Cold Roof

By Todd Fratzel on Insulation, Roofing

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.

Cold Roof over SIP Roof Cold Roof

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.

Cold Roof Air Space Cold 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.

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.

 

About the author

Todd Fratzel

I'm full time builder for a large construction company in New Hampshire. I run their design-build division that specializes in custom homes, commercial design-build projects and sub-divisions. I'm also a licensed civil and structural engineer with extensive experience in civil and structural design and home construction. My hope is that I can share my experience in the home construction, home improvement and home renovation profession with other builders and home owners. I'm also the author of Tool Box Buzz and Today's Green Construction. Please feel free to contact me if you have any questions, suggestions or you'd like to inquire about advertising on this site.

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6 Comments

  1. jeff_williams says:

    What’s used to seal the seams of the sips? Anyone developed a tongue and groove sip (one big T to go in one big G) similar to T&G rigid insulation?

    • Todd says:

      I’ll try to get some pictures this week. Each SIP panel has two slots routed in it the length of the seam. There’s a slot near the top surface of the panel, just under the OSB. That slot is about 1-1/2″ wide on each panel. Once the panels are set we slide a piece of 1/2″ OSB into the slot for the entire length. This acts like a spline. Screws are then driven down from the top of both panels into the spline to keep them held together.

      There’s a second slot about the same size located at the middle depth of the foam. Next we drill a 3/8″ hole every 8″ along the seam. The hole penetrates the lower slot. Spray foam is then injected into each hole until it runs out the top. This seals the slot and helps create an air tight seal. All of the foam is then scraped off the top of the panels before the cold roof is applied.

  2. That’s right, in addition. Proper ventilation is essential. It prevents condensation and dry rot within the attic wood structure and wood substrates, prevents mould, and prevents warping of the plywood. Proper venting also extends the life of the shingles and roof membranes. Provide ventilation in a ratio of approximately 1:300 of the insulated area. Ideally air should enter the vented soffit, rise as heated in the attic, and exit near the peak of the roof.

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