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  1. #14
    Join Date
    Jan 2007
    Location
    N.E. Ok.
    Posts
    1,367
    With ridge vent just make sure you have enough ridge to provide the adequate free opening required for proper ventilation.
    I see a lot of cut up roof lines that obviously don't provide enough ridge to vent properly.

  2. #15
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Quote Originally Posted by printmanjackson View Post
    I found this on another site about venting:

    >The rule with attic ventilation is 1 : 300 with a vapor barrier and 1 : 150 without a vapor barrier. So if you had insulation in the attic with a vapor barrier and the attic was 900 sq. ft., you would need 3 sq. ft. of free venting. If it was blown in insulation without a vapor barrier, you would need 6 sq. ft. of free venting. This is when you have an equal amount of square footage of high and low vents ( gable and soffit).

    Your concern about attic fans is correct. The problem is not the fan but the over sizing of attic fans. Most attics can only handle about 1500 cfm of ventilation because of the vents. Your fan that you have probably moves about 2000 to 5000 cfm. In which case it could cause a problem with drawing air from the home and causing the problems you mentioned.

    Properly sizing the fans is important because of this. You could increase the size of the vents but you're looking at more than doubling the amount. You're much better off putting in the proper size fan. The volume of air in the attic remains constant. So if the vents only allow 1500 cfm and the fan moves 2500 cfm, it will seek to get the air from somewhere, like the home. If the fan couldn't get the air from anywhere else, you would just burn out the fan.<
    The fan doesn't know it is only supposed to draw air through the vents provided for it.
    If you have a fan, regardless of size, drawing air from the attic, it will create a pressure in the attic that is lower than that in the house.
    Where there is a pressure difference, and a path for air to flow, the air will follow it.
    Putting in a smaller fan only reduces the CFM that will be drawn from the house.

    Then there is the whole thing about pulling hot air out of the attic having very little impact on the total amount of heat in the attic.
    Attic ventilation does absolutely nothing at all to reduce the radiant heat transfer in the attic, and the hot air pulled out represents only a relatively small portion of the total heat that is moving through the attic.

    Block the heat from coming in, ideally with a roof system that does so, but more practically with a radiant barrier if your ductwork and/or air handling equipment are in the attic, and provide adequate passive ventilation.

    If your ductwork and/or air handling equipment are up there, it is more cost effective to just blow in more insulation, and not worry about how hot the attic gets, unless you are in an area where winter ice damns are an issue.
    If more government is the answer, then it's a really stupid question.

  3. #16
    Join Date
    Feb 2008
    Location
    Tennessee
    Posts
    91
    >The fan doesn't know it is only supposed to draw air through the vents provided for it.
    If you have a fan, regardless of size, drawing air from the attic, it will create a pressure in the attic that is lower than that in the house.
    Where there is a pressure difference, and a path for air to flow, the air will follow it.
    Putting in a smaller fan only reduces the CFM that will be drawn from the house.<

    Please explain .... how can the pressure in the attic be lower than inside the house when the same amount of air is entering the attic that is leaving? I would think with the proper amount of soffit vents it would offer less restriction than anything else in the house.

  4. #17
    Join Date
    Jul 2005
    Posts
    717
    Printman; installing a continuous ridge vent is quite simple. Remove the existing ridge shingle caps ,cut a 1" piece (strip) from each side of the sheathing at the peak and then fasten the lengths of ridge vent along the peak.
    A continuous ridge vent is best, but you could install a few properly placed "mushroom" type vents close to the peak instead. They will do a good job at exiting air.
    For every 300 sq ft of attic floor area you require a minimum of 1 sq ft of vent ,divided 50/50 between the soffit and ridge vent.
    Example: if attic floor area is 900 sq ft, then you would need a minimum of 3 sq ft of vent. 1.5 for the soffit and 1.5 for the ridge.
    144 sq in = 1 sq ft. (easy math,eh?)
    Remember, the more vent area, the better, and it will also take inanyrestriction that "insect screening" in the vents will create.

  5. #18
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,320
    Quote Originally Posted by printmanjackson View Post
    Please explain .... how can the pressure in the attic be lower than inside the house when the same amount of air is entering the attic that is leaving? I would think with the proper amount of soffit vents it would offer less restriction than anything else in the house.
    As Mark said, the fan doesn't care where the air comes from. The entire attic will be in a negative with a PAV in use. If the typical gaps in construction found in typical American house ceiling are extant, there isn't much resistance for interior air to move into the attic. If you want a visual reference, if your house has ceiling mounted supply air grills for the a/c, pull one or more of those grills off and note the gap between the drywall and the supply duct boot. Nine times out of ten it's significant, and the most overlooked area to consider when tightening up the building envelope.

