Pot Vents vs Ridge Vents


#1

I have a hip roof with less than 10 ft of ridge so I have pot vents (12). Would adding a ridge vent help or would it just interfere with the pot vents working properly?

Is such a short ridge vent better than my 12 pot vents? I might be able to get another 15 feet of ridge on another section of the roof but it’s not as high as the highest (but shortest) ridge.


#2

Even though the total NFVA is less for that length of ridge vent, I still prefer it to static air mushroom vents. But, I overcompensate the Intake Ventilation to arrive at the proper or exceed the Minimum requirements. I also like the fact that the ridge vent sits at the highest point of the roof, while mushroom vents are anywhere from 1 foot to 2 foot or more, below the ridge line.

With both vent products, the Manufacturers claim a short-circuiting effect will occur, so that combination is not advisable.

Ed


#3

Hi,

I would use the mushroon vents. They work no matter what.


#4

These are actually not the spinning mushroom vents - these are the square vents. Sorry if I got my terms wrong.

My roofer strongly recommends the ridge vent but I was concerned about inadequate ridge line(s)


#5

Irregardless of which you choose, you Must have a significant amount of Intake Ventilation.

Has that issue been discussed?

If your soffits are already clad with aluminum panels and even if you have vented, lanced or perforated ones in place, you need to know if the original wood soffit has been cut out completely to allow air to flow through these vented panels.

If not, then i would recommend that you have your roofer install the Smart Vent by DCI Products, which is a shingle over style Fresh Air Intake Vent System.

Intake is actually more imperative than exhaust vents.

Ed


#6

Hi,

Your concerns are legitimate.

We new which you were talking about.

Most hips do not work good with ridge vent. With hips you can have to much intake. It can cross winds in your attic.

I have sectioned off parts of attics to keep from short circiting the exhaust part of the ventilation.


#7

copy/paste from

oikos.com/esb/30/atticvent.html

Ed

Driving Forces
Most people have been taught that warmer air rises and escapes from the high vents, while cooler air enters in lower vents. Thermal buoyancy is a major cause of air leakage from the living space to the attic, but research at the BRC shows that wind is the major force driving air exchange between an attic and the outdoors.

“Our research shows that the role of thermal buoyancy in diluting attic air with outdoor air is negligible,” says Rose.

This requires rethinking about the design and location of attic vents. For example, some ridge vents may allow air to blow in one side and out the other, without drawing much air from the attic. Rose believes that ridge vents with baffles create better suction to draw air out.

Soffit Vents

**Rose considers soffit vents to be very important.

“If a roof had only one type of vent device, I would choose soffit vents,” he says, “because they work well as inlets and outlets.”**

There’s less problem with rain and snow getting in, because soffit vents point downward. To get maximum protection, locate soffit vents as far out from the wall as possible. If rain or snow blow into the soffit, it’s less likely to soak the insulation or drywall.

Soffit vents should always be installed whenever there are high vents. High vents, on ridges or gables, will pull air out of the attic. Without soffit vents, make-up air would be drawn through the ceiling, which increases heat loss and adds moisture to the attic.

Because of the importance of soffit vents, Rose believes that at least 50 percent of the vent area should be low on the roof.


#8

I have heard of ridge vents allowing snow into the attic space.

Several years ago my father got a call on a roof leak on a 1,000sq+ roof he did. When he got into the attic space he found 4 ft snow drifts which got in through the ridge vent. The fix was ripping up the ridge vent, cutting and installing new decking at the peaks, new felt, a row of shingles and ridge. He then installed something like 150 vents which was thought to be adequate at the time. My guess is the ridge vent was a poor design.

A framer I know built a lake home which was 3,500 finished square feet. He had ridge vents put on and thought the attic was much hotter than his home with turtle vents. He first checked to make sure the ridge peak was cut open and the felt and shingles were cut back and sure enough they were. He decided to cut in air vents and as soon has cut the first box he said heat poured out of the opening. After putting in a dozen turtle vents he says his attic is not nearly as hot as before.

I only put ridge vent on by home owners request and on roofs that have full vaults. The only ridge vent I will put down is the 4ft plastic sections which are ridged over the top.

The only turtle vents I will put down are the painted Lomanco metal vents.


#9

Dougger, prior to 1993, I had 4 separate instances over a 2 year period of time which the ridge vent products I was using allowed rain water on 2 occasions and blowing powdery snow on 2 occasions, into the home owners attics.

I was going, WTF, I thought ridge vents were the right thing to do. Then, I attended my first Air Vent Seminar and discovered the reasoning and from my experience, the necessity of a ridge vent needing an external wind deflecting baffle as part of the product.

Since that time that I switched from Cobra Roll Vent and Benjamin Opdykes Roll Vent, both which did not contain an external wind deflecting baffle, I have Never had one single instance of even an inkling of a vent failure allowing snow or rain into the attic.

