Appendix Ridge Vent Sof T Vent Calculator For Standard -Books Pdf

Appendix Ridge Vent Sof t Vent Calculator for Standard
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Section 3 Determining Ventilation Requirements 13, to create a ow of air In addition this standard assumes. Section 3 Determining a proper balance of exhaust and intake venting Unfor. Ventilation Requirements tunately it s probably safer to assume that assumption. rarely holds true, Before the mid 1970s few people thought about establish If you want to install an effective year round ven. ing precise requirements for attic ventilation Homes tilation system follow the steps below which are based on. weren t built as airtight as they are today If a home had any the 1 150 ratio This ratio takes into account that today s. attic ventilation at all it usually consisted of some under homes are built with or remodeled with materials. eave vents In some warmer areas of the country one or doors insulation windows etc that are more energy. more louvers might supplement those vents the purpose ef cient Consequently these homes are more airtight. being as already mentioned to catch the breeze In and need more attic ventilation. especially warm regions an attic fan might be installed. Calculating requirements for an ef cient static vent system. even though there might not be suf cient intake venting. to assure proper functioning The math involved in calculating ventilation requirements. Even if designers and speci ers had wanted to calculate is simple A pad and pencil are all you need. speci c requirements for temperature or moisture, reduction they had little research based information to Note The following process is used to calculate requirements. guide them for non powered ventilation systems If you plan to install a. The Federal Housing Administration tried to close that power fan see calculation instructions on page 14. information gap with minimum property standards for. buildings with one or two living units Since then other 1 Determine the square footage of attic area. standards have been developed An example of current to be ventilated. minimum requirements for ventilation comes from the. 2003 International Residential Code IRC Section R806 To do that just multiply the length of the attic. in feet by its width, R806 1 Ventilation required Enclosed attics and Example For this and the following calculations. enclosed rafter spaces formed where ceilings are applied directly we ll assume the home has a 40 by 25 attic area. to the underside of roof rafters shall have cross ventilation for. each separate space by ventilating openings protected against Calculation. the entrance of rain or snow 40 x 25 1 000 square feet of attic area. R806 2 Minimum,R806 2 Minimum area,area The total,The total net free.
net free ventilating,ventilating areaarea, shall not be less than 1 to 150 of the area of the space. not be less than 1 to 150 of the area of the space ventilated ventilated 2 Determine the total net free area required. except that the total area is permitted to be reduced to. 1 to 300 provided at least 50 percent and not more than Once attic square footage is known divide by 150. 80 percent of the required ventilating area is provided by for the 1 150 ratio That determines the total amount. ventilators located in the upper portion of the space to be of net free area needed to properly ventilate the attic. ventilated at least 3 feet 914 mm above eave or cornice vents. with the balance of the required ventilation provided by eave or Calculation. cornice vents As an alternative the net free cross ventilation area. 1 000 sq ft 150 6 6 square feet of total net free area. may be reduced to 1 to 300 when a vapor barrier having. a transmission rate not exceeding 1 perm 57 4 mg s. 57 4 mg s m2,is installed on the warm side of the ceiling. 3 Determine the amount of intake and exhaust,R806 3 VentVent clearance. clearance Where,Where eaveeave or cornice,or cornice ventsvents. low and high net free area required, are installed insulation shall not block the free ow of air.
A minimum of a 1 inch 25 4 mm space shall be provided For optimum performance the attic ventilation system. between the insulation and the roof sheathing at the location must be balanced with intake and exhaust vents. of the vent, This is a simple calculation just divide the answer. Is adequate attic ventilation now assured by following from Step 2 by 2. this requirement, The intent of the requirement after all is to establish Calculation. minimum standards For example the IRC permits the net 6 6 2 3 3 sq ft of intake net free area and 3 3 sq ft. free area requirement to be reduced to the 1 300 ratio in of exhaust net free area. certain situations That amounts to less than 1 2 of vent. area for each square foot of attic oor area barely enough. 14 Section 3 Determining Ventilation Requirements, 4 Convert to square inches Note For roofs with a 7 12 to 10 12 roof pitch you may. want to add 20 percent more CFM and for roofs 11 12 pitch. The net free area speci cations for attic ventilation and higher add 30 more CFM to handle the larger volume. products are listed in square inches Therefore let s of attic space. convert our calculation in Step 3 from square feet. to square inches To do this simply multiply by 144 2 Determine the amount of intake venting. required The formula is,Calculation, 3 3 sq ft x 144 475 sq in of intake net free area and CFM rating of fan 300 square feet of intake. 475 sq in of exhaust net free area ventilation needed. 5 Determine the number of units of intake Calculation. and exhaust venting you ll require 700 300 2 3 square feet. To make these calculations rst refer to the Net Free To turn that gure into square inches multiply by 144. Area Table below The table lists the approximate net. free area in square inches for common intake and Calculation. exhaust ventilation units 2 3 x 144 331 square inches of net free intake area. To perform the calculations divide the net free area To nd the number of intake vents required use the. requirement from Step 4 by the appropriate gure from Net Free Area Table below see Low Vents Intake. the Net Free Area Table3 For our example we will use. the gures for ShingleVent II and undereave vents Net Free Area Table. Net Free Attic Vent Area,Calculation Type of Vent sq in approximate.
