Do I need an attic fan

Discussion in 'HVAC Heating & Cooling' started by spfrancis, Jun 29, 2018.

  1. spfrancis

    spfrancis Member

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    We have a summer place that is near the beach. It is rambler with a basement. If I had to guess, I would say that it is probably less than 1800 sqft with the basement. We have had it for about 2.5 years now. I have noticed last summer that during the very hot days, the AC can't keep up. The attic is sealed(no entry) and maybe only 3 feet high in the peak. So, not a lot of room there. I've thought about putting an entry door into the attic to possible blow somethign up in there. I do not have an attic fan to disperse the heat. Ive noticed on days when it is 95degrees outside, that my temp will climb for 75 to 79. Not unbearable.

    I have no idea how hot it gets up there(attic) on the hot days. I was thinking of a couple of different things to try, but would like to get some thoughts. I'm wondering if there is a temp probe I can purchase that I can poke a hole in the ceiling of the main level, to get some temp reading of how hot it is getting up there. The other option is to try just try installing an attic fan. Just jump right into a solution, without a clear smoking gun. Since I won't have power, unless I do a lot of work, i thought maybe I can put in a solar attic fan. Is that the wrong approach?
    Any ideas?
    Sony,
     
  2. spfrancis

    spfrancis Member

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    a picture of the house.
     

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  4. Stuff

    Stuff Active Member

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    It looks like there is a vent on the front so possible it is fine. Have you measured the ceiling temperatures in the house?
     
  5. spfrancis

    spfrancis Member

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    I haven't done that before. I see that there are some units for $80 online.
    Raytek (RAYMT6) Mini Temp IR Thermometer
    is it worth getting something like this, or I should really have an HVAC guy use the better caliber equipment to perform this test?
     
    Last edited by a moderator: Jul 15, 2018
  6. Stuff

    Stuff Active Member

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    I think it would be better to get someone out and do the annual service and evaluation. Then they can determine if there are other issues that would prevent the system from keeping up. Leaking windows, bad ducts, etc.
     
  7. Jorgebaloy

    Jorgebaloy New Member

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    Attic fans can either be gable mounted or they can be roof mounted. A typical attic fan works by exhausting hot air and drawing in fresh air from the exteriors. They are accompanied with additional vents which help in drawing in fresh air. Since, the hot attic air at a temperature of about 150°F is replaced by cooler air at 85 - 95° F, your living space remains cooler for a longer duration. Moreover, the load upon your air conditioner reduces, thereby reducing your electricity bills.

    Besides cooling your attic and reducing the load on your air conditioner, these fans also serve several purposes. They draw moisture from the air and dry it out. This inhibits the growth of mold and mildew. Since, they continuously exhaust the air out of the house, they prevent accumulation of moisture in any area of house. They also prevent mold formation at the most moisture prone areas.

    Since attic fans tend to quickly draw air out of the house, they also do not allow dust and dirt to settle in the house. Large amount of air is removed from the attic everyday, which also takes away dust with it. Thus, they are a boon in the hot months of summer.
     
  8. spfrancis

    spfrancis Member

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    Thanks Jorgebaloy. I believe that the little gable vent that you see in the front of the house is not going to really help much. I'm thinking that doing a solar roof fan will get the hot air out of the attic. Given that the attic is pretty limited in size. I am leaning toward a solar roof fan (minimal need to be crawling in attic). I was wondering if I do an attic fan, then do I need to have some sort of venting from the main floor to go up to the attic? My thought is that the AC might be working harder then it needs to, given that it can't keep up when the temps climb to 95 outside.
    The other comment about bringing folks out. I typically haven't' gotten great advice from local guys on things like this. There is a bumper sticker down in Delaware "LSD" lower slower Delaware. It is basically making a jab at contractors down in the area. I had an earlier thread in December where I had what was really a pretty clear cut "troubleshootable" issue, and the one contractor came in said that I need in affect"a new furnace". I always take the approach that doing something my self, like an attic fan is a lot cheaper than having guys come out and start a boondoggle like this could end up being.
     
