Help with Attic Exhaust

[Lost-In-Translation]

I recently installed a gable mount attic exhaust fan to help keep the attic cool. So far it hasn't been much help.

I have the gable vents all around the house, but what appears to be happening is the attic fan is just sucking hot air from outside into the attic, and not helping keep the attick temp down.

I am wondering, is it a good idea to cover the gable vents (mine are 5 inch wide screens running the length of the house) during the hot summer months with say plastic sheeting? Would this help keep the fan from sucking hotter air in from the outside? Would it basically ruin the idea behind the attic exhaust fan?

It just seems that the fan is a waste when all it does is suck in more hot air through the gable vents.

Any insights from anyone with similar experiences or ideas will be tremendously appreciated!

TIA!!!

[Patch]

I would think that on a hot day, the air in the attic would run 120 to 140 degrees. Assuming the outside air temp is 90 degrees, you will still be way ahead by pulling in the outside air. Don't block any of the vents with plastic.

[Art]

Do you really want your house ac going into the attic? Actually next time you re-roof, consider dumping the fan, plugging the hole and putting in a ridge vent instead.

[xrongor]

sucking hot air in? the word hot only has meaning in comparison to something else. put a thermometer up in the attic and check the temp on a 'hot' day. now turn the attic fan off and see what happens to the temp. the result is very predictable. your attic is much hotter than the air its sucking in from outside.

randy

[Bob in CT]

[SQLit]

Attic fans are sized to the size of the home. My old house needed 2. If you block out the vents where is the air for the fan to exhaust going to come from? Does your fan have a themostat on it? Where do you have it set? I had mine set at ~120 degrees. They would generally turn on around noon and run to 7-9 pm. Have you considered metal turbines? They work for free and do quite well.

[Bob in CT]

[Lost-In-Translation]

Can't use the turbines, I have a tile roof and installing them would be a small nitemare. My attic fan is direct wired, has a thermostat and it is set to about 95 degrees. It usually kicks on at about 10AM and can run easily until 10 at nite. I have a thermometer in the attic and the gauge is inside the house. It shows the average attic temp is about 105 degrees. When the fan is not running (I also installed a cut off switch) it only varies, so far, by about ten degrees. When the outside temp is about 90 the attic is running 110, and does come down to around 95-100 but if the fan goes off it climbs right back up. It pretty much has to run all day regardless. Since my air conditioner duct work is in the attic, when the air goes off it is so hot up there that the ducts heat up and when the air kicks on, the first thing that comes out into the house is extremely hot air about about 20 seconds until it starts to cool down.

I feel like I"m robbing peter to pay paul here by trying to decide if it's worth it to try to keep the attic temp down when it heats up the duct work anyway and fills the house with heat every time the AC kicks on.

When considering covering up the "soffit" vents, I had considered covering them all except for the opposite end of the house. My line of think was that at least it would draw air from one end to the other rather than getting all it's flow from the vents nearby and doing nothing to move the attic air on the other end of the house.

Hope that makes sense.

Thank you!

[Bob in CT]

Actually, with an attic fan, that's what's recommended -- the fan should be away from the vents. Do you also have a ridge vent? If you could put a few thermometers up there (like the Radio Shack remote kind), then you could try covering some of the soffit vents and seeing what happens. If you have a ridge vent, then I'd like to see your data. I don't have a ridge vent and no soffit vents and want to install an attic fan with a few soffit vents placed at the far ends of the house. However, I also want to add all soffit vents and add a ridge vent, but am unsure as to whether the attic fan should go.

[John Grabowski]

I'm not an engineer, but did you do some basic calculations to determine how much volume of space is in your attic. Exhaust fans are usually rated in cubic feet per minute (CFM). How many cubic feet of space is in your attic? A small gable fan will not move as much air as a whole house fan. My guess is that your fan is too small to move the amount of air out of the attic necessary to bring it down to a satisfactory temperature. Other factors also come into play such as the amount of sunlight hitting your roof and the amount of insulation you have.

Keep in mind that you will never get it cooler than the outside air that you are sucking in.

Don't seal off your soffit vents. The make up air needs to come from somewhere.

I find it difficult to believe that the outside air is hotter than the air in the attic.

John Grabowski

[Joseph Meehan]

And what did you expect it to do?

The sun on the room normally makes the air in the attic a lot hotter than outside. Passive ventilation (your gable vents along with the soffit vents) bring cooler (but still hot) air from the outside through the soffit vents and lets the hotter air out the gable vents (hot air rises).

Adding a fan usually increases the air flow. Occasionally it may acturally decrease the air flow because of short-circuiting, but I tend to believe that is rare. Over all as long as the fan is working I believe almost all will help decrease the temperature. If the fan fails, which many do as most are cheaply made, it will reduce the air flow and usually cause increased heat.

My personal opinion (from personal experience) is that most of the time fans are not worth the effort.

Are you sure you mean "gable" and not "soffit?" Gables are at the top of the end wall and soffits are along the bottom edge of the roof.

And when you exhaust all the air, you have a vacuum. If you exhaust air out, it has to come in somewhere.

Not only that, without ventilation, you could collect moisture and end up with mold and water damage. You would also end up with a lot hotter attic since the sun on the roof is going to be adding a lot of heat with no place to go.

It brings in less hot air.

Leave the fan as it is or take it out. Make sure you have enough ventilation. Check the fan from time to time if you don't take it out. When it stops working take it out and don't replace it.

