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Discussion Starter · #1 ·
First apologies for the long post, I do want to include as much relative info as possible. My house is getting a new roof put on (GAF) and I am concerned with limited exhaust when choosing a ridge vent (GAF Cobra Snow Country with 18 SqInches of NFA per linear Foot). I did the calculations and thought about having Cobra Hip vents installed. The roofer said it may not look good (?) and then suggested two fans and a ridge vent over the garage (I know, no power fans with ridge vents). I called GAF to ask about their relatively new (Spring 2015) Cobra Hip Vent (9" of NFA per Linear Ft.). The lady I spoke with was not familiar with my type of roof construction (Hip roof without a continuous hip rafter) and told me that since their installing video showed a continuous hip rafter, then that is what I would need in order to have the hip venting. The lady also told me that I should have more exhaust venting than intake, I knew she was misinformed.

So I am trying to determine if I can get away with just ridge venting, or if I will also need hip venting or if I should just get two fans (which I really do not want).

My House with 8/12 pitch roof:
Drawing shows bird's eye view of the house, the colors are as follow:
Tan area is the heated area below the attic insulation.
Grey is unheated and non-insulated area (garage & porch).
Yellow is the false roof over the front entry door (closed off to main roof).
Green line at rear of garage is where main roof sheathing comes down to just about 2 feet above the attic floor (so a tight fit to crawl to above the garage)
Light blue is the soffit area (24 inches deep on rear, 12" on sides).
Red lines are the ridges, hips, and valleys.
Dark red circle is the current power fan (will be removed with ridge vents).
In the picture of the front of the house there currently is a gable vent over the garage, and a gable vent on the false roof over the entry door. The one over the garage will be blocked off, the one over the false roof will stay for venting.

Attic area to garage considerations:
The main roof sheathing comes down to just 2' off the attic floor, which looks to me like there would be an air pocket at the garage ridge. I don't think air coming in from the garage soffits would travel low to the main ridge vent, nor air from the rear soffits travel low to get under the sheathing to climb up to the garage roof ridge. So to me these may possibly be separate venting area (?):vs_worry:
With the above, I've separated the areas

Main attic area 32'x68' = 2176 SQFt
Garage attic 12'x21.5'= 258 SQFt

My current soffit intake:
Sq feet of soffits venting directly into the attic is: 218'
Soffit area into the garage is: 24'

Member Bud9051 kindly provided a number of 4.6 Sq Inch of vent per SqFt of my type of soffit vents.

218' x 4.6"= 1002.8"
24' x 4.6" = 110.4"

Using 300 rule:
2176 Sq Ft attic area' / 300 = 7.25' X 144 = 1044 Sq Inches of NFA needed. A 50/50 would be 522 intake and 522 exhaust and I have 1000 coming in.

Garage Sq Ft area 258 / 300 = .86' x 144 = 123.84" NFA required, mine is 110.4"


The ridge on my main roof is 34' long. With the ridge vent, about 6" on each end is not cut open, so about 33' then available. As above, GAF Cobra Ridge vent Snow Country provides 18" NFA per linear FT, so 33' x 18" = 594" of exhaust venting.

594" + 1002.8" = 1596.8 total NFA. Split 62.8% Intake and 37.2% Exhaust

To give a 50/50 for over the garage, I measured that 5.5' of ridge venting would provide 99" near a 50/50 without going over the rule of not having more exhaust than intake.

Other considerations: I live in Maryland, Eastern Shore, I get a nice breeze and some strong winds along the rear of the house with a relatively moderate seasonal climate.

Your thoughts?

Again apologies for the long post.
Thank You
 

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Another thing to check is to make sure there is venting between the yellow section of roof and the main roof section. Same applies over the garage.
Some times the valley sections are simply laid on top of the main roof and there is not sufficient venting to those areas. Condensation can cause grief.

Are those gable end vents functional? That will add to your top end venting as well, so you might be closer to 50/50.
 

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Discussion Starter · #4 ·
Thanks for the replies. The yellow roof is indeed closed off from the main roof and there is a working gable vent there along with eve venting (at least there are vented material, so perhaps vented). I was advised to leave that working gable vent as is and put no ridge venting over that. There are also eve venting on the front of the garage, as mentioned earlier that gable vent will be closed off.
Thanks
 

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First, don't get too concerned about the exactness of your calculations. "The attic ventilation ratio “1/300” is an arbitrary number selected by the writers of FHA
(1942) with no citations or references"
That's 1942 and the code organizations and roofing industry has latched onto these numbers like they are the Holy Grail.

Here is the source of that quote: http://www.structuretech1.com/wp-content/uploads/2013/12/Early-History-of-Attic-Ventilation.pdf

Unfortunately, the codes and warranty requirements are hard to ignore even if they are unfounded, but the flexibility should eliminate some of the concerns about being a few square inches.

You actually hit the more important point "Other considerations:"The 1/300 or whatever you use for static ventilation literally goes out the window when the wind is blowing. Also, snow and rain can become a concern. Unfortunately, those considerations are hard to engineer so you just have to watch and wait.

