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Yet another request for advice.
I built a two-story 16×24 addition onto our little cabin in Fairbanks, Alaska a year and a half ago. It has a shed roof, a type which I had never built before. The roof is made up of 14″ TJI rafters to allow R-38 FG batts and a 2-inch airspace on top of them (this also parallels the goofy 18″ thick roof on the old cabin- not built by me). Roofing is Norclad ribbed steel on top of 1×4 purlins (standard practice around here). At the peak of the roof, the top edge of the steel is covered by a piece of right-angled steel trim that laps down onto the fascia. The space between the roofing and the trim (i.e. the space between the ribs) is filled with a foam closure made specifically for this type of roofing. To vent the roof, I put in continous soffits vents along the bottom and top eaves. The interior is sheetrocked but not taped yet. The vapor barrier is 6mil poly, with Tremco sealant at all penetrations/ overlaps to seal it. The only place where there is air leakage is where the metalbestos chimney penetrates the roof, where the metal cylinder I installed to provide clearance around the chimney as it goes through the roof is not sealed.
The problem is that the water vapor that does escape from the interior does not appear to be getting out of the upper vent. I have noticed condensation in the soffits, which when it has melted has damaged some of the soffit material and they will have to be replaced. I am finishing the house to sell it this summer, and want to get this problem fixed before we sell it. I think what is happening is that the heat/water vapor that escapes has to go down before it goes out- that is, it travels up the 2″ air space, but because it has to go down 14″ to the upper soffit vent, it is instead condensing on the upper part of the soffit and possibly (although hopefully not) inside the rafter bays. Maybe this is confusing, but I have added a crude disgram to make it more clear.
My question is, what are some ideas for a different venting system? Basically, I want to install something similar to a ridge vent, so that the vapor flows up and directly out. However this is made difficult becasue it’s a shed roof, and therefore there is no proper “ridge”. Any advice would be much appreciated.
Thanks, Olav Ormseth
Replies
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Yet another request for advice.
I built a two-story 16x24 addition onto our little cabin in Fairbanks, Alaska a year and a half ago. It has a shed roof, a type which I had never built before. The roof is made up of 14" TJI rafters to allow R-38 FG batts and a 2-inch airspace on top of them (this also parallels the goofy 18" thick roof on the old cabin- not built by me). Roofing is Norclad ribbed steel on top of 1x4 purlins (standard practice around here). At the peak of the roof, the top edge of the steel is covered by a piece of right-angled steel trim that laps down onto the fascia. The space between the roofing and the trim (i.e. the space between the ribs) is filled with a foam closure made specifically for this type of roofing. To vent the roof, I put in continous soffits vents along the bottom and top eaves. The interior is sheetrocked but not taped yet. The vapor barrier is 6mil poly, with Tremco sealant at all penetrations/ overlaps to seal it. The only place where there is air leakage is where the metalbestos chimney penetrates the roof, where the metal cylinder I installed to provide clearance around the chimney as it goes through the roof is not sealed.
The problem is that the water vapor that does escape from the interior does not appear to be getting out of the upper vent. I have noticed condensation in the soffits, which when it has melted has damaged some of the soffit material and they will have to be replaced. I am finishing the house to sell it this summer, and want to get this problem fixed before we sell it. I think what is happening is that the heat/water vapor that escapes has to go down before it goes out- that is, it travels up the 2" air space, but because it has to go down 14" to the upper soffit vent, it is instead condensing on the upper part of the soffit and possibly (although hopefully not) inside the rafter bays. Maybe this is confusing, but I have added a crude disgram to make it more clear.
My question is, what are some ideas for a different venting system? Basically, I want to install something similar to a ridge vent, so that the vapor flows up and directly out. However this is made difficult becasue it's a shed roof, and therefore there is no proper "ridge". Any advice would be much appreciated.
Thanks, Olav Ormseth