for 4 foot length of ridge vent,High Vents Exhaust. 475 sq in 72 6 6 pieces of vent,FilterVent 8 length 144. or seven 4 foot lengths of ridge vent,ShingleVent II 4 length 72. for 16 x 8 undereave vent Roof louver 50, 475 sq in 56 8 5 pieces of vent Wind turbine 12 112. or nine 16 x 8 vents Rectangular gable louvers,12 x 12 56.
12 x 18 82, Calculations for power fan installations 14 x 24 145. If you plan on installing a power fan you can 18 x 24 150. calculate intake and exhaust requirements using the 24 x 30 324. following formulas Low Vents Intake,16 x 8 undereave 56. 1 Determine the fan capacity needed to,16 x 6 undereave 42. provide about 10 to 12 air exchanges,16 x 4 undereave 28. Continuous Sof t Vent 1 length 9,Vented Drip Edge 1 length 9.
The formula is, Attic square feet x 0 7 CFM capacity Perforated aluminum sof t. One square foot 14, For example using the same dimensions as the previous Lanced aluminum sof t. example One square foot 4 7, Be sure to check speci cations for individual products to. Calculation determine actual net free vent area,1 000 sq ft x 0 7 700 CFM. 3 You can also use the calculation table in the Appendix to. determine the number of feet of ridge vent and sof t vent. required for an installation, Introduction The Year Round Bene ts of Proper Attic Ventilation 1.
Of course the longer these hot sunny conditions last. Introduction The Year Round Bene ts, the more uncomfortable it becomes in the home That s. of Proper Attic Ventilation because an unventilated or inadequately ventilated. attic seldom loses enough heat overnight to compensate. for the heat gained during the day Ironically the effect. is magni ed in modern homes with heavier insulation. What s the purpose of attic ventilation It seems like a see the insulation ventilation connection on page 2. simple question easy enough to answer Unfortunately Eventually this accumulation of heat begins to have. all too often that s not the case Most homeowners more practical and costly consequences. and even some experienced builders and contractors The most obvious are the actions taken by homeowners. believe the purpose of attic ventilation is to remove heat to cool themselves To reduce the effect of the heat. that builds up in the summer not only the daytime heat gain but also the excess heat. That s accurate of course But what that answer leaves being stored in the attic they turn on fans window air. out is just as important as what it includes conditioners or central air conditioning systems As the. If you understand the principles of attic ventilation hot weather continues these appliances run longer and. you know an effective venting system provides year longer a fact well documented by utility companies. round bene ts across the country Homeowners pay for all this added. During warmer months ventilation helps keep energy consumption in higher utility bills. attics cool A less obvious but equally costly consequence. During colder months ventilation reduces moisture can be found on the roof itself Homeowners can t see it. to help keep attics dry It also helps prevent ice dams happening but over time excess attic heat can cause some. We can make that answer more speci c and more shingles to distort and deteriorate The result is premature. meaningful by translating those functional descriptions failure of roo ng materials and perhaps a leaky roof. into a list of bene ts Once that happens the cost of a new roof is the least. Several purposes of an attic ventilation system are to homeowners can expect to pay More than likely they. provide added comfort to help protect against damage also may face added costs for structural and interior. to materials and structure and to help reduce energy repairs related to water in ltration. consumption during all four seasons of the year, Your goal should be to provide those bene ts whenever Figure 1. you design and install an attic ventilation system The. rest of this booklet will show you how,Ventilation During Warm Weather. Dealing with the effects of heat Why on a hot day Roof Sheath. are the upper rooms of a home always warmer Temperature. Part of the answer of course is simple physics hot 140 Attic 115 Attic. lighter air rises while cooler denser air falls But in Temperature Temperature. most homes the vast majority of homes without adequate. attic ventilation a far more important factor comes into. play the downward migration of heat,Consider what happens in such a home on a typical. summer day see Figure 1 Radiant heat from the sun hits. the roof The roof temperature increases and heat travels. technically it conducts through the roof sheathing into. the attic As heat builds up in the attic it radiates to the. attic oor then into adjacent living areas raising,temperatures there.
