Applied Building Science goes to great “heights” to install the proper roof, Soffit and fan venting products to keep the attic cool & dry and the roof structurally sound. A properly sealed and ventilated attic will increase comfort, save on energy costs, improve the durability of the home and reduce entry of moisture, mold, and other irritants or pollutants into your home.

The Problem
In predominantly heating (or cold) climates like Michigan, the primary purpose of attic ventilation is to keep the attic free of moisture build-up during the winter months. While attics in these climates can be prone to wind-driven snow and rain entry in some roof designs, studies have shown that moisture on attic components typically comes from inside the living space 1/. The key to problems with moisture (like mold or rot) is the temperature of the roof sheathing.

Many homes built prior to the year 2000 are deficient in attic ventilation. Some older homes do not have Soffits (or overhangs). They rely on ridge or gable vents for air supply. These homes are most problematic for moisture-related issues because there is inadequate vertical air flow to clear out the attic of moisture-laden heat from below.

Few contractors and sub-contractors – even building inspectors - understand how the components of an attic or roof interact with each other. Every trade may do its job properly, but if nobody is paying attention to the issues of moisture sources and ventilation, a house can end up with mold and mildew problems. Even visible exterior signs of ventilation at the Soffit and ridge do not guarantee a clear path exists.

For example, see if you can determine which trade has overall responsibility for a well-ventilated attic:

• Re-siding contractors sometimes install perforated vent strips under the Soffit, but do not cut out holes through the wood being covered. Others have the vent strips running in one direction but don’t match the openings already there. They never go into the attic to verify the cavities are clear (see the insulator) or check for proper ridgeline ventilation (see the roofer).
• Re-roofers rarely go in the attic to check for signs of poor ventilation like mold on the roof sheathing (see the owner) or whether insulation is blocking the eaves vents (see the insulator). Some don’t even know how to calculate the required roof ventilation, relying instead on what is or isn’t already there.  They are known to cut undersized holes for the ridgeline venting which can significantly reduce it effectiveness.
• Most re-insulators have no clue about the impact of air infiltration from the living space or what the attic needs for proper ventilation. While some do install air chutes at the eaves before adding insulation, few check to make sure the eaves venting is operational or sufficient (see the siding installer). Required ridgeline ventilation is rarely addressed (see the roofer). Many insulators merely blow or roll material over unvented bath fans (see the roofer or fan installer).

Continuous ridge venting has gained in popularity over the traditional can-type vent, both for function & appearance.  Continuous ridge venting is best used on gable roofs with uniform rafter cavities from top to bottom across the entire attic. But some roof designs are not good candidates for this type of venting. Hip-roof construction, for example, has complex rafter geometry of four sides, no gable ends, and a very short ridge.  This type construction can be problematic for moisture build-up.

Over-ventilating an attic can be just as problematic as under-ventilation. Without proper attention to the rafter cavities at the eaves, continuous soffit venting can lead to “wind-washing” across the attic insulation. The increased air currents can “strip” the heat-retaining air pockets out of the insulation, especially fiberglass batts & loose-fill.  This can result in heat loss, cold outside walls and condensation forming on the roof sheathing.

It is not uncommon to find bath fans – even dryers and kitchen fans - dumping moisture-laden air directly into the attic. Why? For one thing, roofers don’t install fans and fan installers don’t install roof vents.  Many fan outlets are either covered up by insulation or have a non-insulated pipe hanging from a rafter. Complaints of cold/hot air & attic contaminants backdrafting into the bathroom, as well as condensation dripping from the fan, are not uncommon. The dampers built into most fans do little to stop air infiltration; some times they are stuck open.

Heating/cooling equipment and ductwork are sometimes located in the attic. Like crawl space locations, this practice in cold climates leaves a lot to be desired. The reasons? Poorly sealed ductwork – especially on the return or suction side – can re-circulate fiberglass insulation particles, dust, attic mold, and other contaminants into the living space. Also, supply ducts that are not sealed and un-insulated can add moisture and heat to an attic that wants to be dry and cool.

The degree of mold infestation in attics subject to poor ventilation and condensation varies considerably. In minor infestations, black blotches can be seen growing on roof rafters and boards very close to sites where moist air is passing through the ceiling. In the worst case, heavy mold infestations may envelop all of the roof rafters and boards, as well as coat insulation and ductwork. 

