ONE SIZE DOES NOT FIT ALL
For the vast majority of roof systems, attic ventilation is required by the four model building codes (BOCA National Building Code, Uniform Building Code, International Building Code, and Standard Building Code). Despite these code standards, attic ventilation is not a "one size ﬁts all" application. For example, in hot and humid climates, air intake can actually increase moisture problems in attics, or if you plan to build a very complex roof design, it many not be possible to ventilate every 'nook and cranny' of the attic. In fact, all of the model building codes allow for the design of vented and unvented roof systems (http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation/). Because of the complexities with this issue, Raynproof Rooﬁng recommends you consult with a local building expert or contact your local building department before modifying your ventilation design.
The National Housing Agency ﬁrst recognized the importance of attic ventilation in 1942. Since then, the amount of attic ventilation required by building codes has steadily increased. During summer months, attic ventilation can reduce excessive heat and humidity build-up, while in the winter, proper ventilation can reduce problems associated with condensation and ice dams. Proper attic ventilation also promotes energy savings by reducing air conditioning costs during the summer and maintaining insulation performance during the winter.
At the current time, the ratio of 1 square foot of net free ventilation for every 300 square feet of ﬂat attic area is the typical standard, with the stipulations that ventilation must be split evenly between intake and exhaust, and that the ceiling must have a properly installed vapor barrier that separates any 'conditioned' air space in the house from the unconditioned air space of the attic. For steeper roofs, or for roofs with more complex designs, sometimes the ratio of 1/150 is required.
TEMPERATURE BENEFITS WITH ATTIC VENTILATION
To dispel a myth, the temperature difference between a non-ventilated attic and a fully ventilated attic is not huge — typically no more than 10 degrees Fahrenheit. The fact is, a ventilated attic is still going to be very hot during a summer day. A similar situation exists when you crack open your car window on a hot day. There will be a small amount of improvement, but overall, the temperature inside car will remain very warm. With this in mind, the importance of attic insulation cannot be overstated. Since a ventilated attic will be very warm during summer months, maintaining a thermal break (with insulation) between the house and attic is crucial for comfort and energy savings.
The good news about attic ventilation is that it allows the attic to cool down fairly quickly once the sun sets. This allows the plywood and rooﬁng materials to cool off as well. Without ventilation, asphalt composition shingles will typically lose 10% of their expected service life. All asphalt rooﬁng manufacturers recognize the link between proper ventilation and shingle life expectancy, and because of this, attic ventilation is required in order to comply with the terms of shingle manufacturer warranties (for example, see page 4 of CertainTeed’s Shingle Warranty
for their clause requiring adequate ventilation).
AVOIDING CONDENSATION WITH ATTIC VENTILATION
Inside the typical home, there are multiple sources of moisture vapor. Activities, such as washing clothes, cooking, and bathing, release many gallons of water into the air. During the winter, this water vapor will remain airborne as long as it does not come in contact with a cool surface or cool air. To prevent this, homes have vapor barriers and insulation to separate wall areas from cold areas. Despite these measures, some of this warm, moisture-laden air inevitably will ﬁnd its way into unheated attic spaces. Without proper ventilation, this warm air will condense into water when it comes into contact with the cold air, and it will collect on top of insulation or onto the underside of plywood roof sheathing. Over time, these conditions will lead to problems with mold, or insulation will become saturated and loose its insulating qualities. On the other hand, the problems can be avoided with proper ventilation, which will remove any warm air from the attic before it has a chance to condense.
Roof ventilation aside, it is important to minimize the amount of water vapor that can leach into the attic. Having a vapor barrier on the ceiling of your home is crucial. In the summer time, this keeps cool air-conditioned air out of the attic, where it might otherwise come into contact with warm humid air and cause condensation. In the winter, a vapor barrier will keep any warm, moist air from the house out of the attic, where it would otherwise come in contact with cold air and condense. Consult you local building experts for more information on the appropriate type of vapor barrier for your region.
