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Old 04-27-2008, 04:02 PM   #1
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attic ventilation design help


I'm still early in the learning curve and would appreciate any help with a bunch of questions .

Now for the house specifics:

Located in southeast MA, the house is a Gambrel cape which has a knee wall on the front with 2 small gable dormers and a full shed dormer across the entire back. The house is 24' x 34' for 816 sq ft of attic space. I'd like to have a minimum of 6 sq ft. of ventilation. If I split intake/exhaust vents 60/40 (I think I read somewhere that this ratio was best, but...), I would need 2 1/2 ft exhaust and 3 1/2 ft intake.

I am thinking of putting Shinglevent II (with 3' ringed nails) across the ridge. Using the manufacturer's claim of 18 sq in net per linear ft, I would need 20' of ridge venting. To feed this, I need a minimum of 504 inches net on the intake side.

The front of the house is high on a very windy bluff overlooking the water. The lower half of the roof (it's a Gambrel cape or Queen Ann), from the gutter to the top of the knee wall was insulated by the builder as an exterior wall The two small gable dormers have never had any ventilation. The house faces northeast. I'd link a picture but my camera still requires film

I am thinking of using Air vent's Edge or Cor-a-vent's In-Vent along the lip of the roof that overhangs the kneewall. There is a piece of trim that runs the length of the house just below this overhang so aesthetically, this is a good place to put it. Since Cor-a-vent claims 10 sq in per ft so the roughly 26 ft (minus 8' for the 2 dormers) gives me 260 of my needed 504 sq in. If I run the In-vent the full 34' of the back roof (there are no soffits), I'm over my target of 504 sq in intake so I could lengthen the exhaust vent accordingly.

As far as the dormers go, I will install soffit vents and ridge vents in proper ratio as well.

Has anyone experienced weather related problems with the In-vent? I get pounded here. Overlooking the water---up high---the rain and snow is merciless.

How about Airvents Edge? Any experiences--good or bad?

Along the back of the house, the Invent would be 3' above the gutter per the manufacturer. But along the front of the house, it would be just above the trim above the kneewall---almost like a drip edge. Has anyone installed it in this sort of location? I will call the manufacturer this week but I was hoping for some reports from the field.

If I install In-vent along all of the back and front edges and then calculate that I can accommodate 24' of ridge vent---which exceeds my "required" 20 ' of vent---how do I install the ridge vent? As opposed to only running 24' of Shinglevent centered on the roof---with dummy vent extending to the gable edges---do I do the math and cut a proportionally smaller opening for the full length of the ridge? It would seem that this would decrease the likelihood of driven rain or snow if the actual opening were even smaller.

Is there some validity to splitting the intake/exhaust to a 60/40 ratio? I've also read a lot of 50/50 recommendations.

I have never seen drip edge along the gable edge of a roof but after reading here, I'm thinking I should ask for it. Any arguments one way or the other?

I read somewhere--in the wee hours--that the stains on my existing light colored roof--which is facing north---is not wear but rather algae or mildew and that I should look for new asphalt shingles with a high content of some material but I don't remember what. Does this ring a bell for anyone?

Is there a special cap-type shingle that lays flatter when covering the shinglevent? The images at the website looks a lot smoother than the capped ridge vents I've seen around here.We are leaning to old fashioned 3-tab shingles because we aren't crazy about the busy look of the architectural shingles

We have a whole house fan that is mounted horizontally on the attic floor. We will be removing 2 huge vents from the gable ends which served as exhaust vents for the fan as well as for venting the attic. Provided we occasionally wanted to still use the fan--on low speed---what effect will this have on the filter material inside the ridge and edge vents? Will it serve to backflush and clean them or bunch them up and screw things up? Any best guesses?Although it is blasphemous, I'm toying with putting in a 24" x 30" gable vent on each end of the house to accommodate any excess backpressure from the fan despite any influence it may have on the ends of the balanced ridge vent system.

Thanks for any help or insights any of you can provide.

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Old 04-27-2008, 06:36 PM   #2
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attic ventilation design help


I will answer inside your quote for you, one topic at a time.

Ed

Quote:
Originally Posted by Noreaster View Post

I'm still early in the learning curve and would appreciate any help with a bunch of questions .

Now for the house specifics:

Located in southeast MA, the house is a Gambrel cape which has a knee wall on the front with 2 small gable dormers and a full shed dormer across the entire back. The house is 24' x 34' for 816 sq ft of attic space. I'd like to have a minimum of 6 sq ft. of ventilation. If I split intake/exhaust vents 60/40 (I think I read somewhere that this ratio was best, but...), I would need 2 1/2 ft exhaust and 3 1/2 ft intake.

