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Ridge vent? Power vent? Both?


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06-30-10, 11:20 PM   #1  
Ridge vent? Power vent? Both?

I am looking to ventilate my hot attic to hopefully reduce my cooling costs over the summer and perhaps even my heating costs in the winter. I have owned my house for 3 months. It's fairly small - 2 bedrooms, 1 bath.

I have been reading a lot of information online and whenever I type in "attic fan" I get a lot of solar attic fan hits. I have some questions about solar attic fans that no one has answered yet, so I am still skeptical.

An electric attic fan w/ humidistat on the other hand is something that I could see giving me a great deal of benefit.

I went over to Lowes to price some today and the salesperson showed me some ridge vents that he encouraged me to look into saying that they are better overall.

I'm more confused than ever. I like the idea that a ridge vent is at the highest point of the roof and it consumes no electricity, but how can it move out the hot & humid air as efficiently as a fan? Anyone here care to chime in to give me some direction? Thanks.

 
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07-01-10, 03:33 AM   #2  
Welcome to the forums! With what you have already heard and read, it is not only the method of extricating the air, but how the fresh air gets in the attic. Proper soffit vents allowing fresh air into the attic space is critical. A full ridge vent is most effective, since it will evacuate all the hottest air that settles at the peak. At the same time, it is drawing fresh air in through the soffits, keeping the air at a tolerable range.
Power vents fail, solar vents fail. Ridge vents are passive and require no power consumption.

 
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07-01-10, 04:48 AM   #3  
Thanks for the reply.

Whom should I contact to assist with the installation of a ridge vent and soffit(s) vent? Is this the job of a roofing contractor?

I walked around my house, and I found what I think are 2 gable vents. I do not see a soffit vent.

 
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07-01-10, 05:24 AM   #4  
If you Google attic ventilation you can find calculators that can help you determine your ventilation requirements. Here's just one example.

I agree with Chandler that a ridge vent for exhaust combined with soffitt vents for air replenishment is probably the mose effective and reliable way to go. I would not waste money on powered ventilation.

Ridge vents are fairly easy to install and relatively inexpensive. A roofer can install them in a day. Soffitt vents are even easier. You can buy the vents and install them yourself.

 
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07-01-10, 08:41 AM   #5  
Wayne-

What do I need? A saw?

Thanks guys. I think this is really going to improve my attic. And my garage.

 
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07-01-10, 10:21 AM   #6  
I had a ridge vent installed by the roofing contractor when I redid my roof. It looked so easy I did my garage myself. Here's a tutorial on installing a ridge vent that will give you an idea of what's required.

There are several different styles of soffit vents. The ones I used were continuos strips. I just drilled a bunch of holes in the soffit between the rafters and put up the vent over the holes. The site linked above also has a tutorial for soffit vents.

 
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07-01-10, 06:03 PM   #7  
My dad told me after looking at my house and garage that there's metal underneath the overhang. He said he's not sure how to put a soffit vent to the attic in with that metal there.

I think I will have to call a professional for an estimate.

Regarding garage ventilation, do I want my soffit vent up high or do I want to put it down close to the ground?

 
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07-02-10, 03:58 AM   #8  
Soffit venting is done at the soffit. Not sure where you would plan on having it at the ground. Post a couple of pix on the metal where you need soffit. There may still be a way you can do it. As far as the garage, unless it is designed weirdly, the soffit is where the roof ends and turns back to the wall.

 
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07-02-10, 04:11 AM   #9  
Will try to take some pics later today. My house was built in 1919. Pretty old.

 
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07-09-10, 06:27 AM   #10  
You're forgetting a critical issue. In that age house, it wasn't built in regards to energy conservation/efficiency. You need to install radiant barrier in the attic. This will keep the heat OUT of the attic and remain in the frame/decking/shingles. It's an easy DIY job, but you may want to wait until fall/winter. Of course, install the ridge vents over as much horizontal ridge as possible.
Radiant Barrier Foil Insulation Do-It-Yourself Radiant Barrier Tips

Use the foil, not the spray on applications that cost more and only block about half the radiant heat. It's amazing when I go into my attic on a hot day and it's just 110*-115*.

 
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07-11-10, 08:56 PM   #11  
Just some more info.

I have 2 gable vents on opposing ends, east and west direction. No soffit or ridge vents.

I've done some checking, and it would seem that the best time to install a ridge vent would be when a new roof is put on, right?

Also, I've looked into vents under the overhang where soffits are in newer houses. I'm not sure if it would be a good move for the airflow. Air is going to take the path of least resistance, correct? My current setup with air moving from one end to the other would be better than the wind coming up from under the overhang and out the gable vent, right?

