Rafter Baffles for Full-Vent Channel
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Several articles on this or other sites advocate a 2″ deep rafter baffle for a “Full vent channel” However, soffit vent such as Hardie have 5 sq in/lf of venting. Assuming a 14.5″ wide rafter bay receives the venting from 16″ linear inches/ 1.33 lf of soffit, at 5 sq in/lf, 6.65 sq inches of venting goes to the 14.5″ wide bay. This requires less than a 0.5″ gap along the 14.5 ” to transfer all the air from the soffit vent up through the baffle. SO – would a 2″ deep baffle even matter? Seems like a big box store +/-1″ baffle would be good enough to transmit all the air the soffit can take up. Am I thinking about this correctly?
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Flow-rate through pipes and ducts is tricky. If you have two sizes (eg a 1" diameter pipe and a 3/4" diameter pipe), then "it's only restricted by the smallest aperture flow passes through" sounds like a good intuitive model - one we adapt to other situations with the term 'bottleneck'. In reality, the pressure drop ("energy wasted on friction squeezing through a space") is proportional to some exponent of the aperture (diameter^3? diameter^4? diameter^5? Something in there, depends on turbulence), multiplied by the length that flow is choked down to that aperture.
So a 1 foot section of 3/4" pipe followed by a 200 foot section of 1" pipe will end up having a lower flow than either section would experience without the other being present, and the 200 foot section is likely a larger portion of the flow restriction than the short 3/4" stub.
Plumbers use this principle all the time with fittings that drop down to half or less the internal diameter of the pipe they're dealing with. It doesn't cause big problems, because that fitting might only be an inch or two long.
You'll see a similar situation here. The soffit is a flow restriction of only a fraction of an inch, but the baffles are several feet long.
Friction matters more the longer the space.
So is there any sort of calculation that can be made to possibly use the big box store vents, or is that so impossible that it drive the recommendation for the 2" ones available only online to get the most depth?
In my example above
a. air goes through small holes in the Hardie soffit (first restriction) at a rate of 6.65 square inches per rafter bay (16" oc) - although probably a bit less if there are joists extensions holding up the soffit...
b. air floats freely around the soffit over the Hardie panels
c. air gets channeled through the rafter baffles. The big box "ProVent" for example, state they have 15 square inches of net free area each, and after the 1" flange each side, are 12" wide.
d. air comes out of baffle and floats freely in attic, hopefully to ridge vent
If the soffit takes in 6.65 si, is a rafter baffle with 15 si free area noticeably worse than a rafter baffle with 28 si free area?
And how much does length impact this?
- Big box ones come 48" long, so I could trim to 2' each and they would work
- 2" deep baffles I can only find from one mfg. They are only 37.5" long, which is how I originally intended to use them. I could try to trim them to 18.75" (which would still work, just more difficult to install) if it would make a difference.
Don't think in terms of "limiting factor" or "size needed to move the air" . The resistances add. It can all be calculated - if you have the numbers. To get a feel for it, use one of the online duct friction loss calculators and compare the resistance of very short small ducts to longer, larger ducts.
As far as I know, this is the source of the 2" recommendation and it's almost "data free" when it comes to support for 1" being a problem in cases of good air sealing and normal snow loads. The same source sometimes recommends diffusion ports - which can be considered a zero inch vent.
Also note the vapor barrier recommendations there.
If your ceiling is well sealed, you need very little ventilation, so seal up the ceiling and install code min vents.
Unless there are local conditions that require more ventilation, or dealing with a low slope roof, there is no point in adding more.
Code minimum vent space is 1”. No point in considering anything smaller than that.
You don’t need the vent channel area to equal the square inches number listed on soffit vents. The channel has other issues like static air layers near the surfaces, flow restriction due to friction, things the soffit vents don’t have to deal with.
A simple analogy is this: consider the case of a hose nozzle on the end of a long water hose. The open area of the nozzle is maybe a 3/16” diameter hole. Now consider a several hundred foot run of hose. Going up from 1/2” hose to 5/8” hose will result in noticeably improved water pressure at the nozzle and much better water flow from the nozzle itself. The difference is in less flow restriction in the larger hose, even though the nozzle’s open area is smaller than either hose’s internal diameter and cross sectional area.
Bill