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09-15-2012, 12:36 PM #1
Passive vs. Powered Attic Ventilation in Florida
Currently trying to get estimates to replace the T-111 siding on my CBS on slab home in Florida. Siding surrounds the attic between the roof and the cement block exterior walls. I'll be replacing the shingle roof at the same time, but none of the roofing contractors I've contacted will tackle the siding replacement.
So far only one contractor is willing to bid the siding job and am awaiting his quote. He's an older gentleman and has a long history as a reputable general contractor in my county. However, one of his recommendations has me concerned as to how "up-to-date" he is regarding building science and makes me wonder if he's a good choice for this job.
Both times he was at my house he told me he would put powered attic ventilator fans in the "gables" on the back half of the house, where the roof peaks in the middle. I told him he couldn't do that because the "gable" at the rear of my house does not back up to the attic, it's actually part of the wall in my family room, which has an open beamed ceiling. After he pondered that awhile, he then (twice) said he would put the fans in the siding at both ends of the front half of the house above the garage and the guest bedrooms. I told him I was concerned that powered attic ventilation would cause negative pressure in the attic and result in higher energy costs not only from running the fan, but also from pulling conditioned air up into the attic. He said that can't happen. When I asked if the fan would exhaust or intake air, he said it would be an intake fan. I didn't discuss it further, but I was concerned how the intake from two powered fans could possibly be matched to the exhaust space available through my off-ridge vents in that part of the attic so as to avoid positive pressure and infiltration of more hot attic air into my conditioned space. Also concerned about pulling in too much humidity.
Everything I've read online advises against powered attic ventilation in Florida. Comments?
09-15-2012, 03:55 PM #2
Well, he could be a great roofer and siding installer, but it sounds like he needs to stay away from fans, far far away...If more government is the answer, then it's a really stupid question.
09-15-2012, 05:05 PM #3
09-15-2012, 07:53 PM #4
Intake, as in pressurize the attic?
I don't think so. Not unless you are absolutely 100% positive that the ceiling between attic and house is 100% airtight.
Even then, if you have blown fiberglass or other loose fill insulation in your attic, blowing a bunch of air across it reduces its insulation value big time. Basic theory of insulation is air entrapment. You can't trap air if you keep blowing against the traps.
All of the above still applies if the fan is made to exhaust vs. blow in.
If you plan to be in this house long term, consider passive options available for a cooler attic: a) radiant barrier applied to the rafters; b) "cool roof" options that greatly reduce heat gain to the attic; or c) insulating the roof deck and making the attic air conditioned space. I am currently pursuing option "b" (cool roof is going in as I type) as my research indicates reflective metal roofing returns the best reduction in air conditioning costs. The Florida Solar Energy Center did an extensive study of this and it is available at their website."In this house we obey the laws of thermodynamics!"
- Homer Simpson
09-15-2012, 09:34 PM #5
Thanks all for confirming my decision to pass on powered attic ventilation. I swear this guy reminds me of something out of the late 1960's when everyone on the block was adding central air to the old homestead. It was all the rage to add roof turbines and/or powered gable vents at the same time in an effort to save on cooling bills. Turns out it was wasted effort, but everyone was doing it!
I've read the Florida Solar Energy Center study and will be implementing several of the recommendations for reducing heat in the attic. The first will be reflective roofing. I'll not be going with metal, but will use Energy Star white shingles. Second will be replacing the old ridge vent with the more effective type with the wind baffle and internal filter, as well as extending it by about 10 feet. For some unknown reason, the previous roofers only cut the vent about 8 ft long even though there's a good 18 feet inside the attic that could be vented. In addition, the three small off-ridge vents in the front half of the house are too short and will be replaced with longer vents. I have soffit vents all around to provide plenty of airflow for passive ventilation once the ridge venting is corrected.
Thank you all for confirming I'm on the right track regarding attic ventilation. And I think I'll keep looking for a siding contractor who's a bit more up-to-date on "building envelope" issues.
09-15-2012, 11:41 PM #6
Unbaffled and internally baffled ridge vents perform poorly at best. Under some wind conditions, some ridge vents don't vent from the attic at all, wind blows in one side and out the other, and doesn't allow any air to vent from the attic.
Externally baffled ridge vents perform better under all wind conditions.