The 60/40 split is not only what major manufacturers, but Building Science Engineers claim as optimum efficiency. They also state that ther can be no such thing as "Too Much" ventilation, as long as different exshaust vents are not mixed. I will answer the gable vent question farther down.



I am thinking of putting Shinglevent II (with 3' ringed nails) across the ridge. Using the manufacturer's claim of 18 sq in net per linear ft, I would need 20' of ridge venting. To feed this, I need a minimum of 504 inches net on the intake side.

20 feet gives you the "Minimum" requirement and you probably calculated this for the 1/300 formula, which usualy needs to be doubled by using the 1/150 formula instead. Install the ridge vent 100 % continuous for the entire ridge length, but do not cut the slot for the portion that overhangs the soffits.



The front of the house is high on a very windy bluff overlooking the water. The lower half of the roof (it's a Gambrel cape or Queen Ann), from the gutter to the top of the knee wall was insulated by the builder as an exterior wall The two small gable dormers have never had any ventilation. The house faces northeast. I'd link a picture but my camera still requires film

I am thinking of using Air vent's Edge or Cor-a-vent's In-Vent along the lip of the roof that overhangs the kneewall. There is a piece of trim that runs the length of the house just below this overhang so aesthetically, this is a good place to put it. Since Cor-a-vent claims 10 sq in per ft so the roughly 26 ft (minus 8' for the 2 dormers) gives me 260 of my needed 504 sq in. If I run the In-vent the full 34' of the back roof (there are no soffits), I'm over my target of 504 sq in intake so I could lengthen the exhaust vent accordingly.

The Edge Intake Vent was just introduced this spring on Air Vents website, but I have been in touch with the Head Engineer from Air Vent on a continual basis over the years and test their products in different scenarios with detailed reports and photo cd's for them to evaluate. I have not had the opportunity to use The Edge Vent yet, but I do not like the 5" above the bottom of the vent for slot cutting postion from what they showed me at a Seminar last month.

I have a standing offer from Cor-A-Vent to use their product for free, but have not taken them up on that yet either. They have 2 sets of tests with the NFVA results. The 10" NFVA is based on a pressurized system, I believe. It is on a blog site they have for comparison.

I like to stick with what has proven to work for me, and the Smart Vent by DCI Products Inc., has worked without problem or failure for about 6 years for me so far and I use it on about 50% of my installations.



As far as the dormers go, I will install soffit vents and ridge vents in proper ratio as well.

Has anyone experienced weather related problems with the In-vent? I get pounded here. Overlooking the water---up high---the rain and snow is merciless.

How about Airvents Edge? Any experiences--good or bad?

Along the back of the house, the Invent would be 3' above the gutter per the manufacturer. But along the front of the house, it would be just above the trim above the kneewall---almost like a drip edge. Has anyone installed it in this sort of location? I will call the manufacturer this week but I was hoping for some reports from the field.

I have installed the Smart Vent at 3-4 foot above the eave edge on open soffit homes and also multiple times as a mid-roof application. No problems, even on a 3 1/2/12 pitch roof as a mid-roof install prior to our recently passed brutal winter with excess snow fall.



If I install In-vent along all of the back and front edges and then calculate that I can accommodate 24' of ridge vent---which exceeds my "required" 20 ' of vent---how do I install the ridge vent? As opposed to only running 24' of Shinglevent centered on the roof---with dummy vent extending to the gable edges---do I do the math and cut a proportionally smaller opening for the full length of the ridge? It would seem that this would decrease the likelihood of driven rain or snow if the actual opening were even smaller.

Is there some validity to splitting the intake/exhaust to a 60/40 ratio? I've also read a lot of 50/50 recommendations.

Building Science Guru, Joe Lstirubek.



I have never seen drip edge along the gable edge of a roof but after reading here, I'm thinking I should ask for it. Any arguments one way or the other?

I would use it, especially from the weather conditions you stated and might even consider using Grace Ice and Water Shield at ALL perimeter edges, just to be better protected.



I read somewhere--in the wee hours--that the stains on my existing light colored roof--which is facing north---is not wear but rather algae or mildew and that I should look for new asphalt shingles with a high content of some material but I don't remember what. Does this ring a bell for anyone?

Copper and Zinc embeddment in time release ceramic coated granules, which are manufactured or patented by 3-M Company, which provides a 10 Year guarantee against the formation of roof algae growth. Shingles with this included in the granule formation are classified as AR shingles, standing for Algae Resistant.