A couple of comments:
How old is the house?
Keep the garage gable vent, it is doing no harm.
I assume the garage attic has an air sealed ceiling, not open to the attic space.
I would go ahead and add a ridge vent over the that front isolated area.

Bud
 

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Discussion Starter · #6 ·
Thanks everyone, I value and appreciate your input and thanks Bud for providing a link to the roof venting document.

To answer some questions and clarify some things
The house was built in December 1999.

The garage roof does open to the main roof, though the main roof sheathing comes down to about 2 feet above the attic floor (pic attached). To me, it looks like dead air could be trapped near the ridge of the garage roof near the main roof.

The second picture shows the enclosed roof over the front entry and side of house with the gable vent.

Thank You
 

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The front seems to have very little soffit area. You can't count the soffits that run up under the gable overhang, that is a closed space up there. Same for the gable overhang on the garage.

When 2 attic spaces are connected as yours are shown, there is minimal pressure across that opening to motivate the air to flow. You will want a ridge vent over the garage along with the soffit vents on each side. However, you may want/need to close off that opening as the garage ridge vent is lower than the main house. Theoretically, if you have a 50/50, half way in between there will be no pressure. We cannot determine your exact ratio as NFA is actually a poor approximation, but whatever it ends up being, the lower ridge is going to have far less pressure. If it gets too low it becomes an intake vent.

As a note, the 18 in² NFA for the ridge vent is one of those poor approximations. They are not counting the bug/snow screen, just a ¾" gap on each side. There are no testing standards for NFA so mfgs can cheat, and they do. (3/4"+ 3/4")(12") = 18 in² per foot.

As for the year built, not many homes in 99 were focused on modern air sealing. If you have air sealed the attic floor and have a vapor barrier, then the 1/300 is the rule. If not sealed, then they go to 1/150. I contend there are steps in between that they should allow, 1/200 or 1/250. All part of the common sense variables not mentioned in the codes.

Bud
 

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Discussion Starter · #8 · (Edited)
OK, lots of things to consider. Why not design a ridge vent system that vents one-way and when needed have a variable speed fan system that can control the air pressure inside the attic relative to outside pressure, taking into account for inside humidity, temps, wind etc.?

The roof fan I have no offers little exhaust when not running (comes on at 100 degrees, no humidity switch) as I've noticed a positive pressure when not on (when I open the attic access panel I've noticed insulation and debris come into the living space with a breeze). I imagine when the fan runs it could create a negative pressure possibly pulling moisture in under damaged shingles and vent flashing, gable vents etc. as well as pulling in heated/cooled air from the living area. I've only been here 6 weeks so have not seen that fan operate.

I'm thinking that if there is more intake to exhaust, then a positive air pressure will be in the attic and help prevent the above. But I guess there are a lot of variables such as temps, insulation, wind etc that change the air flow ratio.

Here's a link I found helpful by American Institute of Building Design: http://www.aibd.org/blog/?p=160

They also suggested as you did Bud about closing off the garage area.

Thank You.
 

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The idea of utilizing one or more (intake and exhaust) powered attic fans will happen, as homes get smarter. Passive ventilation is as simple as you can get and for most climate conditions is can and has done the job. I laugh at the smart thermostats that are a patch to make people feel like they are saving energy rather than them making the much needed energy improvements that allow them to set it and forget it. When it comes to adding technology to our lives, people have no idea what those geeks back in the labs have in store for us. Computers will soon be monitoring EVERY aspect of our lives. Hal is alive and well.

As you can see in Paul's article, once these benchmarks, like the 1/150/300 with it's roots coming from 1942, get established others simply reference others and no one ever goes back to rethink the issue. My hope will be to give them (all) a new way to understand passive ventilation and then I suspect their advice will change.

One more thought on a smart attic or home. Many of the repairs I do on my vehicles are due to sensor failures. That is where we need the advances for our homes and everything else to allow programmed controls.

Bud
 
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Discussion Starter · #10 · (Edited)
OK, so I've decided against a powered fan. Seems to me that the only time that the fan rated NFA is achieved is when the fan is powered on. I also don't like that a powered fan could create a vacuum (if intake is not flowing properly) and pull moisture from under the shingles where there is damage or age deterioration etc. My house's roof has a fan now, it is very hot in the attic and it's pressurized, so I'd imagine there is very little air flow exhaust 99% of the time until that fan spins up.

I think I will go with ridge venting. I see that GAF offers a metal ridge vent on their web page (didn't see it in the printed brochure which is why I didn't include it earlier). GAFs metal ridge vent would provide 21.5" NFA per Linear Foot so that would be 688 NFA (34' ridge, Alu ridge vents require 12" from each end of the ridge not to be cut, so on the 34' ridge, only 32' of venting allowed X 21.5 = 688).

I didn't forget what Windows on Wash wrote, but I thought you weren't suppose to mix both types of venting?

Thank You
 

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Powered vents are way more powerful than natural air flow, so when they turn on, the entire attic goes negative. All vents, soffit, ridge, and anything inbetween will usually become intake vents trying to match the exhaust flow.

