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.
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.
- 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. 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
