Attic soffit vent baffles

For the baffles that allow air to flow from the soffit vents up to the ridge vent, would it be appropriate/advisable to fasten 1/4" ply or underlayment to the 2x4 rafters? The 1/4" sheets would be about 2.5' x 8' and fastened to the rafters where the rafters meet the top plate. This would form a "chute" allowing air to flow and preventing insulation from getting in to the soffit. It would allow more air flow than the types of baffles that you staple to the underside of the roof deck. Would it be too much air flow?

I'm considering adding blown-in insulation to my attic crawlspace. I have additional questions on that topic, but I'll save them for a separate thread.

 

Plywood baffles should work fine. Obviously, airflow is created by the soffit vents so the baffles wouldn't increase it. If anything, baffles reduce airflow versus nothing. But, again, airflow is established by the vents.

 

Thanks Tony P.
I do have vented soffits (CertainTeed Invisivent over screening).
I wasn't sure if that approach would result in too much airflow--reducing the effect of the insulation.

 

It doesn't really matter what materials you use your sizing the amount of free area (baffles) to the room size so adding the wooden ones isn't getting you "additional" air flow.

 

Sorry Marq1 , I didn't explain myself very well.
I was comparing the amount of airflow one would get by fastening 1/4" sheets to the rafters vs. the amount of airflow one would get by stapling commercially available baffles to the underside of the roof-deck.
It seems to me that the commercially available baffles have fairly narrow channels for the air to flow through.

 

I got your though but what I'm stating is the rule of thumb is 1 baffle per 150 sq ft of attic space. So if you have 1500 sq ft you need 10 of the store bought baffles.

f your boxed in shape has a greater area then you might need only 6-7. That is why there really is no extra!

 

"Would it be too much air flow?"
No, you cannot have too much air flow.

 

I think that's like asking, can you have too much money!

It's simple sizing, the size of the area determines the amount of ventilation, your not gaining anything, and possibly defeating, a properly sized system. Why spend time and money on something you don't need!

 

I could find just a many saying there is no maximum but there is a minimum.
Having so many vents that the insulation get blown around on windy days would be too much.

Even then, there are too many variable to say even what a minimum is.
Is there s ridge vent?
Are there roof vents and if yes how many and where are they?
Are there gable vents and what is their size?
How well is the building cavity insulated from the attic?

 

With respect to the "Myth #1" that Marq1 shared, it does sound like it's possible to have too much ventilation.
In answer to the questions posed by manden, there is a ridge vent. No other vents, aside from the soffit vents that I mentioned. The exterior 2x4 walls are insulated with Roxul, I doubt that there is very good sealing where the top of the wall meets the attic floor.

 

Lets stick to the science, here is the GAF calculator so the OP can do their own calculation on gross ventilation.

https://www.gaf.com/en-us/for-profes...ion-calculator

And you might be surprised, ventilation has nothing to do with insulation, even an uninsulated garage/shed needs to be sized correctly!

 

Not sure why you would think that would surprise me.
Ventilation has to do with temperature and moisture control not keeping the space warmer or cooler that the outside ambient.

GAF makes shingles and both the above can effect shingle life hence the calculator.
Also note that even GAF says to round up the calculation.
Also seems science recommends GAF products.

Also I do not like their calculator as it uses square footage and not cubic feet.

 

Marq1 and manden, I appreciate the discussion.
On a related note, I've been looking at indoor/outdoor thermometer/hygrometers. I found at least one that can handle multiple sensors. I'm thinking of buying one and putting one of the sensors up in the attic crawlspace.