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  1. #118
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    Quote Originally Posted by Brian GC View Post
    Though my post was quoted you did not respond to it.

    I'll pose the question again. Isn't reducing the PAV motor speed better in most cases than disconnecting it?

    Brian
    In most cases, the answer is NO. Disconnect it.

    Put in a radiant heat barrier, add more soffit vents, add insulation, seal all of your outlet and switch boxes (with the breaker off!), etc... But - disconnect the PAV.

  2. #119
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    Quote Originally Posted by Brian GC View Post
    Hasn’t it been determined here that not all PAVs are bad…only the ones used with inadequate soffet and gable vents? In less than ideal situations, reducing motor rpm to 200 – 300cfm would make for a perfectly operating PAV. Low power with low cfms. Disconnecting a PAV should never be the first choice, it’s illogical.

    Logic keep us out of trouble, emotions get us into it. Look at the animal kingdom, they’re 90% logic and do very well.

    Brian
    Keeping the term "logic" in mind, which approach is more logical?
    • Install radiant barrier on underside of roof deck to minimize radiant heat gain to an attic. Radiant heat is said to be over 70% responsible for heating the insulation laying on the attic floor, which then transfers that heat to the conditioned spaces below via conduction and radiation
    • Do not install radiant barrier, but rather rely on a powered attic ventilator to reduce attic air temperatures. Heated attic air is said to be under 30% responsible for heating the insulation laying on the attic floor, which then transfers that heat to conditioned spaces below via conduction and radiation
    Logic would dicate the radiant barrier solution provides higher return on investment. It is a once and done proposition. The PAV needs an ongoing power source to be useful. When it is not running, it is useless. The radiant barrier never shuts down, it is always available to work, with no ongoing consumption of energy.

    Appealing to logic, does reducing a PAV's CFM output from 1,200 to 300, either with radiant barrier or without it installed, make sense for increasing the PAV's effectiveness at reducing that ~30% heat gain from superheated attic air? Earlier in this discussion either I or someone else crunched numbers at what volume of air would be needed to keep an attic at outdoor temperatures on a, say, 100 degree day. The amount of air needed was tremendous. Reducing the airflow to reduce air removal from the conditioned space would be countered by the increased heat gain the attic would experience with less ventilation, resulting in greater heat gain to the attic floor insulation and structural members.

    Emotions do not always get us into trouble. Combined with logic, they give us the push to see a matter through, and to do it well. Animal kingdom logic is eat or be eaten...as humans, we can do better than that.
    Last edited by Shophound; 05-13-2008 at 04:06 PM.

  3. #120
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    Aug 2003
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    Quote Originally Posted by mark beiser View Post
    If you are building a new house, I'm a big fan of moving the the insulation to the roof line, and sealing the attic. It makes one hell of a lot more sense than having to deal with super heated air in an unconditioned attic.
    Ventilated unconditioned attics in cooling dominated climates, with the air handling equipment and ductwork in the attic, are a pretty dumb idea, but it has been the norm for decades.
    Since when did tract home builders in Texas ever think of doing anything smart when it comes to HVAC and building thermal envelope design?

    Granted, most builders came of age in an era of cheap energy. Throw the ducts and equipment up into a superheated attic. Skimpy overhangs on south facing facades. A building envelope punctured like swiss cheese. Lofty interior spaces difficult to heat and cool due to gross stack effect. A roofline more complex than college algebra. An entry that could allow a giraffe to comfortably fit in. All done so when the potential buyer rolls up to the curb, he says, "Wow! I want that house!"

    Gotta love it!

    /rant

  4. #121
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    May 2007
    Location
    Naples, Fl
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    889
    Quote Originally Posted by shophound View Post
    Since when did tract home builders in Texas ever think of doing anything smart when it comes to HVAC and building thermal envelope design?

    Granted, most builders came of age in an era of cheap energy. Throw the ducts and equipment up into a superheated attic. Skimpy overhangs on south facing facades. A building envelope punctured like swiss cheese. Lofty interior spaces difficult to heat and cool due to gross stack effect. A roofline more complex than college algebra. An entry that could allow a giraffe to comfortably fit in. All done so when the potential buyer rolls up to the curb, he says, "Wow! I want that house!"

    Gotta love it!

    /rant
    It's not restricted to the industrialized builders. We've had multi-million dollar custom homes fail. One in particular that got demoed had standing water turned mold colony in the elevator shaft. There was some issue with water (condensation not sure) anyway someone had put pie tins on the floor trusses to catch the water.

