# Guidance on efficient attic venting



## Nealtw (Jun 22, 2017)

It is as easy as the square inches of the hole in the roof or in the front of the cap which ever is smaller. Screens make it smaller.
I can't help you with the numbers. Others will be along.
Welcome to the site.


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## Walker1200 (Jul 10, 2021)

Hey Neal, 

Thanks for the welcome! I appreciate the quick response, and look forward to learning more!

-Kerry


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## Bud9051 (Nov 11, 2015)

All vents that use some sort of bug or rain screen have what we call a net free area (NFA). Each mfg should have assigned that NFA for each vent. If yours are olde or otherwise don't show that number, just search for something similar and use their number. The actual numbers for roof venting are far from exact.

As for drilling holes in your soffit, those pop-in vents (my assumption) have very low NFA. One I searched for was only 19%. A picture of your soffits might help with suggestions.

The 300 number is used when the attic to house is well air sealed and there is a vapor barrier in place. A good paint my suffice as that VB.

When not specifically air sealed they use 150. The quick swing is an example of less than exact numbers. Roof pitch and attic height ban also contribute to the decision, although not part of the guidance you are seeing.

Got down to your photos and looks like a reasonable roof pitch.
Maybe a photo of the inside of the attic and maybe a nearby big city so we better know your climate.

Bud
And another welcome.


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## Walker1200 (Jul 10, 2021)

Hey Bud, 

Thank you for the response and the warm welcome! I appreciate the insight. I looked for a similar product and located the NFA (50 sq in). I'll attach a photo just to make sure I'm not way off. In theory then, would I be looking at 350 (7x50) sq inches of exhaust area?

Great info. I'll use the 150 value when calculating, as that seems to pertain more to my situation. I'll get some pictures of my soffits and the attic soon. This is actually for my rental property, which is a few towns over. It was built in the 30s and has vented soffit covers, but underneath there is just solid wood. No daylight reaching the attic. If the pop-in vents have too low of an NFA, would it make sense to drill my holes and cover them with a thinner screen to get more air flow?

I live in upstate NY, so I get hot humid summers and freezing cold winters. Although the winters seem a lot longer than the summers. =] 

I appreciate all of the info, this is really helpful!

-Kerry


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## Walker1200 (Jul 10, 2021)

Also...this might be a dumb question...but at this point, since I have exhaust but no intake which is probably creating negative pressure in the attic and pulling air from the apartments below...would I be wrong in saying ANY more intake is better than the zero that I have now? 

-Kerry


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## Bud9051 (Nov 11, 2015)

Yes, some low venting is better than nothing. Typed up my solution, may not work for you, waiting for pictures.
Not sure if this would work in your case but I installed soffit venting on the back of my cape, low slope extended area I call a dust pan. Under overhang was plywood so I drilled a line of 2" holes and then cut between every other pair creating a series of larger openings. I then covered that line with a 3" strip of vinyl vent. Came out nice.

Bud


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## MTN REMODEL LLC (Sep 11, 2010)

Bud9051 said:


> *Yes, some low venting is better than nothing. * Typed up my solution, may not work for you, waiting for pictures.
> Not sure if this would work in your case but I installed soffit venting on the back of my cape, low slope extended area I call a dust pan. Under overhang was plywood so I drilled a line of 2" holes and then cut between every other pair creating a series of larger openings. I then covered that line with a 3" strip of vinyl vent. Came out nice.
> 
> Bud


If I may coattail on this thread....

I have retired in a hot/dry southern Nevada town. I am familiar with attic venting in Colorado, where I used soffit/ridge and pot vents.

But I'm in a relatively high quality production built stucco home with very neglible attic venting with my main A/C running in the attic area. This is not just my home but the rest of the comminity also.

That attic gets hotter than he11 in our summers...and I cannot believe that they have virtually no venting....just a minor ridge vent...I would think that would be a major concern and even a building requirement (or at least best practice) even back in 1996 build. (The home incidentally is excelently insulated, both structure and attic....but still has major A/C runs in attic.)

I have nominal ceiling can lights and all ceiling registers and none are airtights.

I am not going into any major roofing job to enlarge my ridge vents and have very nominal/small soffits and can not install soffit vents due to virtually inaccesable trusss structure.

I think my only practical option to improve venting would be to go high on a gable vent at one end of the home and install intake "soffit like" vents in an overhanging deck ceiling.

I think I would like to assist this venting with a power vent....but I don't want to depressure my attic with a gable fan and suck interior conditioned air out my ceiling leaks. 

I was thinking that maybe I want to place a power vent at my intake deck vent and presurize my attic blowing out the gable vent......but I don't want to be blowing outside hot air into my ceiling leeaks either.

I'm interested in any comments or possibel suggestions to my ideas.

