My garage is attached to my house and I built it with attic trusses. After installing the metal roofing And doing all the sheathing for a ridge vent, we slapped on a ridge cap during a snowstorm an let it sit for the winter. Now spring is here, If I am doing spray foam insulation, do I need to vent? I am using 16' long by 3' wide "pro-rib" roofing. should there be enough air movement in the ribs of the roofing? The current plan is to make the attic area above the garage a den. the framing is 2x6 so i am concerned that the venting spacers will reduce the insulation value alot. I have seen many cathedral ceilings spray foamed to the roof sheathing but will this reduce the life of the sheathing?
Apr 08, 2013, 12:40 PM
you need air inlets to the attic low, and the ridge vent high. build some framing along there the depth of the foam you're spraying, and temporarily cover the air movement area with plastic. tell the foamers to leave those spots, and get the rest.
the foam insulation mixtures commonly in use now are water-tight and gluey, so they will not let water vapor to the roof panels... no little rivers dripping at the walls of the garage. strip the plastic when the insulators clear out, and you should be fine with airflow. granted, the 'Nard panels are warranteed, but better for all if water doesn't touch them inside.
sig: if this is a new economy, how come they still want my old-fashioned money?
Apr 09, 2013, 07:22 PM
It sounds to me like you are thinking I am still building. the roof is on. the soffit venting is in just the ridge cap hasn't been installed yet. there is a solid one on there now. Any insulation would be done from inside the attic space. I will be using the "chutes" you buy from the home improvement store and running them up from the soffit area all the way up to the ridge cap. I wasn't sure if you still hed to vent this way with the spray foam, but it looks like I will. Thanks for the reply!
Apr 09, 2013, 08:43 PM
Closed cell foam does not require venting. In fact you create a envelope by blocking off the soffit area. The reason for the venting is to allow cold outside air between the batt insulation and the roof so it will not sweat. But with closed cell you are insulating the underneath of the roof so that the warm air will not come into contact with the roof to prevent that temperature change and eliminate that sweating. You will need to apply enough foam to achieve the proper insulation or R-value.