Attic ventilation calculation question


#1

We’re replacing a 6/12 hip roof on our home. Currently our attic ventilation is a nightmare so I’ve been researching the best route to go with the replacement (and boy, has this board been a wealth of info!) since I don’t want this to continue to be my headache.

For both aesthetic and functional reasons, a ridge vent seems to be the way to go for the house. We’ve interviewed 3 roofers so far (all established, legit companies) and each one has spec’d 50’ of ridge vent (total across the top and the two projecting ridges in the front), and varying opinions on soffit vents (ranging from adding 10 to adding none, sigh).

My math, using the NFA formula for 1/300 ratio I’ve seen, comes out different. Here’s what I get:

Attic floor sq. ft. = 2500, going through the math it comes out to providing 600 sq. in. NFA each in the exhaust and in the soffits. Our main ridge is 36 feet, and from reading the installation instructions it seems that stopping 12 inches from the edge is required so I have 34 feet available for venting across the top of the main ridge. Figuring a ridge vent that supplies 18 in. per lineal foot, that’s 612 sq. in. of exhaust, which is close to the recommended number of 600 I get from that calculation sheet.

Why would all three recommend adding ridge vent to the other areas (the attic is completely open, so there are no closed-off places needing special attention)? I got “more is always better” from 2 of them and “well, that’s what we usually do but if you want just the top ridge that’s fine” from the 3rd. I’m leaning toward a combination of 1. it’s more profit to sell and install 50’ rather than 34’ and 2. they just don’t know attic ventilation all that well. Or am I missing something here? I’m not a roofer. If we need 50’ I sure don’t want to cut corners, but from what I’ve read 50’ is overkill, and “more is always better” isn’t the case… especially when not adding enough soffit vents. I keep reading that too much exhaust is bad (and likely what our problem is right now with 8 box vents and a power vent).

About the soffit vents: currently we have 19 4x16s, which gives 427.5 sq. in. of intake. Even with 34’ of ridge vent, I’d need 9 additional ones to get 630 sq. in (everything I’ve read said to have more intake rather than exhaust if you weren’t 50/50). The guy that recommended 10 more soffit vents did so in conjunction with 50’ of ridge vent, so even he’s leaving me with more exhaust than intake.

The short question is: is my math right on this? 34 feet of ridge vent and 9 more 4x16s in the soffits to have good ventilation in the attic? Or is there a legit need for the 50’ and can someone explain the “why” of that to me?

Thanks for reading all that! :slight_smile: It’s a long first post but this is all new info for me and I want to make really sure it’s done right.


#2

[quote]The short question is: is my math right on this? 34 feet of ridge vent and 9 more 4x16s in the soffits to have good ventilation in the attic? Or is there a legit need for the 50’ and can someone explain the “why” of that to me?
[/quote]

Well you can not just put 34 feet up there if they are using good ridge vent. And depending on what is being used will greatly change the ammount needed.

Ok here goes, something i tell all my clients is this: One, just meeting code is a minimum. Not anything more. I ask them when they opened there checcking account if they just put in the minimum amount and continue to say im sure you did not like most people. I do not feel putting the minimum is going to benifit anything other than code. I like to keep my ventilation ratios in the low 200’s to make sure there is no problems.

With that If they are going to lay down 50 feet of soffit you need to bring up the intake. A minimum of 50% intake 50% exhaust. So with 50 foot of ridge vent you will need,
4x16= 33 vents
6x16= 22 vents
8x18= 17 vents

Now this is the minimums. It is reccomended to run 60% intake and 40 % exhaust. So that adds more vents to the sofftit.

Lastly if you have ridges more than 3 feet lower than the main ridge you should not put riidge vent on them. If you have any qeuestions feel free to post them here or you can email me. GTP


#3

[quote=“Snoopygirl”]We’re replacing a 6/12 hip roof on our home. Currently our attic ventilation is a nightmare so I’ve been researching the best route to go with the replacement (and boy, has this board been a wealth of info!) since I don’t want this to continue to be my headache.

For both aesthetic and functional reasons, a ridge vent seems to be the way to go for the house. We’ve interviewed 3 roofers so far (all established, legit companies) and each one has spec’d 50’ of ridge vent (total across the top and the two projecting ridges in the front), and varying opinions on soffit vents (ranging from adding 10 to adding none, sigh).

My math, using the NFA formula for 1/300 ratio I’ve seen, comes out different. Here’s what I get:

Attic floor sq. ft. = 2500, going through the math it comes out to providing 600 sq. in. NFA each in the exhaust and in the soffits. Our main ridge is 36 feet, and from reading the installation instructions it seems that stopping 12 inches from the edge is required so I have 34 feet available for venting across the top of the main ridge. Figuring a ridge vent that supplies 18 in. per lineal foot, that’s 612 sq. in. of exhaust, which is close to the recommended number of 600 I get from that calculation sheet.