You appreciate the effects of that process when you. look at the temperatures involved These are typical. temperatures for a home with no attic ventilation on a Unvented Radiant heat penetrating through roof sheath and. sunny day with an outdoor temperature of 90 F 32 C attic enters living areas of home. Temperature at roof sheath as high as 170 F 77 C Vented With proper ventilation the heat is vented out of the. Temperature at attic oor up to 140 F 60 C attic keeping living areas cooler. Temperature in rooms directly beneath attic,uncomfortable. 2 Introduction The Year Round Bene ts of Proper Attic Ventilation. The insulation ventilation connection Problems arise when the warm moist air from the living. Ef cient insulation increases the need for effective ventilation quarters moves toward the attic where the air is cooler and. Why Because heavier insulation absorbs and holds more drier That moist air is drawn to the attic in two ways. heat That means it s less likely overnight cooling can remove The rst is through a process called vapor diffusion It s. heat that builds up in an attic during a prolonged period of hot a process in which water vapor naturally travels from high. sunny weather humidity conditions to low humidity conditions in. The solution to this dilemma isn t to reduce the insulation in our example from the living quarters into the attic The. an attic That would only create problems at other times of the force of vapor diffusion is so great that moisture even. year Instead the goal is to design an attic ventilation system that travels through building materials such as sheet rock. effectively compensates for the additional heat gain produced. by the high levels of insulation Figure 2, In short effective attic ventilation also helps cool attic insulation. How ventilation helps solve attic heat problems, Ventilation can t eliminate the transfer of heat from roof. to attic but it can minimize its effect To do that a well. designed system must provide a uniform ow of cool air. along the underside of the roof sheathing That steady. ow of air carries heat out of the attic before it can. radiate to the attic oor, It s critical that this air ow is uniform That means intake. and exhaust vents must be balanced for both position. and air ow capacities Otherwise hot spots can develop. under roof sheathing drastically reducing the ef ciency. and effectiveness of whatever ventilation is installed. Ventilation During Cold Weather, Unvented Moisture rising up through the house condenses in the.
Dealing with the effects of moisture buildup When, winter arrives and temperatures plunge you might think attic causing damage to studs insulation and other materials. the movement of heated air would no longer cause prob Vented A vented attic allows moisture to escape. lems in attics But that s not true With seasonal changes. the conditions just reverse Heat doesn t travel from an attic. into the living quarters Instead heated indoor air travels Even vapor barriers for all their effectiveness cannot. from the home into the attic along with moisture totally stop this process The second way moisture travels. Figure 2 illustrates how this process of moisture transfer into an attic is by air moving through openings cut into a. takes place Furnace warmed air circulates through the vapor barrier Such openings are commonly found for. house picking up water vapor generated by activities such example at recessed ceiling boxes and attic entries. as cooking bathing and the washing of clothes and dishes The problems start when moist air hits cooler rafters. The use of humidi ers common in many homes provides trusses and roof sheathing The moisture condenses as. an abundant and continual source of moisture Keep in water droplets or frost Eventually the condensation. mind also that the warmer the air is the greater its drips on the insulation below If too much water soaks. capacity to hold moisture into the insulation its volume can be compressed and its. The problem is especially acute in homes with electric effectiveness reduced The sequence of events that follows. heating Most of these homes were built since the mid is predictable greater heat loss leads to colder rooms. 1970s using advanced insulation materials and methods colder rooms lead to a greater need for heat greater use. As a result most are tight allowing minimal in ltration of the furnace leads to higher energy bills. of outside air In addition electric heat sources do not But that s only the immediate problem and its con. require air for combustion so another common source of sequences As with heat buildup moisture buildup has. outdoor air has been eliminated The positive side of these long term effects That s because not all the condensing. super insulated homes is of course the greater energy moisture drips into insulation The structural elements. ef ciency But because cooler drier outdoor air is kept out of the house absorb some leading to wood rot and the. the indoor air holds greater amounts of moisture deterioration of roo ng materials Other moisture is likely. to soak into the attic oor and eventually into ceiling. materials causing water stains and paint damage in. the rooms below, Introduction The Year Round Bene ts of Proper Attic Ventilation 3. How ventilation helps solve attic moisture problems A heavy snow cover accumulates on the roof This. Although the problems of attic heat and moisture have snow accumulation not only provides the necessary. different causes they share a common solution a high moisture it also acts as a layer of insulation preventing. ef ciency ventilation system that allows a uniform ow heat loss through the roof sheathing As a result tem. of air to sweep the underside of the roof sheathing In peratures in the attic are typically warmer than they are. warmer months such a system exhausts hot air from an on days when the roof is free of snow. attic in the colder months it exchanges warm moist air. with cooler drier air In both cases the result is the When all three conditions are met ice dams form quickly. same less damage to a home Heat high in the attic causes snow to melt near the roof. peak The water from the melting snow ows toward the. Dealing with the effects of ice dams Winter creates a eave area where colder roof temperatures allow it to. special attic ventilation problem in areas where snowfall refreeze If conditions persist over several days this re. and cold temperatures are common occurrences The freezing of