# Too much soffit venting?



## Hogan773 (Aug 22, 2012)

I am trying to minimize ice damming.

Just had pros come and air seal the attic floor, and they added 12 inches cellulose over existing fiber batts.

Now my attic gets "closer" to the outdoor temps than it did in the past, but still about 10-15 degrees higher.

I'm thinking of cutting in a few extra soffit vents because it is an easy job. I have a hip roof so have metal square roof vents, and a bunch of 16x4 soffit vents. Many were partially covered with dust so I scraped them off. 

Thinking of adding in a few 16x8s to open up even more airflow.

My QUESTION is, they put in a bunch of DUROVENTS in the rafters when they did the air sealing. The Durovents have a net free air of 18 sq in or so. Does adding more soffit vents help or am I now dealing with the DUROVENTS as the constraint in my system? Also I am not an expert but are soffits generally open a bit....I mean between the rafters, or if you put a big vent in does it ONLY go up that channel? I can't really reach in there right now to see if the soffit rafter bays are open to each other.

I don't want to get up there with my jigsaw if it aint gonna help because only so much air can get through the Durovents.


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## Windows on Wash (Aug 30, 2011)

Post up a picture and you will get more feedback.

Theoretically you can have too much soffit venting if the system is out of balance dramatically but most homes are out of balance in the wrong direction (i.e. too much exhaust and not enough intake).


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## Hogan773 (Aug 22, 2012)

I cut in one new 16x8 yesterday afternoon. Pretty easy job.....just did it reaching out the window rather than get up a ladder. I confirmed that my soffit is "open" between the rafters which means that there is a triangle-shaped area that is open across all the rafters near where the soffit joins the house. I wasn't sure before, and was thinking that if each rafter bay only flowed up into one Durovent then it wouldn't be useful to add a 16x8 (65 NFA) vent if the baffle only allowed 18 NFA to come through. Given that the whole soffit is "open" in a sense, then I decided to add the larger vent (vs a 16x4) because air should be available to find its way up all the different baffles.

I may add a couple more vents just to ensure that my intake is not the constraint. It's an easy DIY job, while adding more mushroom vents at the peak is not something I'm going to attempt myself for a variety of reasons.


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## Grumpy (Dec 12, 2003)

10-15 degrees hotter than outside really isn't a problem if you ask me, in my unscientific opinion. Too much soffit venting is when the birds can get in. Until that point, I have never read any study suggesting that there can be too much properly installed intake. There shold be at least 3/4" preferrably 1.5" between the bottom of the sheathing and top of the insulation. Too bad they didn't install rafter baffles... Did they?

If you let me know the square footage, I would be happy to run a calculation and let you know approximately how many minimum vents would be recommended. However on a 1,000 sqft foot print you'd want about 18-23 (depending on slope) 4"x16" undereave/soffit vents. 

I have no idea what a durovent is, nor if it will interrupt the intake of the soffit vents. Also I hope they didn't install so much insulation as to block the intake from the soffit vents. And how many is a bunch, when it comes to attic ventilation, we deal with specific formulas not just guess work  I couldn't find a mathematical translation for a "bunch". 

Furthermore what kind of exhaust do you have? Is that the durovent? I'm too lazy to google it  

http://reliableamerican.us/articles/attic-ventilation-ice-daming.htm


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## Hogan773 (Aug 22, 2012)

Durovent ARE the baffles

I have a 40 × 40 house. Some of the attic is taken up by cathedral ceilings. 

I calculated using the 1 to 300 rule. I am just on the edge with the current. 4*16 that are there.


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## Windows on Wash (Aug 30, 2011)

1:300 is great but if the top side of the envelope is tight and insulated, you are usually pretty golden.


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## Gary in WA (Mar 11, 2009)

Page 603-618, I wish it was all there, a good read: http://books.google.com/books?id=Z8...page&q=attic airflow with gable vents&f=false

Long as you are close to balanced or meet other guides, the 300 rule is fine, per code (minimum); http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_8_sec006.htm

Scientifically: http://www.greenbuildingadvisor.com/blogs/dept/building-science/lstiburek-s-rules-venting-roofs

Ice dam prevention, enough vents, (Grumpy will even like this one);http://www.brainerdhomeinspection.com/roofve~1.pdf

Gary


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## Hogan773 (Aug 22, 2012)

Great info Gary - thanks

I especially like the ice dam research and the "22 degrees" number....very helpful. I've always instinctively known that the ice dam problems occur most when it is very cold outside, not just 29 degrees. I'm hoping that with my new insulation sealing and more venting, I have a fighting shot at getting the attic close to freezing when it is 20 degrees outside. Might not get all the way but I figure if I'm a few degrees off it will be ok. Last winter before the air sealing my attic would never get below 41-42 degrees even when it was 15 outside. This year it got down to 35 last week when the temp was about 21. That was before I cut in my new 16x8 vent. Still planning on cutting in another few if the temps stay warm enough to work.


