Attic Fan Or Ridge Vent

dlcj

I have a 2100sq ft 6/12 hip roof, 300' of witch is the open carport and pourches that have a hidden vented vinyl sofit material. The sofits are vented every 4' with normal sofit vent. There are also 2 dormors with fake vented shudders. Plenty of intake volume! I have a 16" (i think) attic vent fan on roof near the center at top. Last summer we had 100 degree days for a month straight and some more days. The fan (set on 115) would run from 9 to 9 every day and i replaced the motor twice in 2 years. Would a ridge vent be better. There is only about 30' (half the house) of flat ridge to use. About half the attic has 12" of blown insulation and the rest got skimped on and averages around 8". This winter i plan on blowing in more and bringing it all up to about 14". Would the extra insulation make the fan not economical anyway.

Ed the Roofer

Firstly, one vented soffit panel every 4 feet probably is minimal, not plenty of intake volume.

The soffit panel range from 6" to 9" per square foot of ventilated panels. Every 144 square inches is one square foot of ventilation and the ideal scenario would be to have 60 % intake and 40 % exhaust.

Is the blown insulation clogging up the intake perforations?

Usually, a ridge vent with and external wind deflecting baffle is more efficient than a power vent, and as you have already experience, less expensive and continually functioning.

Ed

theroofinggod

I would also think the gable louvres on the dormers would be more in line w/exhaust than intake,as they are they probably short circuit your venting system as the fan probably pulls air from them instead of from the soffit vents----I agree w/bringing the r-value of your insulation to r-38,then put the ridge vent,the fan should be on a thermostat so it only works on the really hot days

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mdshunk

What would you consider a "really hot day"?

When I wire these attic fans up, I always struggle with what to set them at. Going from memory, the thermostats have about a 60 to 140 degree range. If I set it at 90, it will pretty much run 24/7 for much of the summer. Is there a rule of thumb on setting these things?

Tom M

I set them at 100. IF that thing is running 9to9 at 115 you definetly need more air. A ridge vent will complement whatever else you got going on. I dont know about the 60 / 40 thing. I have read all the different ideas methods as well and I believe in the old stack effect of high pressure naturally exhausting.

dlcj

There 2 sofit vent panels every 4' all the way around house (hip roof). The dormor vents are only about half way up between the ceiling and peak of roof. The 300sq ft of carport and pourch have a fully vented vinyl sofit but you can only see the holes from inside out, ive never seen this before now, i like it. So i do believe there is enough intake with all this combined.The rafters are sitting on top of a plate on top of the ceiling joist giving me about 12" between top wall plate and roof decking, plenty of room there too. I believe i will ad more insulation and the ridge vent and remove the fan. I am working on closing in an 14x18 section of the attic for storage. I built the floor up with 2x6s so i still have r30 and i am lining the walls and roof with r4 3/4" foam board.The wife wants the room sealed from bugs and dust and this was the easyest way. Sense this room will be sealed from the rest of the attic and will not have hvac, Im thinking of using that attic fan set on about 100 to ventalate this room into the rest of attic with a few intake vents of course. Does that sound like it could work. Thanks for your imput.

Grumpy

I find 85 to be as low as you'd want them set. I think I once read 90 is average setting. If you are replacing the motor, that means it doesn't have good intake. It's struggling to pull air that doesn't exist.

What is a vented shutter? A Gable vent?

DO NOT MIX RIDGE VENT AND GABLE VENT,OR RIDGE VENT AND ATTIC FAN. THAT'S JUST BAD BAD BAD ADVICE. Unless the attic areas are completely seperated.

Patrick

Nice Grumpy!!! Now how can I explain that to all the elderly folks I do roofs for that point to all their neighbors houses with no soffit vents, a gable vent, AND a new ridge vent and demand I do it that way because if the whole neighborhood is like that it must be the properway

Grumpy

Explain nothing! Let the manufacturers talk for you.