    Even without failure they are a challenge: multiple washer and driers, bathrooms that look like car washes, 6 and 8 burner gas stoves and multiple dish washers. Now couple all this with an envelop that is under pressure outside to in 90 percent of the time. Oh yeah they're from up North and want to keep the house at 68F

  5. #122
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
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    Quote Originally Posted by Brian GC View Post
    Though my post was quoted you did not respond to it.

    I'll pose the question again. Isn't reducing the PAV motor speed better in most cases than disconnecting it?

    Brian
    There isn't a way to reduce the speed of the motor in a PAV, it is either on or off.
    Off is best.

    Natural ventilation has no operating/repair cost.
    If more government is the answer, then it's a really stupid question.

  6. #123
    Join Date
    Apr 2007
    Location
    Los Angeles
    Posts
    14
    Sweeping generalizations aside....my actual experience:
    In my Los Angeles house (1600 square feet), I sealed all the penetrations very well, especially when I installed a new ceiling. The attic is very well insulated R30: w. vapor barrier taped.
    I installed two commercial quality sidewall gable vents with gravity dampers from Grainger. On the opposite side of the house I installed two oversized gable relief vents as well as eave vents on the half of the house farthest away from the gable fan. I installed a Honeywell remote bulb t-stat (100-220 degrees IIRC) and a solid state fan speed controller in the furnace closet. I leave the controller set to about 130 degrees and the speed at about 20hz. It works great. In the summer when I forget to turn it on when I go to work the house is over five degrees warmer upon my return that evening.
    I also installed a relay so that when my whole house fans ( quiet cool fan dot com) come on, the gable fans bang on a full speed and help boost the air changes.
    It's a great system, but honestly it takes a lot of interaction, that Mrs. Rizzo could not do.

  7. #124
    Join Date
    Mar 2008
    Location
    Long Beach, CA
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    3,384
    Quote Originally Posted by shophound View Post
    Keeping the term "logic" in mind, which approach is more logical?
    • Install radiant barrier on underside of roof deck to minimize radiant heat gain to an attic. Radiant heat is said to be over 70% responsible for heating the insulation laying on the attic floor, which then transfers that heat to the conditioned spaces below via conduction and radiation
    • Do not install radiant barrier, but rather rely on a powered attic ventilator to reduce attic air temperatures. Heated attic air is said to be under 30% responsible for heating the insulation laying on the attic floor, which then transfers that heat to conditioned spaces below via conduction and radiation
    Logic would dicate the radiant barrier solution provides higher return on investment. It is a once and done proposition. The PAV needs an ongoing power source to be useful. When it is not running, it is useless. The radiant barrier never shuts down, it is always available to work, with no ongoing consumption of energy.

    Appealing to logic, does reducing a PAV's CFM output from 1,200 to 300, either with radiant barrier or without it installed, make sense for increasing the PAV's effectiveness at reducing that ~30% heat gain from superheated attic air? Earlier in this discussion either I or someone else crunched numbers at what volume of air would be needed to keep an attic at outdoor temperatures on a, say, 100 degree day. The amount of air needed was tremendous. Reducing the airflow to reduce air removal from the conditioned space would be countered by the increased heat gain the attic would experience with less ventilation, resulting in greater heat gain to the attic floor insulation and structural members.

    Emotions do not always get us into trouble. Combined with logic, they give us the push to see a matter through, and to do it well. Animal kingdom logic is eat or be eaten...as humans, we can do better than that.
    I’ll start by saying that my first choice is a turbine fan. They’re cheap, free to operate, do not overdraw, easy to install, on all the time, and are easy to service.

    I have never been in an attic with radiant heat barriers but off the top of my head I’d say it would be difficult to install in residential attics. If the +70% figure is correct, then if you took two 100 degree days, one sunny and one overcast, you’re saying the attic on an overcast day (no radiant heat) would be 70% cooler. I do not think so. Or, are you saying the attic will have 70% less effect on the house. Also, I don’t think so. IMO, two 140 degree attics with and without radiant heat would be <20% apart.

    IMO, most of the heat is due to lack of ventilation. Radiant heat is a source but not the main cause of the ambient temp. Like an oven, what causes it to get hot, the hot burners or shutting the door? Answer; shutting the door or ‘lack of ventilation’. Decreasing the heat source and ventilation compared to increasing the heat source and ventilation might be a wash, but as far as ambient temp. goes, my money would be on the turbines. Without ventilation, a little heat goes a long way.

