Open Cell Foam Vented Attic

Open cell foams use water as a blowing agent.

Open cell foam vented attic.

Open and closed cell foam insulation use a blowing agent to forms the cells within it. We removed the wet sweating r 30 batts insulation on an poorly vented attic and installed open cell spray foam sprayed to the underside of the roof sheathing. Closed cell foams use enovate also known as 245fa. There is 300 difference there but the cost they give you is about a third difference and open cell foam at 12 inches will not give you the performance of 2 inches of closed.

While the denser closed cell variety does not readily absorb moisture open cell foam can draw it in like a sponge. Attic ceiling to stop airborne moisture from reaching a cold surface and condensing inside the building envelope in this design insulation effectively separates the interior and exterior spaces while slowing down moisture flow so the dewpoint is not achieved within. Unvented attic assemblies unvented attics rely on an air impermeable insulation installed to the roof deck s underside i e. The myth that open cell is more economical is bullshit.

Open cell foam weighs 1 2 pound per cubic foot and closed cell weighs 2 lbs per cubic foot. Another study s computer model indicated that open cell foam insulation is riskier than closed cell foam in all us climate zones. Building code vapor retarder requirements for unvented attic construction have been in place since the 2009 irc and have resulted in thousands of successful assemblies that contain. An open cell foam low density spray polyurethane is a spray applied to provide continuous insulation and an air sealing barrier.

If you convert your vented unconditioned attic to an unvented conditioned attic by installing open cell spray foam on the underside of your roof sheathing you may be surprised to discover that your attic is now the most humid room in your house. 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. Some spray foam manufactures have tested primer coating products that result in the formulation of a class ii vapor retarder directly on the surface of the open cell spray foam. The problem is that the attic has become extremely moist don t have a reading currently to the point that the ductwork sweats grilles and in attic so badly that the drywall ceiling.