  9. Stuff

    Stuff Active Member

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    At this point you have no idea how much the attic is affecting your system. It is possible that adding a fan will make the situation worse by pulling the cool air out of your house. This causes negative pressure in the house which then sucks hot air in from the outside.
     
  10. WorthFlorida

    WorthFlorida Panama Canal’s new locks!

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    Installing an attic fan or roof ventilators do work good with additional soffit venting. It's an economical way to go. Heat always travel to cold. If you attic is 150 degree and the house is 100, heat from the attic will permeate into the home. A cool attic is always beneficial. Any south & west facing windows that get direct sunlight covered with a solar heat film can greatly reduce the heat transmitting through the glass. https://www.homedepot.com/s/gila?NCNI-5

    If you do not want to run a fan, installing roof vents on one side of the roof and adding soffit vents can make an improvement with no electric cost. There are solar powered roof ventilators available and the price has come down on them. If you go with the fan option, be sure not to run them in the night hours if it's very humid. You don't want to pull in humidity where insulation material can absorb moisture.

    https://www.homedepot.com/s/roof%20ventilation?NCNI-5
     
    Last edited: Jul 2, 2018
  11. Dana

    Dana In the trades

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    More often than not an attic fan will INCREASE rather than decrease the load on the AC, primarily due to the increase in outdoor air infiltration into your conditioned space that occurs when depressurizing / pressurizing the attic. It'll cool off the attic, but not necessarily the house.

    The original though to add attic insulation is the right approach, but the prerequisite to insulating is air sealing the attic floor/ceiling, otherwise you may end up with wet insulation during the winter months as warmer damper conditioned space air leaks into the colder attic. It really needs to be assessed for air tightness first.

    Rather than cutting a hole in the ceiling for access (which would only increase the number of infiltration paths), make one or more of the gable vents into an access hatch (even if you have to make it bigger.)

    In houses with low attic insulation a modest solar powered attic ventilator can knock maybe 5% off the combined cooling energy use. At higher R it hardly matters. But knocking that much off the average energy use isn't taking much off the peak load, which is your primary comfort issue. Read this classic research summary piece done in Florida.

    If insulation is to be added, cellulose usually works better than fiberglass in open blown situations. Fiberglass is semi-translucent to infra-red radiating off a hot roof deck, and it can be hotter an inch into the material than the attic AIR temp as that radiated heat is absorbed, so you're insulating against a higher temperature with less insulation. Cellulose is opaque to IR radiation, and the surface temp will track with attic air temp.

    If there isn't enough space for more than a few inches of insulation, it may be worth stapling radiant barrier to the undersides of the rafters. Use a perforated aluminized fabric type (not overpriced mylar bubble pack). The grid of tiny perforations raise the vapor permeance enough to allow the roof deck to dry into the vented attic, whereas the unperforated fabric or bubble pack types can become moisture traps. Radiant barrier WILL lower the peak ceiling temperature in a low-insulation attic, while increasing the peak roof temperature (but don't sweat the shingle warranty thing.) Dollar for dollar insulation is usually going to beat radiant barrier, if can get at least R30 in there with at least an inch of clearance between the fluff and roof deck (which can be an issue in lower slope roofs, and 2x6 joists).

    Are there ducts in the attic?

    Are the ducts & air handler well sealed at every seam, joint, and duct boot?

    Are the walls insulated? How air tight? Even though it's a summer place, air infiltration can be a significant part of the cooling load that can be controlled by air sealing. (It takes a LOT of air sealing to make a place tight enough that it needs mechanical ventilation- start with the big holes.)

    If the basement is not air sealed there may be sufficient stack effect infiltration adding to your cooling (and heating) loads too.
     
  12. spfrancis

    spfrancis Member

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    Yeah thanks all for the good insight. I am going to open up an opening into the Attic so that I can get in there from the main level. I will be able to see the insulation at that point. My goal is to look at the insulation, and potenitally put the solar attic fan in there. I thought that putting in the attic fan is a must, unless I see that there is no insulation up there. I was looking at doing a blown-in insulation solution with Cellulose. I may wait until the fall to do that. I think that the Attic fan should be quick, and minimal time in the attic to complete that.
     