Is there a special cap-type shingle that lays flatter when covering the shinglevent? The images at the website looks a lot smoother than the capped ridge vents I've seen around here.We are leaning to old fashioned 3-tab shingles because we aren't crazy about the busy look of the architectural shingles.

The cap shingles are sort of a perforated 3-tab shingle anyways. Style is your choice, but the 30-50 year architectural shingles do perform much better than todays 3-tabs sold in most cases. They are Double layered over 50% of the exposed surface and also only have 2 corners on each exposed shingle to potentially reveal any curling or shrinking versus a 3-tab shingle, which has 6 corners and is composed of only 1 layer throughout.

Many of the cheaper Roll Vent varieties of Ridge Vents do not look aesthetically pleasing due to the uneven surfave the cap shingles are nailed to. The Shingle Vent II is a solid cover continuously on the entire product.



We have a whole house fan that is mounted horizontally on the attic floor. We will be removing 2 huge vents from the gable ends which served as exhaust vents for the fan as well as for venting the attic. Provided we occasionally wanted to still use the fan--on low speed---what effect will this have on the filter material inside the ridge and edge vents? Will it serve to backflush and clean them or bunch them up and screw things up? Any best guesses?Although it is blasphemous, I'm toying with putting in a 24" x 30" gable vent on each end of the house to accommodate any excess backpressure from the fan despite any influence it may have on the ends of the balanced ridge vent system.

If you do decide to keep side wall gable vents, get the type with hinged shutters that close when not having the whole house fan on and blowing the internal air into the attic. I have seen no problems with the internal filter being clogged on the Shingle Vent II and I have checked up on jobs over 10 years old to document my beliefs.



Thanks for any help or insights any of you can provide.
Ed

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Old 04-27-2008, 11:56 PM   #3
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attic ventilation design help


Thanks, Ed.

Do you foresee a problem using an In-Vent or SmartVent at the dripedge of the roof overhanging the kneewall on the front of Gambrel roof house? There is no gutter here.

It faces northeast and catches a lot of wind and snow during some winter storms but being a Gambrel roof, you are basically looking at/seeing the roof constantly and I would only want to put it 3' up from the edge if it were necessary.

Since the back of the house has a gutter running full length (and no soffit), would you install the Smartvent at the dripedge over the gutter or 3' up the roof?

Thanks again.

Peace of mind is beginning to creep in :-)
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Old 04-28-2008, 12:03 PM   #4
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A photo might help or a drawing showing where the knee wall is located.

How substantial are the winds and snows you are referring to?

Ed
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Old 04-29-2008, 02:04 AM   #5
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Ed:

My camera takes film Although it doesn't have a full shed dormer across the back, this jpeg http://z.about.com/d/architecture/1/0/E/2/1864-5.jpg shows a situation similar to mine on the front. I also have two gable dormers like those shown.

As you look at the front of a Gambrel cape, from the top of the kneewall down to the gutter, although it has roofing shingles on it, doesn't have attic space behind it---it is a crawlspace accessible from the bedrooms behind.

From the top of the kneewall up to the ridge is all attic space. This is where I wanted to install an IN vent, SmartVent or Edge as an intake vent---the dripedge at the top of the kneewall, just above that piece of trim.

There is no gutter at this location---just a dripedge that hangs over a piece of trim that runs the full length of the front of the house. Although I've seen globs of ice hanging from the gutter at the foot of the kneewall, I've never seen them hanging from the overhang at the top of the kneewall. However, my attic has always been cold due to the oversized gable vents for the whole house fan. Those vents are going away though.

I've seen pictures on one of these forums of Smartvent installed directly behind a gutter with ice hanging from it and evidently no problems. Their roof slot is 6" above the dripedge.

I spoke with someone technical at Cor-a-vent who says to mount IN-vent 36" up from dripedge. This isn't a problem on the back of the house as you really can't see it. On the front of the house, due to the Gambrel roofline, it will be obvious as a 34' stripe across the roof.

At AirVent, someone is supposed to get back to me assuring me that I can mount the Edge at the dripedge on the front of the house.

I guess I want it all I want Shinglevent II on the ridge, IN-vent on the back roof, 36" from the gutter (for peace of mind) and the Edge or Smartvent over the trim at the kneewall. With a black roof going on, the black Edge would be less conspicuous.

In your installations of Smartvent, do you put it right up to the dripedge despite being above a gutter or do you pull it back a bit?

I was concerned as to how far away from the dormers to stop the slit. Each manufacturer has their own specs on this with Cor-a-vent being the most conservative again.

Would you hazzard a guess as to whether their conservative mounting locations---36' from dripedge and 12" from dormers is due to problems with their design encountered in the field?