Under optimal conditions, I would have continuous soffit vents (makeshift in my case) and a continuous ridge vent over the attic. But that may entail an entirely new arrangement and a lot more money than a ventilation upgrade can save.

I greatly appreciate ALL of the feedback so far. Especially with regard to the radiant barrier foil, which I am now looking into (does that work in a mixed climate btw?) If I have stated something incorrectly or if I'm misinformed, I would like to be set straight. Thanks.

 
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07-14-10, 09:35 AM   #12  
I agree that the radiant barrier, given the $130 cost/1000 sq ft, is almost a no-brainer. My attic was on average 20 deg F above ambient by 5 pm and it did not cool down to ambient until 3 am or so. That is why the house (especially upstairs on a 2 bedroom) feels like the heat is on at night. The temps are measured at the top of the insulation - it's much, much hotter at the peak of the roof.

After installation of the radiant barrier I typically see only 6-8 deg F above ambient in the attic (and I could only cover about 75% of the roof). After also beefing up my attic insulation up to ~r60 I only need AC if outside temps reach 90+. My neighbors run theirs several times per week at about 80 deg+ highs in the summer and I hate the noise.

Add a ridge vent in addition to the radiant barrier and insulation (also seal all holes in ceilings) and you'll be amazed at the difference. The previous owner of my house also installed reflective film on the south facing windows but I don't know how much this helped. Once you fix the attic, your major heat source will be windows. Even double pane are only about r3. Trees that shade the windows are ideal.

Downside is it took me several days of work in the attic - it can only be done in the winter. Even then I was drenched in sweat and dust.


Last edited by AlexH; 07-14-10 at 10:13 AM.
 
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07-14-10, 08:14 PM   #13  
Alex-

So with regard to my particular situation, do you think it would be better if I just installed a ridge vent and no soffit vents? Or would it be better to install a ridge vent, soffit vents, and close my gable vents?

I'm definitely in agreement regarding the importance of radiant barrier foil. It seems that you and I both live in mixed climates, so I presume you simply lay the radiant barrier down (no stapling) over top of the insulation?

 
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07-14-10, 08:51 PM   #14  
I'm no expert on attic ventilation so hopefully others will comment but you always need soffit vents since the hot air rises and thus pulls outside, cool air from the soffits.

I have read you don't want both gable vents and ridge vents but I'm not sure why.

I live in San Jose, so my primary concern is heat and thus I stapled the foil to the underside of the rafters.

Read as much info at atticfoil.com and then ask the owner if your not sure. He is a great guy in that he will provide detailed answers for a $100 order.

As I said, while you are up there, seal all holes - here is a great article -

How to Seal Attic Air Leaks | The Family Handyman

 
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07-14-10, 09:21 PM   #15  
Just to add another comment - from my limited research the heating costs reduction is addressed more efficiently by sealing the attic and insulating to the recommended r-value for your area.

There are various radiant barrier products designed specifically to address heating costs (some of these go under the insulation) but I don't know what the cost/savings tradeoff is.

Even the attic/rafter radiant barrier will reduce cooling costs by a maximum of 20% or so best case (1 story ranch house) and probably no more than 10% for most applications. I rarely use AC so I did it to improve the comfort level inside my house. It probably reduced my upstairs indoor temp by 5-8 degs from 6 pm to 2 am or so.

It's just that it's so cheap that the savings don't have to be high to pay for it - if you can stand the pain of install

 
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07-15-10, 04:30 AM   #16  
Posted By: AlexH I have read you don't want both gable vents and ridge vents but I'm not sure why.
If my understanding is correct, it's because the ridge vent will pull air from the gable vents instead of the soffit vents. The air will take the path of least resistance, right? Someone please correct me if I'm wrong.

Thanks for the info. I would like to get some of my attic work done a cool day.

 
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07-15-10, 07:38 AM   #17  
You are correct about the path of least resistance. What you are overlooking is the volume of air within a confine (attic) remains constant. In other words if a certain volume of air leaves the attic, a volume of air equal to that must enter the attic. Temperature and pressure differences causes this movement of air. The soffit (low) vents and gable/ridge (high) vents create what is known as "Free Venting" Adequate Free Venting is achieved when the High and Low vents are equal in "Net Venting Area" and sized according to attic floor area, (1 sq. ft. for every 300 sq. ft. of attic floor) for houses without a vapor barrier the vent area is doubled to 2 sq. ft. This is recommended regardless of your situation.

I personally never recommend ridge vents. Having inspected 1,000's of homes in my career, I have observed the meshes that are used to prevent rain, debris and insects from entering the attic through these vents clog up. At that point they are useless.