There has been a video circulated around that a roofing contractor did showing ridge vents doing nothing. Many people have used that one video to claim that ridge vents don't work.
Perhaps the issue had more to do with the installation, or design of the ridge vents installed in the house they tested.
In most of the homes I see ridge vents installed in, the slot at the peak of the roof is not cut back as far as it should be, often only 1.5" to 2" total, which is not enough.
The University of Florida, and other building science testing organizations, have gotten different results.
Here is a good video demonstrating the vastly superior performance of one brand of externally baffled ridge vent vs. a variety of brands of unbaffled and internally baffled ridge vents.
If more government is the answer, then it's a really stupid question.
09-16-2012, 10:02 AM #7Regular Guest
- Join Date
- Jun 2001
- Moore, Oklahoma
Be VERY careful about ridge vents, in most cases the don't work. The video above is one of the rare cases that it's done right.
Also consider what happens when the filter on the Shinglevent II clogs up. If the ridge vent actually works upon initial installation the filter WILL clog, just like the filters on your home A/C unit do over time. Are you willing to have somebody go into your attic to clear the filters every summer? The filter can be seen in the attached photo, the material looks a LOT like that used in HVAC filters, clogs up the same way...
IMHO the old whirlybird style vents work best over the long term, get the lighter aluminum ones, they won't rust and the bearings last the life of the roof. The do cost more than the galvanized ones but are worth it. Whirlybirds have lost popularity due to their looks and all the hype about ridge vents/power vents, but they DO work.
If you are really concerned about heat in the attic use light colored shingles when your roof is replaced. The difference between dark and light is huge. It's like walking on the sidewalk in summer with bare feet then stepping onto a recently paved street. Dark colors are popular because they normally look better, but light colors are more energy efficient.
09-16-2012, 05:34 PM #8
http://www.coolroofs.org/products/re...pe=select&type=Shingles+or+Shakes&market_type=&company_name=&bra nd=&model=&select_color=select&color=Bright+White&color=Off-White&min_solar=&min_solar_3yr=&min_therm=&min_the rm_3yr=&sri_init=&sri_3yr=&slope=Steep&crrc_prod_i d=Off-White&min_solar_3yr=&min_therm=&min_therm_3yr=&sri _init=&sri_3yr=&slope=Steep&crrc_prod_id=
You may need to cut and paste the above link into your browser to make it work, as it appears to be too long for a hyperlink to work here.
The "ISR" number means "initial solar reflectance" and "EMI" indicates emissivity, or how quickly a material sheds heat once it is heated. With ISR, higher numbers are better. For comparison, a dark asphalt shingle has an ISR of around 0.09, meaning about 91% of the solar energy striking that roof at any given time is absorbed by the material, ultimately ending up in the attic. The best white composition shingle I saw for ISR on the CRRC link was 0.29, which was the CertainTeed Oxford Star White NC XT30 product.
ISR is exactly as it states...the amount of reflectivity the material has when brand new. After three years the ability of the material to reflect heat degrades due to dirt and biological growth. For the product mentioned with the highest ISR, the three year SR number looks good, degrading from 0.29 to 0.28. I wonder if the shingle has some built-in way to resist biological growth, as that is what often fouls composition shingles in humid climates.
I am assuming you mean a composition type white shingle in your reply above. If so, I include what I've said in this post to encourage you to choose the highest ISR and highest emissivity white shingle available in order to reap some benefit from increased reflectivity over standard dark composition shingles. If you have HVAC ducts in the attic, reducing attic heat gain is always a winner in a predominantly cooling climate like Florida, and where I live. I also will choose passive solutions any day over active ones, meaning I'd rather have the material itself deliver a benefit vs. using electricity to accomplish the same end."In this house we obey the laws of thermodynamics!"
- Homer Simpson
09-16-2012, 06:16 PM #9
09-16-2012, 07:04 PM #10
09-16-2012, 07:45 PM #11
I'd like to put a white metal roof on my house, but my house would look silly with a white roof, and I'm concerned about the reflection off of it causing problems for my neighbors when the sun is in certain positions.If more government is the answer, then it's a really stupid question.
09-16-2012, 08:40 PM #12
Thanks for the help! I knew all four Solaris colors were Energy Star rated, but I never would have known the tans were significantly more reflective than the gray.
09-16-2012, 09:23 PM #13