Ed


#10

Simple answer is the 10 feet will not flow enough at all to maintain the roof warranty. 180 inches of exhaust is equal to a little over 3 can vents. Stick with the can vents. You will void the warranty with the ridge vent. Find another roofer since he does not know the basics of attic ventilation.

If you keep the can vents and add ridge vent that would be even worse.

See there is a formula to this. For every 300 sq feet of free attic space you need 1 sq foot of ventilation balanced between sofft and exhaust. Hip roofs can not support such forumlas due to the lack of ridge. If he insists on voiding you warranty why would you go with him is my question.


#11

GTP,

If the Total NFVA being supplied meets or exceeds the FHA 1/300 or 1/150 formula, then the roof is code compliant and will be under warranty.

As it stands now, per manufacturers own studies, between 90% to 95% of al shingle roofs installed do No t meet the Minimum requirements for the manufacturers warranty per the ventilation requirements.

Ed


#12

Thanks guys. Seems there’s some good faith disagreement. I don’t think the roofer is trying to sell me something - it seems it would be cheaper for him to replace the existing pot vents rather than put in ridge vents (since that would require replacing the decking with the holes for the pot vents).


#13

10 foot of ridge vent for any home is not enough period. I have never seen one small enough to support 10 feet. They normally cover the existing holes with aluminum and shingle over with 2 layers of felt. Most contractors do not replace the wood but some do.

So we can agree the ridge vent is not enough how big is your home? If your home is bigger than 750 square feet then ridge vent is not the correct application and will void the warranty. Now if there are all those vents like you said im sure it is bigger than 750 sq feet. Number will not lie. Using the 150 there is no way.

He is trying to up sell you something. Ridge vent is not free and costs more money to install.

I agree most roofs are not done correctly and i have seen them fail and the roofs warranty be voided. Either way i can bet with 12 can vents the home is not 750 square feet. If you use them together ridge and cans then you have another problem. Like i said find another roofer he is trying to sell you something that is voiding the shingle warranty right off the bat.


#14

The total footprint of the home is about 2000SF

The main section of the house (underneath the big hip roof with the short ridge) is about 40x35 and has 5 pot vents. This is the highest point in the house. I’m guessing it’s close to a 12/12 pitch.

Another section of the house (connected to the big hip roof) is about 20x20 and has another 5 pot vents. The ridge over this section of the house is about 10-15 feet lower that the short ridge at the top of the big hip roof. But its about a 20 foot long ridge.

There is another section of the house (connected to this smaller section) about 20x20 which has a cathedral ceiling and has no vents. It seems like it would be a good idea to add a ridge vent over the cathedral ceiling - it has a 20 foot ridge that runs perpendicular to the rest of the house.


#15

On the Vaulted/Cathedral ceiling area, not only is it a good idea, but quite necessary. You still have to ensure that there is a minimum of a 1 1/2" to 2" space on top of the batt insulation in the rafter bays of the cathedral ceiling area, or else air will not flow from the soffit intake vents.

Also, it will be mandatory to have 100% continuous soffit venting to be placed in every rafter bay for air flowage to actually move through via the stack affect.

These points you just brought up are even all the more pertinent as to the rational of increasing the Intake Venting.

As a matter of fact, most of the validated ventilation experts on the laboratory and filed analysis side, state that Intake Ventilation is of greater significance than the exhaust ventilation. If needed, I could probably find which documents reference that point for validation.

Ed


#16

A 20X30 ATTIC ONLY REQUIRES 10FT OF SHINGLEVENT TO MEET MINIMUM. OR 4 HAT VENTS.


#17

Even at the “Minimum” 1/300 code, if all other required conditions are met, you would need 2 square feet of NFVA, which is 2 x 144 sq in = 288 sq inches.

So, if all conditions are met for the 1/300 rule to be applicable, then that would translate to 6 static air vents or 16 feet of 18 sq in per foot version of ridge vent.

Now, how often are all of the 1/300 conditions actually met in the real world?

So, use the 1/150 formula instead.

4 sq ft required instead,

32 feet of ridge vent

or

12 static air vents.

Now, by increasing the amount of NFVA from Intake Ventilation, you can lower those exhaust numbers down to a more manageable and realistic amount of Exhaust NFVA to meet or exceed the criteria.

Ed


#18

I would have to see a picture of the home to see how i would ventilate it. Yes ridge vent will work on the cath area well as long as 2 things happen, 1 there are bafflles going the up the roof and there are soffit vents. It being closed off will allow that. But it must have baffles so the air can flow across the roofing deck with flow being stopped by insulation. Never seen one that did not need baffles yet.

As far as a 20 by 30 home. Um, i have not seen a 600 sq foot home before so i can not comment, SMallest i deal with is around 900. But with 12 pot vents it would indicate that it was larger