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# Soffit and Ridge Vent Calculations

#1
12-16-15, 04:51 PM
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Join Date: Dec 2015
Posts: 11
Soffit and Ridge Vent Calculations

Hello,

We had a nasty attic mold problem that was discovered this fall. Have no idea how long its been going on, but it was making household members pretty sick with sinus and joint pain. An air quality test tipped us off, with high levels of black mold, Aspergillus, and Cladosporium. Included is a nice photo of the mold:

I was a bit nervous about it all, whether or not we would have to replace the roof decking, blast it off and treat it, or settle with just spraying and painting over. Everyone had an opinion and no one had a straight answers. Luckily, our homeowners insurance covered remediation, so we didn't have much choice, but it was done fairly well. Wood was scrubbed and sprayed with mold treatment, the tops of contaminated insulation was cut out and removed and the area was bombed to get any extra spores. It was painted over with an inhibitor.

A few remediators told me that the decking needed to be replaced or blasted, but money is tight this winter and this was free. So far mold tests are negative.

But now we need to fix ventilation. Upon examination, we only have ONE gable vent. The second one (which I found this summer in our shed coincidentally) was never installed because we added on a chimney and the contractor was lazy and didn't put the second one on.

Our plan is to close off the lame gable vent, and install soffits and a ridge vent so this doesn't happen again. I am having trouble figuring out how many soffits to put on. I first did a ventilation calculator here:

GAF | Attic & Roof Ventilation Calculator

The length of the house is 34X26 (attic is as long and wide as house). With a square footage of 884ft, it came up with:

ridge: 10 of their AR10 ridge vents (haven't looked up stats on that yet)
Soffit: 9 16X4 metal soffits

It looks like they are going by the 1/300 rule.

The GAF calculation came up with 212.16 sq inches net free area. Another resource told me to use the 1/150 calculation:

By hand, 884/150= 5.89sq feet net free
Then I'm told to divide by 2 for intake/exhaust = 2.95
2.95X144=424.32sq inches.

So I have no idea really whether I should do the 1/300 and use ~10 16X4 or use the 1/300 rule and use 16 16X4 soffit vents. This seems like a really big difference. The remediator told me that our attic is way over insulated, and even removed some insulation as it was contaminated and we had so much.

I looked up building codes, and it says use 1/300 if a vapor retarder is in use..the house was built 30 years ago and I assume there is no vapor retarder. All I see in the attic is just insulation draped on the floor of the attic and the sides of the house (roof decking is clear of insulation). We live in cold climate zone 5.

My instinct tells me to use 16 of the 16X4, or ~7 of the 16X8 soffits. Does this sound right? I don't want to mess this up as people have gotten ill from the mold and we don't want it to happen again!

#2
12-16-15, 05:24 PM
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A better way to get consistent ventilation from your soffits is to use a lineal soffit vent which can be installed along the entire length of your soffit, ventilating all of it, not just parts of it.

You would also be wise to do air sealing if possible (any penetrations in top plates, vent pipes, chimneys- places thst introduce warm humid air into your attic. And check that bath fans and dryers are correctly ducted outside.

The 1:150 or 1:300 rule is not an exact science but you generally don't need to exceed 1:150. How many feet of ridge do you have on the house, cuz normally you out ridge vent on the entire thing regardless.

#3
12-16-15, 07:23 PM
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Join Date: Oct 2008
Location: New England
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With question marks on the air sealing and the vapor barrier you would use the 1/150.
Your "ridge: 10 of their AR10 ridge vents " I don't know what those are. As X asked, how long are they and what is their NFA per foot? (NFA net free area).
Same NFA concern on the 9 16x4 soffit vents, they should say what their NFA is.

From your calculation, you need 3 ft² up high (ridge) and 3 ft² down low (soffit).

The AR10 ridge vents appear to be 10' long but your house is only 34' long ????