Say as I say... I then pass them brochures and web site addresses. www.airvent.com www.rollvent.com Then if they insist on STILL doing it wrong, I pass the waiver At that point they know you mean business. I have had people ask me to do it wrong. I have been in the same position. It's my warranty, it's my reputation, I'd rather walk than do it wrong.

dlcj

Ok guys, Break it up. I never mix ridge,gable, and or power vents. Only if i install a power vent IN the gable vent will i use both and i never set the stat below 100 (this is Alabama)or it would never stop. I need to know what is the best style ridge vent to use.The house is 60' long but only 30' of horizontal ridge. I like the shingle over vent best but dont know how good they work. Would this still be more efficient than a power fan. The vents on the dormors are rectangle gable vents. They are just mounted to look like closed sudders over a window. There 4 of them. I looked again and the dormor vents are closer to the ceiling than the peak so i concider them intake.

theroofinggod

usually the attic fan should be set at about 105 degrees

__________________
"If it`s worth doing.It`s worth doing right!!!

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Ed the Roofer

From Air Vents, Power Cool fan instructions.

Ed

8. THERMOSTAT OPERATION:

approximately 10% below the thermostat setting.

Patrick

So just my luck after seeing this debate, I'm on a job site today looking over a roof we are starting next week and I am in conversation with the HO who I know pretty well and his "environmental engineer" who was on site overseeing the new HVAC system and venting system. So i say OK smart guy, since your hear how about educating homie here that I am correct and he really needs to block up those gable vents since we are fully venting his soffits and ridge as he requested. Engineer says why???? I have soffit vents, gable vents, and a ridge vent on my house and thats the way it should be. Sadly in the real world people tend to look at you funny if you say "well EdTheRoofer and Grumpy said...." But I did say that myself and other high end contractors strongly favor doing things a particular way.

reveivl

The engineer knows that hot air rises. No-one has explained to me how hot air can lie stagnant at the bottom of an attic (with good soffitt venting), while cooler air moves from gable vent to ridge vent.

The venturi effect alone would cause the lower hot air to be drawn upwards as well as it's natural tendency to rise because it is warmer and therefore less dense than the cooler air.

I haven't done extensive testing of this, but I'm unaware of what magical force is stopping the warm air that is supposedly 'just sitting there' from rising. Maybe it exists, but like your engineer, it doesn't make sense to me.

__________________
From where does knowledge come? If you need to know what is in a box, you could ask someone (not reliable), you could pray, (not useful), you can consult with the scripture (not helpful) or you could open the box (science)

Tom M

I dont believe there is anything wrong with having both. Its all speculation. It is said that gable vents or attic fans alone can cause hot spots in the attic with-outy proper intake but if you have all you need plus its just bonus. Im not convinced that it can short ciruits airflow by having cross ventilation. I looked at the link Ed posted Certinteed continuing education and it was an excellent site to read up on. Something else I should add is this HVAC engineer could be sucking combustible air from his attic with an HVAC unit syphoning air flow. I have to look into it when I have time but new units should take all outdoor air whenever possible.

Ed the Roofer

I know that the user names can sound sort of silly and even lame, but the information I usually post is backed up by research studies and manufacturers requirements.

Regarding: reveivl' comments:
"I haven't done extensive testing of this, but I'm unaware of what magical force is stopping the warm air that is supposedly 'just sitting there' from rising. Maybe it exists, but like your engineer, it doesn't make sense to me."


I have been involved in lengthy debates on some very sophisticated forums with research links thrown about all over the place to continually back up one side of an argument or another, but the fact does always come back to;
"What Is The Manufacturers Specifications?"

If I intentionally deviate from the specifications, I would assume the liability for the products and environments proper functionality. I do not have the financial resources to absorb such a monumental risk.

Maybe the engineer does? Provide the HO a waiver, citing his preference to choose his friends "Opinion" versus the manufacturers specifications and another tune will be heard.

Ed

Patrick

To give you an Idea he has scissor trusses and would like to fill from the roof sheathing to the bottom of the scissor trusses with spray foam

reveivl

Yeah, I follow manufacturers specs, too. That doesn't mean that they are correct, or make sense. It becomes a case of CYA.

But this is a discussion, not an installation to manufacturers specs, and would someone please tell me what force is keeping all this hot air in the lower regions of the attic against every applicable law of physics that I learned in school?

Hey, maybe there is one and I was just playin' hookey that day.

__________________
From where does knowledge come? If you need to know what is in a box, you could ask someone (not reliable), you could pray, (not useful), you can consult with the scripture (not helpful) or you could open the box (science)

Patrick

Well to quote you sir, its a "magical force"

dlcj

I think this is a job for Myth Busters on TLC