Freeze/thaw moisture issues behind roof insulation
I insulated a vaulted ceiling with R30 rockwool between the rafters and 2” foilfaced foam board below rafters to slow thermal bridging. I’m having moisture issues. Small amounts of water dripping down the walls on thaw cycle after deep freeze. I drilled 4” hole through foam to explore, and found only moisture ontop of rockwool and roof sheathing is damp. Do I need to increase ventilation? I have @2” space between rockwool and sheathing. I sliced the tri-flex along the ridge before installing ridge cap, and I installed foam strips on the windy side to prevent rain and snow from blowing under. Should I cut back tri-flex to open gap and create better flow? I only did one slice down the middle because I was afraid of blowing rain getting into the ridge.
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Replies
Your moisture issue in the vaulted ceiling insulated with R30 Rockwool and 2” foil-faced foam board is indicative of inadequate ventilation and moisture management in the roof assembly. The presence of moisture on top of the Rockwool and damp roof sheathing during thaw cycles suggests that warm, moist air from the interior is condensing on the colder surfaces near the roof sheathing. Here are steps to address the problem:
1. Increase Ventilation:
Ventilation Space: The 2-inch space between the Rockwool insulation and the roof sheathing is a good start, but the effectiveness of this ventilation channel depends significantly on proper air intake and exhaust.
Ridge Ventilation: Cutting back the tri-flex underlayment to open the gap at the ridge may indeed improve airflow, provided that you have adequate intake ventilation at the eaves. If you're concerned about rain penetration, ensure that the ridge vent product you're using is designed to prevent rain and snow ingress while allowing air to flow. There are ridge vent systems specifically designed for high-wind or snow-prone areas.
2. Enhance Intake Ventilation:
Soffit Vents: Ensure that soffit vents are clear, adequately sized, and distributed evenly along the eaves to provide sufficient cool air intake. This intake is crucial for driving the ventilation process, allowing warm, moist air to exit through the ridge vent.
Balanced Ventilation: Aim for a balanced ventilation system where the intake and exhaust areas are roughly equal. This balance ensures a continuous flow of air that can carry moisture out of the roof assembly.
3. Air Sealing:
Prevent Warm Air Leakage: It's essential to seal any potential air leaks between the living space and the roof assembly. Warm, moist air escaping from the living area into the roof can lead to condensation problems. Pay particular attention to penetrations, light fixtures, and any gaps in the insulation layer.
4. Review Vapor Barrier Placement:
Vapor Barrier: Ensure that a proper vapor barrier is in place on the warm side of the insulation to prevent moisture from migrating into the insulation and roof assembly. The foil face on the foam board acts as a vapor barrier, but ensure it is properly sealed at all joints and edges.
5. Consult a Professional:
Given the complexity of ventilation and insulation in vaulted ceiling assemblies, consulting with a building science professional or a roofing contractor experienced in dealing with similar issues may provide specific insights tailored to your home's design. They can offer advice on improving ventilation, air sealing, and moisture management based on a detailed inspection of your roof assembly.
Conclusion:
Addressing moisture issues in a vaulted ceiling requires a comprehensive approach focusing on improving ventilation, ensuring adequate air intake and exhaust, air sealing to prevent warm air leakage, and correctly placing vapor barriers. By enhancing the ventilation pathway and ensuring the entire system is correctly balanced and sealed, you can mitigate moisture problems and protect your home from water damage.