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  1. #1

    Default Power ventilators

    I am always behind in reading the Monthly JLC, so I apologize that am just getting to this. I take issue with a part of an article in the October 2013 JLC, "Building science secrets". On page 42, the author states point blank "bottom line: power ventilators are a waste of money and a liability". I disagree. The roof heats from the top down, and you are relying on passive airflow from the lower intake vents to the top ridge cap to cool the plywood and shingles on the roof. There is certainly lag time in achieving an efficient circulation of air on really hot days. In Michigan we have frequent temperature shifts within a day. A little assistance from a power ventilator will keep your shingles from prematurely aging by allowing a little cooling. We have our power ventilator set on 110 degrees. So if the attic temperature ever gets to 110 degrees, the power ventilator automatically kicks on and assists with cooling the the roof deck by moving the cooler air in at a faster rate. We spent way too much money on that roof to allow it to prematurely age from overheating.

    We have R-78 in the attic, and the space is "tightly sealed" with respect to the inside of the house...we are not "air-conditioning" the attic. The air draw has to come from through the baffles on the lower soffit vents. Further, we only run the AC three or four days a year, so again we are not"air-conditioning" the attic. The author needs to broaden her thinking with regard to power ventilators.
    Last edited by Chrisdog; 01-01-2014 at 11:56 AM. Reason: more clarity and details

  2. #2
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    Default Re: Power ventilators

    Chris
    The section you are taking issue with is only a couple of paragraphs long and it is tough to quantify everything involved in so short a statement.
    There is a lot of truth in what was said. If you have a power ventilator that also has a humidity sensor configuration and it operates in the winter this can also lead to added moisture in the attic from the warm air inside you home and can become a real liability.
    This is a complex matter and a couple of paragraphs does not do justice to the issue.
    Fortunately a lot of issues are not as critical as the box we try to put them in when we are discussing them and there is broader range of operational acceptability. If there weren't we would see a lot more failures in a lot less time.
    I would agree that the conclusion in the last two sentences on the captioned picture is a broad stroke but I have seen that to be a correct statement in some of the homes I consult on.
    Mark Parlee
    BESI(building envelope science institute) Envelope Inspector
    EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
    Level one thermagrapher (Snell Training)
    www.thebuildingconsultant.com
    www.parleebuilders.com
    You build to code, code is the minimum to pass this test. Congratulations your grade is a D-

  3. #3
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    Default Re: Power ventilators

    Well, my take on it comes from the points about shingle life. There is no data showing any benefit to the shingles from power venting. Nothing to show a year or a few years.

    What heats the shingles is the sun. Not the attic. What happens in the attic is minor, negligible at best, from a heat standpoint. The attic gets hot, but nowhere near as hot as the shingles. How much is the power vent going to reduce the temperature in the attic? Chances are it's pulling air from walls that are also getting heated by that same sun in addition to whatever part of the shaded walls. So you'll get, perhaps, a few degrees of cooling to the plywood, and the chances are that won't make it to the shingle; the heat drive from the shingle is considerably greater than the cooling presented by the plywood.

    That attic is never going to get close to the temperature of the shingle, even with no venting at all. Meaning that any added shingle life is going to be so small that it's unlikely that you will save anything.

    Look at it this way- you're looking at a power vent that costs from around $100-$350 for a solar version and $80-200 for an electric vent. Add installation costs- anywhere from $100-300- and you've got a total cost of somewhere in the $300-500 range. Those are going to be good for about 1500' of attic. I'd say that roof is going to cost you a max of what, $10k for a 1500 sf roof. That's going to be a "50 year" shingle. If everything works out perfectly, which we're assuming with the "will add a few years of shingle life" you're looking at that roof's value being so low in the later years of the roof that you'd probably need to add 5 or more years to the life of the roof.

    IOW, there's no statistical data proving that power venting adds life to a roof v. regular venting to add enough time to the life of the roof, much less enough life to offset the cost of the fan and installation and (if powered) electricity.