  13. spfrancis

    spfrancis Member

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    @Dana: Are the ducts in the attic. No, the are in the floor for the main level. I will look at the seams, but the basement is mostly finished now. The one thing they did is they added more living space at the back of the house. I'm sure that wasn't in the formulate when they speced the AC unit. I can get to some of the ducts to inspect. The back of the house used to be a deck, and the previous owner did convert it to a living space with a roof line, and drop ceiling. I know that it doesn't keep the same temps as the main house. I don't think there is enough insulation in the floor of that area, and the windows aren't great. I am planning on insulation under the floors of that area. It probably measures 12 feet deep by 25 feet wide. Our plan is to keep the house for another 4-5 years before we do a big remodel where we probably tear itdown to the studs. My goal is to make it comfortable, without droppign thousands of $ into it. I will look at the radiant barrier option.
     
  14. Dana

    Dana In the trades

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    With a gable vented attic using a gable mounted fan on one end, and leaving the opposite gable end open for air to enter (or exit, depending on the fan orientation) is far preferable to putting the fan on the roof deck. You may need to increase the gable vent on the other end to meet the minimum free air requirements of the fan. Undersizing the gable vent opening relative to the manfacturers' specified minimum will excessively pressurize/ depressurize the attic, forcing air to flow between the attic and conditioned space. A typical 25 watter needs a minimum of about 5 square feet of vent area, 8-10 square feet would be even better. Looking at your picture I'd guesstimate after subtracting out the grille area you have a free air of about 2 square feet on that end.

    [​IMG]
    It's best practice to not combine gable venting with soffit/ridge venting, since the gable vents short circuit the flow. (This is primarily a wintertime moisture management issue.) If you're going add some anyway, do soffits-only to start with, since that minimizes the parasitic stack effect pressures that would be sucking air out of the conditioned space into the attic in winter. If adding ridge venting, make sure it's no more than about half the free air of the soffit vents.

    Perforated radiant barrier runs ~$150 (delivered) per 1000 square foot roll (typically 4' x 250') from multiple online sources- shop around. That will usually have bigger effect on attic air temps than a 20-30W solar fan.

    Most AC systems are ~2x oversized for their actual loads. Often it's the DUCT design that is deficient, delivering high static pressures (= low flow), which causes them to underperform on the sensible-cooling (temperature) spec. At some point it will be worth checking all the pressure drops on the system with a dual-port manometer to make sure it's all up to snuff. It's also common for AC systems to under-perform due to lack of a well defined return path for doored off rooms with supply registers but no return register. A door cut at the threshold usually isn't adequate (unless it's big enough for the cat to squeeze under), but utilizing partition wall stud bays as jump ducts, with a grille near the bottom on one side, near the top on the other, will work. A fully commissioned Energy Star duct system will test at room-room pressure differences of no more than 3 Pascals (0.012" water column) under all conditions, doors open, closed registers open, closed etc. (at all air handler speeds if multi-speed).

    A cheap 2-port hand held manometer with a resolution of 0.01" w.c. is usually good enough to find the worst-offender rooms, as well as for measuring the static pressure drops through the duct system (which is the small hand-helds are primarily they're designed for.) When measuring at the very bottom of the instrument's scale it's good to test in both directions to figure out what's going on with rooms with a very low pressure drop since they are not exactly NIST calibrated high precision instruments. (You can't fully qualify the room pressures on an Energy Star duct system with one of those, but for most HVAC purposes it's "good 'nuff".)

    What are the exit-air temperatures at the supply registers? Could there be frost / icing on the coils?

    Low flow can lead to low temps (<<50F) at the registers and frost-inducing temps at the coils cutting into the flow and overall system performance. The severity of that type of condition would be highest on sticky high-dew point days than when the dew points are in the 50s or lower. A dirty filter can cause this type of problem too, when it gets grungy enough.
     