For reducing cooling costs when Adequate Free Venting is present I usually recommend "Gable Vent Fans". First they are the least expensive thermostatically controlled attic fans and simple to install. And more importantly, you cut no holes in your roof. Second is Equilibrium. Let's assume you get the recommended size gable fan if 1800 cfm. The second Law of Thermal Dynamics, High to Low, tells us that an object of lower temperature (outside air) will absorb heat energy from an object of higher temperature (attic air) until the temperature of the two objects are equal in temperature. This is known as "Convective Heat Transfer". Which naturally occurs with Adequate Free Venting but has accellerated heat transfer with the use of the fan.

Let's assume the volume of air in your attic is 10,000 cubic feet. A 1800 cfm fan will replace all the air in the attic in less than 10 minutes. Assuming your attic floor is retangular without any dormers on the roof, the formula to determine volume of a gable roof attic is Length x Width x Height divided by 2.

1800 cfm fans are usually 8 inches in diameter which means it usually will fit over most gable vents, they operate at a low rpm to move that amount of air. This causes the operating costs of these fans to be low.

All this is known as "Induced Attic Ventilation" and works best when "Adequate Free Venting" is present.

 
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07-15-10, 09:30 AM   #18  
Francis,

Thanks for the post - I had researched attic fans that pull air from inside the house through the attic but the install is not practical, especially on a house that already has a ceiling AC cold air return.

Although the radiant barrier reduces peak attic temps, the roofing material retains heat (especially concrete roofing) for 6 hours+ after sunset and by that time the insulation has reached equilibrium thus allowing heat into the house.

Is there a website that has more info on the gable fans?

For example, if you have 2 gable vents, is one fan sufficient?

 
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07-15-10, 11:13 AM   #19  
There are a lot of sites where you can get information about gable vent fans on the web. The biggest problem I see with them is people oversize the fan, thinking bigger is better. Your net vent area must be equal or more than the cfm rating of the fan. In other words if the fan moves 1300 cfm (recommend in your case) then your net vent area must allow the minimum of 1300 cfm. If you get a larger fan such as 1800 cfm, the inadequate net vent area will shorten the life of the fan.

Furthermore there are a lot of factors involving heat transfer. Time is just one. For example, for the air being drawn through your attic to extract heat from your roof decking and floor insulation (thermal mass) takes time. So a high air volume fan will reduce the attic temperature quickly but the temperature of the attic quickly returns to where it was before. This is because the fast moving air did not remove much heat from the thermal mass in the attic. On the other hand, a low volume fan will extract more heat from the thermal mass in the attic because of the time it has to do so.

I recommend one gable fan rated at 1300 cfm in your situation. Provide adequate free venting as much as possible. Simple rule of thumb in determining net vent area is divide by 2. So if I a gable vent that is 1 foot by 1 foot which equals 1 square foot. The louvers in the vent reduce the net vent area to half the size of the vent. So if you have 2 gable vents, you have 1 sq. ft. of net vent area. If in this case there is 600 sq. ft. of attic floor, then all you would need is 1 sq. ft. of net vent area at your soffits to obtain adequate free venting.

You do not have to be so exact, just make sure you don't have less than the minimum net vent area and the high/low vent area does not have to be equal but not less than the minimum net vent area recommended.

 
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07-17-10, 04:51 AM   #20  
Posted By: resercon I personally never recommend ridge vents. Having inspected 1,000's of homes in my career, I have observed the meshes that are used to prevent rain, debris and insects from entering the attic through these vents clog up. At that point they are useless.
This makes me wonder why ridge vent manufacturers do not create replaceable filters. I know it's not very fun going up on the hot attic to replace filters, but it would be worth it for improved performance right? Plus, how often would the vent even need to be changed? It would seem more likely to be years, not months.

Thanks for the detailed info. That was extremely helpful.

 
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07-17-10, 06:18 AM   #21  
Unfortunately clogged meshes is only one reason I never recommend them to clients. As a licensed home inspector I have permanately installed comments concerning ridge vents and insert them in reports to avoid repeatedly writing the same comments. The most common problem observed is the saw depth when cutting out the roof decking goes too far, cutting into every rafter on both sides of the ridge board the vent is over. Second to that is hammer and nail gun damage to the vents. The third is my favorite, they didn't cut out the roof decking for the vent. Laughable but true and you will surprized how often I find it. There are a lot of comments stored in my PDA, in fact 91 comments on ridge vents. I sync the PDA with my lap top on site where there are even more comments on ridge vents. These comments on my lap top are less common. From my point of view creating replacement meshes for these vents is the least of my concerns.

 
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07-22-10, 06:38 AM   #22  
You should look at some hard research about attic ventilation. Those high CFM fans can cause issues you have not taken into consideration. Proper air sealing at the ceiling and then adding insulation will reduce the impact of the heat from the attic.


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