Ridge Vents w Box Vents

[Snipe Hunter]

That "No" was a little confusing at first. It took me a moment to realize that you thought I meant a ridge vent without vented soffit.
I have partly vented soffit, but it's ugly and probably grossly insufficient and so I will have that replaced with full vented soffit all the way around the house.

thanks for your responses to all

[Snipe Hunter]

Interesting conversation. A few questions from a non-builder;

In South Florida (near Ft. Lauderdale) the roofers do not use ridge vents and seem unfamiliar when I discuss it. 

Due to the heat buildup in attic, I had several vents added and a thermostat controlled attic fan near the top of ridge.  Keeps the attic down to 90+/- degrees in summer. 

Any reason ridge vents are inappropriate in hurricane areas? or high humidity?

I followed most of the conversation, except the mention of baffles when discussing soffit vents. Is that the internal blocker so the insulation doesn't block the vent?  

Thanks,

People do things differently in different parts of the country. Attic fans are great.. when they're working. They're less effective for big attics unless you install several. Ridge and soffit vents are great because the heat moves the air for you and hot air is free. I'm blessed with being able to visit a lot of attics and there's a huge difference in the temperature with inadequate venting.

There's really 2 things that need to happen. Air movement to keep moisture levels down and air movement to remove heat. Heat kills asphalt shingles. Moisture causes mold and it plays hell on your insulation especially if it's insufficient also. All that wet air condenses in the coolest place in the attic... that's your ceiling drywall and insulation in the day time and at night your joists and sheathing soak up a lot of moisture.

I wish I had some pics of some of the old attic fans (and attic pull down ladders) I see. Lots of ornate cast iron. Everything was well made and pretty back then (Think Stanley Planes). Form and function. I inspect a lot of old houses. Some of them have a big motor bolted to the joists with a belt running to a fan mounted in a gable vent. Some of them still work. These are 1/2 to 3/4 HP motors. These are the old heavy mid 1900s motors. I was given one of the motors just because I thought it looked cool.

[JosephP]

Calculate the square footage of "open roof" with your ridge vent vs. with box vents.  Generally boxes get your more venting.

I'm unclear why leaving the boxes WITH the ridge vents is a bad plan.  Is too much venting a common problem?

[Snipe Hunter]

Calculate the square footage of "open roof" with your ridge vent vs. with box vents.  Generally boxes get your more venting.

I'm unclear why leaving the boxes WITH the ridge vents is a bad plan.  Is too much venting a common problem?

It interrupts the air flow to the ridge vent. Too many box vents will prevent hot air from flowing all the way up the sheathing and out through the ridge.

Think of the problems that would be caused by poking holes in your HVAC duct or in the side of your chimney. The hottest air is at the ridge and it's needed to pull air from the soffits. ... heat rises. So if you have box vents, soffit vents and a ridge vent, the ridge vent will pull some air from the box vents and some from the soffit vents. If the soffit vents are inadequate, it will draw very little from the soffits. So wet, hot air is stuck below the box vents. The heat will rise but not the air. Sure, some will get out but not like it would if it had a straight path from the soffit to the ridge.

I had a graphic but cant find it. The air movement from the soffit to the ridge creates kind of like a venturi effect which moves the air in a circular wheel like fashion. So all that air is tumbling, pulling air from the attic floor, rolling it up the sheathing and out the ridge. Air is moving in the entire attic preventing moisture and mold problems. That's why gable vents aren't very efficient. They only pull heat out of each end of the attic but since there's no make up air, the moisture stays in the attic and most of the heat. Put a powered fan in one end and you get real air movement but the air comes from the other gable vent. It doesn't come from the insulation or attic floor which is what were are really trying to cool and dry. You get a straight line wind from one vent to the other. It only really cools and drys the peak of the roof.

Ideally, we want as wide a ridge vent as possible and as much venting in the soffits as possible.

I can go into an attic and the heat is unbearable and another attic where it's very tolerable.

I did one last year in a 1900s church with a slate roof. There were 2 furnaces and 6 air conditioner air handlers in it. The only ventilation was a gable vent at each end, maybe 80 ft apart. Zero air movement. I had to leave the attic and cool off quite a few times before I got every thing inspected. Temps were about 140 in the middle of the attic. The church has added soffit venting and powered one end of the gable vents. It's a lot better but still hot.

[JosephP]

So that is the "why" for sofit vents. Assuming those are adequate, why does it matter if the moving/vented air goes out of a box or a ridge?

Have you seen a circumstance where the soffit vent was adequate but there was a moisture buildup between boxes and the ridge (with or without ridge vents)? I'm assuming the boxes are within a couple of feet of the ridge...

[EatenByLimestone]

If the box vents aren’t being used, then they are just spots for wildlife to enter.

[DogwoodTales]

If the box vents aren’t being used, then they are just spots for wildlife to enter.

No more then they are when they are being used. At any rate, I will be boarding them up on the inside and most likely leaving the box in place until the roof needs replacement.

[Snipe Hunter]

So that is the "why" for sofit vents.  Assuming those are adequate, why does it matter if the moving/vented air goes out of a box or a ridge?

Have you seen a circumstance where the soffit vent was adequate but there was a moisture buildup between boxes and the ridge (with or without ridge vents)?  I'm assuming the boxes are within a couple of feet of the ridge...

Because if it goes out the box vent... hot, wet air is still stuck above the box vents. If it had box and a ridge vent, the ridge vent is sucking air from the box vents instead of the soffit vents. The attic doesn't fully vent.