Whether installing a new roof material or simply evaluating a roof’s current condition, paying close attention to attic ventilation is critical for several reasons. Some of the reasons adequate attic ventilation is important include:
- Keeping the home cooler in the summer. An abundance of summer heat trapped in the attic due to little or no ventilation can make the home more expensive to cool.
- Limiting ice dams in the winter. Ice dams are caused by warm (usually improperly ventilated) attics that allow snow on the roof above to melt. As the melted water runs down the roof, it reaches the overhanging eaves. Since eaves are generally not above the attic or living space and therefore not heated, cold winter air can refreeze this melted snow allowing for it to find its way back up under the shingles. This can lead to rotted soffit, fascia, and trim boards, water infiltration into the home, and possibly presenting an attraction to insects.
- Damage to the shingles themselves. Intense summer attic heat from an improperly ventilated attic can actually bake the roofing materials (shingles, sheathing, etc.) from underneath over time, thus causing premature deterioration of the various roofing materials.
- Moisture condensation on the underside of the roof sheathing can lead to mold and/or damaged framing. Attic moisture can also lead to roof decking expansion and waviness. If there are unsealed or uninsulated openings between the attic and living space (such as the attic access panel/scuttle, where wiring passes through framing, etc.), moisture from the living space can enter the attic readily.
The purpose of properly balanced attic/roof ventilation is to allow cool air to enter the attic at the soffits (eaves), rise uniformly up the underside of the roof sheathing, and exit at the ridge venting. In perfect ventilation conditions, the attic’s air temperature should be closely match that of the exterior air. Proper attic insulation also helps keep the living space comfortable.
Most roofing material manufacturers have explicit installation procedures regarding minimum required attic ventilation. This ventilation criteria must be met in order to maintain product warranties and allow the roof system to work (and last) as designed.
HUD and most building standards require a minimum 1 square foot of attic ventilation for every 150 square feet of attic floor space, however when half of the ventilation is at the ridge (peak) and half is at the eaves (soffits), this ratio may be reduced to 1 square foot ventilation for every 300 square feet of attic floor space. It is best to have balanced ventilation (intake and exhaust) in the form of proper ridge and soffit venting, however, when this is not possible (such as when the home has a hip roof), studies have shown it is best to have more ventilation at the soffit. There are ridge vent-like exhaust vents designed for hip roofs, however, which will allow for needed exhaust.
Overall, ridge venting provides approx. 18 square inches per linear foot of Net Free Area (NFA). For balanced air flow, the installed soffit venting should also provide 18 square inches per linear foot of Net Free Area (NFA); this would be split between the home's front and rear (each soffit vent should provide 9 square inches per linear foot of Net Free Area (NFA).
The different types of exhaust attic/roof venting in the order of function (in order from worst-to-best) are:
- Gable Vents (worst)
- Box Vents
- Turbine Vents
- Power Fans
- Ridge Venting (best)
Certainteed®, a major roofing material manufacturer, requires that when ridge and soffit venting is added to a Certainteed® shingled roof that has gable, box, turbine, or power vents, these old vents must be covered or removed. This requirement is explicitly stated in the CertainTeed® Shingle Applicator's Manual. Who wants to pay $5,000 ~ $20,000 for a new roof only to find out later that the roofer cut corners and did not install adequate attic/roof ventilation and the new roof warranty is now void?
Having one or more box, gable, or turbine vents, and/or power fan(s) causes non-uniform ventilation and allows hot spots to occur at the roof's underside. This will likely cause non-uniform shingle wear over time since proper balanced attic ventilation is prevented.
On multiple occasions in my travels, I have seen relatively new roofs installed with little or no ventilation, an issue arises due to premature deterioration and the shingle manufacturer is called. In both cases, a shingle manufacturer’s representative visits the home to evaluate the situation and determines that the minimum amount of attic ventilation wasn’t met upon installation (per installation instructions) and now voids the shingle warranty on the spot!
Quite a few times, I have inspected homes with fairly new roofs, ridge venting is installed on the exterior of the roof, but when I get into the attic, I find that the roofer never cut the groove in the roof sheathing to allow the ridge venting to serve any purpose. In some cases, I find that only a 1/2" wide ridge vent opening exists instead of the needed 2~3" wide opening (see photo below).
Again, the purpose of venting is to allow for a uniform air flow along the underside of the roof. When box vents, turbine, or power fans, for example, are also present, the uniform air flow can be interrupted creating hot spots in the roof. Under certain conditions, these other vents may actually allow hot air to enter the attic through the ridge venting. The overall roof and attic ventilation can be adversely affected and, as previously stated, may void the shingle manufacturer’s warranty.
