We're re-roofing this spring and I'd like to replace the roof vents at
the same time.

We currently have turbine-style vents which were installed decades ago
when the attic was insulated and vented.

The turbines have moving parts and I'm certain that they'll start to
fail soon, probably at the worst time. Last winter during a nasty cold
spell following a heavy snow, one did start screaming as it turned, but
quieted down as soon as the sun came out the next day. We probably won't
be so lucky next time.

On the other hand, the turbines are tall and stick out above the snow
on the roof. We often have a foot of snow on the roof for months at a
time.

Other than trying to avoid turbines, does the style matter, so long as
they're big enough and there's enough of them?

--
Bert Hyman St. Paul, MN

Bert Hyman wrote:
We're re-roofing this spring and I'd like to replace the roof vents at
the same time.

We currently have turbine-style vents which were installed decades ago
when the attic was insulated and vented.

The turbines have moving parts and I'm certain that they'll start to
fail soon, probably at the worst time. Last winter during a nasty cold
spell following a heavy snow, one did start screaming as it turned,
but quieted down as soon as the sun came out the next day. We
probably won't be so lucky next time.

On the other hand, the turbines are tall and stick out above the snow
on the roof. We often have a foot of snow on the roof for months at a
time.

Other than trying to avoid turbines, does the style matter, so long as
they're big enough and there's enough of them?

I little oil or grease would probably extend their lives indefinately.

In "Bob F"
wrote:

Bert Hyman wrote:

Other than trying to avoid turbines, does the style matter, so long
as they're big enough and there's enough of them?

I little oil or grease would probably extend their lives indefinately.

Maybe so, but

A) I don't do roofs.
B) I'm interested in hearing about the pros & cons of other styles

--
Bert Hyman St. Paul, MN

Bert Hyman wrote:

We currently have turbine-style vents which were installed decades ago
when the attic was insulated and vented.

The turbines have moving parts and I'm certain that they'll start to
fail soon, probably at the worst time. Last winter during a nasty cold
spell following a heavy snow, one did start screaming as it turned,
but quieted down as soon as the sun came out the next day. We
probably won't be so lucky next time.

On the other hand, the turbines are tall and stick out above the snow
on the roof. We often have a foot of snow on the roof for months at a
time.

Other than trying to avoid turbines, does the style matter, so long as
they're big enough and there's enough of them?

I'm a fan of turbine vents, especially where the summers are very hot.
They work better than passive vents, and are straightforward to replace
if they get noisy and/or rusty.

In "John Weiss"
wrote:

They work better than passive vents, and are straightforward to
replace if they get noisy and/or rusty.

As I mentioned in my other post, I don't do roofs, but ...

Is there a replaceable bearing or some such, or is the entire rotating
assembly replaced?

Are they of standard size, so that I can wander into my nearest
home-improvement store and pick up a replacement?

--
Bert Hyman St. Paul, MN

Bert Hyman wrote:

In "John Weiss"
wrote:

They work better than passive vents, and are straightforward to
replace if they get noisy and/or rusty.

As I mentioned in my other post, I don't do roofs, but ...

Is there a replaceable bearing or some such, or is the entire rotating
assembly replaced?

Are they of standard size, so that I can wander into my nearest
home-improvement store and pick up a replacement?

I've never tried to replace the bearings, but have "dropped in" several
replacements in the past. There are several sizes available, but I
believe they are standard.

Bert Hyman wrote:

We're re-roofing this spring and I'd like to replace the roof vents
at the same time. We currently have turbine-style vents which were
installed decades ago when the attic was insulated and vented.

The turbines have moving parts and I'm certain that they'll start to
fail soon, probably at the worst time. Last winter during a nasty
cold spell following a heavy snow, one did start screaming as it
turned, but quieted down as soon as the sun came out the next day.
We probably won't be so lucky next time.

On the other hand, the turbines are tall and stick out above the
snow on the roof. We often have a foot of snow on the roof for
months at a time.

Other than trying to avoid turbines, does the style matter, so
long as they're big enough and there's enough of them?

Bert Hyman St. Paul, MN

What kills your shingles is heat.

Heat happens in the summer - not winter.

The worst heat happens when there is no wind. If there is no wind, your
wind-powered vents will not turn, and the temperature in your attic will
rise - probably to 140f or more.