Fill in some details and we will continue with the math. No rush on closing off the one remaining gable vent as we may be able to include it, despite the roofing industry's advice.

Bud

#4
12-16-15, 08:48 PM
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Join Date: Jan 2011
Location: USA
Posts: 4,855
Add a ridge vent and more soffit vents.
Make sure the attics air sealed and there's baffles to keep the vents clear.
In your area there should be a bare minimum of R-38 of insulation.
Any bathroom or dryer vents need to be run out the roof or side wall not into the soffit or attic.

#5
12-17-15, 08:09 PM
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Join Date: Dec 2015
Posts: 11
I am very confused on what type of soffit to get. The small circular ones I figure are out, because Id have to install dozens.

Masterflow sells an 8 foot aluminum under eave soffit vent at home depot with a net free area of 68sq inches. With an attic area of 884...I assume I would buy 6 of those, 3 for the front and 3 for the back?

Honestly I don't know what the difference is with all of them. Do people generally buy the continuous ones? Right now there are NO soffit vents on the house. The house was built in 1985 and the contractor was an idiot jerk. Parents didn't have a choice with him because he was the only contractor in town awarded a certain type of mortgage back then from his politician buddies. Did a lot of things wrong on the house. Apparently he didn't understand attic ventilation, cause all he put on was one gable vent. We didn't know until the mold grew.

The dyer vents out the basement window, and we have no bathroom vents into the attic. There is nothing up there from the house. Its just an attic with insulation. We do have a chimney.

The length of the house is 34 feet. We also have an addition with a cathedral ceiling that is 12 by 12 feet. I'm thinking we should put some sort of soffit on that as well. Would the small round ones be ok as there is no attic to vent?

I don't know the exact length of the ridge, I assume its about 34 feet as that is the length of the house. Home depot sells a low profile cobra ridge vent and a Builders Edge 11 in. x 48 in. Ridge Vent. The cobra looks nice as it won't alter the look of the roof much, as we have a nice shingled cape. I don't quite understand how it works..i assume you cut the hole, install it then shingle over?

#6
12-17-15, 08:28 PM
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Join Date: Dec 2015
Posts: 11
To make things less confusing, the house is 34 feet long and 26 feet wide.

Only one vent, a gable vent, is present.

We have an addition on the side of the house, with no attic. It has cathedral ceilings...and obviously no type of venting at all.

I am currently trying to figure out what type of soffit vent to use: Round, 16x4 or 16X8, or one of the continuous masterflow 8ft aluminum one. Also, advice on spacing between the soffits would be great!

We are going to install a ridge vent, as I think it makes more sense then cutting a second gable vent on the other side of the house (it would also be oddly offset due to the placement of the chimmney). From what I have read, gable vents aren't the best and I am not interested in any type of power vent. Ridge seems the best option, and less labor intensive than cutting into the side of the house.

I suppose a ridge vent could just be install the entire length of the roof? Or if the roof is 34 feet..would 30ft of ridge vent be good?

I'm also wondering if the cathedral ceiling addition should have a soffit AND a ridge vent. I don't have any idea what the best way to vent that is.

The house is a cape, pretty standard for new england. Attic is tall enough in some space to stand at the center, then gets smaller as it is a side gabled house. We have eaves along the front of the house. No mold in those thankfully!

#7
12-17-15, 09:40 PM
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we have no bathroom vents into the attic
I don't think you understood what I meant by, seal up all the penetrations. When a hole is drilled in your top plates for electrical wiring (heading to lights) or plumbing vent pipes (which exit out the roof) the electrical wire or the vent pipe do not completely fill the hole that was drilled. There is a gap around it. The same thing could be said for any ceiling light that is not IC / air tight. Warm moist air from the house can come right up through those penetrations unless they are sealed with spray foam. Generally, a fire rated spray foam is used where that is required by code, unless they require fire caulk around penetrations.

If there is any ducting in the attic that is coming from your furnace, that can be a huge source of air exchange if not sealed correctly.

