JAG Construction - Vented vs. Unvented Attics
Since the 1970s, we have all become accustomed to the ventilated roofing system (vented attics). This 45 year old technique is exposing some fundamental flaws when we experience things like high energy bills, cold and drafty homes or ice damming. The alternative system is with the use of high density, closed cell spray foam, directly to the roof deck which eliminates the need to ventilate.
Vented Attic System: Ideally, the vented attic system is used to remove warm, humid air from the attic so moisture doesn't condensate on the bottom of the roof deck helping to prevent any rotting of the substrate or plywood. This system can fail due to restricted or unbalanced air flow from the soffit's, with hips, valleys or a complicated roof design. In these cases, the venting system can pull conditioned air from the living space; the heat or air conditioning that you are paying for.
Air leakage from the living space occurs through cracks & joints in the ceiling, around electrical & plumbing penetrations, duct work or especially around recessed "can" lights. These features greatly increase the likelihood of moist air leaking into attics. When the attic ventilation is restricted, condensation can form on the underside of the roof deck. Heat loss can cause snow melt on the roof surface, leading to ice damming and/or roof leaks and in some cases deck rot.
Unvented Attic: When closed cell spray foam insulation is applied directly to the underside of the roof deck and gable end walls, it creates an unvented attic space. When high density spray foam is greater than 2" thick, then it is classified as a vapor barrier and is also air impermeable. These two aspects are the reason that you do not need to ventilate.
This will eliminate almost 100% of the heat loss, and completely stop all of the air loss from the conditioned space to the outdoors. The vapor barrier will stop the moisture from getting to the roof deck. The spray foam is now protecting the roof from rotting from the inside. This means we are no longer losing valuable heat or air conditioning. The highly efficient spray foam has also proven to be extremely effective against reducing solar heat gain in the summer and reducing ice damming due to thermal loss in the winter.
The unvented attic system has far more benefits than its counterpart. It now has a confirmed track record that has held up to extreme conditions. When the older vented attic technique fails, consider the unvented roofing system for a proven, final solution.
Written by Michael Jagodzinski
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