We’re reaching the time of year where entering the attics some homes takes on all the charm of a dry sauna with itchy fiberglass strewn throughout. Already this year, I’ve had an attic over 130 degrees and a bunch that were more than 40 degrees above the outside temperature. Since high attic temperatures lead to degradation of the roof shingles, this is a problem but not the sole one and not the actual cause. It’s symptomatic of poor attic ventilation.
Poor attic ventilation presents itself differently depending on season, weather, and homeowner behavior. The high temperatures that I see now, and will until the end of summer, are one indicator. Another, in winter, is the formation of ice crystals on the sheathing, typically on the north face. Excessive humidity, regardless of season, is the third factor I look for since the attic space should maintain the same relative humidity that is present outdoors.
High temperatures can cause cracking and blistering in the shingles and greatly increase the cooling costs for the home in summer. The purpose of the granulation on the shingle is to protect the asphalt which protects the fiberglass backing materials. The cracks and blisters expose the asphalt to sunlight. The asphalt is volatile and breaks down under the impact of UV light. Obviously anything that impacts the integrity of one part of the shingle affects all of them.
Ice crystals and high humidity (same issue, different seasons) are the principle forces that drive mold growth in attics in this region. The excessive moisture also leads to deterioration of the sheathing materials in the attic. Recognizing the humidity issues early can forestall the concerns. Something as simple as rust on the roofing nails can be a good indicator of possible problems. More often than not, the insulation or ductwork below the nails will have drip spots to further confirm the condition.
All of these devolve back to ineffective ventilation. Or, to use CertainTeed’s expression, inadequate ventilation. Per their warranty information, “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 the SureStart Protection.” In other words, that 30- or 40-year shingle is now only warrantied for a small fraction of the original and on a limited basis at that. Most, if not all, manufacturers have similar provisions.
Interestingly enough, inadequate ventilation is not defined in their literature. I had to call them to try and nail down the answer. Carl from the Technical Support department was great and dispatched a copy of their ventilation requirements from the Applicator Manual. For a balanced system where the air intake at the soffits represents 50 percent or more of the ventilation, they recommend one square foot of Net Free Ventilation Area. NFVA is basically how big a hole we need to get the proper amount of air flow.
The calculations are pretty easy. Execution of the project, though, tends to fall short. The two most common causes for poor ventilation, assuming the proper calculations have been made, are covering the soffit vents with insulation (or sealing with paint) and not properly cutting the ridge vent.
Blocking soffit vents most often happens when people add insulation to the attic space. In the attempt to improve the home’s efficiency, they inadvertently makes matters far worse, doubly so if the homeowner creates a damp atmosphere in the home by showering without using the ventilation fans or does a lot of cooking that releases moisture. Without the input pressure of air, the attic space retains the moisture.
Painters don’t help when they paint over the soffit vents. Remember that the criterion is Net FREE Ventilation Area. Fill all the little gaps with a good quality exterior paint and we no longer meet the definition.
I’ve had attics with blocked soffits measure at better than 70 percent relative humidity while the exterior of the home measured under 30 percent. You can smell the problem in these attics as soon as you pop the access cover. This is usually more discoverable in the winter and spring seasons here—our summers and falls are typically too dry to make the humidity a reliable guide. Mold, if present on the sheathing, makes the call easy. Ditto for ice crystals on the sheathing. While pretty, they need to go away.
Ridge vents have their own issues. The most common installation error is failing to cut the vent wide enough. CertainTeed (not picking on them – they’ve been most helpful!) requires an opening of 1 ½ inches for trusses construction and 3 inches for rafters. Most failures I see here are undercutting the required width. This restricts how fast hot or wet air can exit the attic, even assuming the lower vents are doing their job.
Unfortunately for the home inspector, there is no recognized standard for how hot is too hot, so we end up with roofing contractors arguing that they built to code (defined as the worst house you’re legally allowed to build) while we argue about actual performance. I use a level of 20 degrees of differential in the outdoor and attic temperatures. At that point, improving the attic ventilation becomes a recommended upgrade unless I see worrying signs of moisture build-up. At 40 degrees, it becomes a recommended repair as there is almost always a significant defect in the system at that point.
Some helpful resources: