The short answer…
Keep the conventional attic vents in place. Your roofer is right, in the sense that he’s repeating the ridge vent manufacturer’s instructions. But the ridge vent manufacturer is wrong.
The very long answer…
There’s a huge amount of marketing in favor of ridge vents. Some of that marketing includes the claim that ridge vents pull more air out of the attic because of the way those ridge vents are designed. In order to gain the benefit of the ridge vent design, the manufacturers say you need to seal all other attic vents, except the soffit vents. They have laboratory testing that says so. But it’s one thing to get a certain result in a laboratory under controlled conditions. It’s another thing to get the same results in real life circumstances.
The lab results don’t apply, if…
If your ridge line isn’t perpendicular to the direction of the wind, the lab results don’t apply.
If the wind isn’t blowing a certain speed, the lab results don’t apply.
If you have some trees around or near your house, the lab results don’t apply.
If your attic is cluttered with things being stored, the lab results don’t apply.
If it’s a hot day with hardly any breeze, the lab results definitely don’t apply.
On top of all of that, even if all of those circumstances were in favor of laboratory conditions, the difference in how much air would move through your attic due to the ridge vent design is minimal. It’s simply not that much of an improvement, under the best laboratory conditions. The marketing only tells you, under controlled conditions, that air is drawn out of the ridge vents, more than conventional vents. What about the difference in attic air transfer when you compare “only ridge vents” to “ridge vents plus other vents.” They don’t publish the difference in total air transfer between only ridge vents or ridge vents plus other vents.
On a warm day, when the air is fairly still, the heat in your attic is going to be much higher because you closed off the additional conventional vents.
I’m not against ridge vents. I like them. I just don’t like the huge amount of marketing that says you’ll have better results by sealing all your other vents. That’s foolishness. If you’re locked in your attic, and you have a choice of only ridge vents or ridge vents plus several conventional vents, you know what you’ll choose.
The bottom line is that warm air rises. As warm air goes up it finds openings and it leaves the attic. As it leaves, outside air is drawn into the attic through the soffit vents. The more attic vents you have for the air to flow out, the better your ventilation will be.
On a whole other aspect of the ridge vents issue…
If your rafters are the pre-manufactured trusses, then you don’t have a ridge beam in the attic. But if your rafter system was built as the carpenters were framing the house, then you have a ridge beam. That ridge beam closes off at least half of the ridge vent opening. That doesn’t make the ridge vents useless, but it dramatically cuts down how much air can escape through the ridge vents.
On yet another aspect of the ridge vent issue…
Your roofer needs to do a good job, otherwise the function of the ridge vents is diminished. For maximum benefit, the opening that the roofer cuts in the sheathing, along the ridge, needs to be as wide as possible. Some roofers don’t measure the gap, chalk a line and cut the sheathing. Some just eyeball it as they move their power saw across the roof. If they make that total cut out 1/4" narrower than it could have been, on each side of the ridge line, that adds up to 1/2". When you multiply that across the entire length of ridge, you’ve lost quite a bit of vent space. On a simple house design with 120 linear feet of ridge, that means you would lose 5 square feet of vent space. Visualize a 2 foot by 2.5 foot rectangle being cut out of your roof and the difference that would make in how much air flow you have, out of the attic.
You don’t want roofers building your kitchen cabinets. They generally aren’t known for their finesse and attention to detail. It’s very common to have the roofer short change you on the amount of vent space, just because they busted it out in a hurry. You would never know that when you look at the ridge from the ground.
To sum it up, ridge vents are good, but I recommend against closing your other vents. The value in ridge vents is the total amount of open vent space you have near the top of the attic for the rising air to flow out.
I hope this helps. Feel free to follow up in the future.
Your roofer with a keyboard,
I thought this was pretty good summary.