Ridge Vent . . . and Nothing But the Ridge

Can anyone convince me that this picture shows anything other than the fact that the ventilation provided here is effective only in the first five inches down from the ridge? And yes, there is continuous soffit venting that is baffled and open to the outside.

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Comment by Bob Blanchette on January 14, 2012 at 10:25pm

Agreed, ridge vents simply don't work. The major problem is the "filters" used in their construction. The air must pass through a mesh material similar to what a cheap A/C filter is made of. At the relatively low pressure differences in an attic very little air is able to pass through the filter. If any air does manage to pass through the filter it will quickly clog, just like the filter in your HVAC system.

For southern climates the turbines work best, they pull a LOT of air with the slightest breeze. Even when there is no breeze present they act like a 12" hole in the roof. On windless days the hot air escaping from the attic is strong enough to actually turn the vent!! Light one of your "smoke sticks" in an attic vented with turbines, you will see exactly what I'm talking about !!

Comment by Ed Voytovich on December 30, 2011 at 5:56pm

My theatrical smoke generator uses basically the same stuff we use in the Wizard Sticks and also in kids' electric trains.  There is a bit of slightly sticky residue, but it doesn't last long.  It sure does show you whether the ducts are sealed.

Note, however, that fog juice can set off hard wired smoke detectors.  Ask me how I know.

Comment by Bud Poll on December 30, 2011 at 5:47pm

Air Vent has some good graphics http://www.airvent.com/professional/index.shtml but yes, deflected or channeled might depend upon the mfg chosen.  But it can't go straight up as the top is covered and must go to one side or the other, depending upon the wind.  Analyzing air flow with wind is so specific to each house it is hard to predict the exact results.

As for the smoke, it all depends upon what is going on outside that day, but it is surely the picture of a thousand words.

On the topic of smoke, are you happy with the residue level of what you are using?  Any related problems?


Comment by Ed Voytovich on December 30, 2011 at 5:08pm

Would the air be "deflected" or would it be channeled out the side of the vent with the more negative pressure? 

I'd like to go back up in that attic with my theatrical smoke machine. 

Comment by Bud Poll on December 30, 2011 at 4:36pm

Ed, thinking about that picture, those first 5" also correspond to the vent cap on top where the warm exiting air is being redirected back down and out, thus keeping that small segment of the sheathing slightly warmer.


Comment by Ed Voytovich on December 30, 2011 at 8:32am

If the three control layers (conductive, convective, vapor) are established in the same place at the attic flat, the mildew/mold will be denied liquid water and will be dormant and non-destructive. 

I recommend that everyone in our industry read Jeffrey May's The Mold Survival Guide and My House is Killing Me.  Jeff sometimes presents at ACI.

Comment by Bruce Navin on December 30, 2011 at 8:12am

Good posts, Gentlemen. I know that the comment regarding lawyers & anxiety broached the question, but in reality-what do you advise a customer to do about the mold?

It seems to me that once you see it, you own it until you reveal it to the customer and offer advice to fix it. As auditors, it's our job to give the customer the plain truth, not to appease anxiety. Otherwise, we could be paying for the Mercedes. I'm not saying that we should scream bloody murder, but if we see mold-we should tell them that it is there, it is "alive" and that the way to remove it is with new sheathing. Explain that we have solutions to fix the source of the problem (improved ventilation and airsealing), and offer what the experts & science has concluded to date regarding mold. I don't know off hand, but would the ventilation & airsealing cause the mold to "die off" due to lack of moisture supply? Would it be really that harmful to health & durability at that point? If it's not so bad, some customers would conclude that they will ventilate & air seal and decide to not replace the roof & sue the builder.

It seems to me that the right approach is to inform the customer, tell them what the science says, point them to some resources, document accordingly and mark the situation as CYA'd. 

Comment by Bud Poll on December 30, 2011 at 6:51am

So you already knew the actual problem was the bath fan vented into the attic, and that was compounded by your mistrust of ridge vents.  However, my recent work can help shore up your concerns about ridge venting.  Basically, the upper vent should be large enough and as high as possible and nothing besides a power vent equals a properly done ridge vent.

Here is an adjustment to the thinking on attic venting.  When you calculates the minimum vent area, you must remember, that is a minimum and the switch between 150 and 300 is not a hard switch, there are many levels in between.  Once you decide on a number, say 8 sq ft, then divide that in half assigning half to the upper and half to the lower.  Now, here is the change.  Each of these numbers is the minimum for each area and should only be increased, never decreased.  There should be no trade-off where increasing one allows you to decrease the other.  Reducing one end of a pipe while increasing the other does not provide the same flow.

For the attic above, if the venting met the minimum (1 per 150) requirement and was not compromised by the insulation, then the majority of the fault lies with the person or persons who vented the bath fan into the attic, especially with three teenage kids and two adults, I'll assume the dog doesn't use the shower :).  If the less than suggested opening at the ridge reduced the upper vent area to below the minimum, then that installer is also responsible.

As for the nicely washed 6" on each side of the ridge, that confirms that the incoming cold air does not "wash" the bottom of the roof deck as often described, but spills out onto the attic floor with the mix gradually filling the attic as it pushes the warmer (lighter) air up and out.  As it approaches the peak, the flow rate picks up as the area decreases and thus the last few inches does benefit from the washing effect (moving air).

Great picture, what is/was the outcome of the search for the villain?


Comment by Ed Voytovich on December 30, 2011 at 5:51am

The 2200 ft² house is in Central New York, Climate zone 5.  It was an early winter day with low outdoor humidity, and the temp at the time was in the mid-thirties.  Three teenagers, two adults, and a large dog live in the house with at least one bath fan ventilated into the attic.  The house was built in the 1980s and there is no air sealing.  The long side of the house faces west, into the prevailing wind.  This amount of thriving mildew clearly did not spring up over night.

It doesn't take a blower door to figure out why there is moisture in the unconditioned attic or why there is mildew on the OSB.  It's more interesting to think about why that's the extent of the area upon which the ridge vent appears to have an impact upon the moisture in the attic.

Hint:  I don't believe that ridge vents work worth a darn, and this extreme example appears to me to support that opinion. 

Also:  my title may be misleading . . . it was not meant to say that there is no supply ventilation air . . . it was actually a reference to a Monty Python skit from some years ago that pops up frequently around our house but is otherwise lost in the mists of time.

Comment by Bud Poll on December 29, 2011 at 10:01pm

More information is needed here. Temperatures where this house is located.  Moisture levels, ie is this the Oregon coast with an onshore breeze, or just cold and dry?

There are two details we need to look at, one the source of the moisture and two, the effectiveness of the venting.  From the picture, both are in question.  What is the RH inside the home, its age, and do the walls and ceiling have an functioning vapor barrier.  Has the house been air sealed to minimize warm moist air from inside leaking into that attic?  Have you checked to be sure bath, kitchen, and dryer vents are vented to the outside and there are no disconnects dumping moisture inside the home.  Bath vents in the soffits tend to deliver the moisture right back to the attic, but it is often indicated by a localized moisture problem.  This is so widespread that I would suspect some moisture is entering the attic and simply not being vented out.

When you say continuous soffit venting, we need, as Bob asked, the net free vent area.  A row of 3" pop-in vents looks like continuous venting, but amounts to almost nothing.  You will need a good look at those soffits and verify that the cellulose we see was not blown right under the baffles to fill them.  Was this part of an energy audit with a blower door test?  If so, some zone pressure readings can give you an idea as to the amount of attic venting.  More on that if a blower door test is in the future.

I'll let you catch up before I continue.


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