Passive vs. Powered Attic Ventilation in Florida

[Florida Joy]

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?

[mark beiser]

Well, he could be a great roofer and siding installer, but it sounds like he needs to stay away from fans, far far away...

[jimj]

Everything I've read online advises against powered attic ventilation in Florida. Comments?

Not only in Florida.

[Shophound]

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.

[Florida Joy]

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.

[mark beiser]

Second will be replacing the old ridge vent with the more effective type with the wind baffle

You are on the right track with the externally baffled ridge vents.
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.
http://www.youtube.com/watch?v=rE6fyWHfTXc

[54regcab]

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.

[Shophound]

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.

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.

You might be interested in the information at this link by the Cool Roof Rating Council regarding white composition shingles:

http://www.coolroofs.org/products/re...pe=select&type[6]=Shingles+or+Shakes&market_type=&company_name=&bra nd=&model=&select_color=select&color[4]=Bright+White&color[5]=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.

[Florida Joy]

You might be interested in the information at this link by the Cool Roof Rating Council regarding white composition shingles:

http://www.coolroofs.org

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.

I am assuming you mean a composition type white shingle in your reply above.

Thanks for the link! Actually, the Energy Star shingle I've already selected for my house is the CertainTeed Landmark Solaris Crystal Gray. It matches the color of my existing roof and it has the same 0.29 rating on the CRRC link as the CertainTeed Oxford Star White NC XT30 product.

[Florida Joy]

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.

CertainTeed Landmark algae-resistant (AR) shingles have the additional attribute of resisting the growth of algae especially in damp regions.

[mark beiser]

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.

[Florida Joy]

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.

I ran an open search for any brand, any color shingle rated with ISR of 0.29 or greater. It seems the CertainTeed Solaris shingle in Sierra Buff, Coastal Tan, and Santa Fe Blend have the highest reflectivity, significantly higher than either the white or gray, i.e., 0.41, 0.40, and 0.40 respectively. Since a light tan would actually be a better match to the color of my walls and siding, I think I just changed my mind! Changing the roof color will require submission, with fee, to my HOA Architectural Board for review and approval, but I believe the end result will be worth it.

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.

[Florida Joy]

Thanks for the link! Actually, the Energy Star shingle I've already selected for my house is the CertainTeed Landmark Solaris Crystal Gray. It matches the color of my existing roof and it has the same 0.29 rating on the CRRC link as the CertainTeed Oxford Star White NC XT30 product.

FYI, the CertainTeed XT30 series shingles are only warranted for 70MPH winds. Insurance in Florida requires shingles warranted for 130MPH winds. The Landmark Solaris series meets this requirement when installed as part of a CertainTeed roofing "system", including ridge vents.

[vstech]

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.

I bought my house in 99 because it had great shade, and an aluminum roof... yup, bare aluminum sheeting, the stuff was installed in 1949, and it's in perfect condition... there is no ventilation in my attic other than the small 14x20 gable on one end... basically useless, without soffet or a matching gable on the other end, and my attic is rarely above outside temp... bare aluminum is VERY REFLECTIVE of radiant energy. the reason there is only one gable is due to the other side of the attic being finished... sortof... the room is inclosed, but the roof sheeting and rafters are visible... 1900 tree limb rafters that is... and cedar shakes for sheeting... someone put shingles on top of those sometime after the 20's and then the aluminum was put on.
I get comments all the time that the rain must sound awesome with a metal roof, but the aluminum doesn't make any noise from the rain, it's a dull metal that's absorptive of impact sound.

[Shophound]

Mark Beiser wrote:

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.

We looked at other metal options besides the white standing seam. One of them was a "stone coated" metal shingle that is installed on top of battens, allowing the shingles to vent from the eaves to a ridge vent. I liked the idea, and several colors were available that weren't stark white (that would, such as in your case, be more neighbor friendly). However, I could not locate any data stating what the reflectivity was, or if there was reflecitivty/emissivity data, was it, combined with the back-venting, comparable to the standing seam options we were considering?

If I had no option but to use a "shingle-like" metal product due to neighbor or HOA concerns, I'd seriously consider laying down at least one thickness of foil faced foam insulation sheathing board, then the battens, then the metal shingles. That would likely make for one seriously cool performing roof, even with darker shingle colors.

[mark beiser]

We looked at other metal options besides the white standing seam. One of them was a "stone coated" metal shingle that is installed on top of battens, allowing the shingles to vent from the eaves to a ridge vent. I liked the idea, and several colors were available that weren't stark white (that would, such as in your case, be more neighbor friendly). However, I could not locate any data stating what the reflectivity was, or if there was reflecitivty/emissivity data, was it, combined with the back-venting, comparable to the standing seam options we were considering?

