Some chimney install issues in another thread reminded me to post this question.
My cathedral ceiling support box is installed between 2x12 rafters (with 2x6 pieces framing the box between the rafters). With the rafter depth and a steep roof slope, and with adequate support box left exposed under the ceiling, the top of the box on the low side ends up just flush with the roof sheathing, while on the high side the top edge of the box is a few inches below the sheathing. The end result is that the rafter vent on the high side (running up to the ridge) is visible / exposed - basically a space above the edge of the box and below the sheathing.
I have lots of clearance all around, roxul insulation, etc. So I'm satisfied that it's safe, but I noticed last weekend that the light film of fresh snow over this particular rafter bay, upstream from the chimney, was melting, while other areas were still frosty.
It's pretty clear to me now that I have some of the warm air from the stove pipe support box area that's moving thru the rafter vent space and out the ridge vent (not all air is venting up thru the flashing / under the storm collar).
The air is circulating meaning the rafter vent is working, but it's warm air, not cold "soffit" air.
I never really noticed this before as I'm a weekend burner and would show up on a weekend to find a foot or 2 of snow - then the couple days out there I guess wasn't long enough to melt enough to make this really obvious (with lots of snow up there). Just happened to catch the ideal conditions for it to show up.
For those who install chimneys with support box in a rafter bay in a cathedral ceiling with soffit and ridge vents - do you block off that rafter bay completely (no vent, no air flow from box to the ridge), or so you leave any exposed rafter vent space open on the high side and let warm air flow thru to the ridge vent?
I can rectify this easily (small piece of sheet metal or flashing to block the rafter bay / vent), or I can leave it alone with some warm air venting right out. Not sure what to do...?
Sorry for rambling - wish I had a pic but hopefully my description isn't too fuzzy. Any thoughts are appreciated.
Many thanks...
My cathedral ceiling support box is installed between 2x12 rafters (with 2x6 pieces framing the box between the rafters). With the rafter depth and a steep roof slope, and with adequate support box left exposed under the ceiling, the top of the box on the low side ends up just flush with the roof sheathing, while on the high side the top edge of the box is a few inches below the sheathing. The end result is that the rafter vent on the high side (running up to the ridge) is visible / exposed - basically a space above the edge of the box and below the sheathing.
I have lots of clearance all around, roxul insulation, etc. So I'm satisfied that it's safe, but I noticed last weekend that the light film of fresh snow over this particular rafter bay, upstream from the chimney, was melting, while other areas were still frosty.
It's pretty clear to me now that I have some of the warm air from the stove pipe support box area that's moving thru the rafter vent space and out the ridge vent (not all air is venting up thru the flashing / under the storm collar).
The air is circulating meaning the rafter vent is working, but it's warm air, not cold "soffit" air.
I never really noticed this before as I'm a weekend burner and would show up on a weekend to find a foot or 2 of snow - then the couple days out there I guess wasn't long enough to melt enough to make this really obvious (with lots of snow up there). Just happened to catch the ideal conditions for it to show up.
For those who install chimneys with support box in a rafter bay in a cathedral ceiling with soffit and ridge vents - do you block off that rafter bay completely (no vent, no air flow from box to the ridge), or so you leave any exposed rafter vent space open on the high side and let warm air flow thru to the ridge vent?
I can rectify this easily (small piece of sheet metal or flashing to block the rafter bay / vent), or I can leave it alone with some warm air venting right out. Not sure what to do...?
Sorry for rambling - wish I had a pic but hopefully my description isn't too fuzzy. Any thoughts are appreciated.
Many thanks...