As for mixing types of vents, under normal natural ventilation that isn't a problem, as the pressures will balance so the infiltration and the exfiltration match. Using a powered exhaust fan can be a problem as you mention, with the rain and snow coming back through one of those vents. There are rare occasions when a ridge vent leaks and the vast majority of them do an excellent job. Natural ventilation will never give you a cool attic as it is the temperature difference that moves the air. As it cools, it creates less flow.

But it is free and works basically 24/7 and ridge and soffit vents are compatable with all other vent locations.

Bud
 

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Can't add anything to what Bud has already laid out for you.

Big things are going to get those soffit vents cleaned out as I pointed out in the first post in the other thread. I think someone posted a link that effectively repeated what I had already said about 10 days later.

The baffled ridge vents are quite good at keeping sideways rain and snow out as long as they are installed properly.

Despite what was posted in another informationally incorrect link, warm air doesn't rise so the mixing of ventilation along the ridge line will not scavenge air and "short-circuit" the venting. Be sure the soffit vents are open and clean, air seal and insulate, and vent passively and you should be all set.

Good luck and keep us posted.
 

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Your attic picture of sheathing appears in pristine condition. Usually when you add more insulation without adding more venting is when problems arise. Air sealing will stop moisture from below, keeping the 1/300 (probably close to what you have now) will help dry it out without adding excessive moisture from the soffits per your Maritime climate. A couple of turtle vents at the top of the hips with some ridge vent on the gables should do it, IMHO. When you cut into the upper portion of the small gable for access, sheet over the lower portions that are now open to maintain the shear-flow resistance of the shear resistance of the roof diaphragm. It all acts as a unit. http://www.awc.org/pdf/codes-standards/publications/wfcm/AWC-WFCM2001-HWG90B-0610.pdf (90 mph)

Scroll down to "high winds" per your location-120 mph; R301.2.4; http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_3_par005.htm?bu2=undefined

Sim modeling (stack effect, no wind) shows different soffit vent sizes (ratios) (to prevent ice dams) effect roof heat gain and insulation thickness effects ceiling heat gain; http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1059&context=mechengfacpub&sei-redir=1&referer=http%3A%2F%2Fwww.google.com%2Furl%3Fsa%3Dt%26rct%3Dj%26q%3Dvelocity%2520of%2520rising%2560%2520attic%2520air%2520from%2520passive%2520ventilation%26source%3Dweb%26cd%3D39%26ved%3D0CGkQFjAIOB4%26url%3Dhttp%253A%252F%252Fdigitalcommons.unl.edu%252Fcgi%252Fviewcontent.cgi%253Farticle%253D1059%2526context%253Dmechengfacpub%26ei%3DqKtiT5ClBMXe0QHf6eDFCA%26usg%3DAFQjCNEttRcYqHsDOwb6ZhW2A5Hx8ncVIA#search=%22velocity%20rising%60%20attic%20air%20from%20passive%20ventilation%22

To control the three circulating convective loops (mentioned in gable roof) in loose-fill low-density blown insulation; cover with 3" of cellulose insulation (or housewrap). R-20 would be the least R-value, IMO, to use before cost comes into play;http://digitalcommons.unl.edu/cgi/v...y rising` attic air from passive ventilation"

Though your location calls for R-38 in attic; http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=Maryland

Be sure to foam at the drywall ceiling/wall joint at top plates because the wood framing moves with the humidity changes seasonally, creating gaps for air/heat from wall cavities to attic for the ice dams, etc.

Passive attic ventilation with soffit/ridge vents is very slow (1/300= .60 air change per hour without wind if I remember correctly...) ;

Fans are effective though have their own risks, let us know...; eg.

And the bigger vents exhaust more air faster;

Gary
 

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"The attic ventilation ratio “1/300” is an arbitrary number selected by the writers of FHA
(1942) with no citations or references"
That's 1942 and the code organizations and roofing industry has latched onto these numbers like they are the Holy Grail.
Do you suggest some better numbers as a general starting point?
A couple observations about that statement...
Just because they seem to lack verifiable data doesn't mean they are wrong. Some early Greeks claimed the earth was a sphere 300-400 years before any science verified it.
If you believe these numbers could be wrong why do suppose they are a "holy grail" to the code organizations? Its not like codes don't change when new ideas come along. They change regularly. Why do you suppose code organizations refuse to update ventilation numbers when they don't have a problem updating other things? Is somebody paying off somebody?
 

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There is no question that the 1/150/300 has served us well in the past, but newer homes and more importantly home renovations that get only part of the needed improvements, need to consider tweaking the code recommendations. A vaulted ceiling is a good example of a feature we are seeing more. What number would you use for the 300, attic floor (house footprint) or area of the sheetrock below.

As for what people should use, I would tend towards the 1/150 unless I'm certain I have sealed the ceiling plane and have a good vapor barrier in place. But there should be a few steps between 150 and 300.

As for what the code organization choose to use, they don't do the testing, so rely on the best available information. Since Mr. Rose has done some digging I suspect we may see some fine tuning of current requirements from NREL, DOE, or LBLN or other research organizations.

For now, we must meet or exceed whatever code the local authorities are looking for. Codes are strict, but we must remember that some of the numbers they use are based upon many wild guesses, NFA being one of them.

Bud
 
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