Confused about soffit vents

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Old 03-28-20, 04:31 AM
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Confused about soffit vents

100 yo home in North NJ. Inconsistent framing, lath and plaster walls.
2 story with very badly finished attic we are starting to redo. Basically no insulation so it is brutally hot in summer and our #1 goal is to get the insulation right. Step 1 in that is venting.

Attic has ridge vent but nothing else. The soffits as they extend beyond the top edges of the walls are simply enclosed with vinyl siding (although a bit open and covered with steel mesh at each end of building). So how and where do we put soffit vents to ensure good airflow before we start insulating? To complicate this, we have a problem with critters getting into the soffit spaces (outside the building; hence the steel mesh) and want to be sure we do not exacerbate that issue.

Help!

 
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Old 03-28-20, 06:21 AM
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You'll need 1 square foot of vent for each 150 square feet of attic. Calculate the square footage of your attic and divide by 150. Thus if your attic is 1,500 square feet, you'll need 10 square feet of vent. That is, 5 square feet of soffit (staggered reasonably evenly) and 5 of roof vent. This is the minimum. It's OK to have more but match the soffit vent area to the roof vent area. You'll need baffles to connect the soffit vents to the attic area. Baffles are very important. Don't assume air from the soffit will find it's way to the attic area because insulation will probably block airflow.
 

Last edited by Tony P.; 03-28-20 at 07:44 AM.
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Old 03-28-20, 07:00 AM
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The need for soffit ventilation is largely a result of today's building methods. Houses built 100 yrs ago were not built with the idea of needing soffit ventilation and so usually it is not even possible to do so because of the way the rafters and soffit area was constructed.

What Tony P says is true, but that is usually only possible more modern houses, with larger rafters,that are built with soffit ventilation in mind. Most houses with 2x4 rafters that sit on or die into the top plate were not. Would need to see your rafters and soffit to know for sure.

Adding ventilation to houses built in this manner will usually require a major roof, soffit and fascia remodel, that will likely also require siding repair and gutters. Now you are talking big $$$.

Add to that, that often when people finish their attics, they have created their own problem. The space was designed as an attic and they have largely taken that space over, expecting that hot attic to be nice comfortable living space. That's not reality. Reality is that your room is now right next to the 150 degree roof.

100 years ago, ventilation was done by opening the attic windows. Insulation and ventilation were not as important as it is now and most houses had none. They also didnt have air conditioning. So finished attic rooms are just not going to be comfortable 70 degree rooms like you have in a well designed modern home.

So my thought is that you probably are trying to do the impossible, and maybe should look into other ways to add ventilation. With no photos or diagrams of the construction of your soffits, I'm just guessing. (Add them if you have them) But in my experience, these are the facts of working in older houses.

Unique ways to ventilate that might be an option are made by a company called cor-a-vent. A product like InVent can be used, assuming you want to reroof the house in order to add ventilation from above. They also have strip vents that can be added continuously across your soffit and/or fascia, but these require a complete rebuild of the soffit and fascia. And if you download their product brochure and look at the diagrams, you might get an idea or two.
 
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Old 03-28-20, 07:10 PM
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Thanks

I should have been clearer about what I was asking.

Sleeper your answer is more inline with what i was looking for. This is a duplex with a very poorly finished attic. The extra space is highly desirable, though so just trying to make it bearable without creating a mold problem. We can easily do baffles as Tony suggested, but I just can't figure out placement of vents. No pictures as we haven't ripped out the crappy sheetrock yet.
 
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Old 03-28-20, 07:35 PM
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If it's easy to put in baffles and you have a clear path over the top plate and out to the fascia then you have it made. But you will probably need to demo any existing gutter, soffit and fascia cladding in order to put your soffit ventilation in.

Tony P already gave you the numbers, just do the math. I kind of prefer 60% soffit and 40% ridge. It's not an exact science. But try to be at least that close if not 50-50.

The best soffit ventilation IMO is continuous vent, whether it's Coravent or one of the aluminum strip vents. Both of these require a complete soffit rebuilt, but the idea is that just like you have a 2" slot at the ridge for ridge vent, you make a 2" slot down the entire soffit for intake.
 
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Old 03-29-20, 06:38 AM
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Thanks again

Ugh. Sounds like demo is most likely.

Crazy idea BUT there are tons of air gaps...is there a way to harness that for ventilation purposes?
 
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Old 03-29-20, 06:57 AM
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No. You need to decide whether you are going to tackle this via the roof (https://www.cor-a-vent.com/blog/INVent-vs-SmartVent.cfm) or via the soffit (https://www.cor-a-vent.com/s-400.cfm) or fascia (https://dciproducts.com/fascia-low-slope/) or by using traditional soffit strip vents, or 8x16 louvers that are spaced evenly down the soffit.

You would just need to do the math on each one to figure out how much ventilation you need in sq ft, divide that in half, then divide that by how much ventilation each product could give you, as they are all different.

Then factor in the price of installing each one.

the least invasive option might be these... since you just have to drill a hole and pop them in. You might be able to do that without much demo. But you would need a lot of them. The 8x16 louvers are similar but larger... just cut a rectangle hole and screw the louver on over the hole.
 
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Old 03-29-20, 07:07 AM
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Crazy idea BUT there are tons of air gaps...is there a way to harness that for ventilation purposes?
Not so crazy but not exactly correct. Home construction invariably isn't perfectly sealed and becomes less so over time. If the gaps you're referring to are from the living area they should be sealed as much as possible to maximize your conditioned air. Your problem is you're assuming air will travel in the direction you want it to go but the opposite is most likely true. That's part of the reason your living space is hot; air is traveling from the attic to the living area because air pressure is greater in the hot attic.

Instead, you want cooler than attic air from outside to enter at the soffit vents, travel up the baffles into the attic and push hot air out the ridge vent. There is nothing magical about placement of soffit vents as the air will mix in the attic. More important is to balance the incoming and outgoing air so air pressure works to move the air in the direction you want.
 

Last edited by Tony P.; 03-29-20 at 09:10 AM.
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Old 03-29-20, 09:35 AM
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Thanks

Much appreciated on all counts.
 
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