Attic mold growths can have a variety of causes. Inadequate ventilation is one of the major contributing factors. In northern climates like Michigan, warm moist air from people and their activities rises up (called Stack-Effect) toward and through the ceiling (around light fixtures, trap doors etc.). If the attic is well ventilated it passes to the outside. If the attic is poorly ventilated, the moisture will condense on cool/cold surfaces. When the temperature is very cold you may even have frost or icicles growing in your attic. Stack-effect pressures can also draw moisture out of crawl spaces through plumbing and HVAC cavities that run to the attic.

Wind-effect can also have an impact on moisture levels in the attic. Wind blowing across the roof and around the building can create positive and negative air pressures which can deliver moist air into the attic. These pressures can accentuate air infiltration and exfiltration to and from the living space as well.

How We Fix It
Applied Building Science’s Attic and Roof Protection System addresses all of the air infiltration, ventilation, and insulating requirements to keep the attic dry.  We address attic ventilation requirements in the following manner:

1. We advise the client to seek the services of a mold professional if the attic has an excessive amount of mold on roof boards or in the presence of ductwork.
2. We calculate & upgrade any deficiencies in eaves or ridgeline ventilation
3. We unblock openings between the Soffit and attic that are obstructed by insulation.
4. We Install foam air chutes above the Soffit to provide an unobstructed air flow path from the eaves to the ridge.
5. We install fan vent roof cap (with positive-closing damper) and insulated flex pipe’ if needed.  

Calculating Attic Ventilation
A vent’s effectiveness is measured by its Net Free Vent Area (or NFA). The NFA is the portion of the openings in the vent and bug screen that actually ventilate. Generally speaking, 50% of required ventilation should be installed at the ridge (called high ventilation) and 50% at the eaves (called low ventilation). Ideally, the low ventilation will be distributed evenly around the eaves to create a balanced flow to the ridge.

Edge Venting
Many older homes do not have overhanging Soffits. The rafter terminates at the outside wall. This transition is covered by a fascia board, followed by the gutter. Homes such as these (e.g.: bungalows, Cape Cods, some hip roof construction, etc.) are very susceptible to moisture build-up, ice dams and attic mold. Without an upward flow of air to the ridge, there is no means of evacuating moisture and heat that collects under the roof.

Applied Building Science uses an “edge venting” product to efficiently counteract this problem. Air is able to flow through the edge of vent panel, down a slot that is cut across the roof boards and up the rafter cavities to the ridge. Once the shingles are installed over the vent panel, there is little visual evidence of the edge vent with the gutter in place.

Ice Dams
http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/96/961109.html

Powered Attic Ventilators and Whole House Fans
Applied Building Science does not install powered roof ventilators (PAV) or whole house fans, nor recommend their use.  The benefits of a well sealed, ventilated, and insulated attic far outweigh the perceived value and negative operating costs of their use.

Studies have shown that a temperature difference between the attic and the outside of 10-20 degrees F in the summer months and 5-10 degrees F in the winter months is acceptable. Attic temperature should be monitored to determine whether it fits within these boundaries, as well as the presence &performance of the natural ventilation strategy currently in place. Alternate means of ventilating the attic should be wisely considered only if the geometry of the attic or roof structure defies the physics of using natural ventilation techniques.

PAVs can create as many issues as people think they are solving. The attached article (Attic ventilation.pdf) is quite compelling, in that PAVs can create serious health, safety and energy efficiency issues because they actually de-pressurize the attic.

Whole house fans do move a lot of air. But, they do require a sufficient source of incoming air to match the air being exhausted. Otherwise, the fan can backdraft the water heater or even pull car exhaust from the garage. They can also pressurize the attic and actually force hot, humid air down the gaps in wall cavities, dropped ceilings, recessed lights, etc.), bringing with it insulation fibers and other attic pollutants.  They also leak a lot of air into the attic around the louvers and housing when not in use in winter. This requires Applied Building Science to construct insulated boxes around the fan in the attic to prevent heat loss from living space and cold air infiltration from the attic.

Footnotes
1/   “Venting of Attic and Cathedral Ceilings” by William B. Rose and Anton Ten Wolde from the ASHRAE Journal October 2002