TIA


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## Nealtw (Jun 22, 2017)

MTN REMODEL LLC said:


> If I may coattail on this thread....
> 
> 
> TIA


You should start a new thread, thts will just add confession.


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## Nealtw (Jun 22, 2017)

Walker1200 said:


> Also...this might be a dumb question...but at this point, since I have exhaust but no intake which is probably creating negative pressure in the attic and pulling air from the apartments below...would I be wrong in saying ANY more intake is better than the zero that I have now?
> 
> -Kerry


Is there a problem you are trying to solve like ice dams or are you looking at just making it right?


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## Bud9051 (Nov 11, 2015)

With high only attic venting the natural stack effect is already negative at the floor (wrto) with respect to outside, that is as measured horizontally to the outside. While the ceiling inside is positive (again wrto) giving you a maximum pressure across the ceiling. When low venting is added (assume equal high and low) the pressure in the attic shifts with half high and half low, thus less pressure across the attic to ceiling below.

Now, introduce a powered exhaust vent and it totally dominates the attic pressure being far greater than natural stack. Think 50 or greater pascals vs 2 pascals. For those who think in terms of inches of water column, ¼" wc equals 60 pascals. The result will be +4 pascals at the attic ceiling (it varies with temperature) and -50 pascals at the attic floor. To reduce the effects of this huge pressure difference you need an extremely well sealed ceiling plane.

Your thought of using a powered intake is a bit less problematic except now you would be forcing hot attic air into your home which might be more noticeable.

Ventilating our attics has always been to eliminate winter condensation and ice dams, that is until builders got stupid and started putting ducts and mechanical equipment up there. Since yours is already there you have few solutions.

I have read discussions about push/pull approach with 2 fans one intake and the other exhaust. In theory it works just the cost of running 2.

The other solution is massive insulation on attic floor and all ducts.

Attics will get hot but shingles are supposed to be able to survive, per Florida research, link available.

When it is time to replace those shingles, venting and solar reflective shingles are an option.

Bud


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## MTN REMODEL LLC (Sep 11, 2010)

Bud9051 said:


> With high only attic venting the natural stack effect is already negative at the floor (wrto) with respect to outside, that is as measured horizontally to the outside. While the ceiling inside is positive (again wrto) giving you a maximum pressure across the ceiling. When low venting is added (assume equal high and low) the pressure in the attic shifts with half high and half low, thus less pressure across the attic to ceiling below.
> 
> Now, introduce a powered exhaust vent and it totally dominates the attic pressure being far greater than natural stack. Think 50 or greater pascals vs 2 pascals. For those who think in terms of inches of water column, ¼" wc equals 60 pascals. The result will be +4 pascals at the attic ceiling (it varies with temperature) and -50 pascals at the attic floor. To reduce the effects of this huge pressure difference you need an extremely well sealed ceiling plane.
> 
> ...





Nealtw said:


> You should start a new thread, thts will just add confession.


Neal...You are correct...I just thought this was so close a discussion issue....Sorry

BUD....Thanks for your thoughts.....!!!!!! It's pretty tricky screwing with it and determining in advance the net effect...net effect being both the venting stack effects of air flow and AND the cooling loss of ducting in a hot attic.

(My whole thought process has to be reversed as cooling is our major issue and not heating as in Colorado.)

Maybe I should not play with the venting....and best worry about what sealing I can accomplish to my envelope...which is not much given my circumstance.

In general....would you expect much change in either airflow or temp reduction of attic with just a minor passive lower vent installed under my exterior deck ceiling and an added high gable vent......*Just your experienced gut feeling.*

Our summer temps run well over 100 dailey in june/july/august. I have vaulted ceilings at probably 3in 12 and roofing at 6in12.

Thanks again.

Interestingly...our roofs are tile....and I think we may have underlayment deterioration...because I have had neighbors develope minor roof leaks....not sure if we have felt or what as underlayment....and ii has not been a substantial problem as we have VERY little rain.


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## Walker1200 (Jul 10, 2021)

Nealtw said:


> Is there a problem you are trying to solve like ice dams or are you looking at just making it right?


Hey Neal, 

Its a little bit of everything. In general, I'm trying to make it right, but in doing so I hope to reduce the chances of mold, reduce ice dams and also reduce the utility bill for the whole house.


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## Bud9051 (Nov 11, 2015)

Mtn here is a link on all sorts of air sealing. Although all leaks are important, the big ones are the priority.
https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/TBC_Guide_062507.pdf

"would you expect much change in either airflow or temp reduction of attic with just a minor passive lower vent installed under my exterior deck ceiling and an added high gable vent"

Hard to predict. In general it takes a lot of minor improvements to make a noticeable change, this is just one, but it moves the issue in the right direction.

I'll let Neal continue with OP thread.

