Ventilated Cold Roof
Ventilated roofs are nothing new, however ventilated cold roof are used on SIP roofs in order to prevent ice dams, damage from trapped moisture and a second layer of defense for roof leaks. In this article I’ll show you how we use ventilated cold roofs on timber framed homes that have SIP’s roof panels.
In the photo above you can see a timber frame home that we built. The roof consists of traditional timber framed rafters and purlins which support structural insulated panels (SIP’s). The SIP’s are 6-1/2″ thick Urethane foam panels that create an amazing insulation layer and a great vapor barrier. While Urethane insulation can stop interior water vapor from hitting the cold roof surface these panels do have joints that could possibly let air through. If water laden air passes through joints in the SIP’s it could hit the cold roofing surface and condense leading to possible rotting problems on the SIP panels.
In order to avoid that situation we use a ventilated cold roof. The cold roof is very similar to traditional ventilated roofs in the sense that air is allowed to enter the eaves of the roof through a vent. The air travels up the roof pitch and exits at the roof peak. To accomplish this we install 1×3 pine strapping along the roof every 18″. Then we install a layer of 5/8″ sheathing which will be our final roof surface under the shingles or metal roof.
As you can see above the trapping creates air voids between the 5/8″ roof sheathing and the top of the sip panels. The soffit detail for this house will include a 2″ wide continuous strip vent. The ridge of the roof will include a traditional ridge vent.
In the photo above you can see two pieces of fascia. The lower piece is a 1×8 attached directly to the end of the SIP panel. The upper 1×3 is nailed to the end of the 1×3 strapping, it’s flush to the top of the upper roof sheathing, and hangs down below the strapping. We installed a piece of aluminum strip vent on the bottom of the strapping tight to the face of the lower fascia, and tight to the back of the upper fascia. This detail also creates a very nice shadow line.
Without this detail the roof could be prone to ice dams from sun induced melting and possible water damage from leaking water vapor hitting the cold surface of the SIP’s. This may seem like an extra, expensive step but it’s absolutely necessary in order to prolong the life of the roof and provide a maintenance free roof.