    Not to mention the life of the fans. They appear to have about a 10 year limited warranty on them at best, meaning you might get 15 or 20 years out of one. So you're most likely going to have to buy a couple of them.

    As Martin pointed out, if you really want to extend the life of the roof, money is better spent in adding insulation. If you don't want to spend more money, buy a light colored roof- that will do more than any number of vent fans.
    http://www.lavrans.com

    "He uses statistics as a drunken man uses lamp posts; for support rather than illumination." -Andrew Lang

  4. #4
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    Default Re: Power ventilators

    Lavrans, yes, but that's the summer argument.

    In northern climates, much has been made of ice damming. What isn't usually discussed and hasn't been studied (to my knowledge) is shingle deterioration from freeze-thaw cycling. But common sense tells you those little granules are going to get dislodged with freeze-thaw.

    Looking out of my window today I could see that every house around me has its ridge vent completely covered with snow, so zero venting. There is warm air in those attics no matter how well insulated and that will aggravate the freeze-thaw at the shingles.

    My solution (for winter) is to install a very small cfm solar vent and let it run continuously. This of course in addition to the regular passive venting.

  5. #5
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    Default Re: Power ventilators

    Sure- and that sounds like a good point in cold weather areas. I don't really know enough about ice dams and what makes them happen. I can see that warm air in the attic could cause that- I'm not sure that a fan could move the air out fast enough in a poorly insulated house, although anything built after the '80s it sounds good.

    The OP was back on the heat issue though, and that's all I'm confident enough to talk about.

    I am curious about something- most of the houses I've seen with ridge vents have snow melt on the ridge when there is poor insulation and sealing. Seems like, as much as anything, an argument for side-wall venting as much as power vents...

    It sounds from what I've read that air sealing and keeping the HVAC venting inside the envelope is the main line of defense against ice dams from warm air in attics (I know there's an element of sun and orientation in it, too, but as for construction methods...)- I'm betting plenty of those houses have their HVAC runs pulled through the attic, etc. Can you pick out those houses?
    http://www.lavrans.com

    "He uses statistics as a drunken man uses lamp posts; for support rather than illumination." -Andrew Lang

  6. #6

    Default Re: Power ventilators

    Mark, I have no concerns about the power ventilator drawing air from the house during the winter. It is temperature controlled and the attic won't hit 110 degrees in the winter. The attic is also tightly sealed, so no air will draw through the house in any case.

    if the statement had said " IN most situations...." then I probably would not have taken issue with the statement. I just know that the concerns presented do not apply in our situation, so the broad stroke brush does not either.

    I did read Lavrans reply...and I agree, the "added lifespan" of a roof shingle due to the presence of a correctly installed power ventilator, would be tough to quantify in the short term, though I expect that they could probably successfully analyze this in a lab setting.

  7. #7
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    Default Re: Power ventilators

    Quote Originally Posted by Lavrans View Post
    I am curious about something- most of the houses I've seen with ridge vents have snow melt on the ridge when there is poor insulation and sealing. Seems like, as much as anything, an argument for side-wall venting as much as power vents...
    How do you sidewall-vent an attic? That's a new one for me.

    Quote Originally Posted by Lavrans View Post
    It sounds from what I've read that air sealing and keeping the HVAC venting inside the envelope is the main line of defense against ice dams from warm air in attics (I know there's an element of sun and orientation in it, too, but as for construction methods...)- I'm betting plenty of those houses have their HVAC runs pulled through the attic, etc. Can you pick out those houses?
    No, that's not how it happens. In my area we have almost zero houses with ducts in attics but plenty ice damming.

    Many older houses have 1) very little or no overhang, and/or 2) very small (vertical) distance between the top plate and the roof deck. In both cases there is insufficient insulation (or room for it) and no soffit vents. Ice dams form at that location mostly, that's why we put ice & water at the lower part only.