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## Hardway (Dec 28, 2011)

I have 40' of Soffit and Ridge vent on both sides of the roof.. The attic temp is the same as it is out side. No ice dams.


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## Grumpy (Dec 12, 2003)

GBR in WA said:


> Ice dam prevention, enough vents, (Grumpy will even like this one);http://www.brainerdhomeinspection.com/roofve~1.pdf
> 
> Gary


Sooo many words! Just skimming my brain hurt LOL. I'll have to read it later when my brain is not in hyper-drive.


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## jagans (Oct 21, 2012)

I really dont think that you can have too much ventilation through your soffits, because you can only get what you can get based on net area intake. Whatever you do dont use typical bug screen for soffits, it will become blocked with atmospheric dirt in about 4 months. If you are really having trouble with ice damming, consider using a rigid insulation with a high resistance value like Polyisocyanurate at the pinch condition near the eaves. It has a rated value of around 6.5 per inch of thickness. Styrofoam EXPS is good too at 5 per inch and it does not absorb moisture. You could cut in 3 inch iso, then foam the edges tight. continue back 4 feet from the eaves, then go from there with blown in whatever. Fiberglass, Cellulose, or whatever. 

I already explained ice damming in another post, not sure if it was here or not. Read it if you can.


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## Hogan773 (Aug 22, 2012)

jagans said:


> I really dont think that you can have too much ventilation through your soffits, because you can only get what you can get based on net area intake. Whatever you do dont use typical bug screen for soffits, it will become blocked with atmospheric dirt in about 4 months. If you are really having trouble with ice damming, consider using a rigid insulation with a high resistance value like Polyisocyanurate at the pinch condition near the eaves. It has a rated value of around 6.5 per inch of thickness. Styrofoam EXPS is good too at 5 per inch and it does not absorb moisture. You could cut in 3 inch iso, then foam the edges tight. continue back 4 feet from the eaves, then go from there with blown in whatever. Fiberglass, Cellulose, or whatever.
> 
> I already explained ice damming in another post, not sure if it was here or not. Read it if you can.


Will search for your post.

I sort of agree re soffit....I figure I should open up the soffits enough that whatever air is rising out from the mushroom vents on the roof has NO PROBLEM with getting replaced somewhere in the soffit intake system.....and more would allow easier intake and also give a fudge factor should some of the vents become partially blocked over time, etc. At this point I'm going to add another 3 16x8s outside of windows where I can easily access without getting way up on a ladder and killing myself. 4 total 16x8s is an added 240 NFA which when added to the existings should be plenty. I have a complicated hip roof with 4 mushrooms way up top (240 NFA), 3 more in slightly lower peaks, plus 8 feet of ridge vent in an area of the roof that is above a cathedral room and somewhat isolated away from the rest of the open attic. Hopefully the 3 mid-level mushrooms are acting to exhaust warm air and not becoming intakes for the top 4 mushrooms, although I know that is a possibility. I do see air moving up from the soffits through the baffles as I can see spider webs swaying in the air so I know some air is coming in from the bottom.

Re: foam board insulation are you saying I should use that at the very edges along the attic floor? I think its probably too late now. The guys put in the baffles then stuffed in fiberglass batts at the edges which help to hold in the blown cellulose.

_*Here is a question......is there any benefit to somehow tacking up polyiso foam boards DIRECTLY TO THE ROOF DECK UNDERSIDE in specific areas where I'm trying to prevent melting, ie on the front area where I've had problems? Basically would adding a foam board to the plywood deck help to keep that decking more like the temp of the outside? Or would it just help insulate the deck from the bottom while the snowpack is insulating it like a blanket from above? Also not sure how I could get the foam board on in an airtight seal ....if you didn't then any attic air would just work up into the gap and it wouldn't do anything.*_


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## Windows on Wash (Aug 30, 2011)

If you can't get it sealed up, it may serve no purpose. If the outside top plate is seal up tight and you have good R-value at the narrowest point in the roof, you should be fine.


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## Hogan773 (Aug 22, 2012)

Windows on Wash said:


> If you can't get it sealed up, it may serve no purpose. If the outside top plate is seal up tight and you have good R-value at the narrowest point in the roof, you should be fine.


