powered roof vent noise

[rieferman]

Hello,
I installed a powered roof vent per the manufacturer's instructions in the roof of my barn. Ceiling in the space is finished with sheetrock in a vaulted fashion up until the collar ties where the ceiling is flat (tray) creating a mini-attic cavity near the peak. Baffles installed under the insulation allow air to travel from the vented soffits on both eaves to that cavity. The roof vent is then installed centrally close to the ridge to draw the hot or moist air from that cavity (the roof vent has a humidistat as well as thermostat).

The good news is that it's working. When it's hot or humid, the fan kicks on, runs for awhile and then kicks off. Repeat as conditions require.

The bad news is that is sounds like a helicopter is flying far overhead. It's not so annoying as to drive me absolutely crazy, but it's noticeable for sure. I can hear this through the insulation and the glued/screwed sheetrock. The vent is at 12 feet above where I'm listening from.

My hypothesis is as follows: The barn roof rafters are 24" apart with 1/2" plywood over ancient lathe strips. The 14" fan cut out is centered in the 24" space, and I think it's basically turning the plywood/lathe into a drum. That drumming is what I hear (and sounds like a helicopter).

My proposed solution: From the underside (through the hatch I thankfully had the sense to leave myself) stiffen the area supporting the fan so that the vibrations is absorbed. I was thinking something like a 3/4" layer of rigid foam against the roof deck (glued) followed by a 3/4" layer of plywood (also glued). Block those both in place with 2x4 framing between the existing rafters.

What do you all think?

Thanks in advance!

 

[pattenp]

Without being there to see it and hear it it's hard to say. My first thought was the space below the fan is to short and you are getting bad air turbulence which causes the sound.

 

[rieferman]

To test that theory, I should be able to leave the hatch (which is immediately below the fan) open - this would allow unlimited amounts of air flow. Would you agree with that test approach?

edit: And, if that does prove to be the case, is there a way to back off the fan speed, or would you recommend increasing intake somehow else?

 

[pattenp]

If opening the hatch eliminates to noise then yeah you need to find a way to allow better air flow or slow down the fan. Slowing down the fan may be the easiest solution.

 

[rieferman]

Cool, thanks Patenp, I'll give it a try this weekend and report back.

Note to self: google how to slow the fan down

 

[pattenp]

Here's a fan speed control. You just need to verify your fan's amps.

http://www.amazon.com/dp/B004JYM2ZY/?tag=atomicindus08-20

 

[kbs2244]

If the more air experment works, then I would suggest more sofffit vents.
It will allow more air to the fan, and may even allow so much more circulation that the fan dosn't come on as musch.

 

[pattenp]

I know it's a wild guess but I think if it's an air turbulence problem because of the ceiling being too close to the fan then more soffits won't help. Opening the hatch directly below the fan may allow a more direct air stream where as more soffits may not provide the same result. I'm just theorizing.

If the more air experment works, then I would suggest more sofffit vents.
It will allow more air to the fan, and may even allow so much more circulation that the fan dosn't come on as musch.

 

[rieferman]

Good point, I should have mentioned that the soffit is completely vented (i.e. it's not a solid eave with a few holes drilled in it, it's a box framed eave with vented soffit the entire length) so intake should be sufficient. With the baffles, space where the lathe keeps the roof deck up off of the rafters, and the space behind the insulation (R-19 installed in true 6" spaces, leaving about 1/2" space once the material settled) I think that enough air is getting up there. So I think I'd agree that the turbulence may be more of a proximity to the hatch issue rather than amount of intake air issue.

I'll know shortly, heading out to the barn now for some work. When it gets hot enough to start kicking the fan on, I'll do some tests and report back.

Thanks again guys!

 

[bd8134]

Some reports say an attic fan consumes more energy than it saves.
I had a fan in my house attic that turned on at 115. During the summer it used to come on a lot and your could hear/feel the vibration down 2 levels of the house.
Having a humidity sensor is also supposed to be a bad idea for the winter.
I have removed my fan now and gone with an insulated rafters in the attic as my a/c air handler is up there.

 

[rharman]

Maybe look at rubber washers/grommets to isolate the fan brackets. When I had to replace a gable vent fan, the reviews I read mentioned it was quite loud until someone tried that. I mounted the new one with rubber washers and it's very quiet.

 

[rieferman]

bd8134 wrote:

Having a humidity sensor is also supposed to be a bad idea for the winter.

In a case like mine (a cathedral ceiling) detecting humidity and removing it before it can condense, soak the insulation, and bleed through the sheetrock is important, so being able to sense the humidity is very useful. In general, I can't see much down side to removing humidity or heat from an isolated non-living space, but I'm always willing to learn a new take on any topic.

---
As for the noise update... I started very simple this weekend... When we cut the hatch for the ceiling access, we used a roto zip against the hatch framing to make a perfect cut, and then used that cut out piece as the hatch covering. It was held in place by sitting it on top of a few drywall screws in the framed opening. Later, the plan was to trim out the opening properly and all the cut out to sit on the trim lip instead.

To pattenp's point about turbulence, it occurred to me that perhaps the loose sitting cut out hatch covering was bobbling on the sheetrock screws as the air pulled through. So, I simply shimmed it tight in place before beginning my work for the day to see if I could notice a difference.

Now, it wasn't that hot that day so it's possible it just didn't kick on, but in 8 hours of working, I didn't hear the fan making the helicopter noise. On similar weather days recently, it kicked on throughout the day, so I'm feeling optimistic. However, I will watch it for another couple weeks as the weather warms up and report back with a final verdict.

 

[Mike in Ohio]

I have that type of fan in my attic. The original one was whisper quiet. The motor went bad and I put a new motor on it. Now I can hear it on the second floor when it is running, not real loud but noticeable. At least I know it is working!!!

 

[rieferman]

Update: Stopping the access hatch from being able to vibrate took the noise from helicopter to just a regular "a fan is on" sound. Thanks for the tips and troubleshooting!!