snow melt can form an ice dam see Figure 4. problem begins with the formation of ice dams literally The weight of the dam itself can damage gutters and. barriers formed of ice that prevent melt water from fascia When it eventually falls it also can damage struc. running off a roof The map in Figure 3 shows areas of tures or shrubbery below But the greatest damage occurs. the U S where average winter conditions can lead to when the water pooling inside the dam begins to in ltrate. the formation of ice dams under shingles The shingles themselves are damaged. if not destroyed Far more serious however is the damage. Figure 3 Areas of snowfall in the U S caused at the plateline area Insulation can be soaked. reducing its effectiveness Plus water can in ltrate into. 35 average January temperature both exterior and interior wall cavities leading to structural. damage and the deterioration of painted surfaces At the. very least mold and mildew can form creating unpleasant. odors and mold spores resulting in poor indoor air quality. Melted snow and ice,penetrate roof,snowfall Ice damages. line roof structure,Above the 6 8 snowline concern should be given to. the prevention of ice dams forming on the eave of the roof. Ice dams can form when the following conditions exist. Warm air accumulates near the peak of an attic This. condition is much more common than people think, It occurs because most attics experience some heat loss.
from attic insulation And because warm air rises the. upper portion of an attic is always the warmest Normally. that pocket of warm air won t result in problems,that is until the following conditions are met. Lower areas of the roof remain cold Once again, this is a common condition especially in the area just. above the eave where temperatures may not be much, higher than the ambient outdoor air If the outdoor. temperature is well below freezing conditions are, favorable for the formation of an ice dam Unvented Heat entering attic from the home melts the snow on. the roof and forms destructive ice dams, Vented Heat is vented out of the attic creating a cold roof.
4 Introduction The Year Round Bene ts of Proper Attic Ventilation. How ventilation helps solve ice dam problems When A defense against ice dams. homeowners set out to eliminate ice dams their typical To reduce the possibility of ice dams use a three step approach. response is to add more insulation to attics But no amount 1 Install adequate attic ventilation Because ice dams. of insulation if used alone can eliminate the formation form when a roof has warm upper surfaces and cold lower surfaces. of ice dams An ef cient attic ventilation system must be the solution is to equalize temperatures over the entire roof The. part of any solution most effective way to equalize temperatures is to create a cold roof. A properly designed ventilation system creates a cold To do that you need a well designed attic ventilation system. roof a condition where the roof temperature is equalized that will supply air ow along the entire underside of the roof deck. from top to bottom An equalized roof temperature in That s critical because only a uniformly distributed air ow can. turn helps eliminate the conditions that lead to the reduce variation in roof temperatures from peak to eave. formation of ice dams see Figure 5 One of the most ef cient and effective systems from both cost. By now you probably know how ventilation creates and performance standpoints uses ridge vents and an evenly. a cold roof it allows a ow of air to sweep along the distributed layout of sof t vents. underside of the roof sheathing minimizing temperature 2 Install adequate attic insulation Attic insulation serves. differences By now you also may have a valid question to two purposes First it reduces heat loss from a home s living. raise Aren t we talking about a uniform ow of cold air quarters Since that heat loss is a key factor contributing to the. sweeping through the attic creation of ice dams stopping it at its source is critical Second. Exactly and that s why ventilation alone isn t a complete adequate attic insulation diminishes the energy impact of having. solution either For maximum comfort reduced structural cold air owing through the attic. damage and optimum energy conservation ventilation When installing insulation or checking existing insulation. must be used with a waterproo ng shingle underlayment be sure to install adequate amounts around electrical xtures and. and of course with insulation Ample insulation1 is wiring and plumbing chases These areas often contribute to. required to minimize heat losses and high ef ciency air signi cant heat loss With existing insulation also check for. movement is required to remove any heat that enters the water damage and for areas compressed by foot traf c or stored. attic Figure 6 illustrates insulation recommendations objects Finally make certain existing insulation meets today s. based on geographic zones R Value requirements,3 If possible install waterproo ng shingle. Figure 5 underlayment WSU Even the most ef cient attic ventilation. system may not be enough to eliminate all ice dams A combination. of weather conditions roof pitch building orientation and other. factors may allow ice dams to form under certain conditions If that. happens a WSU barrier can minimize and possibly eliminate. water in ltration into the building structure see Figure 7. Install WSU according to the manufacturer s instructions In. general install WSU at least two feet above the interior wall line. many contractors say a three foot barrier is even better When. working in valleys install WSU three feet on each side of the. valley center see Figure 8, 1 It s dif cult to say precisely how much insulation will be required Many. Top Ice dams besides being unsightly are destructive factors from house design to its orientation to the weather enter into the. Bottom Vented attic with snow melting evenly is much equation A good rule of thumb however is to provide at least 10 to 12. more desirable inches of insulation That s equivalent to an R Value of 38. Section 1 How Ventilation Works 5,Figure 6 Figure 8. Thermal Recommendations,Winter Heating Plus Summer Cooling. The dark shaded areas are the places that waterproo ng. Zone Ceiling Wall Floor shingle underlayment helps protect from the melting water. Insulation Insulation Insulation coming off the ice dam. R Value R Value R Value,1 R 19 R 11 R 11,2 R 30 R 19 R 11 Section 1 How Ventilation Works.
