Insulation, Radiant and Vapor Barriers - vapor barriers - XPS and kraft
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econguy
11-15-06, 05:59 PM
hi
i recently insulated a few rooms of a fully dormered second floor. part of the ceiling is sloped because it's actually the roof rafters, and the remainder of the ceiling is flat, because it's made from the collar ties. i had no problems insulating the flat ceiling to R-49, but had a few questions about how i insulated the sloped portion.
the rafters are only 2x6, so my process was:
- staple ventilation baffles to the underside of the roof
- install kraft-faced R-21 fiberglass insulation in the rafter bays
- attach 2" extruded polystyrene (XPS) foam board next to the kraft-facing, which added R-10
- attach 1x3 strapping for the drywall next to the XPS, with 3.5" screws driven through the strapping and XPS and into the rafters
i stapled the R-21 flanges to the 1.5" side of the rafters, so when i attached the XPS, they were in direct contact with each other.
my question is, since the XPS and kraft-facing are both considered vapor retarders, will they trap moisture? or since they're fitted tightly against each other, will they just act as one vapor retarder?
thanks
i recently insulated a few rooms of a fully dormered second floor. part of the ceiling is sloped because it's actually the roof rafters, and the remainder of the ceiling is flat, because it's made from the collar ties. i had no problems insulating the flat ceiling to R-49, but had a few questions about how i insulated the sloped portion.
the rafters are only 2x6, so my process was:
- staple ventilation baffles to the underside of the roof
- install kraft-faced R-21 fiberglass insulation in the rafter bays
- attach 2" extruded polystyrene (XPS) foam board next to the kraft-facing, which added R-10
- attach 1x3 strapping for the drywall next to the XPS, with 3.5" screws driven through the strapping and XPS and into the rafters
i stapled the R-21 flanges to the 1.5" side of the rafters, so when i attached the XPS, they were in direct contact with each other.
my question is, since the XPS and kraft-facing are both considered vapor retarders, will they trap moisture? or since they're fitted tightly against each other, will they just act as one vapor retarder?
thanks