    Need even more of a visual reference? Check this out, from my own house prior to my efforts to seal it:
    Attached Images Attached Images  

  6. #19
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Quote Originally Posted by printmanjackson View Post
    >The fan doesn't know it is only supposed to draw air through the vents provided for it.
    If you have a fan, regardless of size, drawing air from the attic, it will create a pressure in the attic that is lower than that in the house.
    Where there is a pressure difference, and a path for air to flow, the air will follow it.
    Putting in a smaller fan only reduces the CFM that will be drawn from the house.<

    Please explain .... how can the pressure in the attic be lower than inside the house when the same amount of air is entering the attic that is leaving? I would think with the proper amount of soffit vents it would offer less restriction than anything else in the house.
    1000 small holes, that collectively add up to 1 square foot, will collectively allow the same quantity of air through that a single 1 square foot hole would.

    Pretty much every bit of independent 3rd party research I have seen on the subject comes to the conclusion that PAVs are not worth the investment in the up front cost, maintenance and operating cost, even in attics that are very well sealed off from the conditioned space.

    Since I have gotten into measuring house leakage, I have had the opportunity to actually measure exactly how much air PAV's were pulling out of a number of homes.
    The worst one I found was a 6,000ish square foot house with three 1600 CFM PAVs very little passive ventilation, and a large number of non air tight recessed light fixtures and poorly sealed thermal bypasses. With all of the fans that roofers installed last spring, about 2800 cfm was being pulled out of the house. the main part of the downstairs couldn't maintain a 78 set point on a 85 day, with an AC system that was about 20% oversized for keeping it 75 on a 100 day with normal infiltration.

    The best one I have tested was pulling about 150 cfm out of a 1800 square foot house. They had a 1200 cfm PAV, with 2 gable vents and lots of soffit vents, 8' ceilings with no recessed light fixtures or attic access within the conditioned space. The PAV was causing a little over half an air change per hour, mostly from pulling air in through electrical outlets and light switches through the wire penetrations in the top plates of the walls.
    If more government is the answer, then it's a really stupid question.

  7. #20
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    I think that too many make lots of 'blanket' statements which assumes lots--

    I have posted results for my house during July 20th --
    my PAV seem to help --
    --at least the attic does not get overly hot --
    -- HOWEVER, I do not have the test instrumentation to do a definitive test
    -- 'it is hard to have one's cake and eat it too'
    ----- I have tried to have the PAV on | off on simular 'hot' days [95+]
    -------but, this involves getting onto the roof or into a cramped attic & unplugging the PAV.

    -- btw, I HAVE plugged my penetrations, replaced my regular screening for the gable vents with 0.25" hardware cloth & replaced all of my eve vents.

    be aware, ridge vents should not be used with this house since the roof pitch is too low -- wind would probably drive rain into the attic --

    be aware, here we have high RH [>85%] overnight until 10am, most days! --
    --- during ALL seasons.

    the power consumption of my fan is but pennies per day! so that is not an issue!

    each household has their own lifestyle -- what works for me is probably unacceptable for most -- [my elec billing last 31d = $60 = total elec, = resistance heating. 4y ave = $70/mo.

    this house has LOTS of shade= no spot on my dark asphalt singled roof gets full sunlight more than 2h/d during the summer.

    I have lots of cellulose on the ceiling, covering ~80% of the 8" ceiling joists; a trilevel with a Gambrel roof over the bdrms;2133sf,8ft ceilings. I have tracked my utility billings for 8+y-- can recite btu/DD/cf.
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  8. #21
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,272
    Quote Originally Posted by adrianf View Post
    Go to conclusions for summary and this is with a solar/battery fan.
    ~ 9 year pay back is not attractive to nearly all Americans.

    PV powered fans need to be cheaper and electricity rates need to increase.

    And with updated code requirements for AHUs and ducts in a conditioned space and high ceiling R value,
    the future impact of attic fans will be nil.


