Mixed exhaust vents: Wind turbines and ridge vent
People who know me hear this often: “Don’t Over-Think It”. Things are usually never as complicated as they seem. Of all the questions I get from customers, Attic Ventilation is probably the most misunderstood, and overcomplicated topic in the home efficiency/improvement area. So hear me now: “DON’T OVER-THINK IT”
Attic ventilation is VERY simple. You have HOLES in the bottom of your attic (usually soffit vents) and HOLES in the top of your attic (ridge vent, wind turbines, static vents or power fans). Ideally, air comes IN the bottom and OUT the top. It’s just like a chimney in your home. Pretty simple. Holes in bottom, holes in top…got it?
Think about this. If I came to your home today and started ripping out your soffits and cutting lots of holes in the top of your roof eventually the air temperature in the attic would get close to whatever the outside (ambient) air temperature is. Air temperatures would drop but surface temperatures would not drop much due to the radiant heat gain. A radiant barrier works together with decent attic ventilation to drop both air temperature and surface temps.
Here is where many people screw up attic ventilation. More is not always better. The thought process goes something like this: “If I add an electric or solar attic fan, or some ridge vents, or a few wind turbines or static vents or maybe ALL OF THEM, I’ll move more air and my attic will be cooler” WRONG!!!!
Dirty or clogged soffit vent
Let me ask you. Have you EVER seen a chimney with a fan sticking out the side in the MIDDLE? Of course not, why? Common sense would say that the fan would pull air from BOTH the top and bottom of the chimney. Your attic is EXACTLY the same. Why on earth put an electric fan 15” away from a ridge vent or wind turbine? Yes, there will be great airflow in the top 2 feet of the attic and the rest of the air will be almost stagnant.
Air always travels the path of least resistance. Mixing exhaust vents is a bad thing. The strongest vent will dominate and the other exhaust vents will probably become INTAKE vents. This short-circuits the whole concept of air coming in the bottom and out the top. This can happen many different ways from actually mixing different types of exhaust vents to having ridge vents on EVERY ridge. Usually the ridge vents about halfway up the roof will become the intake vents and totally short circuit any air coming from the soffit vents. All the exhaust vents should be within a few feet of each other from the top.
How to fix it? Start at the bottom with your soffit vents. Have you EVER cleaned your soffit vents? 95% of homes have clogged, blocked or painted over soffit vents. Or, the holes behind the vents are so small virtually no air is coming into the attic. I’ve even seen million dollar homes where they just screwed in the vents and did not bother to even cut holes. I’ve seen many homes where just cleaning the soffit vents (use a dry nylon car wheel brush) will drop the attic temperature 20º in 20 minutes. And those wind turbines that were “broken” start spinning like crazy. Get it? Air comes in and now air will go out.
Hole cut too small behind soffit vent
Next, pick one type of exhaust vent and go with it. By combining a basic strategy of holes in top and bottom your attic will be cooler in the Summer and drier in the Winter. Remember, keep it simple, and don’t over-think it…Holes in the bottom, holes in the top.
Tags: attic fan, Attic Ventilation, attic wind turbine, ridge vent, soffit vents, solar attic fan
I agree completely!!! Ventilate, ventilate, ventilate! when I crawl up into the attic of an older home, I notice that there is never enough air flow. usually there is one or two roof vents and a very few soffit vents. One of the more successful experiments I have done, is to take an AC booster fan (~25watt) and secure it into the existing roof vents while adding more soffit vents. In the middle of summer, there is a difference between passive and active air flow. I prefer passive, but some homes just need a little extra help. If you figure out the basic air-in, air-out flow, then adding a little fan (with a thermostat) can help get rid of the heat faster than waiting for the heat to rise. I don’t like the huge 175 watt roof fans due to the amount of electricity they use and the fact that you can never get enough air into the attic through the soffits to keep the fans from pulling your good AC from the house into the attic.
I usually stick with my passive vents and occasionally use a booster fan when really needed.
Toff
Okay, after reading this, I’m canceling my ridge vent project. I have four exhaust vents a few feet from the top of my roof and my soffits are not clogged, so I’m fine. I cut holes in the radiant barrier by the exhaust vents just as your website instructed. Thanks for the help.
Kevin in Sugar Land
Attic vents? I haven’t vented an attic in over 10 years. Unvented attic… foam insulation against the roof line. Radient barrier with air space under a metal roof.
I totally agree. My first choice is a full foam enclosed non-vented attic with a radiant barrier to the outside as you mention. Unfortunately, this is usually only a cost effective option for new construction. This is for the millions of existing homes that have vented attics. Thanks for the feedback!!
I am now very confused. My understanding is that you must have a ventilated attic for the radiant barrier to be effective. The house I’m working on is essencially a new build. So do I plug the holes or vent the rood?
I am now very confused. My understanding is that you must have a ventilated attic for the radiant barrier to be effective. The house I’m working on is essencially a new build. So do I plug the holes or vent the roof?
What if you bought a house where the owners had a new roof put on using ridge vents. There are holes still cut out in the attic from the wood that used to have the roof vents. How do we close these off so that it is secure for someone to walk on the roof?
Hi,
I am confused on this topic. It sounds like the roof gets hot, radiates heat into the attic, and this does most of the heating up there. I do not have ridge vents, just two wind turbine vents in the roof and an old vent fan that really some idiot installed a long time ago that does little. So I wanted to install a radiant barrier. My trouble is that since I have to ridge vent, the air in the gap between the barrier and the roof is trapped there and stagnant, so it seems to me will just get hot and then conduction will occur through the foil. Am I wrong, or if not, how to avoid this?
