Addressing wind washing in a cathedral cieling
I have been asking a lot of questions about my approach to my ceiling insulation, forgive me.
I am shooting for a 3-4 inch air gap eave-eave below the plywood roof deck in my assemble, leaning toward two layers of fiberglass batts for economies sake.
Now the question is about the vent channel. I have read every article I can find here(the evenings are long these days) about wind-washing and I am wondering the best way to address it in my case.
I am wondering if just addressing the the first 4-8 feet of where the the air enters would have the biggest affect, and perhaps detailing the entire ceiling may be too diminished of returns after that to make the effort worth it? I don’t know, just trying to allocate resources realistically and appropriately.
In this approach I would cut sheets of 3/8 CDX (the cheapest thing I could find) in half lengthwise and attach them 3-4 inches below the roof deck and connected directly to the bird blocking, and sealed. After that the air would exist into the middle of the cavity without a baffle, just the fiberglass, roof deck, and an air gap.
The idea would be that the affects of wind are greatest at the eaves, by the time air reaches the 8 foot mark through the channel its speed has decreased considerably.
Perhaps I’ll get some eye rolls here but to me it’s a constant game of cost/benefit analysis. Thanks for reading and considering!
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Replies
Joe,
Recently in a thread on a similar topic Jon linked to research showing that wind-washing of batts only diminished the R-value marginally. The circumstances of the test were cavity walls, which to me might speak to the effect being a bit higher where there is more active movement - like as you say near the eaves of a roof. A long way of saying I think I agree with you that it only makes sense to worry about air-movement for the first few feet of the roof, although I wish we had more data to confirm what role wind-washing plays in different building assemblies.
If the channel remains the same size from soffit to peak, then the air velocity also remains the same - I don't think that any air enters the soffit, picks up some heat and then exits the soffit. Fiberglass is the most susceptible to wind washing.
Jon R,
Are you saying build a fully sealed vent channel the whole length of roof or nothing at all?