3 R 38 R 19 R 13,4 R 38 R 19 R 19,5 R 49 R 19 R 25. Ventilate comes from the Latin word for to fan,the action of causing air to move. Snow And that s exactly how ventilation works it provides. the conditions that allow air to move, Melted snow and ice For our purposes however we have to get a little. penetrate roof, more technical because ef cient ventilation requires a. very speci c type of air movement We re not interested. Ice damages in moving air just to create a breeze that cools us by. roof structure speeding evaporation Instead we want ventilation. that provides year round bene ts, If you ve ever walked into the stuffy con nes of a.
room that s been completely closed for a lengthy period. Waterproo ng you know air tends to stay in place You also know that. shingle underlayment, just opening a door or window doesn t solve the problem. immediately A ow of air must be established to produce. the air changes needed to remove all the stale air. That s what an ef cient ventilation system must do. too provide a steady high volume of air movement, That means the system components must be sized and. positioned to provide a constant ow of air moving in. a constant direction, Top Water can penetrate to an unprotected roof sheath We can create air movement in one of two ways. causing the roof sheath to rot using natural ventilation or mechanical ventilation. Bottom Waterproo ng shingle underlayment helps prevent. water from penetrating to the roof sheath,6 Section 1 How Ventilation Works. Using natural ventilation Natural air movement is a home s exterior surfaces A wind driven ow of air. created by two key forces thermal effect and wind creates areas of high and low air pressure see Figure 10. see Figure 9 High pressure forces air into the attic while low. pressure draws air out, Wind passing over the baf ed ShingleVent II ridge vent.
creates a low pressure area at the vent s openings which. causes air to be lifted or pulled out, Thermal Flow effect whereby cooler air falls warmer air rises. and Natural Flow effect due to wind come together to ventilate How to put these natural forces to work A properly. an attic designed ventilation system requires balance That. balance is achieved in two ways, Thermal effect We ve already mentioned thermal effect 1 Air ow capacity must be balanced between intake and. brie y It s the inherent property of warm air to rise A exhaust vents In general the net free area2 of intake. well designed system takes advantage of that movement venting should be equal to or greater than the net free. in two ways area of exhaust venting To determine how much net. First since warm air rises an effective system will include free area a particular home requires see Section 3. exhaust vents at or near the ridge That placement allows. the hottest air to be removed from the attic most ef ciently 2 Intake and exhaust vents must be positioned to create. Second the thermal effect creates a natural circulation a proper high low balance That balance is achieved. of air because as warm air rises cooler air falls A well when two conditions are met. designed system assists this momentum by placing intake. vents at the lowest point in the attic typically in the sof t a Half the vent area must be high in the attic with. The cooler air entering these vents cooler as compared the other half low in the attic Without that balance. to the attic air speeds this circulation of air the area of effective ventilation is limited to the lesser. of the two vent areas For example if 75 percent of. Wind By itself however thermal effect cannot create the venting is high and 25 percent low ventilation is. the high volume of air movement needed for effective limited to the air moving through the lower vents. ventilation That s why the in uence of wind is the key. element in the design of a non powered ventilation system b The vents placed high must act as exhaust vents. Wind after all is a natural ow of air So when designing while the low vents act as intake vents That place. a ventilation system you want to make the wind work ment assures a continuous ow of air moving in. to your advantage the desired direction,To use the power of wind you have to understand. how wind force affects ventilation It isn t the velocity of. the wind by itself that causes air to move through an attic 2 Net free area means the total unobstructed area usually measured in. Instead it s the wind s speed as it moves against and over square inches through which air can enter or exhaust a non powered. ventilation component,Section 2 Types of Vents 7, In planning the location of intake and exhaust vents Figure 11. two other factors must be considered, 1 Intake and exhaust vents must be positioned so they Insulation.