As I mentioned before, roofers usually install ridge vent on the entire length of the house. The vent starts at the outside wall and runs to the other outside wall. The only part that doesn't need it is your overhangs.

Can't tell you which soffit vent to get... I would suggest continuous. If you think it's easier to put in individual 6x12's that would work too, just put in the right amount and space them out so they don't look uneven and stupid.

Cathedral ceilings, depending on how they were built, often cannot be vented because they are often completely packed with insulation.

And there is nothing wrong with having gable vents on each end. Or having gable vents and ridge vents. Dry snow probably isn't a problem in your area.

#8
12-17-15, 09:54 PM
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Join Date: Oct 2008
Location: New England
Posts: 10,524
X posted while I was typing in word but I will add what I have even though it may duplicate. I'm just slow .
I like the Masterflow 8 foot aluminum under eave soffit vent. Your math is correct, 3 in front and 3 in back. Plus, perhaps 4 for the addition. The soffit forms an open space along the eave so just spacing the 3 out will provide good air flow. Something like 2' on each end and 3' between the vents would cover 34'. You could use 4 on each side and run them continuous. If you go with those we can describe how to install.

Yes, that builder was an idiot and that mold is/was the proof. You want to make sure that doesn't happen again and that cathedral ceiling on the addition definitely needs ventilation, both soffit and ridge.

For a ridge vent, they remove the cap and cut back the sheathing on both sides per the vent instructions. Then install the vent and re-install a cap. Either of the two you mentioned will work and 30' would be sufficient. The NFA is typically stated to be 18" per linear foot.

I'm a little lost with the description, but I'm assuming the addition should have a ridge vent as well. Cathedral ceilings are supposed to have an air gap above the insulation all the way from the soffit and ridge and it should have an air baffle to channel the air above the insulation. Probably not one there, but hope there is a space above the insulation. Insulation in direct contact with the bottom of the roof can lead to what you just removed.

Bud

#9
12-17-15, 09:59 PM
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I think our replies complemented each other nicely, Bud. There is just no way for us to tell how his cathedral ceiling was constructed or insulated, but you are right. I've seen water pour out of the north sides of poorly constructed/ poorly ventilated cathedral roofs when the spring thaw finally let all the frost/ice melt off the bottom of the sheathing.

#10
05-05-16, 03:00 PM
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Join Date: Dec 2015
Posts: 11
Finally fixed

Hello,

Thank you all for your help. We had an leaky gutter as well on the front of the house, so we replaced the gutter and they installed a continuous soffit vent along the entire front of the house.

We don't currently have any soffit vents on the eave on the back of the house. Is this strange? The contractor said with the continuous venting on the front, we don't need the extra on the back. Mathematically it works out, I just didn't know if correct air flow needs front and back soffits?

We are installing a ridge vent along the whole roof next week, and closing the one lousy gable vent and sealing anything in the attic that could draw warm air up there during the winter.

The cathedral ceiling, thats a question mark for me. I believe it is completely packed with insulation and cannot be vented.

My reply is months later as it took this long to get someone out to do the work once the snow was gone and the weather was better.

Thank you everyone for your replies!

#11
05-05-16, 03:51 PM
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Join Date: Oct 2008
Location: New England
Posts: 10,524
Hi Moxy and thanks for the summary. I applaud your endurance digging through this forum and the endless opinions on the internet it IS confusing. Where your end result may not be exactly what we suggest, you are closer to the project and in control of all priorities. So, if we have moved you in the right direction I consider that a win.

As for the rear soffits I do agree you need some incoming air back there. As for the ugly gable vent/s it is still optional. I'm in snow country and have seen many snow storms that totally cover the ridge vents. Yes, they eventually melt their way open, but there is no negative side to leaving a working gable vents in place. If removing or covering it is a separate project you might wait to see how the attic performs.

Best,
Bud