The science of proper ventilation.
Roof vents can only exhaust the air your soffits let in. Here's what the codes require, what it costs when the intake falls short, and how to find out where your own attic stands.
Check your attic in 30 seconds▶Watch first
The right soffit changes everything.
Two minutes on why intake is the half of attic ventilation almost everybody gets wrong. Then keep scrolling for the numbers behind it.
The Right Soffit Changes Everything — Episode 1. Nothing loads from YouTube until you press play.
Already know your roof size? Run the calculator →01Ventilation Crisis
It's estimated that
70%
of homes don't meet basic ventilation requirements
Research shows most homes don't meet basic ventilation requirements and are in violation of residential building codes, and approximately half of all U.S. homes have evidence of improper ventilation.
The Lawrence Berkeley Laboratory states that poorly ventilated homes have been reported to be one of the leading causes of moisture build-up, mold, mildew and rot in attics — which has contributed to an alarming rise of many illnesses, including asthma, allergies and other respiratory issues.
Lawrence Berkeley Laboratory
iaqscience.lbl.gov/dampness-summary.html
Overheated.
Dangerous.
Damaging Air.
In order to have a balanced system, you need more intake than exhaust.
02Proper Ventilation
In order to prevent attic air from overheating, pressurizing and drawing into the home, there must be equal amounts — or more — air entering through soffits than exiting through roof vent systems.
The U.S. Department of Energy recommends using a continuous roof ventilation system at the rooftop, and a continuous vented soffit system that provides more intake, or net free area (NFA), at the eave edge than what is offered through the roof vent system.
The problem is that most soffit panels today don't offer enough NFA to work with the amount of roof ventilation required for the majority of homes.
U.S. Department of Energy
Attic insulation fact sheet
03Power of Proper Ventilation
What the code
actually asks for.
Continuous soffit and ridge vent
The Department of Energy and Habitat for Humanity state that a combination of continuous ridge vent along the peak of the roof and continuous soffit vents at the eaves provides the most effective ventilation.
The 1:150 rule
The National Roofing Contractors Association recommends a minimum ventilation opening area of 1/150 of the space being ventilated. Natural convection drives the upward flow of air through an attic, and that current is sustained when intake vents placed low make cooler air available to replace the heated air exhausted high.
A balanced attic
International Building Code Sec. 1203.2 requires net free ventilating area of not less than 1/150 of the space ventilated, with 50 percent provided by ventilators in the upper portion of the space at least 3 feet above the eave or cornice vents, and the balance provided by the eave or cornice vents.
Work the 1:150 rule by hand
Skip the arithmetic.The calculator at the bottom of this page runs the 1:150 rule against your actual roof and your actual soffit panel.
Run my numbersProper ventilation can reduce the temperature in your attic by
50°
04Power of Proper Ventilation
Proper ventilation — more intake through soffits than exhaust through roof vents — can, according to the U.S. Department of Energy, lower the attic temperature by as much as 50°F, which significantly reduces the home's heating and cooling demands.
Just because a soffit product has vents doesn't mean it works. A typical one-foot soffit overhang should have at least an 11 NFA, so that when used on both sides of the home it delivers a combined NFA of 22 — enough to draw the airflow a ridge vent system needs.
U.S. Department of Energy
eere.energy.gov — soffit vents
05Energy Costs
Cheaper soffit,
expensive attic.
Residential building code requires ventilation through soffit systems that offer enough net free area to create the updraft that pulls air out through the ridge vents. Disher Engineering estimates energy savings on a 2,000 square foot attic at $292 annually — $4,380 over a 15 year mortgage. When you evaluate ventilation systems, be careful not to be enticed by products that cost a little less up front and significantly more over time.
| Soffit choice | Initial cost | Year 1 | Year 2 | Year 3 | Year 7 | Year 15 |
|---|---|---|---|---|---|---|
| Quality Edge soffit | $650 | $358$650 − $292 saved | $66$358 − $292 saved | −$226 | −$1,394 | −$3,438 |
| Soffit with insufficient NFA | $450 | $742$450 + $292 cost | $1,034$742 + $292 cost | $1,326 | $2,786 | $5,122 |
Summer assumptions
- 2,000 sq ft attic
- 21 NFA vs 2 NFA
- Outside 95°
- Roof 175°
- Inside 65°
- $0.11 / kWh
Winter assumptions
- 2,000 sq ft attic
- R-20 vs R-10
- Outside 30°
- Inside 70°
- 80% furnace rating
- $0.77 / ccf gas
Improve
Indoor air quality
06Indoor Air Quality
According to the EPA, indoor pollution sources that release gases or particles into the air are the primary cause of indoor air quality problems in homes. Inadequate ventilation can increase indoor pollutant levels by not bringing in enough outdoor air to dilute emissions from indoor sources, and by not carrying indoor air pollutants out of the home. Dampness and mold in homes are estimated to account for 21 percent of current asthma cases in the United States.
Based on an analysis of 49,130 indoor air quality tests conducted in homes across North America, Air Advice reports that people in 9 out of 10 American homes are breathing unhealthy indoor air, and that over 91% of homes tested showed elevated particle allergen levels.