Do yourself a favor and scrap the wind-powered turbines and replace them
with electric fans controlled by a thermostat. You can have a powered
vent as well as a few passive vents. All these vents are low profile
and it's not uncommon for them to end up covered in snow by mid winter.
There's no real harm in that.

You should also have styrofoam baffles or shields placed under the roof
decking where the decking passes over your header plate and out over the
soffit. Most people jam insulation in that area leaving no air gap
where air can pass freely along the underside of the roof deck into the
soffit. Your shingles will deteriorate the most in that area, and you
will form an ice-dam in the winter because of heat conduction through
the insulation into the roof deck.

http://products.construction.com/swt...55/E715704.jpg

http://www.houserepairtalk.com/f106/...od-attic-9704/

You should also have a completely ventilated soffit along the entire
length of the soffit. If you have soffit clad with aluminum trim, the
soffit face should be the ventilated type (with small holes in it) and
it should be this way for the entire length of the soffit - not just
every 5 or 10 feet. If your soffit was originally wood then you will
have 1/4" plywood as the soffit underside and you will need to cut holes
in it between each rafter to allow for complete ventilation. A 6" hole
is all you need.

Depending on the past history of water penetration through your existing
shingles, age and weather conditions, it's not uncommon to have to
replace some (or many, or most) of the roof decking around the perimeter
of your roof during a re-roofing job.

And by the way, if you're planning to just lay a new set of shingles
over top your existing shingles, then you're a bone-head.

In Home Guy wrote:

The worst heat happens when there is no wind. If there is no wind, your
wind-powered vents will not turn, and the temperature in your attic will
rise - probably to 140f or more.

What "killed" our shingles was time.

Convection moves air from the soffit vents to the roof vents.

--
Bert Hyman St. Paul, MN

On Feb 25, 10:24*am, Bert Hyman wrote:
Guy wrote:

The worst heat happens when there is no wind. *If there is no wind, your
wind-powered vents will not turn, and the temperature in your attic will
rise - probably to 140f or more.

What "killed" our shingles was time.

Convection moves air from the soffit vents to the roof vents.

--
Bert Hyman * * *St. Paul, MN *

high attic humidity from lack of ventilation can also shoten roof
lives, and worse damage the decking

" wrote:

high attic humidity from lack of ventilation can also shoten roof
lives, and worse damage the decking

You solve high attic humidity by having a proper vapor barrier between
your ceiling and the attic space above it, as well as having proper
soffit ventillation.

"Home Guy" wrote in message
...
" wrote:

high attic humidity from lack of ventilation can also shoten roof
lives, and worse damage the decking

You solve high attic humidity by having a proper vapor barrier between
your ceiling and the attic space above it, as well as having proper
soffit ventillation.

Soffit vents are for "intake" of ventilation, where the cooler air
infiltrates. You need additional vents to "exhaust". Power vents are much
like your information you have, USELESS. They don't work in the winter.

A vapor barrier does nothing for the attic as far as heat loss to the attic
in the winter. Heat WILL still escape into the attic, even with PROPER
INSULATION. The vapor barrier is for the "interior" or "thermal envelope"
of the structure.

At least read up on this stuff, since you have no real experience with it.
Here's a hint, read about condensation. You probably think a cold glass of
iced tea in the summer, is leaking tea through the glass.

Bert Hyman wrote:

What "killed" our shingles was time.

Convection moves air from the soffit vents to the roof vents.

Dream on.

Convection will not move enough air during the dog days of summer to
keep your attic cool.

I have properly ventillated soffits, several passive vents AND a powered
roof vent that I manually control. When I forget to turn the fan on
until noon on some summer days, it's not uncommon for my $15
battery-powered temperature sensor to read a temperature of 140 F in the
attic.

15 minutes after I turn the fan on, the sensor will be reading 120 f and
still dropping.

This is for a roof section that is about 35 feet by 30 feet with a low
pitch. I'm in the same climate zone you are - I'm about 120 miles more
south than you, and a few hundred miles to the east.

Wind-powered roof turbines are a crock of ****. Maybe you have some
sort of romantic or nostalgic attachment to them, but they will not do
squat to keep your attic cooler as compared to just having a hole in the
roof. And during the dog-days of summer when there is no wind, the
turbine blades will actually HINDER convective air flow because of the
resistance to air flow they cause vs just having an open hole.

