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Non Vented Roof Assembly | Using Spray Applied Foam | Conditioning the Attic.

Many folks ask why would you insulate the roof deck rather than the attic floor? Simply put it is the best way to insulate your home.

 

 

The traditional practice of insulating the underside of the roof in the attic has raised much debate in the building industry because "standard" roofing and design techniques call for the attic to be ventilated in order to reduce moisture problems and heat build-up in the hot summer months. However, a vented attic situation it will become approximately 130-degrees in the summer. There's no reason for your air-conditioning and vent-ductwork to have to work in that type of severe conditions. There is also opportunity for moisture to form due to condensation on these appliances.

By applying spray foam directly to the underside of the roof deck, it now insulates the attic space from the extreme heat that once radiated thorough the hot shingles sheathing and roof.  The severe temperatures no longer exist in the attic. In short, the attic now becomes a "conditioned" space of the house that is just as comfortable as any other room in the home.

A roof system insulated with spray foam reduces energy several ways. Energy loss from ducts located in the attic is essentially eliminated. The top of the building is much tighter resulting in less infiltration and exfiltration, so excess moisture isn't pulled into the attic. Infiltration through the ceiling is also reduced. In addition, the attic temperature is lower, which further reduces energy loads.

In a standard insulation system, ceiling insulation reduces the transfer of heat from the attic to the living space (in the summer). Attic temperatures can often approach 140F during the day. Most of this heat enters the attic space through a multi-step process. First, solar energy warms the shingles and sheathing. The hot sheathing then transfers heat to the rest of the attic through conduction, convection and radiant heat transfer. The 140F temperature of the underside roof surface drives the heat transfer process.

By insulating the roof surface with spray foam, the surface temperature exposed to the attic (the temperature driving the heat transfer) is reduced by as much as 40F. Both conduction and convection heat transfer are proportional to a temperature difference, so that heat transfer will be reduced proportional to a drop in surface temperature.

In cold climates, the primary purpose of attic or roof ventilation is to maintain a cold roof temperature to control ice dams created by melting snow, and to vent moisture that moves from the conditioned space to the attic. Melted snow, in this case, is caused by heat loss from the conditioned space through air leaks.
In hot climates, the primary purpose of attic or roof ventilation is to expel solar heated hot air from the attic to lessen the building's cooling load. Control of ice dams, moisture accumulation and heat gain can also be successfully addressed by unvented attic or roof design. 

Spray polyurethane foam (SPF) can be applied in sufficient thickness to satisfy local energy code requirements, directly to the underside of roof sheathing between rafters or joists of any slope in all (heating, mixed and cooling) climates. This configuration controls the entry of moisture-laden air into the insulation and also eliminates dew-point occurring at the underside of the roof deck and anywhere in the insulation, in all (heating, mixed or cooling) climates. Due to the fully adhered, properties air and moisture are displaced out of the insulated space - including at rafters and sheathing. Moisture cannot enter the insulated space from any direction, eliminating the requirement for roof venting.

The benefits of including the attic in the insulated space are:

Duct leakage and heat loss/gain from ducts is much less of an issue.

Air sealing is easier in the roof than in the ceiling.

Dust and loose insulation are less likely to migrate down to the living space.

Tests show energy costs are lower when the attic is sealed.