  15. spfrancis

    spfrancis Member

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    @Dana Thanks for all the information. We went out of town for the 4th, and didn't get back until last night. I did buy a manometer for another issue with the furnace last December. I will have to look into how to use that to test pressure at the registers. I do notice that I don't get the same cold airflow at all vents on the main floor.. I did find an old pic that I took that shows an internal shot of the attic. It looks like there isn't any type of barrier on the inside of the roof. I completely forgot that I had a picture, as this was done back in 2016. I have verified the filter. I will look for freezing over the coils. The one thing that I'm
    wondering about. So there is a vent like seen in the photo on the frnt. The structure used to be a rectangle. When they enclosed off the deck to make itliving space, it make the design more of a "T" shape. So there is a vent on the front, but I can't find a vent anywhere else on the house. I'm wondering if the addition to the back of the house was not designed properly. Screenshot from 2018-07-10 15-08-36.png


    wondering wondering thumbnail_A56085E5-FC43-4010-AE88-139097EC556C.jpg
     
  16. Dana

    Dana In the trades

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    In order for a gable fan to be effective at cooling the attic with outdoor air there has to have a sufficiently sized air intake on the opposite end of the attic as the fan. That arguably COULD be achieved by adding enough soffit vent on the far end of the attic. A roof deck fan mounted in the middle would probably need soffit vents everywhere, and closing off the gable vent.

    Is the attic space over the deck even connected to other attic space, with free air flow between them?

    Radiant barrier doesn't need full coverage to be effective, if its awkward to install it over unconventional framing. Even 75% coverage should make a difference. But the more insulation you have at the attic floor, the less it matters. If going that route, start with attic over the room(s) that seem hotter than the rest.

    In the picture it looks looks like the attic joists are 2x6(?), and the rafters are 2x4(?). (Or is that 2x8 joists, 2x6 rafters?)
     
  17. spfrancis

    spfrancis Member

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    I will have to check when we go out there next week if there is any type of venting from the primary house to the addition on the back of the house. Since the addition has a drop ceiling, it will be easier to check. Does it just need a small opening between the two areas? So thanks for giving me a couple of approaches for how to look at the problem, and possibly address. I may have to make a bigger focus on determining why the one bedroom is not getting goof airflow to the registers. Maybe addressing that problem would let the house cool more evenly. I will also have to look at a radiant barrier.
    Yes the joist are 2/x6, and the rafters are 2x4.
     
  18. Dana

    Dana In the trades

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    The connection/opening between the two attics has to be at least as large as the free-air opening specs for the fan being installed. eg: the installation instructions for the 25 watt AB-255 solar gable fan specify:

    "Intake Vent Area: You need to have at least 650 square inches of net free air intake area (gable or soffit vents) per 20W fan and 750 square inches per 25W fan to ensure proper ventilation."

    It's a 25 watter, so it needs a minimum of 750 square inches (5.2 square feet, such a 28-3/8" x 28-3/8" square ) of free-air vent intake for the attic(s) to the outdoors, and if most or all of the soffit/other intakes are on the other attic the opening between the two attics should also be at least that big to ensure good flow.

    With the tapered 2x6 joist over the top plates of the exterior wall it looks like you won't get more than about 4" of insulation even over the deepest part of the top plate. Code requires that you keep at least 1" clearance between the insulation and roof deck to provide a drying path for the deck. If you cut some 1" polyiso into 1" strips and tack them on both sides of each rafter as spacers, you can use 1" polyiso as the chute/baffle to keep any fiber insulation from the roof deck. if you stack cut up polyiso over the top plates of the walls to that depth it'll give you about 50-60% more R than you would with just cellulose. At 4" stacked you'd get about R25, which is at least half the current IRC code minimum for your climate zone. If stacking polyiso is deemed too big a PITA, 1" of polyiso chute + 3" of cellulose filled in under it would be about R17, which is still better than squeezing and R19 batt into that space.
    [​IMG]
    To hit the R49 code-min with cellulose takes about 14" of depth, so extending your 1" polyiso roof protector chutes up to where the upper edge is about 16" above the ceiling gypsum would be good, and if installing radiant barrier the lower edge of the RB can stop where it's 13-14" above the ceiling gypsum. The foil facers on polyiso are also radiant barriers, so with the chutes in place you'd have nearly 100% coverage even if the perforated fabric RB isn't running all the way down to the joist tops.