    A radiant barrier would work against you in the winter when you want solar heat. And, would reflecting that heat back at the roof causing excessive heat and wear on the roofing material.

    Nobody is trying to keep attic air at an outside temperature, they’re just trying to reduce it 20 degrees or so.

    Logic tells me to always question figures like the 70% / 30%. It may be a fact that is more applicable in places other than attics.

    Brian

  8. #125
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    May 2007
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    ....
    Last edited by adrianf; 05-14-2008 at 04:03 AM.

  9. #126
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    Quote Originally Posted by Brian GC View Post

    IMO, most of the heat is due to lack of ventilation. Radiant heat is a source but not the main cause of the ambient temp. Like an oven, what causes it to get hot, the hot burners or shutting the door? Answer; shutting the door or ‘lack of ventilation’. Decreasing the heat source and ventilation compared to increasing the heat source and ventilation might be a wash, but as far as ambient temp. goes, my money would be on the turbines. Without ventilation, a little heat goes a long way.

    A radiant barrier would work against you in the winter when you want solar heat. And, would reflecting that heat back at the roof causing excessive heat and wear on the roofing material.

    Nobody is trying to keep attic air at an outside temperature, they’re just trying to reduce it 20 degrees or so.

    Logic tells me to always question figures like the 70% / 30%. It may be a fact that is more applicable in places other than attics.

    Brian
    Stand outside on a nice hot summer day. The air temperature in the shade is exactly the same as the air temperature standing in the sun. Radiant heat from the sun makes all the difference in how hot you feel - and in heat gain.

    The biggest impact on shingle temperature is the color of the shingles. Radiant barriers raise the temperature two to five degrees. Going from white shingles to black shingles raises it by tens of degrees.

  10. #127
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    Quote Originally Posted by Brian GC View Post
    I’ll start by saying that my first choice is a turbine fan. They’re cheap, free to operate, do not overdraw, easy to install, on all the time, and are easy to service.

    IMO, most of the heat is due to lack of ventilation. Radiant heat is a source but not the main cause of the ambient temp. Like an oven, what causes it to get hot, the hot burners or shutting the door? Answer; shutting the door or ‘lack of ventilation’.


    Brian
    Unless you stop the radiant heat, adding some ventilation to an attic just converts it from an oven to a broiler. You are going to cook either way.

  11. #128
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    Fort Worth, TX
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    I have never been in an attic with radiant heat barriers...
    You may wish to do the following, all on the same day. Go to a new housing tract where the homes being built use radiant barrier that is integral to the underside of the roof decking, and go into the attic. Preferably in a house that has progressed beyond framing, and has the drywall and insulation in place. Measure the temperature in this attic.
    Next, go find another house nearby, on the same day, where the builder has chosen not to use a radiant barrier product of any kind on the underside of the roof decking. Measure the temperature in this attic and then compare it to the one that had radiant barrier (RB). You will note a significant difference.

    An experience similar to what I just described above was my first introduction to the effectiveness of radiant barriers. Admittedly some of my observations could be interpreted as subjective, since I was not measuring anything other than how "hot" I felt when going into the upstairs regions of homes that were in an advanced state of framing (roof decks on, windows in, exterior sheathing in place, but no drywall). That being said I could not help noting that the house with the RB had little noticable temperature difference between the upstairs and downstairs regions. The one that did not have RB, the moment my head rose above the level of the top floor, the heat became much more intense.

    but off the top of my head I’d say it would be difficult to install in residential attics.
    For residential new construction, it is as easy as making a different choice for roof decking. Opt for an OSB sheet with RB on one side...done. For existing residential attics, the spray-on paints are effective, but at reduced levels compared to the OSB/RB products. I have the spray-on product in my own home, and it is effective at reducing cooling demand and lowering attic temperatures.

    If the +70% figure is correct, then if you took two 100 degree days, one sunny and one overcast, you’re saying the attic on an overcast day (no radiant heat) would be 70% cooler. I do not think so. Or, are you saying the attic will have 70% less effect on the house. Also, I don’t think so. IMO, two 140 degree attics with and without radiant heat would be <20% apart.
    An overcast day is not an effective illustration to compare RB against PAV ventilation. A hot, sunny day would be more appropriate, since these are the conditions both RB and PAV's are designed to work against. My 70/30% ratio, admittedly rough, was a statement of breaking down total heat gain to an attic (heat gain to the house is a separate but related subject). 70% is from radiant heat gain, 30% is from the air in the attic itself becoming heated, which occurs primarily due to radiant heat! The air itself would not heat over outdoor ambient if it was not overwhelmed by radiant heat from structural members that have no radiant barrier in place.

