January 24, 2017
Do metal roofs bring special ventilation needs or condensation concerns when installed on homes? The general answer is “no” but there is more than a simple answer.
Unless a home has been built specifically in a manner which does not require attic ventilation (such as a conditioned space or hot attic, or insulated panel construction), there are 2 reasons why good ventilation is important with ANY home – regardless of the roof covering.
2 Reasons Why Good Ventilation is Important for Any Home
- Energy Efficiency. Ventilation is used during the summer to vent heat out of the attic, reducing air conditioning costs. For many years, building science experts were focused on reflectivity as a way to keep heat out of the attic. While reflective or “cool roofs” are still a good idea, tests have shown that good ventilation which quickly removes any gained heat back out of the attic is equally if not more effective at home energy efficiency.
- Condensation. A great deal of moisture is generated inside of a home’s living space. It is generated primarily by laundry, showers and bathing, houseplants, cooking, and ventless stoves. That moisture, originating in the living space of the home, migrates upward and, if a 100% reliable vapor barrier is not in place behind the home’s ceilings, it ends up in the attic. Once it’s in the attic, if that warm, moisture-laden air reaches a cool surface, it condenses. (Think of that cool glass of lemonade sitting outside on a hot, humid day.) The resulting condensation can create a very unhealthy environment prone to support mold, mildew, and other biological growth. A well-vented attic also helps to keep your attic’s insulation dry which increases its effectiveness. As it gains moisture, insulation loses considerable R-Value and effectiveness. The general calculation for this is a 2.5% decrease in R-Value for every 1% increase in insulation moisture content.
With traditional roofing choices, there is a 3rd reason why attic ventilation can be helpful: good attic ventilation can help increase shingle life. By keeping rooftop temperatures a bit cooler, shingle life can be preserved. This benefit does not really have an impact on roofing materials like metal, tile, and slate which are unaffected by high temperatures.