If you ask me, box vents are more fussy to install (and shingle around) vs ridge vents, and they don't give you as much ventilation area as you think. In your case, the box vents you're being quoted for actually have a circular opening of about 8" diameter (on the bottom) and the perforated area on the outer side may not add up to a whole lot of actual ventilation area.
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Your vents have a specified ventilation opening or area of 50 square inches. Every running foot of ridge venting will give you between about
24 quare inches ventilation (about an inch worth on two sides) and be faster to install - but I don't know about the cost of ridge venting.
Your aluminum Lorenco vents come in a box of 6, and can be had for as little as $20 for a box if this ebay vendor is any example:
http://www.ebay .com/itm/140627349384
Bottom line is that a ridge vent is very easy to install, and gives more ventilation capability and more even ventilation than box vents.
However, be aware of this:
Summer is notorious for having hot days WITH LITTLE OR NO WIND.
Any passive vent system really needs a good wind to help ventilate your roof. So you might (or should) consider not having too much passive venting AND have some power vent fans.
Going with the lightest-colored shingles you can manage is also better than dark shingles.
Nowadays 16" or more attic insulation is required by code. It may be difficult to prevent the insulation from blocking the air flow from soffit vents. If you install baffles to hold the insulation back you may be putting a flow of cold air next to your ceiling. A cold spot on your ceiling allows mold to grow. Might be better to install roof vents just above the insulation at the bottom of the roof and again at the top. When you have continuous venting at the soffit and ridge the venting can be reduced by 50%. Since you do not have continuous soffit venting a ridge vent may be undersized by 50%.
I agree with the above. I'd seriously doubt the qualifications of any roofer who gave the story Derby got.
And I disagree with this. The vent system relies on hot air rising. Wind will help IF it's blowing in the right direction. But with the proper amount of passive vents, for most applications, that's all that's needed.
The baffles are up against the roof sheathing, not the ceiling. If you do the geometry, before they get to the beginning of the ceiling sheetrock they have many inches of clearance for insulation. They are routinely used that way with no problems. It's precisely what they are designed for.
Reduced 50%? As compared to what? With continuous venting you almost always wind up with more venting area. And even if you had venting area with continuous vents that was just equal to that of box vents, why would it be reduced by 50%?
Let's think logically about this. You're being told three things:
A) your current soffit situation is suitable for box vents, but not for a ridge vent.
B) ridge vents need MORE intake than box vents.
C) box vents let out MORE air than ridge vents.
Look closely at B and C.
Can they both be true statements?
Are these roofers saying that the volume of air going out of the attic doesn't have to match the air coming into the attic? Because that's the only way that both A and B can both be true.
If someone determines that you need a certain amount of volume-air-flow OUT OF your attic, don't let anyone tell you that it can't be done equally well with either a ridge vent or a suitable number of box vents. (this is assuming your roof has a long ridge line to begin with
- which any gable-end roof will have).
Now, it's EASY to give your roof enough venting to meet any target outward air flow. The question is -> will it have enough INWARD or intake air flow capability. If it doesn't have enough intake capacity, then it won't matter how much outward venting you give it (or what type of outward venting you have - box vs ridge).
If someone is claiming that box vents can perform some sort of magic and overcome a deficiency of soffit intake venting, and that a ridge-vent can't perform this same magic, then get them to explain this magic to you.
The price per vent is not broken out. There is an overall price for the job which includes the soffit and box vents, zinc strips to control moss, ice/water shield, etc.
Earlier in this thread you guys (someone) discussed how different types of soffit vents allowed for more (or less) air flow in.
I assume the same holds true for box vents vs. ridge vents on the way out. If ridge vents impede the outflow more than box vents for the same amount of intake, then could that mean having easier outflow could overcome less intake vents? Let me explain what I'm thinking...
As an example, let's say I open the glass door on my patio but leave the screen closed. I also (just slightly) open a window on the other side of the room. I then use a fan to try and move air out of the door through the screen. I'll get some intake at the window, but since the outgoing air is impeded by the screen, the intake will be minimal. If I now open the screen, I assume I'll have a lot more intact at the window since the air can move out faster.
So if, as this company seems to be saying, the box vents will let more air out, more air will be drawn in through the smaller soffits - more than would be drawn in by the impeded outflow of the ridge vent. However, with bigger soffit vents, there is more air to be easily drawn in, so the ridge vents aren't laboring to pull air through them.
The baffles are up against the roof sheathing, not the ceiling. If you do the geometry, before they get to the beginning of the ceiling sheetrock they have many inches of clearance for insulation. They are routinely used that way with no problems. It's precisely what they are designed for.
At the top of my wall I have 3 1/2 inches of height between my roof sheeting and my wall to provice circulation from my soffits area to my attic. The baffle is going to use half of that height leaving room for a tiny amount of insulation.
Reduced 50%? As compared to what? With continuous venting you almost always wind up with more venting area. And even if you had venting area with continuous vents that was just equal to that of box vents, why would it be reduced by 50%?
The mandatory minimum attic ventilation area is 1/150 of the area of the space ventilated for all enclosed attics or rafter spaces. This minimum can be decreased to 1/300 if either (i) at least 50% and not more than 80% of the required ventilation is at least 3 feet above eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents or (ii) a Class I or II vapor barrier is installed on the warm-in-winter side of the ceiling.
I agree with you and HomeGuy. This isn't rocket science. You want adequate intake and adequate exhaust venting. You gave a formula of 1 sqft vent for every 150 sqft attic space. I've seen that used before. DD can do some googling to see if there are other ratios or confirm that one. Whatever amount you need, you should have something at least close to that. If the soffit venting is inadequate, the solution is to fix that, not try to compensate with more exhaust venting.
Let's say it's 85F inside your house and now it's night so it's 70F outside. To cool it off best, would you:
A - Open 8 windows upstairs and 8 downstairs
B - Open 8 windows upstairs and 4 downstairs.
C - Open 12 windows upstairs and 4 downstairs
The roofers are suggesting C is the solution, even though from their proposal they don't have enough box vents on the exhaust to do C. I vote for A.
Also, we've gone through the math on a ridge vent. Because it's continuous, you get a lot of opening, far more than from the # of box vents they are proposing. There are a variety of ridge vent designs. Some allow air to flow more freely than others. It's possible the roofers just haven't looked at all that is available and are basing their conclusions on some crappy design. As far as continuous vents, I'd prefer that for the soffits as well for the reason that you get more area and I think they look better.
From aesthetics, what would you rather have? A bunch of box vents sticking out of the roof, or a ridge vent that isn't very noticeable?
Sometimes the geometry helps the aesthetics. My house had a ridge parallel to the street. Beyond that ridge, on the downside which is invisible to passer-bys, are four turbines.
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