[John Grabowski]

I had a similiar problem with my air conditioning ducts. Hot air used to come out initially. I insulated ALL of them (Return and supply) with duct wrap and no longer have that problem.

[Joseph Meehan]

Insulate the ^&$$ out of those ducts. It will not be 100%, but I'll bet it will be a lot more bang for your buck.

BTW those turbines seem to cause more problems that good. There are some areas and situations where they can be useful, but far more times they are more trouble than they are worth.

[Travis Jordan]

/snip/

Skip the attic fan and invest in some additional insulation around the duct work and in your ceiling (particularly if the ceiling insulation is less than about R-30).

Ventilation

Tests continue to show that attic ventilators have little effect on home cooling costs or indoor comfort if the home has adequate attic insulation. Studies conducted by the National Bureau of Standards and Progress Energy uniformly support this conclusion. Some surprising facts were revealed by Progress Energy research conducted over two consecutive summers in Florida. The studies involved 30 different homes and outdoor temperatures ranging between 88 and 95 degrees Fahrenheit. During the tests, attic temperatures generally did not reach excessive levels. The highest recorded temperature at the peak of a roof was 134 degrees Fahrenheit and the average temperature was 127 degrees Fahrenheit. Temperature immediately above ceiling insulation reached 119 degrees Fahrenheit, and averaged only 103 degrees Fahrenheit.

With a turbine ventilator operating, attic temperatures fluctuate - sometimes decreasing about six degrees and, surprisingly, sometimes rising by six degrees. These relatively small changes in temperature had little effect on air conditioning energy use since attic insulation retarded heat flow from the attic into the home.

Most homes already have some type of attic ventilation such as screened eaves or gable vents since it is required by the state building code. Such ventilation prevents moisture build-up and minimizes the possibility of wood decay. Progress Energy research also indicated that additional ventilation is unnecessary and would not significantly affect cooling costs if the home is adequately insulated.

[Bob in CT]

[cut]

That's a nice study, but I wonder about it. My sole source of ventilation is two gable vents. I think if you have soffit vents and a ridge vent, along with good insulation, then I think you have no need for an attic fan. While I plan to add insulation, I haven't yet done so. Moreover, some of the attic simply won't be able to have enough insulation over it, if it's used as a storage area (which mine is). I'm going to use some additional insulation in that area, but not much. I'm also going to minimize the amount of area.

I've been redoing my bathroom fans (vented into the insulation in the attic -- ick!), and it's not been that hot outside, but it's absolutely brutal in the attic. It was about 70-75 outside, and it was so hot in the attic that I instantly started to sweat when I went in there, and I lived 15 years in Arizona. If I can use an attic fan and get the temperature down to close the outside temperature, I think it'll be worth it. Then, when I save enough, I'll insulate the attic, add soffit vents, and add a ridge vent and ditch the attic fan. But adding all of that stuff when the roof doesn't need to be redone is expensive. So, I'll wait 'till the roof does need to be redone and then redo the roof the right way.

[Travis Jordan]

Bob in CT wrote: /snip/

Good plan - venting the bathroom fans vented either through the roof, an outside wall, or the soffit is the first thing to do! I can't imagine what bad things come of all that humid air being introduced into the attic. Particularly on those days when the attic temperature is lower than the indoor temperature. Sounds like mold heaven .

My experience tells me that a gable-end fan (even a good sized 1/4 hp one) will only reduce the attic temperature about 10 degrees during the average day. And that is with soffit vents - with a single gable intake vent it might be even less, since all you'll get is pull-through air.

[Chet Hayes]

Check the instruction booklet for the gable fan. It should indicate what size attic it's suited for and how many square inchs of air inlet it needs, ie soffit, other gables, to work properly. Make sure the fan is the right size and there are sufficient soffit vents. If the fan is too big, or there is not enough air inlet area, then you will suck up cool air from inside the house through random holes and openings.

Also, what is in front of the gable fan? Is it being blocked by fixed louvers or do you have movable ones that open? It's important to have as clear an opening for the fan as possible. Among fixed louvers, I've seen considerable variation in how they are designed and how much air they will let a fan push through.

Most houses could benefit from more soffit vents than what builders typically install. The best solution is a ridge vent combined with soffit vents, however I realize with a tile roof, that's probably out. I certainly wouldn't fool around with blocking off any soffit vents, as what you want is more air coming in, not less.

Even a 10 degree difference in attic temp is not all that bad. Say the attic was 110 without and 100 with. That means the temp delta between the attic and house at 70 was 40 and is now 30. That translates into roughly 25% less heat flow through the insulation into the attic.

[HVAC IsFun]

Youre supposed to bring outside air in...thats the whole point of having an attic ventilator, to cause cooler outside air to become make up air for the hotter attic air thats being exhausted. ON a 90 degree day with the sun out, your attic will be at least 130 f and higher without an attic ventilator .

Dave

[Joseph Meehan]

Don't expect to get it close to the outside temp during the day and evening. That roof is going to be one very large radiator. Vents do help, it would be a lot worse without them, but even the fan is not going to make it comfortable.

Maybe you should consider checking with a construction contractor to see if they have some suggestions for upgrading the ventilation.

[Lost-In-Translation]

I'd love to. Unfortunately, I'm in South Florida. Finding an honest contractor would be about as easy as finding WMDs in Iraq!