Good News at Home Hankey & Brown Inspection Service June 2013 Last month we discussed air conditioner maintenance. This month s topic is: Attic Ventilation. Next month s topic is fire protection in homes with attached garages. Most Twin Cities homes have an attic below all or portions of the roof. The attic can become quite hot in the summer if it is not adequately ventilated. An overheated attic creates two adverse conditions: 1. The heat can damage and/or shorten the life of the roofing. (Excess heat can bake the volatiles out of asphalt shingles.) This the reason that asphalt shingle manufacturer s will often not honor a shingle warranty claim if it can be shown that the roof - attic was not ventilated to their specifications. 2. Excessive heat in the attic can increase the cooling costs for the home. Attic ventilation is also beneficial in winter. A well vented attic helps keep the roof sheathing close to the temperature of any snow on the roof. This reduces the potential for ice dams by minimizing roof snow melt. Proper attic ventilation on simple roofs is not complex. A straight gable with good overhangs is a good example of ideal roof ventilation. The 1/300 ratio shown in the drawing assumes the ceiling has a good air and vapor barrier. Figure 1 Unfortunately, many Twin Cities homes do not have a simple gable roof with one attic. One common difficult to vent house is the typical 1-1/2 story design popular from1930 to the 1960. These homes have multiple attics.
Figure 2 Figure 2 discusses insulation, but each of the 4 attics shown must be ventilated unless the roof itself is insulated. The decision of how to insulate and ventilate a story and a half house is based by whether or not a space below the roof is to be unconditioned attic, or a conditioned finished space. Figure 3 shows two ways the attic can be insulated. Note that option B creates a warm space that is NOT ventilated. What is NOT shown in option A is a vent for air to enter the attic. The only vent shown is near the top of the knee wall. This vent lets air out, in order for that vent to work; there must be an air inlet. Traditionally, the air inlets installed for knee wall spaces were simple louvers known as gable vents, places in the gable end walls of the knee wall attic. Often these gable vents do not work well since they often can not be low enough to create a convective loop of cool air in low and warm our out near the top of the knee wall. Fortunately one roof vent manufacturer has created a Smart Vent that integrates into the roof and works as either an inlet or outlet vent depending on where it is placed on the roof. See Figure 4.
Figure 3 Figure 4 Several design factors are important in successful venting. 1. The best vents are continuous along the soffit (eave) and ridge, since this helps all areas of the roof receive uniform air flow.
2. The ventilation must be balanced such that the air flow out is matched by an equal amount of air coming into the attic. (Most manufacturers recommend more air intake than exhaust to ensure the exhaust vents do not create negative air pressure in the attic.) 3. Types of vents should not be mixed. Standard box vents (roof hats) should not be placed below ridge vents as this will interrupt air flow to the ridge. 4. Gable vents typically do not establish a convection flow of cool air in and warm air out. 5. When having the house reroofed, discuss venting carefully with the roofing contractor. Ridge vents generally look much better than multiple box vents, and usually work better. There are numerous types of ridge vents. Most authorities agree that those with wind baffles tend to perform better in windy conditions. 6. Turbine vents tend to concentrate air flow in a few places and may leave other areas with little air flow. Further, turbine vents depend on bearings, which can fail with age and/or become noisy. Also turbine vents are vulnerable to powder snow entry. 7. When replacing box vents, be sure the roofer centers the vent in a clear opening, and be sure to specify bird resistive vents. (Most roofers use these now.) Vents require little maintenance; however their appearance can indicate their effectiveness. 1. Most soffit vents have small openings and screening. If the vent is more than a year or so old and has NO dust in it, it probably isn t working. Modern perforated metal soffit panels are often installed over old wood soffits. We occasionally find that old wood soffits were not cut open to provide a suitable path for air to reach the attic. 2. Those small openings in the soffit vents are highly vulnerable to PAINT. Any
paint other than the original factory finish will typically clog the vent openings. Replacement would be the best remedy. 3. Attic re-insulation will clog the opening from the soffit into the attic unless insulation baffles are installed prior to re-insulating. Previous issues of our newsletter are archived at our website: http://www.hankeyandbrown.com/goodnewsathome There is also a link to subscribe to the newsletter at the same site. Have a great month.