    IMO, most of the heat is due to lack of ventilation. Radiant heat is a source but not the main cause of the ambient temp. Like an oven, what causes it to get hot, the hot burners or shutting the door? Answer; shutting the door or ‘lack of ventilation’. Decreasing the heat source and ventilation compared to increasing the heat source and ventilation might be a wash, but as far as ambient temp. goes, my money would be on the turbines. Without ventilation, a little heat goes a long way.
    Noone is suggesting that an attic with radiant barrier should not be ventilated. A radiant barrier with natural ventilation is, IMO, more effective than an attic with no RB but has a PAV installed. As for winter solar heating, radiant barrier is most effective in primarily cooling climates, where I and many other homeowners that frequent these forums live. Passive solar heating is a hit and miss affair, due to clouds, sun angle, nearby obstructions to direct sun (in winter the sun angle is low, even at noon, so a neighbor's two story house or an evergreen tree can inhibit solar heat gain), etc.

    And, would reflecting that heat back at the roof causing excessive heat and wear on the roofing material.
    In winter? Nonsense. The ambient air above shingles in winter is a far better heat sink for those shingles than in summer. The sun angle is also much lower in winter, days are shorter, air temperatures colder. The main wear on shingles in cooler months is rain, ice, snow, swings in temperature, and ultra-violet light during days that are sunny.

    Shingles in summer have a less effective heat sink to ambient air, but any wind will help with heat transfer, along with natural drafts of heat rising off the shingles into cooler surrounding air. With a cooler attic below due to radiant barrier, the undersides of the shingle aren't being as cooked as they would be with a superheated attic, which provides little heat sink for the shingle undersides to lose heat to.

    Nobody is trying to keep attic air at an outside temperature, they’re just trying to reduce it 20 degrees or so.
    Attic temperatures at outdoor ambients are the ideal. If RB and natural ventilation can get you close, you're doing very well. If the best a PAV can do is get you 20 above ambient, vs. RB and natural ventilation getting you 10 above, which is more viable...and uses less energy to boot?

    Logic tells me to always question figures like the 70% / 30%. It may be a fact that is more applicable in places other than attics.
    I admit I was recalling those figures from memory. If I find the source where I read that from, I will post it. Logic dictates that any posted figures are bolstered by published data.

    Logic also dictates such figures be correctly understood in context, and properly applied.

  12. #129
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    Aug 2003
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    Quote Originally Posted by RIZZO5150 View Post
    Sweeping generalizations aside....my actual experience:
    In my Los Angeles house (1600 square feet), I sealed all the penetrations very well, especially when I installed a new ceiling. The attic is very well insulated R30: w. vapor barrier taped.
    I installed two commercial quality sidewall gable vents with gravity dampers from Grainger. On the opposite side of the house I installed two oversized gable relief vents as well as eave vents on the half of the house farthest away from the gable fan. I installed a Honeywell remote bulb t-stat (100-220 degrees IIRC) and a solid state fan speed controller in the furnace closet. I leave the controller set to about 130 degrees and the speed at about 20hz. It works great. In the summer when I forget to turn it on when I go to work the house is over five degrees warmer upon my return that evening.
    I also installed a relay so that when my whole house fans ( quiet cool fan dot com) come on, the gable fans bang on a full speed and help boost the air changes.
    It's a great system, but honestly it takes a lot of interaction, that Mrs. Rizzo could not do.
    That's a heroic effort at powered attic ventilation, for sure! You are the exception to the rule, taking the extra steps necessary to isolate the conditioned spaces from the non-conditioned attic spaces.

    That being said, if you could have significantly reduced heat gain to an attic that cost you nothing more than the initial product installation, with no ongoing maintenance or energy consumption needed, would you opt for that over your present arrangement?

  13. #130
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    May 2007
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    Naples, Fl
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    Quote Originally Posted by paul42 View Post
    Unless you stop the radiant heat, adding some ventilation to an attic just converts it from an oven to a broiler. You are going to cook either way.
    Actually it's a convection oven for even cooking.

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