  8. #8
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    Default Re: Power ventilators

    Quote Originally Posted by dgbldr View Post
    How do you sidewall-vent an attic? That's a new one for me.
    Through the gable ends.
    Last edited by S.Joisey; 01-03-2014 at 06:14 PM.

  9. #9
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    Default Re: Power ventilators

    Thanks SJ. Never heard gable vents called sidewall vents.

  10. #10
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    Default Re: Power ventilators

    From what I understand the whole purpose of venting an attic is to keep the dew point temp high as possible on the inside roof deck surface common to the attic. Moisture at this location causes mold/dry rot. In winter keeping the deck cool or from melting ice higher on the roof that freezes at the lower roof causing ice dams. In summer to push hot air out lowering the building cooling load.

    Quote Originally Posted by Chrisdog View Post
    The roof heats from the top down, and you are relying on passive airflow from the lower intake vents to the top ridge cap to cool the plywood and shingles on the roof. There is certainly lag time in achieving an efficient circulation of air on really hot days.
    Heat rises in open air cavities. Flow through any vessel sees a heat rise (not cooling) at the turbulent boundary (deck) from friction and is a function of velocity, pressure, and heat. The center or laminar flow in the attic has the highest velocity and is coolest. You’ll find that true in fuel, air, hydraulic, lines too. So air flow causes boundary heat which raises the dew point at the decking. You can also get there by adding rigid foam below the shingles, a sandwich construction of wood skins and foam core, or insulating the deck interface, that keeps the dew point higher on the wood surface common to the attic. Quantifying is very complex due to thermal, aero, fluid dynamics interchange. You’d need to run some test at your specific location and configuration, thermosisters, CFM, etc, instrumentation. You can get just a feel for the instrument engineering complexity by looking at the latest “BSC Metric test report on wall cavities” here: http://www.buildingscience.com/docum...t_20131021.pdf

    Here is what I found depending on you radiant barriers, speculation and general imo,

    Effect on Shingle Life
    In general, shingles installed on unvented attic assemblies operate at a slightly higher temperature. This has impacts on the durability of roof assemblies. A 2 or 3 degree F. rise in average temperature is typical for asphalt shingles and a corresponding 10 degree F. rise in average temperature for sheathing (Parker & Sherwin, 1998; Rudd & Lstiburek, 1998; TenWode & Rose, 1999).
    All other things being equal, applying the Arrhenius equation (Cash et.al, 2005), a 10 percent reduction in useful service life should be expected. This is comparable to the effect of the installation of radiant barriers. What is more significant to note is that the color of shingles and roof orientation have a more profound effect on the durability of shingles than the choice of venting or not venting (Rose, 1991) – double or triple the effect of venting/non venting.


    Quote Originally Posted by Chrisdog View Post
    A little assistance from a power ventilator will keep your shingles from prematurely aging by allowing a little cooling. We have our power ventilator set on 110 degrees. So if the attic temperature ever gets to 110 degrees, the power ventilator automatically kicks on and assists with cooling the the roof deck by moving the cooler air in at a faster rate. We spent way too much money on that roof to allow it to prematurely age from overheating.

    We have R-78 in the attic, and the space is "tightly sealed" with respect to the inside of the house...we are not "air-conditioning" the attic. The air draw has to come from through the baffles on the lower soffit vents. Further, we only run the AC three or four days a year, so again we are not"air-conditioning" the attic. The author needs to broaden her thinking with regard to power ventilators.
    The problem with power vents is they can cause depressurize the attic, a pressure drop to the outside and sealed living space that cause infiltration. As that differential pressure increases from an out of balance ventilation (more high than low) in the attic the need for tighter sealing occurs. That’s why it is better to have a slightly pressurized attic by more venting low than high. A 60/40 split is good.

    You could apply a similar line of reasoning to the side wall by venting the siding by use of furring strips or cor-a--vent popular in cold climates: http://www.cor-a-vent.com/siding-vent-sv-3.cfm
    Last edited by CASHCOW; 01-04-2014 at 10:40 AM.
    Terry

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