On Feb 25, 10:50*am, Home Guy wrote:
Bert Hyman wrote:
What "killed" our shingles was time.

Convection moves air from the soffit vents to the roof vents.

Dream on.

Convection will not move enough air during the dog days of summer to
keep your attic cool.

I have properly ventillated soffits, several passive vents AND a powered
roof vent that I manually control. *When I forget to turn the fan on
until noon on some summer days, it's not uncommon for my $15
battery-powered temperature sensor to read a temperature of 140 F in the
attic.

15 minutes after I turn the fan on, the sensor will be reading 120 f and
still dropping.

This is for a roof section that is about 35 feet by 30 feet with a low
pitch. *I'm in the same climate zone you are - I'm about 120 miles more
south than you, and a few hundred miles to the east.

Wind-powered roof turbines are a crock of ****. *Maybe you have some
sort of romantic or nostalgic attachment to them, but they will not do
squat to keep your attic cooler as compared to just having a hole in the
roof. *And during the dog-days of summer when there is no wind, the
turbine blades will actually HINDER convective air flow because of the
resistance to air flow they cause vs just having an open hole.

I upgraded some years ago from 2 tiny vents at each end of house to
ridge vent plus those existing vents.

Put my fluke recording temp probe up there before and after.

Before top temp 146 degrees

after 116 degrees

Incidently home inspectors say the spec is the attic shouldnt ever be
more than 15 degrees warmer than the outdoor temp

90 degrees outdoor 115 TOPS in attic my home for sale was 119 degrees

at the home I was attempting to sell the inspector said I had to add 2
large attic exhaust fans.,

The buyers wife didnt want the fan noise so that issue faded away
the house had a ridge vent but no soffit vents and no place for soffit
vents

On Feb 25, 11:00*am, " wrote:
On Feb 25, 10:50*am, Home Guy wrote:



Bert Hyman wrote:
What "killed" our shingles was time.

Convection moves air from the soffit vents to the roof vents.

Dream on.

Convection will not move enough air during the dog days of summer to
keep your attic cool.

I have properly ventillated soffits, several passive vents AND a powered
roof vent that I manually control. *When I forget to turn the fan on
until noon on some summer days, it's not uncommon for my $15
battery-powered temperature sensor to read a temperature of 140 F in the
attic.

15 minutes after I turn the fan on, the sensor will be reading 120 f and
still dropping.

This is for a roof section that is about 35 feet by 30 feet with a low
pitch. *I'm in the same climate zone you are - I'm about 120 miles more
south than you, and a few hundred miles to the east.

Wind-powered roof turbines are a crock of ****. *Maybe you have some
sort of romantic or nostalgic attachment to them, but they will not do
squat to keep your attic cooler as compared to just having a hole in the
roof. *And during the dog-days of summer when there is no wind, the
turbine blades will actually HINDER convective air flow because of the
resistance to air flow they cause vs just having an open hole.

I upgraded some years ago from 2 tiny vents at each end of house to
ridge vent plus those existing vents.

Put my fluke recording temp probe up there before and after.

Before top temp 146 degrees

after 116 degrees

Incidently home inspectors say the spec is the attic shouldnt ever be
more than 15 degrees warmer *than the outdoor temp

90 degrees outdoor 115 TOPS in attic my home for sale was 119 degrees

at the home I was attempting to sell the inspector said I had to add 2
large attic exhaust fans.,

The buyers wife didnt want the fan noise so that issue faded away
the house had a ridge vent but no soffit vents and no place for soffit
vents

This will not make shingles last longer but will cool the space. I got
some perforated
aluminum/polyethylene sheeting and stapled it to the joists below
roof.
I used to walk into my separated garage in the summer and feel the
extreme heat from
the roof. Now, I don't feel any heat from the roof. A major
improvement.

greg

" wrote:

Wind-powered roof turbines are a crock of ****.

I upgraded some years ago from 2 tiny vents at each end of house to
ridge vent plus those existing vents.

Put my fluke recording temp probe up there before and after.

Assuming you can actually compare 2 similar situations:

- same time of year (or similar solar irradiance levels, angles,
exposure times)

- same ambient external air temperature, winds, etc

Before top temp 146 degrees
after 116 degrees

Incidently home inspectors say the spec is the attic shouldnt
ever be more than 15 degrees warmer than the outdoor temp

Because - why?