EPA — IAQ in Homes · epa.gov/iaq/homes
Air Advice · airadvice.com indoor air report
07Building Material Life Cycle Costs
Heat ages a roof
from the inside.
$4,680
Additional shingles
If your roof lasts 15 years instead of 30.
$1,200
Roof deck
Cost of replacing dry rot on the roof deck.
Improperly vented and overheated attics are a primary cause of premature aging of roofing materials. The Asphalt Roofing Manufacturers Association states that recent research has reinforced the theory that prolonged exposure to extreme heat accelerates the aging of asphalt roofing products, and that properly ventilating the underside of the roof deck reduces heat build-up and its related problems.
It is one of the main reasons shingle manufacturer warranties are voided when proper ventilation is not present. In financial terms, if a roof lasts 15 years instead of 30 and the average roof costs $9,360, that is $312 annually, or $4,680 over 15 years in shingle replacement alone.
GAF
"What Is Not Covered. (1) Damages resulting from anything other than an inherent manufacturing defect in your shingles or accessories, such as: (c) Inadequate attic ventilation."
CertainTeed
"Any shingles applied to inadequately ventilated or non-ventilated decks… are subject to a reduced limited warranty period of ten (10) years and do not qualify for SureStart Protection."
Owens Corning
"Our warranty does not cover damage to the shingles due to any cause not expressly covered herein… 6. Inadequate ventilation or roof drainage."
Asphalt Roofing Manufacturers Association
asphaltroofing.org — technical bulletin 209
Savings
Insulation
$5,505
Increased energy costs associated with wet insulation over a 15 year mortgage.
Mold remediation
$3,000
Average mold remediation claim.
Improper ventilation is also a primary cause of increased moisture in the attic, leading to potential insulation damage, mold, mildew and dry rot. Fiberglass loses up to 40% of its labeled R-value when outside temperatures fall to 20°F, and 50% when they exceed 85°F. On top of that, research shows as little as a 4% increase in moisture or humidity will reduce the effectiveness of fiberglass insulation by as much as 80% — costing as much as $367 annually, or $5,505 over the life of a 15 year mortgage, before you count mold remediation, poor indoor air quality and possible replacement of the roof deck.
The U.S. Department of Housing and Urban Development indicates that inadequate ventilation will lead to fungus growth and very costly repairs.
T(NFA) = Toutside + (Troof − Toutside) e−0.235(NFA)
U.S. Department of Housing and Urban Development
Assumptions: 2,000 sq ft attic · 20 NFA · outside 95° · roof 170° · inside 65° · $0.11/kWh · 50% R-value reduction with moisture on a 2 NFA soffit.
ROI
08ROI of Ventilation
ROI formula
Take the difference in cost between soffit products — the one that works versus the one that doesn't — and divide it by the annual savings. If the up-front difference was $89 and the savings $220, divide $220 by $89: a 320% ROI in year one, and 3,200% by year ten.
Payback formula
Divide the annual saving by 12 to get Y, then divide the up-front cost difference by Y. With $292 in annual savings, Y = 24. Divide an $89 difference by 24 for a four month payback.
| Annual | 15 year mortgage | |
|---|---|---|
| Energy savings | $292 | $4,380 |
| Roofing materials | $312 | $4,680 |
| Wet insulation | $367 | $5,505 |
| Mold remediation | — | $3,000 |
| Replace roof deck | — | $1,200 |
| Air quality | — | Priceless |
Estimates provided by Disher Engineering.
What's the payback on your house?Enter your roof size and current soffit to see the 15 year number for your own attic.
Calculate my payback09The Benefits of Proper Ventilation
In review
It reduces the temperature in the attic
Prolongs the life of your roof and other building materials
Reduces energy costs
Can help improve indoor air quality
It saves you money by reducing your energy bills and is a wise investment
When you enhance ventilation you enhance everything.
10Attic Ventilation Calculator
Is your attic actually breathing?
Enter your roof footprint and the soffit panel on your house. This runs the 1:150 rule and Disher's temperature formula against your own numbers — no email required.
Length (ft)
Width (ft)
NFA is net free area — square inches of open air per lineal foot of panel.
What that adds up to over a 15 year mortgage
| Where it shows up | 15 year cost |
|---|---|
| Heating and cooling | — |
| Shingles replaced early | — |
| Insulation wet and underperforming | — |
| Total | — |
Roofing and insulation figures scale from Disher Engineering's estimates for a 2,000 sq ft attic and apply once intake falls short of code.
Short on intake? TruLine 12″ delivers 19.6 NFA.
See the soffit lineIntake required = attic area ÷ 150 × 144 ÷ 2, per IBC 1203.2, which splits ventilation between the eaves and the upper roof. Attic temperature modeled as T = Toutside + (Troof − Toutside)e−0.235·NFA at 95°F outside and a 170°F roof deck. Panel NFA figures measured by Disher Design & Engineering. Estimates only — confirm requirements with your local code official.
