Back when I first started doing home inspections, I was under the impression that roof ventilation was the cure-all for everything. I would look at a lot of problems and quickly point to insufficient roof ventilation as the cause, and recommend more roof ventilation as the cure.
Blistered shingles? Not enough roof ventilation.
Ice dams? Not enough roof ventilation.
Frost in the attic? Not enough roof ventilation.
Today I’m much more ho-hum on roof ventilation. Take it or leave it. Asphalt shingle manufacturers require roof ventilation to help preserve the life of the shingles, despite the fact that the color of shingles will have a greater effect on their life expectancy than roof ventilation will. An attic with insufficient ventilation will get warmer than a well ventilated attic, which may increase the temperature of the shingles, which may decrease the life of the shingles… just a little. Proper ventilation will also help to keep the attic space cooler during the winter, which may help to prevent ice dams. Let me say that again; proper ventilation may help prevent ice dams. I’m not saying it will, but it might. The same thing goes for frost in the attic; as I mentioned last week in my post about frost in the attic, proper ventilation may reduce frost accumulation in attics, but it won’t prevent it.
In other words, roof ventilation certainly isn’t a cure for any condition, but it’s still required. Roof vent manufacturers publish installation instructions that are easy to read and should be easy to follow, and roof ventilation is required in section R806 of the building code, but a lot of folks either don’t read the instructions or they don’t care. Today I’m going to go over a few of the most common roof vent installation errors and issues.
Mixed Exhaust Vents
For proper ventilation, both high and low vents should be installed. On paper, the high vents are supposed to act like exhaust vents while the low vents should act like intake vents. Convection is supposed to help make this happen. In reality, it all depends on how the wind blows, convection has little to no effect, and it’s never perfect. The intake vents will typically be soffit vents, while the exhaust vents may consist of ridge vents, turbine vents, box vents, or powered vents… but only one of those. The photo below shows an example of these different types of vents, all installed on the same roof, which is a no-no.
When different types of roof vents are installed, there is an increased potential for air in the attic to basically short-circuit. In the photo above, the power vent would probably end up sucking in air from all of the other high vents in the photo, while pulling in just a small amount of air from the lower soffit vents. The solution here is to install only one type of exhaust vent.
Power vents shouldn’t be used because they create more problems than they fix. I blogged about this earlier this year: Attic Fans Won’t Fix Ice Dams (or anything else). I use the terms ‘attic fan’ and ‘powered roof vent’ interchangeably. I also use the terms ‘roof vent’ and ‘attic vent’ interchangeably.
Crooked Turbine Vents
I’ve never been a huge fan of turbine vents because I have it in my head that they may end up causing some of the same problems that powered roof vents do, but the fine folks at Complete Building Solutions swear by ’em, and I trust those guys, so I won’t complain about turbine vents. The one thing I’ll mention is that turbine vents need to be installed perfectly level; when they’re not installed level, they don’t turn. In the photo below, the vent on the left wasn’t level. Do you see anything else that’s wrong in the photo?
The other thing about turbine vents is that they really do pull air out of the attic; if air sealing hasn’t been performed in the attic, they’ll pull air into the attic from inside the house, and shouldn’t be used. That bears repeating: do not install turbine vents if the attic has not been professionally air-sealed.
Insufficient intake vents
Current standards specify a 50/50 split between high vents and low vents, but how are low vents supposed to be installed in a house with no soffits?
Without any low vents, the high vents will tend to pull conditioned house air into the attic through attic air leaks. One solution would be to install fascia vents, and another less desirable option would be to install a bunch of box vents low down on the roof. I could go on and on with these roof vent installation errors or shortcomings, but I never make a huge deal about any of this stuff because I don’t think it makes a ton of a difference. As I mentioned at the end of last week’s blog post about frost in the attic, focus on sealing attic bypasses before addressing ventilation. Ventilation mostly helps to hide other problems.
For more reading material on roof ventilation, check out the links below:
- Green Building Advisor: All About Attic Ventilation – this article looks at all of the reasons that attic venting is required, and goes on to say they’re all ho-hum reasons. I completely agree.
- Building Science Corporation: Understanding Attic Ventilation – a discussion of vented vs. unvented attics.
- Building Research Council-School of Architecture: Early History of Attic Ventilation – A long explanation of why our current rules for attic ventilation are arbitrary. This is a fascinating read.
Author: Reuben Saltzman, Structure Tech Home Inspections
December 24, 2013, 7:16 am
Keep in mind that the “fan curve” of a turbine is very different than a powered vent. A turbines “static pressure” doesn’t increase much when airflow is low.
On the other hand a powered vent pulls MUCH harder when airflow is reduced, static pressures get high really quick. Oddly enough when static pressures are low and there is a mind breeze of 5-10mph both vents pull about the same amount of air.
Most ridge vents are basically junk and won’t move any significant amount of air. The “filters” that most of them use are simply too restrictive when dealing with the relatively low static pressures. If they do work, they won’t for long because the filter will soon clog up with the dusty attic air.
