed into them.
BENEFITS: Easy to install and attractive.
PROBLEMS: The holes are too small (see photo below) to pass sufficient air to let the attic aspirate naturally. Most often, these are covered with insulation or partially plugged with dust, pollen, or spider webs over time.
This vinyl soffit covering is very popular because it completely eliminates the need for painting and/or maintenance on your home. Great idea! Unfortunately, it starves the attic for air and costs you money and comfort.
The best solution for a venting problem is to install InvisiVent. Cutting into the existing soffit, covering it with InvisiVent, and wrapping the existing fascia with aluminum, as you can see below, will give you the ventilation and looks you need for your home.
Proper Ventilation Is Vital…Triple 3-1/3″ InvisiVent Has the Highest Ventilation Performance
The Triple 3 1/3″ InvisiVent soffit offers at least 50% more ventilation than most standard vinyl soffits, providing more than 10″ of net-free intake area. This means that not only will Triple 3 1/3″ InvisiVent provide a balanced ventilation system, but will do so with an overhang as short as 10″. The solid panel is an ideal solution for accent areas or where a vented panel is not necessary.
The flat face design creates a straighter panel with the look of smooth painted wood.
- InvisiVent panel is for soffit use only
- Solid panel can be used for soffit or vertical application
- Can be applied 24″ on-center
- Heavy-duty .044″ thickness for rigidity
- Low gloss, matte finish looks like freshly painted wood
- Class 1(A) fire rating
- Lifetime limited warranty
WHIRLYBIRDS
The correct term is “air motor” and is also called “roof turbines.” It is thought that the spinning turbine creates some sort of suction that induces air movement. This has not been proven. These are called “air motors” because they are used in large commercial applications for venting of smoke and fumes from a manufacturing plant. The wind spins the turbine, which is connected by a shaft to a fan blade below which spins and pulls the air from the building.
Turbines used in the residential market do not have these fans and therefore can exhaust only what the roof opening dictates. Pi times the radius squared equals the area. A 10″ unit offers 78.5″ of free net area, 12″ 113″ square inches. Venting an attic using natural aspiration is based on free net area. Use this to calculate the sizes of intake vents (soffit) and exhaust vents (at the top of the roof). We recommend a new vent system for natural venting
Despite the small area of these vents, the vertical stack of this vent allows for the fastest flow of air from a hot attic. For best performance, these vents should be placed as close to the peak of the roof as possible. Because our reflective attic insulation products increase attic temperature by the rejection of infrared heat, attic venting has played an important aspect of looking at the big picture. Getting rid of this heat is very important to reduce energy consumption and increase interior comfort. We have found through experience that homes with ridge venting are historically the hottest, and emails from our customers who have read this website reinforce this over and over.
Puffing tracer gas inside the attic at 2pm just under the opening shows the air in the attic to be stagnant. Stay away from ridge venting unless your home is built where there is a constant breeze because the air moving over the roof will tend to create a vacuum on the lee side of the roof, which will draw the hot air out.
If you see these balls of sap boiled out of your rafters, your attic is an oven due to inadequate venting. This makes your home hot, dries out your structural framing members, and is an indication you may be spending too much to maintain interior comfort.
DORMER VENTS
A 20″ x 20″ opening (400 sq. inches or 2.77sq ft) is made for this 21″ x 24″ dormer or “eyebrow” vent. The actual net-free venting area is 97 square inches.
We do not carry dormer vents but some Home Depot locations have them. We now recommend the Aura vent roof vents because they allow for better air flow in either wind or no-wind conditions, and are specifically designed for use with our German-made powered attic fans.
Ridge Venting vs. Dormer Vents
Having installed our reflective radiant barrier in homes for more than 18 years, we have found that the rejected heat, previously absorbed by the insulation (and passed on later), winds up in the attic, making the attic hotter. It is much hotter, in fact, than an attic without a radiant barrier. This means the attic venting must be properly sized or a quality attic fan should be installed (seen below) to exhaust the excess heat. A properly vented attic, be it vented by natural aspiration (adequate soffit vents along with properly sized and placed exhaust vents) or a powered attic fan should be only a few degrees higher than outside ambient temperature.
