Ventilation required for attic solar power vented fans.
I went to a product seminar last week and one of the presenters was showing an attic solar power vented fan claiming exhausting air up to 1300 cfm per 1600 sqft of ceiling area (Volume didn’t matter, he said). The rep. said that no additional venting was required other than the 1/150 sqft per code on ventilated attics.
I thought the intent of the code was for passive ventilation. So if I have a 2,000 sqft house I would need two of his fans which at full blast it would exhaust 2600 cfm and leaving the venting area at 1/150 sqft I would create a negative pressure in the attic and potentially draw moisture from the inside. I’m wrong thinking this way? How much eave vent area should I install? Do all power vents have same issue? I thank you kindly for your thoughts.
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Yes, power vents create negative pressure problems which increase exfiltration from the conditioned space. The code standards ARE for passive ventilation (1:150 with no ceiling vapor barrier and 1:300 with), and there is nothing better (on a simple roof geometry) than continuous soffit and continuous balanced ridge vents, as long as the ridge vents have external wind baffles.
Armando,
Powered attic ventilation is always a bad idea. It's a solution in search of a problem. Moreover, studies have shown that powered attic ventilators are potentially dangerous -- they can cause such strong depressurization that they draw air through cracks in the ceiling and cause the backdrafting of water heaters downstairs. Even when they don't cause backdrafting problems, they raise homeowners' energy bills by pulling conditioned air out of the home through ceiling air leaks.
Read more:
https://www.greenbuildingadvisor.com/blogs/dept/building-science/are-solar-powered-attic-ventilators-green
https://www.greenbuildingadvisor.com/blogs/dept/musings/martin-s-useless-products-list
Thank you guys. 1st thing I learned from your responses was to check on past blogs if the question has been answered before.
Reading through all that material plus other found on the internet, I can clearly see that power venting an attic does produce negative pressure and unless you have a very tight ceiling and good insulation on top plus enough net ventilation area, you would be creating problems by exfiltrating conditioned air. I can see how homes that are/were not built with good green building techniques would “suffer”; but having said that, it seems that a very tight home built with all the right techniques can take advantage of power vented attics by helping it to keep cooler. Another question is IF power venting becomes cost effective vs. good passive venting and making sure you have a tight and well insulated ceiling.
I’m going to email all these links to the seminar presenter in hopes he can contribute his expert opinion or his company’s claims.
Armando,
If you do a good job designing and building your house, there should be absolutely no need to lower the temperature of your attic.
You've got a tight ceiling, deep insulation, and of course you have no ducts or HVAC equipment up there. Good job! So there's no need to worry about your attic temperature -- and certainly no need to add expensive electrical gadgets that wear out, need maintenance, and have been known to cause safety problems.
You are perfectly correct, water vapour is created in the home by people. Cooking, washing, breathing, sweating, we all create water vapour. Grown ups about 1.5 litres each per twenty four hours, old people less, active children and animals a bit more.
This water vapour is comprised of such tiny molecules that it can pass through most things.
A well designed and built home, has an almost water vapour proof membrane fitted just below the upstairs joists to stop the water vapour rising into the attic.
On homes where this membrane is missing, it is recommended that you have attic ventilation.
However, ventilation only works when the wind blows, and it is never at the desired pressure. It is either too strong and strips the heat from the home, or there is no wind.....and the idea is a failure.
The best solution is to deal with water vapour in the home, use extractor fans in the kitchen and bathroom, to send it outside, the air outside in nearly always drier, so cold dry air will come in and dilute the warm wet air, or use extractors with heat exchangers.
Open a window for five minutes in the morning and evening
Perry,
Unfortunately, your comments are filled with errors and misconceptions.
1. When water vapor travels from the interior of a home to the attic, the mechanism is not vapor diffusion, as you imply. The water follows air leaks. If the air leaks are sealed, the vapor permeance of the ceiling is irrelevant.
2. The purpose of a ceiling air barrier is to stop air movement, not to stop vapor diffusion.
3. Passive attic ventilation using soffit vents and ridge vents works; your statement that "the idea is a failure" is incorrect. It is true, on the other hand, that the value and importance of attic ventilation is often exaggerated. In any case, wind is not necessary for passive attic vents to work. The usual mechanism driving air flow through the vents is a difference in air temperature between air near the ridge vents and air near the soffit vents.