Bud


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## Walker1200 (Jul 10, 2021)

Hey guys, 

I apologize for my delay. I've finally gotten over to the house and was able to take pictures and get the square footage. The calculation is an estimate, but the number I came up with was 788. So let's assume between 780 - 800 sq feet. I'm attaching some pics of the layout and one of the roof vents. I'll also include some pics of the insulation. Some of which looks to be a mix of yellow fiberglass and something else. Some of the rafters have the amber resin that I've seen in other posts. Is that normal or indicative of something else? Thanks for all of the info!

-Kerry


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## Walker1200 (Jul 10, 2021)

I'm curious if anyone sees anything that sticks out in the attic pictures. Also, can anyone identify the type of insulation?

Thanks!

-Kerry


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## SW Dweller (Jan 6, 2021)

About Attic Ventilation


Natural Attic Ventilation At first it may seem odd to add insulation for warmth and then purposely allow cold air to enter the attic through vents, but this combination is the key to a durable and energy-efficient home.




www.energystar.gov


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## Walker1200 (Jul 10, 2021)

Thank you, that is a helpful article. Forgive me for asking, but I dont see a #17 in the article. Is there a specific link within it that I should be clicking on? 

-Kerry


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## Walker1200 (Jul 10, 2021)

Disregard that. I saw the post number and thought that was a reference. Forgive my confusion. =)

-Kerry


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## Bud9051 (Nov 11, 2015)

I'm glad I'm not the only one who gets lost at times 
Bud


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## SW Dweller (Jan 6, 2021)

I went into a turmoil over this years ago. I almost cut a hole in my roof for a solar powered vent.
Then one of the folks I worked with at the University told me about the energy information from the feds. This department is not trying to sell or scare you of anything. Made sense to me;
When I bought my home in Tucson there were no side vents. Lots of low vents nothing up high.
So I wandered the area I live in and none of the homes had vents. I cut vents as big as could go with standard vents. Installed and never looked back.


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## Walker1200 (Jul 10, 2021)

Hey guys, I wanted to revisit this post about venting my attic to get feedback on going a different direction. This house has no Gable vents, but inside the attic i can see 4 areas where they would go (one on each side). I've read many different opinions about Gable vents, but I'm wondering if i put in 4 Gable vents, instead of drilling holes in the soffit, would that be an efficient means of getting better airflow in the attic. In this case they would be for intake, but I understand that they're used for exhaust in many cases. They seem low enough to the attic floor for air to enter and then exit through the roof vents.


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## snic (Sep 16, 2018)

What I've read about gable vents is that they're less efficient at being your intake vents because they are only at the ends of the attic. So, if you expect air to flow in through the gable vents and out through the ridge vent, the middle of the attic between the gables won't get efficiently ventilated.

But it used to be that the only kind of vents were gable vents, one on each gable. The idea is that if your house is situated such that the prevailing winds blow air in one gable vent and out the other, you'll get nice ventilation. The problem is that not everyone's house is situated that way, which is why they came up with the whole ridge-and-soffit vent scheme - it should work regardless of winds. If I understand correctly, you have no soffit vents (but you have eves?) and no ridge vent, but you do have high roof vents. If the roof is relatively new (so you aren't going to install a ridge vent anytime soon), and the high vents provide enough NFA to make up half the required vent NFA, it would make sense to just install soffit vents. But if you can't get enough ventilation that way, adding gable vents is probably worth a try. If you do that, and you feel a breeze when you're in your attic on a moderately windy day, you'll know they're working.

PS: Is that mold on the insulation? If so, it's important to figure out what the moisture source is and fix it. Inadequate ventilation is certainly a possibility, and if you've got mold in one spot you might very well have rot somewhere (e.g., the roof deck). Of course a leaking roof is another possibility, or moisture intrusion through whatever vents you have.


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## Walker1200 (Jul 10, 2021)

Hey snic, 

Thanks for taking the time to post your reply. To answer some of your questions...yes the roof is relatively new and I don't have plans to put in a ridge vent any time soon. You are correct that I do not have soffit vents, but I do have eves. And the attic currently has a spot for a gable vent at all 4 sides of the house. I assumed that this would increase the chances that the prevailing winds would make it into the attic. Having the soffit vents drilled, I think, would be the best option, but I estimated that it may amount to drilling about 90 sq inches of vent space in each of the 4 eves. That seems like it would be a lot of effort vs putting in gable vents (assuming that the result was close to being equal). The way I was thinking about it was that the exhaust vents in the roof would pull the hot air out, and the air would get pulled into the 4 gable vents, aided by any prevailing winds. Am I way off on that?

Regarding mold - at one point there was a leak in the roof, which I'm assuming contributed to that. That was one of the main drivers for getting the roof replaced. One of my goals is to reduce the chance of additional mold by increasing air flow. 

Thank you again for your thoughts!

-Kerry


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