I really like the way they look, but the good stone coated metal shingles are to expensive for my budget on this house.

Even with the extreme hot weather the last 2 summers, and 2 computers burning $500 worth of electricity a year doing distributed computing projects for disease research, I use less than $2000 a year worth of electricity at current rates.
This is in a house with only 2-4" of insulation in the attic, 46 year old single pane aluminum frame windows, and a 15 year old over sized 10 SEER AC system.

I have to balance the cost of what I'd like to do to the house against the relatively low potential return in real energy savings, and the limit to how much potential to improve the value of the house there is.

Basically, the problem I have with this house is that if I do much more than a radiant barrier, good passive ventilation, and R-36 insulation, the ROI flips to infinity.

[Shophound]

I really like the way they look, but the good stone coated metal shingles are to expensive for my budget on this house.

Even with the extreme hot weather the last 2 summers, and 2 computers burning $500 worth of electricity a year doing distributed computing projects for disease research, I use less than $2000 a year worth of electricity at current rates.
This is in a house with only 2-4" of insulation in the attic, 46 year old single pane aluminum frame windows, and a 15 year old over sized 10 SEER AC system.

I have to balance the cost of what I'd like to do to the house against the relatively low potential return in real energy savings, and the limit to how much potential to improve the value of the house there is.

Basically, the problem I have with this house is that if I do much more than a radiant barrier, good passive ventilation, and R-36 insulation, the ROI flips to infinity.

With the work already in place on my house prior to the new roof going on, I don't expect a huge ROI for the roof in terms of energy payback. What we do have is a roof we will never need to worry about again unless a very strong force of nature or a fire damages it (and the house, for that matter). The roof has made the interior of the house considerably quieter against outdoor ambient noise, something we did not expect ahead of time. I have no plans to move ducts out of the attic, so the reduction in heat gain will improve a/c performance, and when it comes time to replace equipment I'm expecting when I run a Manual J again that the result wil indicate I can downsize tonnage. We are also looking at future solar panels, and standing seam is friendly toward installing those units.

If my decision re: roofing material had been based strictly in ROI for energy payback, I would likely have gone with white colored asphalt with a reflectivity of 0.29 or greater, or the less expensive reflective metal shingle options. I realize standing seam metal is not a viable option for all, and many do not like the look of it. For us it was an opportunity to enhance the appearance of the house, reduce attic heat gain, increase high wind resistance (which may increase structure survivability in a tornado F2 or less), and reduce risk of storm damage in one fell swoop. And...frankly, we needed a new roof...this wasn't a boutique decision.

[mark beiser]

I do want some sort of metal roof when it is time to replace what I have.
When the time comes, it will be somewhat expensive either way because I'll need to replace the decking too.
The decking is ok now, but has some delamination, and is a little spongy in some places when I walk on it, so it won't really take another reroof.

I need a good hail storm so I can get some of my money back from my insurance company to help pay for it.

[Florida Joy]

Well, he could be a great roofer and siding installer, but it sounds like he needs to stay away from fans, far far away...

Finally decided to cut this guy loose over the weekend. I've been waiting on his proposal since July 25th. So far, all I've received is his "commentary" on how happy I'll be with the improved energy efficiency of my home if I hire him and approve all his "suggestions".

Contacted another siding contractor through the James Hardie website last Monday. He visited on Tuesday, his roofing "sub" visited on Wednesday, and by Friday evening I had detailed proposals in hand for both the roof replacement and new siding, including specs regarding enhanced off-ridge venting to replace the inadequately sized existing ridge vent.

The siding proposal was quite reasonable, including new vapor barrier, whole house re-paint (primer plus 2 coats), and re-installation of screening that has to be removed to install siding in the atrium. The roofing estimate was a bit higher than I'd received from others, but it does include a 15 year workmanship warranty from the installer compared to 5 years from the others. Unfortunately, the roofer has been in business less than 10 years, so the warranty may not be worth what they're charging.

I may still shop around on the roof and ask the siding contractor if he will consider using a different sub.

[mark beiser]

The roofing estimate was a bit higher than I'd received from others, but it does include a 15 year workmanship warranty from the installer compared to 5 years from the others. Unfortunately, the roofer has been in business less than 10 years, so the warranty may not be worth what they're charging.

This reminded me of something from a few years ago.
A friend of mine bought a new car from a dealership that was having a going out of business sale, got a great deal too.
They even gave him a discount on their in house extended service plan....

I still haven't let him put that episode to rest.