assure continuous air ow along the underside of the blocking. roof sheathing As we learned in the introduction this air ow. is where ventilation is most effective During summer. air ow along the sheathing removes heat before it can. radiate to the attic oor During winter air ow along. the sheathing removes moisture before it can condense. into water droplets or frost, 2 Intake vents must be located so there is little possibility. of rain or snow in ltration Obviously if wind driven air chute. moisture were allowed to enter an attic one reason for. a ventilation system would be negated We would,simply exchange one problem for another similar. problem To prevent this from happening intake, vents should be placed in protected areas the most. convenient being in the sof t area underneath the,eave of the house. Placing intake vents in the sof t doesn t assure that Top Insulation blocks undereave intake vents preventing. a strong wind won t drive moisture into the openings proper air ow into the attic. But should that happen the area around the sof t is. Bottom An unblocked undereave vent allows a passage. less likely to suffer major damage For one thing,for air to move through attic.
insulation isn t installed in the sof t so the problem. of wet insulation is avoided In addition rain or snow. entering a sof t vent is more likely to drain back. through that opening At worst the moisture would, be con ned to the sof t area where it can evaporate Section 2 Types of Vents. quickly without causing permanent damage, Note To assure optimum performance of intake vents. you must make certain the area above the intake opening. isn t blocked by dirt building debris or attic insulation. see Figure 11 In describing how ventilation works we discussed intake. and exhaust vents in general terms perhaps giving the. impression that a single type existed to serve each function. In fact however you can choose from a wide range of. intake and exhaust components allowing you to tailor. ventilation systems to the speci c characteristics of. every home,In general ventilation components can be divided. into two broad categories intake vents and exhaust vents. Within each category there are various styles Furthermore. ventilation components are either xed also called static. or powered,Fixed ventilation, Fixed ventilation components are exactly that units that. don t require moving parts or power assistance for proper. functioning But don t let that description lead you to. believe xed ventilation is a low tech alternative to high. ef ciency systems Just the opposite is true Fixed ventilation. components form the core of all attic ventilation systems. from the simplest to the most sophisticated In fact in most. cases your initial goal should be to try to design a ventilation. 8 Section 2 Types of Vents, system that uses only xed vents or ridge vents a special Figure 12.
high ef ciency type of xed ventilation, Obviously since xed ventilation can be used to create. an entire system units are available for both intake and. exhaust functions, Intake vents The best place to install intake vents is in. or near the roof eave That location provides two key. advantages, 1 The vents are better protected from rain and snow. in ltration Vents mounted in the eave provide almost. total protection We qualify that statement only to. acknowledge the possibility that winds at or near, hurricane force could drive moisture into an undereave The undereave vent an intake vent allows needed air to enter. vent In normal conditions however undereave the attic It is located on the underside of the eave of the house. vents don t allow moisture in ltration, 2 Usually when undereave vents are placed on both Figure 13.
sides of a roof as they should be there s always an. equal distribution of high and low pressure areas, Note This doesn t mean there s an adequate distribution. of high and low pressure areas A ventilation system that. uses only undereave vents violates the principle of a balanced. system intake vents and exhaust vents What this means in. practical terms is that the system will provide a continuous. ow of air along the attic oor but not along the underside. of the roof sheathing where it does the most good, Intake vents are available in many designs In choosing. the right unit for a particular job you have to consider the Continuous sof t vent takes in outside air and is located. structure of the home the area where the units will be on the underside of the eave. located and the net free area provided by each unit. The most common types of intake venting are, Undereave vents which are mounted in the sof t Units Exhaust vents Exhaust vents are designed to permit. vary in size from 16 x 8 to 16 x 4 Naturally net free an ef cient unobstructed out ow of attic air In addition. area varies according to unit size see Figure 12 because they re installed high in the attic where there s. Continuous sof t vents which are also mounted in the greater exposure to the weather these units must be. sof t These units vary in length with the typical length designed to prevent or at least minimize rain and. being 96 see Figure 13 snow in ltration, Vented drip edge which is used on homes without an Exhaust vents must be used with intake vents to. eave area provide a balanced system and thus an adequate ow of. Mini louvers which are typically used with other air through an attic It s also worth repeating another point. types of intake venting they re too small by themselves made previously for maximum ef ciency the net free area. to provide suf cient net free area of intake In most of intake vents should be equal to or greater than the net. applications they re installed in an exterior wall to help free area of exhaust vents. eliminate moisture that collects in the wall cavity To be As with intake vents exhaust vents are available in. effective mini louvers must be installed below the source different designs Two commonly used xed exhaust. of humidity such as a bathroom or laundry area That vents are. placement allows a ow of air to collect the humidity Roof louvers which are installed as close to the roof. and carry it into the attic ridge as possible to allow maximum release of moisture. and overheated air Because they re installed near the ridge. they provide a continuous air ow along most of the. underside of the roof sheathing The air ow pattern isn t. Section 2 Types of Vents 9, uniform however so for maximum effectiveness vents absolutely no intake venting can be installed at low points in.
