How to vent eaves if no overhang, metal roof, Texas?
I'd like to replace my shingle over 5/16" OSB (excellent
condition, but cheap materials) roof with a standing seam
The "attic" interior is inaccessible, just 18" high at
the ridge, down to nothing high at the eaves. There is
no eave overhang; the roof stops an inch past the
vertical plane of the exterior wall. This is a new
manufactured home, so I guess I don't need to have
overhang and gutters to keep water from the foundation,
because I don't have a foundation.
I'm willing to spend good money now to minimize costs,
both roofing and cooling (it gets warm in Texas) over the
next 30 years. I'm assuming costs of energy will go up
dramatically over that time span, as will inflation in
general. So best to do it now, for maximum benefit,
lowest cost, over time.
Currently the tiny "attic" is vented through the roof
with low profile cheap plastic vents -- about 12 of them
in all, six of them high up near the ridge, six of them
low down. The roof is a simple gable, 3:12 pitch, 76 ft
long over a 16 ft wide home, so should be about 8.5 ft
from ridge to eave on the slant. That should be about
1290 sq ft of roof.
If I stayed with asphalt shingles, then this would be a
natural for SmartVent on the eave inlet vents.
Main question: But how in tarnation can I get inlet
ventilation with a metal roof, when I have no eave
overhang or soffits?
Related questions: What sorts of underlayment, vapor
barriers, radiant barriers (above the deck, blown in
chips over the blown in fiberglass, ...?), air gaps
below the metal, and such would you recommend.
I supposed the outlet ventilation would be one of a
ridge vent or a bunch of dormers. I am concerned that
a ridge vent will impede the airflow too much.
Why would a Ridge Vent "Impede" the airflow?
I'd have figured that the Smart Vent would get entirely covered up by the metal roof and associated flashing along the eaves. The best it could do would be to connect the "attic" airspace with the thin layer of air under the metal roof; nothing would connect to the great outdoors.
If I'm figuring wrong, it won't surprise me. I'm no roofer.
For the flashings, they can extend about an inch farther than the leading butt edge of the Smart Vent to still allow air flow to enter, if installed on top of the Smart Vent, otherwise it can be installed under the Smart Vent just like the Gutter Apron Drip Edge Flashing, just like it is shown on their website illustrations.
I have read that website link before.
"Sounds logical except for one basic fact: Hot air rises, it doesn't go down. A ridge vent design dictates hot air must fight gravity, travel down from the peak of the roof in order to escape. This of course is unrealistic and so is expecting this type of vent to be effective. The only "driving force" that makes hot air leave an attic is the differential density of the 140F attic compared to the more dense ambient outside air."
The air does not have to move downhill very much. It is almost a horizontal lateral movement. Fo to the Air Vent website instead and watch the videos they have reflecting the smoke canisters in comparison case studies by an independent laboratory showing the movement out of the attics under various wind conditions.
The hot, thermal buoyancy will push the warmer air upwards. The air already at the ridge vent slot area is not just going to slam on it's brakes and prevent the additional and continual force of more rising air from allowing it to be expelled by the interior forces at work.
The reason is that the "attic" has about zero inches vertical height out at the eaves, which is totally stuffed with the blown in fiberglass insulation. Short of removing the roof deck so that I can put in those plastic chutes (what are they called?) to keep the insulation out of the way, I doubt that there is practical way to have any useful ventilation in or out of that roof within a foot of the eaves.
That roof will only ever have ventilation via penetrating vents, gable vents or ridge vents.
Which does remind me of an alternative -- remove all the roof penetrating vents and put in two powered fans, one on each gable, one blowing in and one out. Just blow a steady stream of air from one end of that long tunnel to the other.
I posted a reply on Roofing.com
How about this.
:thumbup:Build a new truss system SUPPENED OVER the current roof. Close in the new eves, add intake, sheetmetal, and exust your done. Now you have shaded your home, lower engery cost. LOOK GOOD TOO. I have done this mant times here in CENTRAL TEXAS. :thumbup:
Anyhow, I think I figured out a different way to skin this cat, with what I hope will be just as long lasting (all I need is about 30 years, given my age ;)) and cool results. See further my reply http://www.roofing.com/forum/sutra55866.html#55866 over on a thread at roofing.com, and my post over there "THERMAL CONTROL MEMBRANE (TCM) -- any good?" http://www.roofing.com/forum/about7944.html
In short, I am considering venting it from gable to gable, not eave to ridge (this trailer is real long and skinny.) For the actual roofing construction, I'm considering using THERMAL CONTROL MEMBRANE (TCM). I'd put the following layers, top down, on my roof: 1) standing seam metal in some "cool" color, 2) this TCM stuff, 3) the existing cheap asphalt shingles, almost new, 4) existing unknown cheap underlayment, 5) existing 5/16" OSB decking. That's it.
The primary risk I see is that I'm expecting too much of this TCM stuff. Well, that, and the fact that a month ago I couldn't even spell the word "roof". :wink:
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