On 04/08/12 5:48 PM, firstname.lastname@example.org wrote:
It is compared to my 6" soffits.
12" or more was the number that a roofing estimator used when he
recommended box vents for my house.
He mentioned box vents, I asked why not a ridge vent, he said my soffits
are too narrow.
He then went on to explain that ridge vents only work well with wide
soffits, 12" or more.
The principle is that hot air rises. So, you want intakes
near the bottom and outlets near the top. Whether the
outlets are ridge or box vents isn't going to matter.
What's more important is the square footage of each.
As to needing soffit vents that are greater than 12" wide,
are we talking CONTINUOS soffit vents? If so, the
roofer is nuts. Vents even 3 or 4" wide, run continously,
are going to provide a lot of intake area.
I'd go with the ridge vent. And if the soffit vents aren't
big enough, then I'd enlarge them, if possible. Many
cases all it takes are one of the small cordless circular
saws run down the length. Plus ridge vents, IMO, look
a lot better.
That can be done with smaller soffits, too. My last house had 6" soffits and
a ridge vent. The soffit vent was continuous, however. It wasn't the
perforated aluminum that I see elsewhere, some of which only has 1/3
That's a lot of horse shit.
I've installed a few ridge vents, and you usually cut maybe a 6 to 8"
wide gap in the decking at the very peak of a roof line (3 to 4 inches
on either side).
To theoretically balance out the size of that ridge opening with a
similar soffit opening, then a 6" fully-perforated soffit would work.
Also note that sometimes even a 2-foot-wide soffit isin't what it seems,
because the bottleneck will be the gap or opening between where the deck
passes over the wall header. Usually, the wider the soffit - the
lower-slope the roof is, making for a not-so-large header gap.
A ridge vent will give you a more even airflow pattern along the
underside of the decking compared to a few box vents.
It seems to me that there might be some failure to communicate here
(the forum). The OP is saying (taken literally) that his SOFFITS are
6", not his soffit VENTS. That leaves very little room for vents and
maybe the roofer thinks he will need fans to pull enough air through?
I don't know enough to offer an opinion on whether that would be
Thanks for correcting that. Even with 6" soffits, if you ran a 2 or 3
inch wide continuous soffit down it, that adds up
to a lot of intake space, 24 or 36 sq inches per foot.
The only reason that wouldn't work is if there were
something in the construction blocking it. That something
in many cases is insulation that gets shoved too far, right
over the vents. To do it right, there should be plastic
baffles that you can get at HD that get stapled to the
underside of the sheating, at the soffits, that form an
open channel for a few feet from the soffit so that air
A continuous soffit vent together with a continous ridge
vent, IMO is the optimal solution. The ridge vent is
typically a couple inches wide. A similar size opening
along the soffit and you're all set.
Basic rule: You can't have too much soffit vents.
At a minimum, you should have one sq ft of soffit vent for each 150 sq ft of
floor space. This is one sq ft of unobstructed venting. Some screening
material consumes as much as 2/3rds of the space. Refigure accordingly.
If you have a 1500 sq ft house, you need 100 sq ft of soffits, minimum.
Assuming the screening material is hardware cloth or similar that consumes
less than 20% of the opening, you should plan on 120 sq ft of soffit
venting. Further, assuming small, six inch soffits on a 50 x 30' house, you
have 160 feet of perimeter. That means only 80 sq ft, maximum, for available
Obviously this is woefully insufficient for the appropriate amount of
passive ventilation. You'll have to move up a step in addition to ridge
vents. Turbines are the next step and beyond that powered ventilators.
What do you klowns think is happening?
You think there is some big vacuum cleaner sitting on your roof, sucking
air through your ceiling just because you have too much venting?
What are the odds that someone is going to have powered roof fans *and
no* soffit vents?
And think about this: Even IF you are somehow creating a negative
pressure in the attic that IS pulling air through the ceiling - SO
WHAT? This is going to be happening ONLY IN THE SUMMER. ITS NO BIG
DEAL IF IT HAPPENS IN THE SUMMER. It's in the winter when you don't
want that to happen, and who's going to turn on their roof fans in the
in the summer with AC sucking cooled air from the air conditioned
space is a poor choice, but at the same time its when having a cool
attic is most important....
beyond which sucking on the living part of the home will create paths
for the air from the living area to follow all the time.....
a real lose lose kinda situation.
roof fans in the winter help keep the humidity low in the attic which
helps prevent mold growth
In the summer, if I had a choice between:
a) pulling 1000 cfm of air out of my attic using powered vents - with
950 cfm coming from the soffits (and/or gable-end vents) and 50 cfm
coming from ceiling leaks - or
b) having NO powered vents (only passive vents) and therefore only
getting 200 cfm of passive air movement out of the attic (and 0 cfm from
ceiling air leaks) -
It's a no -brainer which one is better for the house and the roof.
And if I can pull more than 50 cfm from ceiling leaks, then the answer
is NOT to turn off the fans. The answer is to fix the ceiling leaks (or
just live with them) - not sacrifice the life of the shingles by letting
attic temperatures soar.
I would bet that the vast majority of roof fans don't have humidity
sensors - only thermostats (which means they only come on in the
I know the theory of ventilating the attic in the winter to prevent
mold, but explain to me how discharging cold, humid air from the attic
and replacing it with cold, humid air from the outside is going to
If the air in the attic is more humid than outside air in the winter,
it's because you have ceiling air leaks (or you're venting bathroom and
kitchen exhaust fans directly into the attic). It's a no-brainer that
you fix those situations first, eliminating the need for winter attic
ventilation in the first place.
That cost might be more than offset by the heat load I'm taking away
from the interior ceiling area by ventilating the attic with 950 cfm of
outside air vs 200 cfm of passive air.
Numbers like 1000, 200 and 50 (cfm) are all just hypothetical, but I
still assert that interior air leaks into the attic are trivial (for any
home built in the last 50 years) - not counting any bad practices like
venting bathroom or kitchen fans into the attic.
Yep, you're right. My bad.
I appreciate the correction and congratulate you for preventing someone from
making a grievous error based on my calculations.
Still, my answer was closer to being right than someone who announces
"purple" as the solution.
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