should be spaced equally along the roof see Figure 14 the attic a louver only installation is preferable to no. ventilation at all, A roof louver is an exhaust vent located near the ridge. With wind blowing perpendicular to the ridge the louvers act as. Gable louvers which are typically installed in the gables Two both intake and exhaust vents. types are available rectangular and triangular In most instal. lations a unit is placed at each gable end see Figure 15. With wind blowing parallel to the ridge air ow dips. A gable louver an exhaust vent allows unwanted air to towards the attic oor leaving the hottest air still on the. ow out of the attic These are located at the ends of the attic underside of the roof sheathing. Note Sometimes louvers are installed in opposite gable ends Ridge vents. without intake venting in the mistaken assumption that a As mentioned above ridge vents are a special type of xed. good cross ow of air can provide adequate ventilation exhaust venting That distinction is warranted because. What typically happens however is illustrated in Figures 16 ridge vents offer unique advantages when compared to. and 17 If wind direction is perpendicular to the ridge the other xed venting units Those advantages include. louvers act as both intake and exhaust vents providing vent Maximum ef ciency The best ridge vents use an. ilation only in the areas near the vents If the wind direction is external baf e designed to draw heated air from an attic. parallel to the ridge a cross ow of air is established although regardless of wind direction or force Figure 18 shows how. the ow tends to dip toward the attic oor leaving the hottest that happens. air still at the underside of the roof sheathing Of course if. 10 Section 2 Types of Vents, Figure 18 Maximum air movement Externally baf ed ridge. vents work better because they take advantage of two. natural forces thermal effect or the fact that warm air rises. and low air pressure that is created at the vent openings as. wind is de ected by the baf e see Figure 10 on page 6. Uniform air movement Because ridge vents run the, entire length of a roof they provide a uniform ow of air. along the underside of the roof sheathing That air. movement helps eliminate hot spots that can develop. with other types of exhaust vents even powered vents. No other exhaust vent provides this type of air ow pattern. Maximum visual appeal Most ridge vents offer a, low pro le design that minimizes its appearance on a roof. Shingle over designs allow optimum blending with other. roof materials see Figure 19 and Figure 20, A baf ed ridge vent creates an area of low pressure on both Figure 19.
sides ridge,of the vent,ridge creates,vent an area. It literally pullsofair,lowoutpressure on both,of the attic. sides of the ridge vent It literally pulls air out of the attic. When wind direction is perpendicular to the ridge it. strikes wind direction,the external is perpendicular. baf e and jumps over to the ridge it,ridge That,movement the external. creates abaf e and jumps,Bernoulli effect over the low.
causing ridge That,to develop on creates,botha sides. of theEffect causingthat,vent When lowhappens,air develop. from theonattic bothis sides of out,lifted the vent When. in much thethat,samehappens,lowfrom the attic,pressure is lifted. created above an out in much,airplane wingthegives.
to thepressure created,plane refer above,also an airplane. to Figure 10 onwingpage gives,the plane refer happens. same thing also to Figure,when 10 theon pagedirection. parallel to thething happens,ridge It moveswhen the wind. up and direction,over the ridge is,creatingto the ridge.
a low pressureIt moves,area onup andsides,both overofthetheridge. ridge vent,creating a low pressure,In addition when littleareawind. forcesides ofridge,exists the ridge,take addition whenoflittle. advantage wind force,the thermal exists,effect ridge vents. to maintain air,take full advantage,circulation across the of underside.
the thermal of effect to maintain,the roof sheathing air. circulation,Warm air rises across underside,to the ridge of the roof. and exhausts sheathing,through the vent,Warm air rises. That allows to the ridge ow,a continuous andofexhausts through. cooler air to enterthe vent,through allows,the aintake.
continuous ow ofridge,vents Only cooler air use,vents to enter. through the intake,effect ef ciently andvents Only ridge. effectively because vents,onlyuseridge,provide ef ciently. continuous andand,effectively,uniformbecause only ridge. air movement vents,provide continuous,the full length and uniform air movement along.
the full length of a roof, Note To provide this ef cient air movement ridge venting. shouldTo beprovide,balanced thiswith,netmovement,free area ofridge. intakeventing,For bestberesults,balanced withventing. intake equal should,net freebearea of intake,divided venting. equally along, sidesresults intake venting should be divided equally.
of a structure,along both sides of a structure, Top Ridge vent installed shorter than the ridge length. presents an awkward broken appearance, Bottom A ridge vent should extend all the way along. the roof for a smooth unbroken roof line,Section 2 Types of Vents 11. Figure 20 Figure 21,Ridge Vent, A roll vent with an internal baf e or without any baf e at all. does not pull air from the attic,Roof Louvers,Figure 22 ShingleVent II.