Static vents are only slightly better if the temperatures in the attic are warm enough. Do a smoke test on different ventilation types and you will quickly see what works and what doesn’t.
December 27, 2013, 5:00 am
@Bob – do you have any photos of clogged filters at ridge vents? I’ve heard claims of clogged ridge vents dozens of times, but I’ve never actually seen a clogged ridge vent myself. For some nice videos comparing different types of ridge vents along with smoke tests, check out Lomanco’s web site. Here’s one such video:
December 24, 2013, 8:27 am
Great post! During our recent second floor remodel the insulation guy offered blown in for the same price as rolled fiberglass so we went with that. However it covered up all the vents. Do you think that was a poor decision? We never had soffit vents so I think we never had decent ventilation to begin with. It has really paid off in my heating bills, they have dropped 50%. Also, what color shingles last the longest and shortest?
December 27, 2013, 5:19 am
@Dave – blown in insulation is far superior to rolled fiberglass. Check out this post: http://www.structuretech1.com/2011/03/my-beef-with-fiberglass-batts/
Also, light shingles last the longest and darkest the shortest. Here’s a recent report on that subject: http://www.professionalroofing.net/archives/past/mar02/feature2.asp
December 24, 2013, 10:02 am
Reuben, this is so true. WAY more important to deal with why the moisture is getting into the attic than trying to get it out. I have seen many perfectly vented roofs with huge moisture issues.
December 27, 2013, 5:03 am
@Charles – same here my friend. I’ve also found plenty of pristine attics with very little ventilation provided; I’ll bet you have too.
December 26, 2013, 3:37 pm
Dave, I’m going to make an educated guess that the lighter the better for shingles. They reflect light better so absorb less heat.
Reuben, been reading a lot of old posts and really appreciate them. Too bad I only started reading after my recent buyer’s inspection. Next time, I’ll give you a call!
December 27, 2013, 5:21 am
@Mark – you got it. Thanks for reading!
December 27, 2013, 7:37 am
I’ve only seen a few that were clogged, most don’t flow any air to begin with. Note that the Lomaco video does NOT include static vents or roof turbines. I’d like to see how those do in a smoke test, preferably on a real house. This is how most ridge vents work in the real world:
December 27, 2013, 7:44 am
I’d like to see a test NOT sponsored/done by a manufacturer showing that roof vents actually work. Every video I’ve been able to find showing they work is done by a manufacturer, most on “test buildings”, not actual houses…
Another example of ridge vents that don’t work:
December 27, 2013, 7:52 am
Tim Carter doesn’t believe in ridge vents either.
The list goes on, I’d like to see an independent test of a ridge vent that actually works (with video on a real house preferably). Get the brand/model/design if you find one that works.
Another scam is the solar attic fans, ridiculous CFM data from their tiny motors. No way a 20W motor is moving 600+ CFM!!! Yet you will see no independent testing showing the contrary.
December 27, 2013, 8:05 am
Notice in the test you linked to the blower was only across the top of the ridge, not the entire house and the back side soffit vents were omitted from the video. Of course this isn’t real world, the fan should be on the entire surface of the house to show how the attic air actually blows OUT the soffit vents on he back side of the house.
Another thing is in the static tests (no blower) he didn’t just leave the roof “open” for very long. It would have shown how much more air was moved with NO VENT than with a ridge vent.
And that’s just the “tricks” I was able to see in the sales presentation. Other manufacturers have similar videos showing their product is superior with similar “magic tricks”.
I’d like to see independent testing of ALL types of vents to show what really works and what doesn’t. I will say roof COLOR and the addition of a radiant barrier under the roof decking make BIG differences in attic temperature. My favorite attic to go in during the summer is one that has been spray foamed under the roof decking, it’s cooler in the attic than the outdoor temperatures!! If it just wasn’t so costly to have it done…
January 11, 2014, 9:05 am
why i am having sending u email
January 11, 2014, 9:12 am
i did check my attic and insulation around base of vent pipe seems dry.u mentioned in your article that condensation can be caused by bathroom /kitchen fan,how do i locate the bathroom fan bypass,do i need to go to the attic or take the bathroom fan assembly out within the bathroom,how to check if fan is working properly
January 16, 2014, 6:02 am
@pervaiz – you’ll need to go in the attic to look for it. http://www.homeenergyresourcemn.org/stream_document.aspx?rRID=5980&pRID=5959
February 21, 2014, 1:42 pm
Help help Please. Desperate. My 7 yr old son thought it would be a good idea to put twigs and sticks in the roof sewer vent of our home. Do I need to state how much of an issue this is to our system?? Omg nothing is unclogging this. How do I get these sticks clear???? Please anyone. And we dont have the cash for big bucks but is that where this is going???
February 21, 2014, 2:17 pm
Go rent a big sewer snake and clean out the drains. If that doesn’t do it, hire a drain cleaning service to do it professionally.