    " Photovoltaic Attic Ventilators

    Two PV attic ventilators were installed on the house's asphalt shingle roof on August 6th, 1997.(3) Both were installed near the peak of the A-frame roof with one on the east face and another on the west face as shown in Figure 2. The fans are designed to provided between 600 and 800 cfm of attic ventilation at peak solar irradiance (1000 W/m2) depending on the free soffit ventilation area. The ventilators consist of a 19.5" x 16" mounting with a 10 Watt thin-film PV module. A five bladed radial fan and a direct coupled DC motor provides attic air exhaust. The units were purchased for $300 each; installation would typically add another $100 - $150 per unit. "

    " ... Comparing periods with similar weather conditions, the test revealed that the PV vent fans have the potential to reduce measured peak summer attic air temperatures by over 20oF. However, the impact over the cooling season is fairly modest with well insulated attics. Measured space cooling reduction was approximately 6&#37; - worth about 460 kWh annually at the test home. "
    Last edited by dan sw fl; 03-30-2008 at 08:46 AM.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  9. #22
    Join Date
    May 2007
    Location
    Naples, Fl
    Posts
    889
    "9 year pay back is not attractive to nearly all Americans."

    Agreed. I haven't been a FAN of pav(s) for many years now.

  10. #23
    Join Date
    Mar 2008
    Location
    Long Beach, CA
    Posts
    3,378
    Quote Originally Posted by mark beiser View Post
    1000 small holes, that collectively add up to 1 square foot, will collectively allow the same quantity of air through that a single 1 square foot hole would.
    I believe a thousand small holes that add up to one sq. ft. will have more resistance than a single hole of the same area. The perimeter surface area of the holes would be much greater, slowing the air flow. An example of this would be water flow within pipes. Ten small pipes that add up to the area of one large pipe would definitely have a slower flow rate. Also, distance from the FAV would reduce flow as well.

    Brian

  11. #24
    Join Date
    Feb 2008
    Location
    Tennessee
    Posts
    91
    ok, so too much airflow (CFM) would pull conditioned air out of the house but what if you slowed the fan down to say half or even less speed. This would be easy to do and you could still get air flow through the attic and not pull conditioned air from the house. Would this not be better than having nothing? like a ridge vent?

  12. #25
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,320
    Quote Originally Posted by Brian GC View Post
    I believe a thousand small holes that add up to one sq. ft. will have more resistance than a single hole of the same area. The perimeter surface area of the holes would be much greater, slowing the air flow. An example of this would be water flow within pipes. Ten small pipes that add up to the area of one large pipe would definitely have a slower flow rate. Also, distance from the FAV would reduce flow as well.

    Brian
    Nevertheless, it is evident the combination of these holes work in concert to result in conditioned air within a house being lost to an attic that is using a PAV. Sure, there might be more air being pulled through the attic via soffit or gable end vents, but a hole in the ceiling to the conditioned space, large or small, will see a transfer of air into the attic from the conditioned space if a pressure difference exists.

    Until one has studied stack effect and pressure differences in buildings, it can be a difficult concept to grasp. I'm still learning. I wouldn't mind following in Mark's footsteps at some point to actually measure these aspects. Nevertheless I do not dispute what he observes, for it's mere physics, which aren't easy to work around.

    Since I am a strong advocate for much better efforts to seal an attic from the conditioned space, I suppose if a ceiling is sealed well enough, a PAV could be operated without large loss of conditioned air to the attic. However, most ceilings are NOT sealed well, and with the panacea mindset we Americans tend to have, thinking a PAV is THE solution for overheated attics. To point blank state they are universally beneficial is misplaced.

    That being said, one must still weigh the cost of operating a PAV, even with a well sealed ceiling, against the savings it renders on cooling costs. I personally prefer intelligently designed passive systems over a PAV any day. Passive systems do not tend to break down, nor do they require much more energy than was used when they were built or installed. We're a gadget freak society that tends to overlook more long lasting, sustainable methods all the time. I readily admit I have been guilty of this mindset myself, but I am coming around to see things differently.

  13. #26
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,320
    Quote Originally Posted by printmanjackson View Post
    ok, so too much airflow (CFM) would pull conditioned air out of the house but what if you slowed the fan down to say half or even less speed. This would be easy to do and you could still get air flow through the attic and not pull conditioned air from the house. Would this not be better than having nothing? like a ridge vent?
    Speed it up, slow it down, stop it altogether. As long as there is a pressure difference between house interior and attic interior, and that difference is negative on the attic side, transfer of air from house to attic will occur.

    I will grant that when an attic heats up considerably, a house can actually experience infiltration of attic air into the house through gaps in ceiling construction. The heated air in the attic expands, and is warmer than the house air, so the air moves toward the lower pressure, lower temperature region. Flip on the PAV and this is then reversed, house air exfiltrates into the attic. Either way is a losing proposition. Seal the ceiling!

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