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Insulation and Attic Ventilation
by Dr. Oneida Cramer
In an effort to get a handle on the mold issue plaguing our homes, building experts are taking a long and close look at home ventilation systems.
?It appears to me that in the area of building science, the zeal for more energy efficient buildings recognized the need for air tightness while ventilation was overlooked,? reported president of Creative Building Solutions, Barry Holloway, at a meeting held recently at AIA Dallas, a Chapter of the American Institute of Architects.
Basically, mold requires three conditions for growth?(1) food, (2) warm temperatures between 45 and 120 degrees, and (3) moisture, according to Holloway. In an ideal environment, mold can grow in as little as 24 to 48 hours.
Simplistically speaking, all buildings are like boxes of air with the inside and outside air trying to reach equilibrium, reported Holloway. As air passes on a wall, it cools down and, if the air actually reaches dew point (100% humidity), it will condense on the wall. Condensation on fiberglass insulation will cause moisture build up and, eventually, mold growth because fiberglass is a good food source for mold. Also, the insulation tends to reduce ventilation, locally.
?In the case of fibrous insulation materials, thermal performance is in direct proportion to the thickness of the material,? reported Holloway. ?Therefore, the thicker the fiber material, the more resistance there is to air flow effect.?
Insulation and carpet are two of the major household culprits that support mold growth in this area of the country, according to Christine Flage, president of Icynene Insulating System. Vinyl-coated wallboard has also been found in every wallboard mold infestation that Holloway has inspected.
?What I find remarkable (and it?s kind of frightening) is where individuals have had mold problems, and when they repair that mold damage, they put the same system back in place that was originally contributing to the cause in the first place,? said Brian Flage, vice-president at Icynene Insulation System. ?That?s where the confusion is?what caused this to happen in the first place? In a lot of cases, it?s a plumbing leak or something like that. But, in other cases, the more difficult ones and usually the more severe ones, there is a condensation issue, and those are a lot more difficult to pinpoint, and correct and control.?
Finding the best ventilation system that retards mold growth could become an issue of design, perhaps.
Using the traditional method of attic ventilation, air flows from the outside through (vents) soffits, along the bottoms side of the attic floor, and then up into the attic space, according to Bill Ellis with TTL Products. This airflow cools the radiant heat that builds up underneath the roofing and also helps retain coolness in the air-conditioned ductwork that is in the attic. And the ventilation tends to retard condensation and avoid mold growth.
The highly geometric roofing designs on many new homes impede the flow of air through the attic, thus reducing ventilation. Venting space is also blocked at the overhang of the attic by the practice of constructing 10-foot ceiling interiors in homes with 8-foot exterior walls, according to Ellis. Even the large decorative crown mouldings can sometimes cut into the space on the soffit vents. PPL products (972-263-6033) often must open the vents behind soffits and clear paths for airflow before they can install their solar powered roof ventilators.
?With the proper attic ventilation, you reduce your cooling costs because you?ve reduced the load on your air-conditioner,? Ellis said. ?For an energy efficient house, there are three main keys and that?s (1) attic ventilation, (2) attic insulation, and (3) a good HVAC unit.?
?In most cases the HVAC mechanic wants zero call-backs, so he over-sizes the unit,? reported Holloway. ?This means the interior temperature can very quickly be controlled; but short cycling of the unit occurs. Over sizing can also cause the unit to not operate long enough to dehumidify (the air). In addition, the interior house temperature can be lowered to a point far below the ambient temperature dew point. And this can lead to condensation on interior surfaces.?
Then, about 10 to 15 years ago, emerged a new idea?the un-vented, completely sealed attic thermal envelope.
?There?s a building science community that?s pushing this very hard,? Brian Flage said. ?What we suggest is doing an un-vented attic and actually bringing the (HVAC) ductwork into the air-conditioned space of the house. There are two ways of doing that: (1) the best overall design for a house would have the air handler somewhere downstairs like in a utility closet or something like that and all the ductwork would be within the traditional conditioned space of the house. But with slab on-grade construction and the other vast majority of the plans and just the way people are accustomed to living, all that stuff goes in the attic. So, (2) the other alternative is to move that thermal envelope from the ceiling (house interior) up to the actual roof deck thereby including the ductwork within the conditioned space.?
?The only downside to that approach is (1) an increased volume of the house, in theory, adding maybe 3 to 5% to the load requirements of the AC system. In reality, if using Icynene (insulation) foam though out the house, the overall difference is a reduction in AC because the entire structure is tighter. We actually see a 30 to 50% reduction in tonnage requirements of the AC system. (2) The only other potential downside to inflating the underside of the roof deck is an increase in shingle temperature. It?s not an issue with tile or metal roofs or even wood shingles. But with traditional asphalt shingles, there is a 3 to 5% increase in shingle temperature, which, in theory, can cause a reduction in shingle life. Actually, we have a statement from Elk Corporation that they honor their full warranties if you use Icynene on the under side of the roof deck.?
Icynene insulation comes as two separate sprays that are water blown into the frames of the walls and ceilings. Wet when the chemicals hit the wall, they undergo a volatile reaction that within ten seconds forms the dry Icynene open-cell air foam insulation. This insulation looks like angel food cake; but it is inorganic and will not support mold growth. For more information, call 972-831-1100.
?The other thing that we strongly recommend that you don?t see very often is some sort of mechanical ventilation system to bring in the correct amount of fresh air from the outside,? Brian Flage said. ?We?re saying build a very tight envelope. But we?re not saying live within that plastic bag. Bring in the correct amount of fresh air?but again, since it?s a controlled amount, you can dehumidify that air and filter it.? Programmable mechanical ventilators are available and they generally go on a separate circuit that cycles independently of the fan on the heating/air conditioning unit.
Dr. Oneida Cramer
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