I guess I could spray a little layer of foam on the underside of the sheathing in select areas....but still trying to figure out if that is a good or bad thing. Let's say the attic temp is 35 when the outdoor air is 22. Will a layer of foam keep the roof COLDER because that 35 degree attic air can't "touch" the plywood sheathing.....or will it actually mean that the roof is "blanketed" on the top from snow (with good R value) and the bottom from foam insulation, meaning the actual roof will not stay cool.

BTW I like the idea from another thread about trying to use a metal flashing to prevent gutter ice from working back under the drip edge. I believe that is my problem.....a valley and gutter fills with ice and there is water in the soffit that then works its way down the outside brick veneer.


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## jagans (Oct 21, 2012)

*To Hogan*



Hogan773 said:


> Will search for your post.
> 
> I sort of agree re soffit....I figure I should open up the soffits enough that whatever air is rising out from the mushroom vents on the roof has NO PROBLEM with getting replaced somewhere in the soffit intake system.....and more would allow easier intake and also give a fudge factor should some of the vents become partially blocked over time, etc. At this point I'm going to add another 3 16x8s outside of windows where I can easily access without getting way up on a ladder and killing myself. 4 total 16x8s is an added 240 NFA which when added to the existings should be plenty. I have a complicated hip roof with 4 mushrooms way up top (240 NFA), 3 more in slightly lower peaks, plus 8 feet of ridge vent in an area of the roof that is above a cathedral room and somewhat isolated away from the rest of the open attic. Hopefully the 3 mid-level mushrooms are acting to exhaust warm air and not becoming intakes for the top 4 mushrooms, although I know that is a possibility. I do see air moving up from the soffits through the baffles as I can see spider webs swaying in the air so I know some air is coming in from the bottom.
> 
> ...


No, the whole idea is to keep the underside of the roof deck (Including the roof deck at the eaves) close to the outdoor temperature. did you read my post on ice damming? Most of the guys posting on here don't seem to understand what is going on, so it is easy to be misled.


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## Hogan773 (Aug 22, 2012)

jagans said:


> No, the whole idea is to keep the underside of the roof deck (Including the roof deck at the eaves) close to the outdoor temperature. did you read my post on ice damming? Most of the guys posting on here don't seem to understand what is going on, so it is easy to be misled.


Yes I read your post and it agrees with my existing understanding of ice damming which is why I paid for air sealing and insulation already. 

What I'm struggling with is whether there is an opportunity on a localized basis to further help the roof avoid taking on heat by actually applying foam to the decking. Not a fully enclosed attic but just to help a certain area that leads to a valley stay really cold so I don't get melt down into the valley.


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## jagans (Oct 21, 2012)

*Taking on heat?*

You absolutely do not want to do that as all it will accomplish is to cook your roofing system. If you have insulation in your attic floor,that is where you want to stop hot air/moisture loss. Your roof deck should stay ventilated. to stop moisture migration, you should have a vapor barrier on the warm side, usually, drywall ceiling, vapor retarder, insulation. You want the dew point to fall in the middle of the insulation

You want outdoor air to flow up through the soffit, along the underside of the deck and out the ridge vent.


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## Hogan773 (Aug 22, 2012)

jagans said:


> You absolutely do not want to do that as all it will accomplish is to cook your roofing system. If you have insulation in your attic floor,that is where you want to stop hot air/moisture loss. Your roof deck should stay ventilated. to stop moisture migration, you should have a vapor barrier on the warm side, usually, drywall ceiling, vapor retarder, insulation. You want the dew point to fall in the middle of the insulation
> 
> You want outdoor air to flow up through the soffit, along the underside of the deck and out the ridge vent.


Sounds good. I have already seen some improvement with the air sealing and insulation, so fingers crossed for when the snow falls. Last winter I couldn't get the attic below 42/43 even when outdoor was 15. This year it seems to be holding about 10-15 above, so when temps went to 20, the attic got down to 35. In warmer temps it gets closer (tonight it is 46 outside and the attic is 50.5) presumably because the closer you get to indoor temps the easier it is to be the same. Part of the issue also is that I have a furnace and ductwork in the attic, in a separate drywalled room but still it creates heat. But the better insulation now means it doesn't run as much, and with less heat coming up thru the can lights etc, it is easier for the roof exhausts to get rid of the heat that makes it out of the furnace room and into the attic. I know - bad design - but the builder did it that way before I bought the house, and I ain't ripping out my zone 2 HVAC.