Because it's bad for your roof/attic/house, or because it's hard to
attain such a differential given unrestricted venting airflow?

90 degrees outdoor 115 TOPS in attic my home for sale was 119
degrees at the home I was attempting to sell the inspector said
I had to add 2 large attic exhaust fans.,

the house had a ridge vent but no soffit vents and no place for
soffit vents

I don't get what you're saying.

You're saying that you were able to keep your attic temp no higher than
115 (or 119?) with just a ridge vent and some hokey twirly vents, EVEN
THOUGH you have no soffit vents ???

I assume you have a two-sided roof (inverted V) where you can have a
continuous ridge vent from one side to another. The outside air you're
pulling into the attic is coming from two vents mounted on the opposite
sides of the attic - probably located 1/2 way or 1/3 of the way from the
top of the roof line. The air currents inside the attic are such that
most of your space and roof is baking in the heat, while the ends of the
roof and attic that are in the path of the air currents will be cooler.

You might also have light-colored shingled, or have some trees shading
your roof.

Either way, not having soffit venting is a major obstacle in keeping
your attic cool and properly ventilated.

In Home Guy wrote:

Wind-powered roof turbines are a crock of ****. Maybe you have some
sort of romantic or nostalgic attachment to them,

Maybe you should try reading my post.

--
Bert Hyman St. Paul, MN

On 2/25/2011 9:50 AM, Home Guy wrote:
Bert Hyman wrote:

What "killed" our shingles was time.

Convection moves air from the soffit vents to the roof vents.

Dream on.

Convection will not move enough air during the dog days of summer to
keep your attic cool.

I have properly ventillated soffits, several passive vents AND a powered
roof vent that I manually control. When I forget to turn the fan on
until noon on some summer days, it's not uncommon for my $15
battery-powered temperature sensor to read a temperature of 140 F in the
attic.

15 minutes after I turn the fan on, the sensor will be reading 120 f and
still dropping.

This is for a roof section that is about 35 feet by 30 feet with a low
pitch. I'm in the same climate zone you are - I'm about 120 miles more
south than you, and a few hundred miles to the east.

Wind-powered roof turbines are a crock of ****. Maybe you have some
sort of romantic or nostalgic attachment to them, but they will not do
squat to keep your attic cooler as compared to just having a hole in the
roof. And during the dog-days of summer when there is no wind, the
turbine blades will actually HINDER convective air flow because of the
resistance to air flow they cause vs just having an open hole.

Just curious as to what type of vents you have. I have ridge vents
running the entire length of all my roof sections. I have heel trusses
with lots of room for insulation above the outside walls plus a good 5
inches of opening to allow air to enter the attic through the soffits.
I have been up in the attic when the sun has been shinning all day
and the outside temp are in the high 90's. It's warm up there but not
hot. I would estimate the temp up there to be about 5 deg above outside.

My neighbor has the regular square vents on his roof. According to him
he has the required amount of vents per code. He does have well
ventilated soffits but his attic is hot.

You can work in my attic with little discomfort. I wouldn't even go up
into his.

LdB

LdB wrote:

Just curious as to what type of vents you have.

Pretty much exactly like this:

http://www.single-family-home-remode...roof-vents.jpg

It's about 12 inches on a side.

I have 2 roof sections on my house. One is almost square - 35 ft by 30
ft. It's got 4 sides (like a pyramid). If I had a ridge vent, it would
only be about 4 or 5 feet long.

So I have 2 vents (like in the picture) on 2 sides of that roof, and a
powered vent fan on the third side. The fourth side has no vents - you
can see that side from the road (you can't see the other 3 sides).

The powered vent looks like this:

http://www.atticvents.org/wp-content...ttic-Vents.jpg

The other roof section is about 25 feet by 50 feet. It's got 2 sides.
I could put a 25-foot ridge vent on it, but because it's lower it
doesn't get as much direct sun in the afternoon. I also have 2 passive
vents and a powered vent on that roof.

I have been up in the attic when the sun has been shinning all
day and the outside temp are in the high 90's. It's warm up there
but not hot. I would estimate the temp up there to be about 5 deg
above outside.

I have tan or light-brown colored shingles (the lightest color I could
find that matches the brick on my house).

In late july or early august, on a 90 degree day outside with full sun,
with my fan running from dawn til dusk, I can barely keep the
temperature at 125.