    It's pretty easy to cut 1" foil faced polyiso into 1" wide strips using a 4" steel wallboard taping knife that has been sharpened on the edges, and a 4' level as a straight edge. Pre- cutting a bunch of the spacer strips & chutes can save a lot of time & trouble in the attic. You might even tack the strips to the chutes with dabs of foam board construction adhesive ahead of time, once you've figured out how much tail to leave off the spacer strips for the taper at the bottom edge, then just toenail/tack them to the rafters once you're up there. Cut the chutes about 1/2" narrower than the space between the rafters to make it easy to get them in there. A shot of can-foam in the gaps at the rafters is enough to glue it in place- it doesn't have to be a perfect air seal- just enough to keep blown cellulose from getting by it into the vent channel.

    The picture below looks like they went for deeper than code-min on the vent channel, not that you would need to. The low depth of your joists & rafters needs as much insulation over the top plates as you can get, so stay at 1". (Plank roof decking like yours is somewhat less susceptible to moisture issue than OSB or plywood roof decks any.)

    [​IMG]
     
  19. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    I put in a radiant barrier (stapled to the underside of the roof rafters)...there was an immediate decrease in the ceiling temperatures. Prior to doing that, the ceilings were hot to touch by the end of an afternoon and into the evening. AFter the radiant barrier was installed, they were room temperature and there was no appreciable heat being radiated from the ceiling. Pretty simple, cheap, and quick once you've got access to the attic. Additional insulation is still a likely good idea. Fans can create some problems with air infiltration in the living spaces, potentially making the a/c have to work harder by pulling in hot, humid air.

    If you're toying with replacing the shingles on the roof, consider a metal roof. There are some shaped like wooden shakes that have about a 3/4" air gap underneath (required for a radiant barrier to work), that will radiate over 90% of the incoming radiant energy which is even better than letting it get inside, but blocking it from getting further beneath the barrier.
     
  20. spfrancis

    spfrancis Member

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    My first plan is to use a Radiant barrier to see if that makes a difference. I may have to wait until the temps get a little cooler. My goal is to staple up radiant barrier, and see if that can move the needle enough. That seems like a cheaper approach, and I would possibly not have to put holes in the roof. I am also going to investigate why the one bedroom is not blowing cool with a lot of force.
    Thanks for all the help.
     
  21. Dana

    Dana In the trades

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    Assuming you have R13 (or less) in the attic, aluminized fabric radiant barrier will cut the heat flux through ceiling by about half. See Table 3, p6 detailing an experimental test under controlled conditions at the Oak Ridge National Laboratory in 2011 using their Large Scale Climate Simulator.

    Attic #1 is R13 on the attic floor, no RB. Attic #3 is foil RB stapled to the underside of 2x4 rafters. With less than R13 the effect of the RB would deliver more than a 50% reduction in peak heat load, with more than R13 the effect would be less. (At a true R49 the RB hardly matters.) With 2x6 joists you probably don't have more than R19, and if the R19 isn't well installed the average performance could be less than R13.

    If you look at Table 2, the summertime daytime ceiling gypsum temperature drops by about 2F, which amounts to about a 4 BTU/hr per square foot drop in sensible cooling load. For a 1800 square foot hour that's a 7200 BTU/hr reduction in load- more than a half ton. At typical " a ton per 750 square feet" cooling load ratios for low-R houses that's on the order of a ~25% reduction in load, dropping from about 2.5 tons to less than 2 tons for the whole house.

    The tightened up version of the house with R30 or more in the attic, blown insulation in the walls, etc. you'd likely come in between 1-1.5 tons of peak load. This graphic was compiled data from dozens of careful Manual-J heating/cooling calculations by a company in Atlanta, plotting square feet per ton against house size:

    [​IMG]

    You're probably in the lower half of the sub-2000' house cluster right now- the RB would probably bring you up into the mid range.
     
    Last edited: Jul 16, 2018
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