4. While exhaust fans are often useful in bathrooms and kitchens, you are confusing two issues. Bath and attic ventilation has nothing to do with attic problems. If you're attempting to solve an attic moisture problem by running exhaust fans in your bathroom and kitchen, you are either misguided or living in a house with very serious ceiling air leakage problems. If your ceiling is that leaky, fix your ceiling!
Armando - I agree with Martin and Robert - stay away - one other problem with the attic fans is the majority of the heat in your attic is radiant heat - ventilation will not move radiant heat. Studies from Advanced Energy show that attic fans only reduce the attic temperature by about 6-10 degrees. Insted of two fans at $400 - $500 each - put that money towards a radiant barrier - will get much better performance.
I recently went and looked at a house that is about 50 years old - never had any problems - they just added two attic fans last summer and suddenly had a huge mold problem in their crawl space. It appears that more than the attic had a negative pressure - as the conditioned house air was being pulled into the attic, it was being replaced with the air from the crawl space - the crawl air was being replaced with the humid outside air and they had condensation on all of the ductwork and insulation. Great example of house as a system.
Danny,
Oh boy — a post discussing my two favorite products, powered attic ventilators and radiant barriers! Seriously, Danny — I don't really want to go off-topic here — but spending money on a radiant barrier, just like spending money on a powered attic ventilator, is usually a waste of money.
It's better to spend your money on air sealing or, if necessary, thicker insulation for your attic floor. Study after study has confirmed this.
Your example of the 50-year old house with powered attic ventilators depressurizing the house only proves my point. Those homeowners needed air sealing work, not a radiant barrier.
Yes Martin I agree that air sealing is obviously the first step - kind of thought that was a given on a site like this. I was not suggesting that I would install a radiant barrier before air sealing - since the topic was attic temperature was merely giving a solution for that particular issue. Should have been a little more clear - thanks for pointing out the confusion. Although I do stand by radiant barriers (especially radiant barrier roof sheathing on new construction) as a good "phase II" measure after all the basics are taken care of especially if the HVAC unit is in the attic.
Martin,
While I agree with you on the absurdity of radiant subslab "insulation", a radiant roof barrier in a hot climate has been documented by the Florida Solar Energy Center as significantly beneficial, particularly when used in conjunction with a well-vented roof.
Light colored and high-mass roofings are the best way to minimize summer attic heat gain and consequent ceiling heat flux and heat gain into attic-based AC ducts. But with a dark-colored roofing, both ventilation and radiant barriers are essential.
The downside of a light colored roof is that it retains a higher consistent moisture level because of the reduction of heat flux.
To clarify my position on radiant barrier sheathing for new construction: it really only makes sense if the builder insists on installing ductwork in the attic. I agree that, if there are any ducts up there, radiant barrier roof sheathing makes sense.
It's just a little bit tragic, however, that radiant barrier sheathing is the best approach that Florida builders can come up with to address the design insanity of attic ductwork.
Assuming a well designed house with ductwork inside the conditioned envelope of the house, it's hard to justify the use of radiant barrier sheathing. It's harmless enough, however, and I did recommend its use in my blog on hot climate design.
If a sub-roof radiant barrier can reduce attic peak temperatures in hot climates, then regardless of whether ductwork is in the attic, the heat flux through ceiling insulation into the conditioned space will be reduced.
In a hot, sunny, AC-dominated climate it makes sense to use a sub-roof radiant barrier in conjunction with other temperature limiting strategies such as light-colored and high-mass roofing and attic ventilation.
Even in cold climates, radiant barriers have their place. One such is under the floor joists over an unconditioned crawl space or basement.
So I have attic ductwork. My house was built in 2002. I am looking to put solar panels for electricity but the Co tells me to first put solar powered fans in the roof at a price of $2100 for three of them.
I have a hip roof. Would not be easier and cheaper to doublewrap the duct work; make sure the suffix are free of insulation and maybe put a couple more vnets in the roof? plus double the insulacion on the attic floor?
George,
Run, don't walk, away from any company representative that tells you that the first solar investment you should make is $2100 for solar-powered attic fans.
You have my condolences on your attic ductwork problem. Solutions include:
- Installing insulation along the plane of your roof to bring your attic within your conditioned space. (Unfortunately, this is an expensive solution.)
- Carefully sealing the seams of your ductwork with mastic and improving the duct insulation. (A more affordable solution).
And you're right -- installing $2,100 worth of insulation on your attic floor is a much better investment than installing $2,100 for solar-powered attic fans !