Ridge vents are very attractive, easy to install, and make all the sense in the world They just don’t work! Everyone will agree that hot air rises. Where best to put an attic exhaust vent? At the peak where the attic air is the hottest?
That sounds logical except for one basic fact: Hot air rises, it doesn’t go down. A ridge vent design dictates that hot air must fight gravity and travel down from the peak of the roof in order to escape. This, of course, is unrealistic and so is expecting this type of vent to be effective. The only “driving force” that makes hot air leave an attic is the differential density of the 140F attic compared to the more dense ambient outside air. If you have any doubts (as we did) as to the function of this type of vent, do what we did: Buy a 2000 CF smoke canister, place it in a coffee can in the center of the attic, and light it. Then go outside and wait for the smoke to come out of the ridge vent. Better bring a lunch, plenty of water and a good book — you will be there awhile.
The roof you see above was built using a continuous ridge vent. When the hot weather came in April to this home, the attic was like an oven despite the fact the intake venting (soffits) were doubled to ensure good flow.
We cut the first hole in this roof at about noon to install the dormer vents that you see above and the air came out so fast that if you kicked the sawdust into the hole, it would hit you in the face! Proof that the hot, stagnant attic air is not being vented properly and being trapped by the ridge vent design. Furthermore, the steeper the roof pitch, the less the ridge vent will work. This observation is based on fact, physics, and common sense. Hot air does not fight gravity. Additionally, in cold climates, it only takes a few inches of snow to render ridge vents completely useless — when you need them the most to keep the roof from getting warm, melting the snow and creating ice dams. Inadequate venting is a major contributor to costly ice dams.
Dormer vents installed on the back side of a home where the hottest attic air temperature was ever recorded by the NRG auditor. There were two reasons:
- The underside of the roof decking was sprayed with silver radiant barrier paint, thus lowering the emissivity and increasing the convective currents (and energy use) in the attic space.
- Ridge vents trapped the hot air inside. The dormers shown here dropped the attic temperature by 42 degrees. Solution: Over the insulation and increased venting.
SOFFIT VENTING
Many homes do have soffit vents but are blocked by improperly installed insulation. Proper attic ventilation is necessary and the products listed below will help.
This is what you may see when you look at the venting around the perimeter of your home. “My attic has hundreds of holes around the perimeter, we don’t need any more venting.”
Remove the blocking and you can see the fiberglass blocking the vent holes. This is a very common problem found during NRG audit inspections.
This is what is recommended on the outside of the home if you do not have a soffit. If you have a soffit, add 8″x12″ screened vents for every 8 feet of soffit. These also can be obtained at Home Depot outlets.
This procedure gives your attic the air it needs to vent properly. In cold climates, this equates to adequate moisture removal and the elimination of the associated problems like mold growth and wood rot. In hot climates, the superheated attic air can flow out the upper vents (if you have them) and draw cooler air in around the house. This is what your architect and builder expected. However, in most homes the insulation contractor may have plugged up some or all of your soffit vents. Our home energy auditors have found that most homes have this problem. Check yours for your home. If you have questions about your existing attic vents, take a few digital pictures and send them to us for evaluation. This is a free service as we fine-tune our online NRG Audit coming soon. Take advantage of our knowledge and free advice! Please, no more than five pictures. Take one of each side of the home — photos of the soffits vents (if any), gable vents (if any), and one or two shots of the attic.
Step-by-Step Soffit Venting
The soffits on this home were completely blocked, making the attic like an oven, increasing energy costs, and making the upstairs unbearable during the summer. This is very common in homes of all ages, even brand new homes across the country.
Cutting holes in the plywood soffit allows intake air to cool the attic. Be sure and snap a chalk line to center the holes that you plan to cut or it will not look nice once you are finished.