Unless you put a temperature sensor positioned just under the decking
(but not touching it) you can't really do a good job estimating
temperature by waving your arm in your attic.

And if you have an attic hatch in your ceiling, and you are estimating
the temperture in the attic by standing on a ladder with your head
poking through the hatch, then you will be fooled by the cooler interior
air flowing past you up through the hatch and into the attic while the
hatch is open. You will not get a true sense of attic temperature that
way.

My neighbor has the regular square vents on his roof. According
to him he has the required amount of vents per code. He does
have well ventilated soffits but his attic is hot.

Code is crap. It doesn't mean additional insulation wasn't added since
the house was built - insulation that is blocking airflow from the
soffits to the attic space.

Codes are well known for skimping out on soffit venting.

You can work in my attic with little discomfort. I wouldn't
even go up into his.

What color are his shingles? What color are yours?

Does he have anything to shield his roof at mid-day? A tree, a
building, some other structure? What about your roof?

On 25 Feb 2011 15:24:28 GMT, Bert Hyman wrote:

In Home Guy wrote:

The worst heat happens when there is no wind. If there is no wind, your
wind-powered vents will not turn, and the temperature in your attic will
rise - probably to 140f or more.

What "killed" our shingles was time.

It's the integral of heat over time.

My front shingles facing north still looked good after 25 years, when
my rear shingles facing south looked terrible. I could only see the
roof from the front, or from the rear if I was up on a ladder.

Convection moves air from the soffit vents to the roof vents.

On Feb 25, 10:27*pm, mm wrote:
On 25 Feb 2011 15:24:28 GMT, Bert Hyman wrote:

Guy wrote:

The worst heat happens when there is no wind. *If there is no wind, your
wind-powered vents will not turn, and the temperature in your attic will
rise - probably to 140f or more.

What "killed" our shingles was time.

It's the integral of heat over time.

My front shingles facing north still looked good after 25 years, when
my rear shingles facing south looked terrible. * I could only see the
roof from the front, or from the rear if I was up on a ladder.





Convection moves air from the soffit vents to the roof vents.- Hide quoted text -

- Show quoted text -

family in the sun drenced phoenix report roofs dont last long there,
the sun bakes them out.

metal roofs arent common either cause they tend to warp.

ceramic tile roofs there are supposed to be the best and last a 100
years if properly installed to begin with

Home Guy wrote:
Bert Hyman wrote:

We're re-roofing this spring and I'd like to replace the roof vents
at the same time. We currently have turbine-style vents which were
installed decades ago when the attic was insulated and vented.

The turbines have moving parts and I'm certain that they'll start to
fail soon, probably at the worst time. Last winter during a nasty
cold spell following a heavy snow, one did start screaming as it
turned, but quieted down as soon as the sun came out the next day.
We probably won't be so lucky next time.

On the other hand, the turbines are tall and stick out above the
snow on the roof. We often have a foot of snow on the roof for
months at a time.

Other than trying to avoid turbines, does the style matter, so
long as they're big enough and there's enough of them?

Bert Hyman St. Paul, MN

What kills your shingles is heat.

Heat happens in the summer - not winter.

The worst heat happens when there is no wind. If there is no wind,
your wind-powered vents will not turn, and the temperature in your
attic will rise - probably to 140f or more.

Do yourself a favor and scrap the wind-powered turbines and replace
them with electric fans controlled by a thermostat. You can have a
powered vent as well as a few passive vents. All these vents are low
profile and it's not uncommon for them to end up covered in snow by
mid winter. There's no real harm in that.

I'll disagree to the extent of scrapping the turbines. You are correct that
they don't work as efficiently when there's no wind - although some heat
escapes through those 18" holes - but when there IS a wind, they will move
an enormous amount of hot air. Plus, they do it at no electrical cost!

As for cosmetics, they can sometimes be installed below the ridge line so
they're not easily visible from the street.

And speaking of ridges, be sure to install a ridge vent. These are dirt
cheap and will probably give you the biggest bang for the buck.

Note: None of these suggestions are meant to be exclusionary. Do 'em ALL.


You should also have styrofoam baffles or shields placed under the
roof decking where the decking passes over your header plate and out
over the soffit. Most people jam insulation in that area leaving no
air gap where air can pass freely along the underside of the roof
deck into the soffit. Your shingles will deteriorate the most in
that area, and you will form an ice-dam in the winter because of heat
conduction through the insulation into the roof deck.

