Basics for an Efficient Roof
In a “cold” roof system, the air barrier and insulation are in contact with each other and located at the ceiling/attic floor. This design prevents heat from leaving or entering the living space while moisture is vented. “Warm” roofs are not vented and have the insulation installed directly on the roof sheathing. Image: https://www.spacetwo.co.uk
Bob Tortorice
Roof covering materials, whether they are basic shingles, solar shingles, architectural shingles or metal coverings, have very little effect on the energy efficiency of one’s home. Real energy efficiency, along with a more comfortable living space and lower heating and cooling costs, is found in the structure of the roof and whether it’s vented or insulated, not in the covering material.
Traditionally, building professionals have used the time-honored practice of venting northern attics to prevent temperatures of up to 140 degrees from entering a home’s living space (usually the bedrooms), and prevent moisture from condensing in the attic. This construction process is known as a “cold” roof. With a “cold” roof, the air barrier (typically sheetrock) and fiberglass insulation are in contact with each other and located at the ceiling / attic floor. Its purpose is to prevent heat from entering and leaving the living space while allowing moisture to leave the living space and be vented to the exterior through ridge or gable end vents.
A “hot” roof, on the other hand, is not ventilated but keeps the heat out of the attic because the insulation is installed directly onto the roof sheathing, or attic rafters. A “hot” roof keeps the heat out of the attic space, thus allowing this normally unused space to be part of the home’s living space.
With that in mind, should a homeowner opt for a “hot” or “cold” roof when they build or retrofit their home? In either case, with the 2015 energy code slated to take effect September 15, 2019, the R value for attic floor insulation is R49 whereas the hot roof or vaulted ceiling is R38. These R values are for both climate zone 5 and 6. These examples from a home we insulated in the Conway, NH area, with two separate attics, may help you decide.
One is a full-height walk-up attic, where the floor of the attic is insulated, and typical soffit and ridge vents are installed, which created a “cold” roof. The second attic is much smaller, no higher than three feet, and has typical drop down stairs. In this space, we chose to insulate the rafter / roof of the attic, thus creating a “hot” roof.
On an 85-degree day, we inspected both spaces. First, we opened the door to the full attic and the four of us on the tour were literally pushed back by the heat. The bedroom below was comfortable, so the attic floor insulation was doing a decent job, but being in the attic for more than a minute or two was unbearable.
When the stairs to the smaller attic were dropped down, we immediately noticed no temperature difference between the attic space and the temperature on the second floor. The home’s owners were delighted with their new temperature-controlled storage space.
From these examples it’s easy to see that when insulating your attic a “hot” roof is far more energy efficient than a “cold” roof when you want to add usable space to your home, no matter what roof covering you choose. Along with excellent energy efficiency, you will gain extra living space that’s comfortable to be in.
Bob Tortorice has over 31 years of green building experience. He is the owner of Building Alternatives, Inc. and Alternative Energy Audits in Franconia NH. Bob is a member of the Efficiency VT program. Call 603- 823-5100 or visit www.buildingalternatives.com.
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