In-depth studies performed by building science experts have shown that attic (roof or attic gable wall mounted) power vent fans tend to pull moisture and conditioned air up through the home's living space into the attic. This can also lead to mold growth in the attic in some cases due to excessive moisture getting trapped in the attic. This movement of air also results in wasted energy dollars as conditioned air (which you paid for when the A/C was running) is now lost into the attic. Hot air may actually be pulled back into the attic under some circumstances. Also, back-drafting of fossil fuel appliances (such as furnaces, fireplaces, and water heaters) and the introduction of potentially dangerous exhaust gases, such as carbon monoxide, into the home may occur because of the negative pressure caused by the attic/roof fan. When air is vented out of the home, make-up air must be supplied from somewhere and this air is normally from within the home's living space.
Think of a hot summer day and your home's A/C system is running. Since your attic is also hot, your power vent fan is also running since it is normally on an attic thermostat. Since this vent is essentially sucking air out of the attic, make-up air needed to fill the attic (to replace the hot air leaving the power vent) enters the attic from the home. This make-up air is the very same air that you already paid to cool in the living space. So, in essence, you're paying to cool your home, then paying to suck hot air out of the attic, then paying more to cool your home even more because the air you already paid to cool is being sucked out through your attic. The solar-type of attic vent fan causes the same pattern, although the fan itself doesn't consume electricity.
For more info on roof/attic mounted powered vent fans, please see: Don't Let Your Attic Suck, Power Attic Ventilators Are A Bad Idea
As well as: Powered Attic Fans - Attic Insulation Facts
To the best of my knowledge, only Certainteed actually requires [in their installation literature] that gable, turbine, or box vents and power fans be removed or covered. The other major manufacturers (namely, Tamko, Owens Corning, and GAF) strongly recommend doing this. When I run across power vent fans (gable vent or roof installed), I routinely recommend removal or permanently disconnecting power from these fans.
While taking actual mechanical measurements to determine if the minimum required attic ventilation is met in a given home is outside the scope of a home inspection (based upon the Standard of Practice of the American Society of Home Inspectors [ASHI]), it is prudent for the home inspector to make general comments and recommendations regarding an observed lack of or apparent insufficient ventilation. The home inspector has a duty to his client to report on possible issues such as this.
Some signs to look for in attics regarding insufficient ventilation can include attic mold, condensation on sheathing or rafters/trusses, wet insulation, or excessive attic heat. Most inspectors carry laser thermometers, so measuring a hot attic’s temperature takes only a few seconds.
A related issue of attic/roof ventilation pertains to unsealed openings between the home's living space and the attic. Attic access doors or panels (aka 'scuttles') should be properly weather stripped and insulated to help limit warm or cool air from the living space from getting to the attic. In my years of experience inspecting homes, I find maybe 5~10% of attic access locations (doors or panels) are actually insulated. Without insulating these areas, you surely know where a large portion of your home's heat gain (summer) or heat loss (winter) will be.
Drywall and plaster are not air tight so air and moisture will pass through these materials. Also, openings in framing such as for running wiring, plumbing venting, ductwork, etc. are often not air tight and are common locations for air and moisture from the living space to find their way to the attic. Ideally, all of these small openings should be sealed with an appropriate material (such as expandable foam); this is standard practice in newer construction nowadays.
Another important note is that attics should be thoroughly and properly insulated. Modern standard now in this area of the country would call for R-38 insulation in attics (corresponds to approx. 11" of blown-in cellulose, 13" of rolled fiberglass batt, or 15" of blown-in fiberglass). The attic access panel (aka scuttle) or attic door should also be insulated. Keep in mind that soffit venting should not be blocked (that will prevent proper air flow into the attic thereby affecting the attic ventilation) by insulation. Baffles may be needed between the roof framing if not already installed. A properly insulated attic can help prevent heat movement (which includes moisture movement) between the attic and living space.
© 2014 Matthew Steger
Matthew Steger, owner/inspector of WIN Home Inspection, is a Certified Level 1 Infrared Thermographer and an ASHI Certified Inspector (ACI). He can be reached at: 717-361-9467 or firstname.lastname@example.org.
WIN Home Inspection provides a wide array of home inspection services in the Lancaster, PA area. This article was authored by Matthew Steger, ACI - owner of WIN Home Inspection in Lancaster, PA. No article, or portion thereof, may be reproduced or copied without prior written consent of Matthew Steger.