AVOIDING ICE DAMS WITH ATTIC VENTILATION
In very cold climates, warm attic air in the winter time can lead to problems with ice damming, whereby ice and snow on the surface of the roof are warmed by air inside of the heated attic. This causes the snow and ice over the attic spaces to melt and run downhill. This water re-freezes when it encounters the unheated roof eave overhangs. Over time, large ice dams can build up on the eaves of a roof. In the right conditions, water may pond behind the ice dams, running back under the shingles and into the house. Again, by reducing the amount of warm air in an attic, proper ventilation can prevent this condition from occurring.Image Source: http://www.extension.umn.edu/distribution/housingandclothing/images/1068p01.jpg
HOW TO CALCULATE ATTIC VENTILATION
For roof pitches up to 8/12, the majority of building jurisdictions specify an attic ventilation ratio of 1 to 300. To satisfy the code requirements, there must be a 50/50 split in ventilation between the top of the roof and the bottom of the roof. Also, there must be a vapor barrier in the ceiling which separates the conditioned air of the house (either heated or cooled) from the unconditioned air of the attic. For steeper roofs, complex roof designs, or to compensate for inadequate intake ventilation, sometimes a ratio of 1 to 150 is specified.SAMPLE CALCULATION (1 to 300)Step 1 Determine attic square footage
Step 2 Calculate Venting requirement with the 1 to 300 ratio
- HOW: Multiply length of attic (in feet) times width of attic (in feet)
- 32' X 46' = 1,472 square feet
Step 3 Convert square feet of venting to square inches
- HOW: Divide attic square footage (Step 1) by 300
- 1,472 square feet divided by 300 = 4.91 square feet of venting needed
Step 4 Split the amount of needed ventilation
- HOW: Multiply square feet of venting (Step 2) by 144
- 4.91 square feet X 144 = 707 square inches of venting needed
(Step 3) equally between intake (bottom of root) and exhaust (top of root)
Step 5 Determine which type of venting
- HOW: Divide total square inches of needed venting (Step 3) by 2
- 707 square inches divided by 2 = 354 square inches needed for intake and exhaust
products and which quantity of these products will satisfy the intake and exhaust (Step 4) requirements.
INTAKE VENTILATION PRODUCTSContinuous Sofﬁt Vent
The amount of ventilation will vary depending upon the width and style of continuous sofﬁt vent used.Rectangular Intake Vents
The amount of ventilation will vary, depending upon the size and quantity of vents used. The most popular size is 6"X16" (96 sq. inches of ventilation)Bird Block Ventilation
The amount of ventilation depends on the quantity and diameter of the ventilation holes. In Seattle and King County, builders are now required to provide four, 2" diameter vent holes for 24" of exterior wall. Each of these holes provide 3.14 square inches of intake ventilation.
EXHAUST VENTILATION PRODUCTSCobra Rigid Vent II
(18.5 sq. inches per foot)
Example: 354 square inches (Step 4) divided by 18.5 = 19.1 linear feet of ridge vent requiredCor-A-Vent Ridge Vent
(13.5 square inches per foot installed)
Example: 354 square inches (Step 4) divided by 13.5 = 26.2 linear feet of ridge vent requiredTurbine Vents
Ventilation amount depends upon size of unit and air speed. Check your local building department for recommendations. NOTE:
Because turbine vents have moving parts, many require an annual inspection for lubrication and/or general maintenance. Turbine vents can often squeak over time, and as a result, they are a leading source of callbacks for rooﬁng installers.AF 50 attic vents
(50 square inches per foot of venting)
Example: 354 square inches (Step 4) divided by 50 = 7.1 vents requiredRVO-38 attic vent
RVO-38 attic vents
(38 square inches per foot of venting)
Example: 354 square inches (Step 4) divided by 38 = 9.3 vents required
The purpose for attic ventilation depends upon climate and season. In most climates, proper attic ventilation can maximize a roof's longevity, reduce heating and cooling costs, and avoid moisture-related complications with your home. However, ventilation by itself is not going to solve all problems in your attic space. Adequate ventilation works in conjunction with insulation and vapor barriers, and these three components together serve to minimize condensation problems, promote energy savings, and improve overall living comfort.
It is also very exciting to see a real break-through in solar-powered attic ventilation that Solar Blaster Corporation
has brought to market recently. Their products are focused on boosting the effectiveness of a home’s current passive vent system. They enhance the eave-to-ridge air circulation that all passive venting systems are based on. Rather than installing one large solar attic fan which can leave hot spots, the idea behind Solar Blaster’s products is simple: installing multiple smaller fans spread evenly across a home’s entire roof line will indeed enhance, and not circumvent, the science behind passive venting. They offer a ridge vent fan (RIDGEblaster
) option as well as an RVO-30 fan (RVOblaster
).About the Author
David Buckles was the sales manager at Raynproof Rooﬁng in Seattle, Washington. He has over 19 years of experience in the residential construction industry. David has been employed as a Journeyman with the Carpenters Union, and he has three years of high school teaching experience in the construction trades.