Why would all three recommend adding ridge vent to the other areas (the attic is completely open, so there are no closed-off places needing special attention)? I got “more is always better” from 2 of them and “well, that’s what we usually do but if you want just the top ridge that’s fine” from the 3rd. I’m leaning toward a combination of 1. it’s more profit to sell and install 50’ rather than 34’ and 2. they just don’t know attic ventilation all that well. Or am I missing something here? I’m not a roofer. If we need 50’ I sure don’t want to cut corners, but from what I’ve read 50’ is overkill, and “more is always better” isn’t the case… especially when not adding enough soffit vents. I keep reading that too much exhaust is bad (and likely what our problem is right now with 8 box vents and a power vent).

About the soffit vents: currently we have 19 4x16s, which gives 427.5 sq. in. of intake. Even with 34’ of ridge vent, I’d need 9 additional ones to get 630 sq. in (everything I’ve read said to have more intake rather than exhaust if you weren’t 50/50). The guy that recommended 10 more soffit vents did so in conjunction with 50’ of ridge vent, so even he’s leaving me with more exhaust than intake.

The short question is: is my math right on this? 34 feet of ridge vent and 9 more 4x16s in the soffits to have good ventilation in the attic? Or is there a legit need for the 50’ and can someone explain the “why” of that to me?

Thanks for reading all that! :slight_smile: It’s a long first post but this is all new info for me and I want to make really sure it’s done right.[/quote]

The most likely reason for your contractors bidding all ridge lines is for continuity. Unless he’s charging you some exhorbitant rate he’s not making very much off of that extra 16 feet. We charge about $6.00 per foot. at least half of that goes for material and application.

For your 25 sq you need about 8.3 sq ft of ventilation split 50/50 or 60/40 intake/exhaust. At 50/50 that’s 597-2/3 sq inches. So 34 feet of ridge vent is what you need to achieve the 1/300 ratio to meet code. You need the same for intake.

Empirically speaking I have never seen a problem as a result of too much venting, intake or exhaust, and I’d be curious to know what your symptoms are, that make you think it stems from too much exhaust. My own experience leads me to believe that your problem may stem from not enough intake venting, not too much exhaust. It’s a funny distinction I know, but I don’t completely buy into what the product manufactures have to say about things like not mixing types (i.e. gable end and ridgeline vents), not putting vents on more than one section, and so forth. I’m sure it helps them sell product, but as far as function goes…I tend to play it a little by ear, using their recommendations as guidelines, not hard and fast rules.

The other thing I would mention, and as simple as it sounds I see it all the time, is you need to make sure that all your bathroom exhaust fans are ducted (preferably with insulated vinyl—not 4" galv. pipe) to a stem vent with a backdraft damper, same with a dryer vent if that happens to duct out the attic, although that’s probably more common in an apartment or a condo. Often these ducts are run up and pointed to a roof vent which may cause the exhaust to blow back into the attic.

Another thing you might check is to make sure your existing soffit vents are baffled and don’t have insulation blocking them.


#4

Thanks for the replies :smiley:

Hmm… one is 4 feet lower and the other is about 7 feet lower. What’s the issue with the differences in height? That’s something I’ve not read anything about but sure would like to learn.

Thanks for your advice gtp - I’m quite willing to go above and beyond code for ventilation. The ice and water shield is also going to be handled in the same way since we have the occasional insanely harsh winter along with intense thunderstorms (Central Indiana). I was concerned about the 50’ only because the estimate given wasn’t paired with adequate intake, and that didn’t seem to concern the one roofer, despite the fact that he’d already pointed out our inadequate intake. If we can do the 50’ (that would involve the two ridges that set more than 3 feet below the main one, is there a workaround for whatever problem that presents?) I’ll make sure I add the extra soffit vents.

3-style,currently our attic is a rainforest. The roof decking is mildewed and wet on the west and north sides of the house, it’s gone through the ceiling in 4 rooms, the insulation is useless in half the attic since it’s been soaked in areas (we had a lovely ice pond up there last month) - fortunately the other half has a floor. The house is 37 years old, but the attic issue just reared its ugly head in early 2005 (I brought it up then when it wasn’t anywhere nearly this bad, but it fell on deaf ears until the living room ceiling started leaking).

Someone sold Dad a power vent a few years back, but didn’t address the intake issue, and he has a habit of painting over things so when I checked the soffit vents out they seem to have a, ah… “generous” helping of paint and a good amount of dust and debris visible in the screens so I plan on replacing all those. We don’t have enough soffit vents in the first place and the ones we do have likely are not working as they should, so we’re mostly exhaust with very little intake, but yes, when I was saying “too much exhaust” I was meaning in relation to the amount of intake. I agree it is too little intake.

Of all the roofers we’ve spoken with at home shows and the ones we’ve had out to look at the roof for a quote, only one addressed the ventilation issue, and even he wanted to set us up with a system that provided more exhaust than intake.


#5

Sadly alot of contractors dont beileve in ventilaion that much. Even tho it depeneds on the warranty the man. gives to you. You want 60% intake and 40% exhaust if you can.

More than three feet below can cause an imbalance resulting in one of the ridge vents turning into an intake. Air will follow the path of least reisitance. Hoep that answers your questions.