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## Windows on Wash (Aug 30, 2011)

jagans said:


> You absolutely do not want to do that as all it will accomplish is to cook your roofing system. If you have insulation in your attic floor,that is where you want to stop hot air/moisture loss. Your roof deck should stay ventilated. to stop moisture migration, you should have a vapor barrier on the warm side, usually, drywall ceiling, vapor retarder, insulation. You want the dew point to fall in the middle of the insulation
> 
> You want outdoor air to flow up through the soffit, along the underside of the deck and out the ridge vent.


+1

As jagans has said, your envelope and insulation layer should be the attic floor and should be continuous and unbroken. If the air sealing contractor did their job effectively, the improvement over the original construction should be significant. 

Depending on where you are located, the painted drywall is the vapor retarder level and there is no additional barrier required.

Air movement carries the bulk of the moisture so if you got that buttoned up you are way ahead of the curve.



Hogan773 said:


> Sounds good. I have already seen some improvement with the air sealing and insulation, so fingers crossed for when the snow falls. Last winter I couldn't get the attic below 42/43 even when outdoor was 15. This year it seems to be holding about 10-15 above, so when temps went to 20, the attic got down to 35. In warmer temps it gets closer (tonight it is 46 outside and the attic is 50.5) presumably because the closer you get to indoor temps the easier it is to be the same. Part of the issue also is that I have a furnace and ductwork in the attic, in a separate drywalled room but still it creates heat. But the better insulation now means it doesn't run as much, and with less heat coming up thru the can lights etc, it is easier for the roof exhausts to get rid of the heat that makes it out of the furnace room and into the attic. I know - bad design - but the builder did it that way before I bought the house, and I ain't ripping out my zone 2 HVAC.


HVAC in the attic is one of the biggest issues in today's construction. Hopefully the air sealing crew addressed all the ductwork for air tightness and insulation value.


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## jagans (Oct 21, 2012)

*Hot Air Loss*

You are losing heat from that attic unit somewhere, Is the plenum chamber insulated?


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## Hogan773 (Aug 22, 2012)

jagans said:


> You are losing heat from that attic unit somewhere, Is the plenum chamber insulated?


The furnace/AC unit is basically boxed into its own little drywalled room in the attic, with a sprinkler head for fire suppression, etc. The little room is insulated with fiberglass batts and tyvek on the outside. There is a sliding door to close it off, although I must leave it cracked open a bit so the furnace can intake its combustion air. The ductwork in the attic is in good shape, and is covered in R-6 sleeve insulation.

I probably would have asked if there was an alternative but I bought the house after it was already mostly built, and I'm not changing it now.


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## jagans (Oct 21, 2012)

*Open door?*

Why dont you bring combustion air in from the outdoors through PVC Piping, and properly seal that door? Heated air is of higher pressure and is forcing its way out into that attic. The vapor retarder should have been on the inside of the wall on the warm side. Under the drywall.


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## Hogan773 (Aug 22, 2012)

jagans said:


> Why dont you bring combustion air in from the outdoors through PVC Piping, and properly seal that door? Heated air is of higher pressure and is forcing its way out into that attic. The vapor retarder should have been on the inside of the wall on the warm side. Under the drywall.


The tyvek was just something that the air sealing guys added to help seal around the fiberglass batts on the walls of that space. I'm not sure how much water vapor there is coming from that room - it is not an inhabited space. It is just a box around the HVAC unit - standup height for someone to be in there if they need to change the filter or work on the unit.

I've thought about adding another intake to the room but that requires more $ and another hole in the roof. Also the intake needs to be 100 square inches as my furnace is 110,000 BTU. How you gonna do that with a PVC pipe?


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## jagans (Oct 21, 2012)

*Soffit*

Can you bring fresh air in through a duct that runs to the soffit?

The differential temperatures you stated meant that you are getting quite a bit of heat into that attic. Its coming from somewhere.


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## Hogan773 (Aug 22, 2012)

jagans said:


> Can you bring fresh air in through a duct that runs to the soffit?
> 
> The differential temperatures you stated meant that you are getting quite a bit of heat into that attic. Its coming from somewhere.


I thought 10-15 degrees was somewhat normal?

Before the air sealing I ran a test last year and left the heater off all night. The attic temp got about 3 degrees cooler that night than it had the night before. Last year it NEVER got below 42 with the heater, as I believe there was significant 68 degree air leaking up through the cans and attic floor. After the air sealing, the temp made it down to 35 on a cold night with the heater still active. 

Your idea of a duct is possible I guess.


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## Gary in WA (Mar 11, 2009)

8" duct is 50.27". Make sure the screening/louvers are subtracted for only NFVA. 

Where are you located?

Gary
P.S. "first condensing surface"; http://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion


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## Hogan773 (Aug 22, 2012)

I am in Chicago

So I'd need to run 2 parallel sections of 8" duct?


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