In the most recent issue of Fine Homebuilding, Martin Holladay wrote a piece on Insulating Unvented Roof Assemblies. Unfortunately, the term hot-roof is a misnomer as the roof really is not that much hotter than a normal roof..
Spray foam in unvented attics intended for regular use helps:
Unvented roof spray foam. Unvented roof strategies with open-cell and closed-cell spray polyurethane foam insulation sprayed to the underside of roof sheathing have been used since the mid-1990's to provide durable and efficient building enclosures. Don't install closed-cell foam. An alternate to spray foam insulation in an unvented attic is plenum trusses which allow you create a reverse bulkhead above the ceiling plane that is insulated.
In the case of spray foam, unless the rafters or trusses are fully coated with foam, heat will. This allows you to install your bulkheads above the ceiling plane without the need for spray foam insulation. Unvented attics have been designed and constructed for some 30 years.
In that vein, we would like to look at one of the best options available for creating a conditioned attic using the hot-roof method. I am planning an addition on my home in Seattle with an unvented roof assembly. Kraft paper is a tough one.
Foam insulation above the roof deck (to keep its temperature above 45°F throughout the year) or spray foam insulation under the roof deck (to keep interior moist air from contacting it). It has the highest R-value I have looked at the costs and I am not considering using spray foam of any kind.
To some, unvented attic construction using spray foam insulation is a fairly new approach in home building. For an unvented, cathedral ceiling, the local code calls for R-38. Open-cell spray polyurethane foam or closed-cell spray polyurethane foam (ccSPF) insulation can be sprayed along the underside of the roof sheathing to provide a conditioned and insulated attic space that can be durable and efficient in.
High-density urethane spray foam provides an airtight seal and high R-values, making it a good choice for unvented cathedral ceilings. Because the drying capacity of unvented roof assemblies is inherently lower than vented ones, it is especially critical. There have been isolated moisture related incidents that raise potential concerns about the overall hygrothermal performance of these systems.
When insulating an unvented roof assembly, prefers closed-cell to open-cell foam because it’s both an air barrier and a vapor retarder. Remove ventilation points when converting from vented to unvented attic Chapter 6.2 Vented and Unvented Roof Assemblies in Hammer & Hand's Best Practices Manual covers construction detailing for roof assemblies.
DuPont RoofLiner is a vapor barrier and therefore cannot be used on an unvented roof. According to the US Environmental Protection Agency (EPA), spray foam is recognized as a premier alternative to the batt insulation of years past. Enhancing roofing material life span by reducing sheathing temperature.
The quest for greater energy efficiency in buildings has led to increasing interest and need for insulation, improvements in HVAC equipment, more airtight construction, new materials and assemblies. Moisture-Safe Unvented Wood Roof Systems The unvented attic space is considered the most effective and up-to-date building practice, particularly by the spray foam industry.
Using either or both spray foam (open and closed cell) and fibrous insulation (cellulose and mineral fiber) can be successful. The idea that you can install one high performance system, such as an unvented roof assembly (with or without spray foam), without addressing other parts of the house is analogous to tossing a turbo charger on a stock Toyota Corolla and then wondering why there are problems. To finish an unvented cathedral ceiling insulated with closed-cell foam, most codes require a layer of 1⁄2-inch drywall or an equivalent thermal barrier (top).
If you don’t want to use SIPs or nailbase, read this article tolearn the four basic approaches to insulating unvented roof assemblies. We're big fans of Fine Homebuilding.
We agree that avoiding sheathing rot is critical, and that air permeable insulation such as fiberglass or cellulose cannot be used. Controlling ice damming by keeping the roof cold. Insulating Unvented Roof Assemblies:
Unvented attics (sometimes called sealed, enclosed, or encapsulated attics) feature insulation on the underside of the roof decking, rather than on the attic floor. Unvented roofs with spray foam applied to the underside of the roof deck have been used since the mid-1990s. Spray foam in unvented attics Spray foam in unvented attics allows homeowners to convert the area into a liveable space.
The design approach is the same for all climate zones. If you want to install rigid foam on top of the roof sheathing, it's OK to install open-cell spray foam underneath, because open-cell spray foam is vapor permeable. Just a general thought on this whole discussion:
However, if you are determined to build an unvented roof, then I think that at closed-cell foam insulation is a better choice. Shingle Warranty After Spray Foam Attic Insulation: Closed-Cell Spray-Polyurethane Foam (ccSPF) at bottom of roof sheathing..
Serving as both an air sealant and an insulator, spray foam is estimated to reduce the standard. Closed-cell spray polyurethane foam can be used to create an unvented roof assembly that works in all climate zones. Removing moisture from roof cavities, structural members, sheathing and insulation.
There are traditionally four main reasons for venting a roof: Using foam board, rather than spray foam, does not provide the air seal of spray foam, so this is a concern. Benefits and Pitfalls of Spray Foam Insulation.
4.1001.6c Elimination of attic venting All gable vents, ridge vents, and roof vents will be covered with suitable backstop material to provide substrate for SPF SPF Spray polyurethane foam application. Common problems found with traditional insulation in an unvented attic assembly can be mitigated with spray foam insulation like Icynene. With it, the orientation and type of insulation applied to the top of the building enclosure has changed from the traditional way, and so temperature and moisture conditions experienced by various assembly elements will be impacted.
Unvented Roofs Vented Roof. For unvented attics, having the spray foam layer applied continuously from the roof/wall intersection to the roof peak, at the specified thickness, encapsulating the entire interior surface area of the attic will hinder airborne moisture entry while providing an energy efficient environment. Senior Engineer Manager at Icynene, John Broniek makes the case for the unvented attic assembly and why insulating the underside of the roof makes sense.
Sealing the board edges with canned foam would probably work but is labor intensive. Foam Free Don't stay stuck in the sci-finightmare of foam, foam, foam. Climate, the solar properties and exposure of the roofing, the air and vapor.