No, you want a ventilated attic. Zark and I were referring to using spray foam an fully enclosing an attic. This is done to move the “thermal enclosure” to the outside and bring all ductwork inside a semi-conditioned space and you would have no insulation on your attic floor. Talk to your builder about this technique or check out: http://www.buildingscience.com.....aled+attic for more info.
I have looked over and read most of the information on your site. The “hybrid” method seems to be the fit for what I have in mind. I am thinking of using two separate inlet/outlet systems for the attic. I would install the radiant barrier, as per the hybrid method, at the eave, and incorporate the flat top method with static ridge vents. This would cause the heat to rise with a vertical vortex. Secondly, after covering the gable ends with radiant barrier to completely close in the attic, I would use the gable vents for the other system. I would probably add unfaced batt insulation on top of the blown-in fiberglass. One end open and the other has a thermostatically controlled fan for the exhaust. Please comment on this arrangement. I am planning to install the radiant barrier next week. Thank you.
I have a house built in the late 70’s I am looking to add a radiant barrier to. The ventilation in the attic consists of 2 whirly birds and 2 end gable vents roughly 2×3’s? Is this enough ventilation to support a radiant barrier or do I need more? (House is rought 1700 sq ft)
Thanks.
Josh,
Do you have soffit vents? Your home may have originally had a wood roof with no soffit vents. If you don’t have soffit vents, the the ideal system would be to install soffit vents to act as intake vents.
Radiant barrier will still be beneficial even with limited ventilation. I would be sure to leave a gap at the bottom near the attic insulation. Air will naturally be drawn in the bottom and head towards the top as it heats up. Here are some install instructions: http://www.atticfoil.com/radia.....uction.htm
Bobby,
Wow, I think you are WAY over-thinking this. Often, there is too much emphasis on attic ventilation when installing a radiant barrier. Let me give you an example: If you park you car in the shade, does it really matter if you crack a window? Here is why. The radiant barrier rejects so much heat that usually whatever attic ventilation you’ve got is usually pretty adequate. I always recommend installing the radiant barrier FIRST. Then, put a thermometer inside the attic below the foil. If the air below the foil gets more than 15-20 degrees hotter then the outside temperature THEN increase the attic ventilation. You do not want to “seal” the AtticFoil, this is not the intention. You want air to flow freely from the bottom of the attic to the top and you want to install the radiant barrier in a way that will not impede, restrict or bottleneck the airflow. Remember, the BEST CASE lowest air temperature inside the attic is going to be the outside air temperature.
Ed,
Thanks for the response. It confirms what I was thinking. I need to add soffit vents. How many would you recommend based on the size of my home of 1700 sq ft? I was thinking maybe 8 total. 4 on the front and 4 on the back with a large enough whole in each one to have plenty of ventilation. Of course I’d make sure to not cover the holes with insulation.
Thanks!
The general rule is about 1 square foot of NFA (net free area) or “hole” for every 150-300 sq. ft of attic space. If you have 1700 ft, then you would need about 5 and 11 total square feet of NFA. This is generally split between topside exhaust vents and lower intake vents either 50/50 or 40/60 with more on the lower side. If you go in the middle with 8 square ft. NFA then you would need about 4 sq. ft of of NFA for the soffit vents. At 144 sq. inches per sq. ft. this is 576 sq. inches.
An 8″ x 16″ vent provides 56 sq. inches of NFA per vent. http://www.airvent.com/homeown.....eave.shtml Note: NFA is different from the vent size. You would think that an 8×16 vent would provide 128 sq inches, but it’s only 56 of OPEN space.
Given all this, you are pretty close with 8 or 10 vents. 8″x16″ soffit vents will do the trick. If you arer using smaller vents 4″x16″ or 6″x16″ you will need more.
I have about 2,600 square feet in my home. I currently have 2 power vents near the top of my roof line. I have 30 (8 X 16) soffit vents around the house. Yes, after two repaintings I am sure that those vents are somewhat restricted with paint, dirt, etc. I have bought 30 new vent covers to replace those old covers. We do need to repaint and/or install vinyl siding on those soffits. I will install the new vent covers if we repaint. However, if we go with the vinyl on those soffits I have noticed that the vinyl covering for the vents are only 12 inches wide with fairly small vent holes. Those would be placed over the current holes cut for the soffit vents. QUESTION: Would I still get adequate venting with the vinyl coverings over those soffit holes with those two power vent fans? I am probably over-powered with the power vents and I do not want to have too little venting from those soffit vents so as to cause the air conditioning to be pulled from inside my home into the hot attic. Your advise PLEASE!
Thanks again Ed for such valuable information. I see another project to add to my growing list!
John,
With vinyl siding, you can just take a circular saw and cut out a bunch of rectangles in the soffit before you install the vinyl siding. You are correct, the vinyl siding that has holes is a little lacking in open area, so you want to install a lot of it. Another option is to replace the existing soffit with a hardy board type soffit that has continuous vent holes. Finally, you can cut a hole in a porch and install an air conditioning vent grill and paint to match. It will act as several soffit vents. It’s easy and rain won’t be able to get up and inside.
I prefer large passive domes to whirlybirds or slantbacks. Whirlybirds have no screening against critters and wasps, and slantbacks are too small for effective ventilation even though they are very weather resistant.
My next project is radiant barrier up there.