Attics come in different shapes and sizes. In older homes and more traditional construction, you’re likely to find a vented attic assembly, complete with soffit and ridge vents. However, in newer construction, a design known as the “unvented roof assembly” or “hot roof” design has become more popular. In this post, we’ll address the following commonly asked questions about roofing methods:
- What is a “hot roof,” and why is it becoming popular in new construction?
- Is a hot roof or cold roof more cost effective?
- Which roof assembly is more energy efficient?
What is a “hot” roof?
A “hot roof” means that insulation is installed closely beneath the roof decking, without space for ventilation. In some cases, this design allows the attic space to be usable like any other room in the house by bringing it into the thermal envelope of the home. Since there is no intentional ventilation between the insulation and the roof decking, the moisture and air control barriers are at the roof deck, allowing the attic space to maintain the same condition as any other living space.
The hot roof assembly is found almost exclusively in new construction. Whether or not a hot roof is a superior setup depends heavily upon climate, homeowner needs, and building plans. In climates that remain cold for a sustained period, unvented roofs can cause more harm than good. The reason for this is because an unvented roof allows more opportunity for conditioned air from the home to permeate the insulation, warming the roof deck and causing ice dams during the winter. This differs from a vented assembly, air from outside flows between the insulation and roof decking, cooling the underside of the roof and reducing the chance of ice dams.
During the summer, a hot roof does not allow for air exchange between exterior and interior air. Depending on the quality of the insulation, hot air from outside can permeate the insulation and warm the conditioned space, raising energy bills and making HVAC units work overtime.
What’s more cost effective: hot roofs or cold roofs?
A hot, unvented roof requires that you pay meticulous attention to air-sealing the attic. An unvented roof typically uses spray foam insulation, which tends to come at a higher cost than batt or loose-fill insulation. You’ll also need to ensure that you install an air barrier and vapor retarder to protect the attic from changes in the weather. Air sealing, proper insulation density, and vapor retarders all come at a cost, but depending on your climate, an unvented roof done right can make your attic space livable and keep conditioned air inside where it belongs.
A vented or “cold” roof has been the traditional roofing assembly for many years. A cold roof implies that the attic is not within the conditioned space of the home. Instead, the attic has insulation on the floor that separates air between the conditioned and attic spaces. For this type of roof, you’ll need to have proper soffit vents, a ridge or gable vent, and ventilation baffles. AccuVent rafter baffles are compatible with all types of insulation, including spray foam. The baffles will prevent any insulation from falling into the soffits, allowing air to flow from the soffit to the ridge or gable vent. Rafter baffles are an inexpensive component of a vented roof.
When it comes to cost-effectiveness, energy bills are the final factor in deciding what type of roof is best. A vented roof will keep a home cooler in the summer, lowering cooling costs, and keep the roof cool in the winter, preventing costly ice dams. An unvented roof will make the attic space livable and keep conditioned air from escaping the home, but the initial installation can be costly.
What’s the more energy efficient option?
This answer also depends on climate. A vented roof will keep the attic cooler in the summer, but also cooler in the winter, preventing ice damming. To ensure the home remains efficient, it’s crucial to air seal the attic, making sure conditioned air isn’t mixing with outside air. This can cause the HVAC system to work harder than it has to.
In most cases, an unvented attic will remain a similar temperature to the rest of the home. Of course, this makes the type and R-value of insulation crucial, since the assembly depends on how well it functions. The attic may become warmer during the summer as the insulation conducts heat into the attic. During the winter, an unvented attic will remain warmer. But without ventilation, the chance for ice damming, mold, and moisture accumulation is higher. With an unvented roof, these damages are less easy to notice.
AccuBlock is designed for unvented roofing assemblies, but it serves a function in vented roofs, too. In an unvented roof, it acts as a template for spray foam insulation to prevent overspray and to allow the insulation blanket to cover the top plate.