Sorry for all the rambling. The roofers around here are not a good source of info. 99% of the ridge vents I see around here stop several feet from the gable edge and most don't have soffit ventilation---let alone something like this.
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Old 04-29-2008, 11:57 AM   #6
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One more question:

Should I use an intake vent product that is sited 5-6" from the gutter, will installing some sort of gutter guard product to keep the leaves out serve as a bridge for an ice dam? The ones I saw clip to the gutter on one side and tuck under the shingles on the other.

Thanks again!
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Old 04-29-2008, 01:59 PM   #7
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Under typical situations, the most common application of the Smart Vent is directly at the eave edge, with the slot cut out fro the 6" to 7" point from the eave, continuously across the entire roof deck, except for at the soffit overhanng, which we still apply the product, but without the slot for a uniform appearance.

I have pictures on ContractorTalk.com in the Roofing sub-forum which show the ice hanging from the edges of the shingles, but not blocking the intake profile of the Smart Vent, and which also show a continuous blanket of snow coverage on the roof after a 14" snowfall the day before. I used the Shingle Vent II ridge vent, which the photos also show that the exhaust trough has melted the snow from the trough area, all the way to the soffit overhangs, where no heat was built up on the inside of the attic.

I will see if I can find that link.

Ed
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Old 04-29-2008, 02:17 PM   #8
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This is not the exact thread I was looking for, but it should do, to ilustrate both the Smart Vent and the Shingle Vent II.

Here are some photos of the Smart Vent for intake, placed, along the eave edge on one side and as a mid-roof application on the other side and using the Shingle Vent II Ridge Vent on the entire ridge line.

http://www.contractortalk.com/showthread.php?t=37659

Ed
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Old 04-29-2008, 07:58 PM   #9
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Here is the thread I really was looking for.

This one shows the house with a fresh 14" snow fall on it and how the Smart Vent remained clear and functional, while at the same time, the Shingle Vent II Ridge Vent allowed the internal attic heat to exhaust out and leave the snow pack unmelted on the roof surface.

http://www.contractortalk.com/showthread.php?t=31798

Ed
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Old 04-30-2008, 02:06 AM   #10
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Thanks, Ed.

When I see those pictures, I have to wonder what makes the DCI product work so well when the Cor-a-vent product---per the manufacturer---is supposed to be 36" from dripedge. Is the baffling in the Smartvent that superior to the IN-Vent?

I'm still trying to get through the "alleged" customer service reps to speak with someone at AirVent knowledgeable about their Edge product.

In some of the pictures, you can see gobs of ice in the gutters gathered at the top of the downspouts---the SmartVent doesn't seem to mind but what happens if you have a leaf-guard type product installed (as I was hoping to do) ? They are supposed to slip under the first course of shingles but if you tuck it under the SmartVent, does the expanding chunk of ice lift the SmartVent?

Thanks again Ed for singlehandedly helping me with this.
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Old 04-30-2008, 11:06 AM   #11
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I have not run into the Gutter Guard scenario yet while utilizing the Smart Vent. DCI Products also manufactures a Flow Free gutter filter, which I have never used, but that style has gotten some positive reviews from the www.AskTheBulder.com web site authored by Tim Carter.

All in all, I don't buy into most of the Gutter Guard products. I have found the plastic sectional pieces with the laminated window screen style filter to work the best and at about 1/20th the cost.

Contact Paul Scelsi at Air Vent. Pscelsi@Gibraltar.com

I didn't look up his e-mail, but I think that is it off the top of my head, and the phone # is 1 (800) Air-Vent

Ed
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Old 04-30-2008, 12:42 PM   #12
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Ed:

I tried to send an e-mail to that address but it was returned.
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Old 04-30-2008, 01:59 PM   #13
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I was close. I forgot the numeral 1, in the address.

Ed



Email: pscelsi@gibraltar1.com


Nickname: Air Vent Seminar Leader
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Old 04-30-2008, 08:10 PM   #14
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Ed:

A million thanks!

I received an e-mail and a phone call from Paul. He answered all of my questions in depth. He was great!

Thanks you, again!
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Old 04-30-2008, 08:37 PM   #15
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Yes he is a great resource to have available.

Did you by any chance mention where you got the contact info from?

I was just discussing some Air Vent features I want to utilize in a forthcoming project and he is helping me out tremendously.

Also, if you have to contact DCI about the Smart Vent, speak with Jack Henderson over there and get the answers about the gutter guard questions directly from the horses mouth. I am conducting some ongoing temperature reading results from using their product also.

Ed

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