Hello,

I have an existing bedroom over a two car garage that is very hot in the summer relative to the rest of the house.  The house is in Central Texas (Austin) and was built in 1994.  The ceiling of the garage is framed using web trusses.  There’s bat insulation (intended for a 2×4 or 2×6 cavity) above the sheet rock ceiling in the garage, but the insulation has all fallen down and is laying on the sheet rock so there’s a substantial air gap between the insulation and the OSB floor deck of the room above.  3 out of the 4 walls of the bedroom above the garage are knee walls that end where the sloped ceiling shared with the roof line starts.  The knee walls are shared with attic storage space that wraps around the bedroom and is accessible from the garage using an attic stairwell below.  The knee walls are currently insulated with fiberglass bats, but there’s no air seal and they’re just strapped up using plastic mesh.  The sloped ceiling above the bedroom has fiberglass bats that fully fill the gap between the sheet rock on the ceiling and the roof deck.  The ceiling joists are 2x6s plus 3/4″ pieces of wood between the end of the 2×6 and the sheet rock on the ceiling of the bedroom.  There’s currently no air channel (aside from what gets around the bat insulation) to take air from the soffit vents up to the small attic space above the flat portion of the bedroom ceiling.  The small attic space above the bedroom ceiling is connected to the main attic above the rest of the house through a small opening near the ridge of the roof.  The main attic has soffit and ridge vents.

I’m planning to remove all of the sheet rock from the garage ceiling and spray ~3-4” of open cell foam on the underside of the bedroom floor deck.  I’m also planning to remove the fiberglass bats on the knee walls and replace with open cell foam around the bedroom to better insulate and air seal the room from the attic storage space.  I’m planning to block under all of the knee walls to ensure there’s no air movement under the room (driven by the soffit vents). 

A few questions:

1.) Is 3-4” of open cell foam sufficient on the underside of the bedroom floor deck where I live?

2.      2.) As part of the project, I’m turning part of the attic storage space on one side of the room into a closet that will become part of the bedroom.  I’ll obviously insulate underneath the floor of the closet just like the rest of the bedroom and add blocking above the outer garage wall to ensure no air transfer under the closet or the rest of the bedroom.  How should I handle the ceiling of the closet?  Should I simply spray open cell foam directly on the roof deck and fill the entire cavity between the 2×6 roof rafters and add blocking to cut air flow off from the soffit vents?  If I do this, should I block off where the spray foam on the ceiling would end above the knee wall shared between the bedroom and closet?  This is where the fiberglass bats that insulate the sloped part of the bedroom ceiling start.

3.)  If I fill the space between the roof rafters above the closet with open cell foam, will I create any moisture issues for the roof deck (I’m in Texas so mild winters)?  Would I be better of using closed cell spray foam for the closet ceiling or creating a channel for air from the soffit vents before spraying the foam (even though there’s no air channel when you get to the sloped ceiling above the bedroom)?

Thank you in advance for any thoughts you can provide.