You should also have a completely ventilated soffit along the entire
length of the soffit. If you have soffit clad with aluminum trim, the
soffit face should be the ventilated type (with small holes in it) and
it should be this way for the entire length of the soffit - not just
every 5 or 10 feet. If your soffit was originally wood then you will
have 1/4" plywood as the soffit underside and you will need to cut
holes in it between each rafter to allow for complete ventilation. A
6" hole is all you need.

Again, I'll slightly disagree. You're correct that one can't have too much
soffit venting.

The usual standard is one sq ft of soffit venting for every 150 sq ft of
living space. If you've got a venting area 4' long and 6" wide on the
soffit, that's 2 sq ft. BUT, you've got to subtract the screening area. If
hardware cloth type screening, subtract maybe 10% of effective venting. If
it's that HardiPlank stuff with itty-bitty holes, subtract 98%.

Note: None of these suggestions are meant to be exclusionary. Do 'em ALL.


actually not.

exhaust fans with ridge vent is the tops for inefficency. the fans
just suck the air from the nearby ridge vent, and that doesnt really
ventilate the attic.........

" wrote:

exhaust fans with ridge vent is the tops for inefficency. the fans
just suck the air from the nearby ridge vent, and that doesnt
really ventilate the attic.........

That could very well happen.

But the same could be said regardless if the fan is wind-powered or
electric-motor powered.

A fan that pulls outside air through the ridge vents and then exhausts
that air right back outside will nonetheless create air currents inside
the attic and it will lower the temperature of the warmest part of the
attic - the area in the peak or top part.

If you have a ridge vent then you probably have an inverted V style
roof, which means you could mount a fan on the side wall of the attic
(or both side walls) and have it blow outside air into the attic,
thereby helping to force the hotter attic air up and out through the
ridge vent.

"Home Guy" wrote in message
...
" wrote:
If you have a ridge vent then you probably have an inverted V style
roof, which means you could mount a fan on the side wall of the attic
(or both side walls) and have it blow outside air into the attic,
thereby helping to force the hotter attic air up and out through the
ridge vent.

This is horrible advice. It demonstrates the understanding of basic
physics.

Forcing air from an "outside wall" inward would be taking in elements such
as water from something God created called rain. The fan would pull water
from the louvers of any vent.

You have absolutely no concept of building practices. You're nothing but a
hack.

HeyBub wrote:

I'll disagree to the extent of scrapping the turbines. You are
correct that they don't work as efficiently when there's no wind
- although some heat escapes through those 18" holes - but when
there IS a wind, they will move an enormous amount of hot air.
Plus, they do it at no electrical cost!

Anyone that likes to get a tan in the summer will know this:

When the sun comes out from behind the clouds, the wind mysteriously
dies down. Then when the clouds come back - so does the breeze.

That fact doesn't help you when your chosen method of active ventilation
is wind powered.

Attic fans (typically 1/8 or 1/10 hp) consume pennies of electricity a
day. Worth the cost when they give you a reliable, dependable CFM of
air movement when you want it.

And speaking of ridges, be sure to install a ridge vent. These
are dirt cheap and will probably give you the biggest bang for
the buck.

Depends on the style of roof.

Again, I'll slightly disagree. You're correct that one can't
have too much soffit venting.

Once upon a time, soffits were faced with 1/4 plywood, and you had to
buy and install screened vents that were visible and you had to do a
nice job of cutting the hole and positioning and installing the vents so
you wanted to install the minimum necessary if you were the builder.

But it was a pain in the ass as a home owner to paint the soffits every
5 or 10 years so you got them covered in aluminum, and with slotted
sheeting it wasn't a hassle anymore to create venting anywhere you
wanted it - including the entire length of the soffit. The only problem
is that unless you cut more holes in the original plywood facing, the
extra aluminum venting will do you no good.

The usual standard is one sq ft of soffit venting for every
150 sq ft of living space.

For new construction, you don't even need to put the 1/4" plywood facing
on the underside of the soffit any more. Just mount your J-channel and
slide the perforated aluminum soffit sheeting into place.

The problem with having one vent every 4 or 6 rafters is that you won't
get much airflow along those rafters that don't have a vent down at
their soffet end. Air will preferentially flow along those rafters that
do have a matching vent where they meet the soffet.