On May 4, 10:59*am, "Bob F" wrote:
DerbyDad03 wrote:
I'm looking for opinions about what I'm seeing when I smoke test my
new ridge and soffit vents.

I had my roof replaced and a ridge vent was installed. I also had full
soffits vents cut and then covered with perforated vinyl. My soffits
are about 6" wide, so there's about 4" of perforated vinyl showing.

The roof is framed such that I have about 3 feet of slanted ceilings
in the bedrooms, and the crawlspace attic itself is about 4 feet from
the top of the ceilings joists to the rim.

There is bat insulation on the floor of the ceiling that extends part
way down the rafter bays towards the soffit. When I pull the
insulation out of the rafter bays, I can see light from the newly cut
soffit vents. The contractor said that the soffits themselves are
free of insulation.
I installed 4 feet of baffles in each rafter bay, pulling them back
about 4" from the soffits, per the contractor's instructions. I then
put the bat insulation back in the rafter bay making sure the tops of
the baffles are open. The insulation does not extend more than 2'
beyond the top of the baffle so I know it's not covering the soffit
vent.
Last but not least, I have a 12" x 12" gable vent at each end of the
house, which I have not yet closed off.

I bought some smoke matches to check the air flow and this is what I
found:
The weather conditions were mid-50's F, thunderstorm, breezy. The
attic itself was fairly cool. I closed the access staircase so the
attic was closed off as it will be under normal conditions.

Holding a smoke match up inside a rafter bay near the ridge, there was
some movement up and out, but also a lot of smoke spreading down and
across the top of the attic. The was certainly no indication of the
smoke being rapidly sucked up and out through the ridge vent.

Holding a smoke match inside a rafter bay near the top of a baffle,
there was movement up the rafter bay towards the ridge, but it seemed
more like natural up flow as opposed to strong breeze blowing it
upwards. If there was a breeze, it was not significant.

Finally, holding a smoke match within an inch of either gable vent
showed significant outflow through the gable vent. It was literally
being sucked out of the vent at a rapid pace. If I backed away from
the gable vent 4 to 5 inches, the smoke just floated upwards towards
the ridge vent.

I suspect that I need to cover the gable vents, but based on what I
have described, do I need to be concerned that the soffit and ridge
vents are not operating properly? Do I need to wait until it's really
hot and try another smoke test?

Without sun on the roof, the chimney effect is going to be minimal. Testing on a
sunny day would be way more meaningful.

Gable vents are known to "short-circuit" the soffit/ridge vent flow. I have
thought about stapleing a layer of aluminum or fire resistant plastic to the
roof 2x4's in my attic to form a flow upward along the roof from the soffits,
and leaving the gable vents to vent the space under that. That might keep the
roof cooler than just blocking the gable vents.- Hide quoted text -

- Show quoted text -

Still seems strange that the gable vents were drawing out, not in.

It could have been caused by Bernoulli's principle since the wind
outside could have caused less pressure outside the gable vents than
inside. However, I would think that the same thing would have happened
at the ridge, yet the ridge vent did not show any significant draw
either in or out.

I'm trying to make sure that everything is working before I pay the
contractor. He still has to install the gutters and the contract is
"payable upon completion". While I trust him to come back if there's a
problem, payment has a way of changing people's priorities.

Anyway, the forecast for weekend is mid-60's and sunny, so I'll
probably block the gables and see what happens. I may still have to
wait for a really hot day to see significant draw through the ridge
vent.