Fifteen roof louvers are required to equal the exhaust venting of. 42 linear feet of ridge vent This clearly demonstrates the. performance and aesthetic advantages of ridge vents. It s important to emphasize that the advantages listed. above apply only to ridge vents that use an external baf e An externally baf ed vent pulls air from the attic from both. design A series of independent tests has concluded that sides of the ridge vent. only an external baf e can direct the wind up and over the. vent That s signi cant because it s that controlled ow of. air that creates the area of low pressure that causes air to Guarding against weather and dirt in ltration. be pulled from an attic see Figure 21 and Figure 22 When choosing a ridge vent look for a design that provides. maximum protection against rain snow dust and insect in ltration. If installed properly standard ridge vents with an external baf e. produce a steady ow of exhaust air at the ridge Usually that. air ow protects against weather in ltration To guard against. unusual circumstances where rain and snow in ltration may be. possible advanced products such as Air Vent s Multi Pitch FilterVent. and ShingleVent II include a patented weather barrier designed to. trap moisture before it can enter an attic, Air Vent also offers variations of its advanced ridge vent design. for contemporary homes For example Air Vent s Peak FilterVent. provides continuous venting at the intersection of a roof peak and. vertical wall Air Vent s Flash FilterVent provides similar continuous. venting at points where a roof meets a vertical wall. 12 Section 2 Types of Vents, Wind turbines Depending on the size of the motor and the ef ciency. Technically wind turbines aren t a xed vent system of the blade design power fans can move more than 1 500. because they use a moving part to help exhaust air from cubic feet of air per minute CFM That high volume of. an attic That moving part consists of a series of specially air movement is critical To ensure adequate ventilation. shaped vanes that turns wind force into a rotary motion power fans must provide at least 10 changes of attic air. As the spinning vanes gain velocity they create an area every hour Some models offer a two speed option that. of low air pressure That low pressure in turn pulls air allows fan speed and air movement to be determined. from an attic by the demand for increased ventilation. Although not as effective as ridge vents wind turbines Although a power fan can move a large volume of. provide a low cost alternative in areas where consistent wind air a single unit generally cannot vacuum all hot air from. speeds of at least 5 mph are typical Without that minimal an attic Usually to provide uniform air movement along. wind speed wind turbines act essentially as roof louvers the underside of the roof sheathing a series of power fans. When the wind is blowing however wind turbines can must be spaced equally along a roof. be effective air movers, To provide maximum ventilation bene ts wind Figure 24. turbines like roof louvers must be equally spaced along. a roof Otherwise ventilation will be focused in the area. surrounding the wind turbines allowing hot spots to. develop in other areas of the attic see Figure 23,Power fans are used to move large volumes of air. a good option for hard to vent hip roofs that have limited. horizontal ridge length available for ridge venting. When evaluating the feasibility of using power fans it s. important to evaluate one factor which is considered to be. Wind turbines are located near the ridge and are used to a major disadvantage namely that power fans cannot vent. exhaust air from the attic away moisture during the winter unless they are equipped. with humidistat controls, If this is a problem in your climate it can be solved by.
Power fans using a power fan that has a humidistat control When. For the most part a power fan is a motor driven version that s done power fans do offer key bene ts For one they. of a wind turbine ensure a high volume of air ow even on days when outside. A power fan uses the rotary motion of blades to pull air is virtually still a common occurrence in inland areas. air into the attic through the intake vents at the sof t and on hot summer days. exhausts it out of the attic near the ridge But instead of In addition power fans provide ventilation in some cir. using wind power to drive the blades power fans use cumstances where xed systems would prove inadequate. electricity to drive high ef ciency motors or sunlight if Most static exhaust vents in a hip roof application fail to. they are solar powered meet ventilation code requirements for high exhaust. Unlike a wind turbine however the effectiveness vents while power fans can provide the air needed to. of a power fan isn t dependent on wind force Instead ventilate the attic properly see Figure 24. a power fan is turned on and off as needed automatically. with thermostat and humidistat controls In some Note To determine the fan capacity and sof t vent. models an integral humidistat control is standard in requirements when using a power fan see Section 3. most models however a humidistat is an add on option. Generally solar powered fans do not have thermostat.

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