Q: Working in my attic on a project, I checked my insulation, and found it to be R30 unfaced … and there is no vapor barrier, paper or plastic.
Everything I have read indicates that attic insulation should be faced to prevent moisture moving from the house into the attic. To correct the problem, I thought of two options.
One option is to buy faced roll insulation, pull the old insulation aside, install the faced insulation (faced side down) and then put the old insulation back on top of the new insulation (with total depth for R38). A second option is to buy plastic membrane (6 mil), pull the old insulation aside, install the plastic between the studs, and then put the old insulation back on top of the plastic. And then I’d add new unfaced insulation for an R38 depth.
I have not noticed any mold in the insulation or on the underside of the roof. I’d appreciate your input for the best option to use.
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A: Moist, warm air generated in the home can travel through the ceilings and walls to the colder outside areas. Moisture travels from warm air to colder air and from a higher humidity or moisture content to a lower content. In cold climates, having a vapor retarder on the attic side of the ceiling may help to control the moisture flow. I say may help because it is almost impossible to stop all air/moisture flow to the attic.
When the warm, moist air reaches the cold attic space, it can form ice crystals on the underside of the roof sheathing. When the attic begins to warm, the ice crystals turn into a liquid, which can cause decay and/or mold to develop. Since the moisture/air flow can travel through ceilings, you need to make sure the attic is well ventilated.
In mild to warmer climates, there is no need for a ceiling vapor retarder. The moisture that reaches the attic continues to rise through the attic, escaping through the roof’s venting system. If you have not discovered moisture problems in the attic and you live in a moderate to warm climate, you can simply apply another layer of unfaced insulation over the existing insulation.
Layer the new insulation perpendicular to the existing layers to slow airflow through the insulation. Another option may be to have blown-in insulation installed over the batt insulation. Consult with your local home store, building inspector or qualified insulator if you are unsure whether or not you need a ceiling vapor retarder.
If you do live in a colder climate, I would not attempt to install a vapor barrier such as plastic on the ceiling in the attic. A vapor barrier blocks all moisture transfer, whereas a vapor retarder allows a minimal amount of moisture transfer. There are different classes of vapor retarders, and the colder the climate, the more vapor-impermeable vapor retarder you may need.
If you are detecting moisture problems with your home, you may have insufficient attic ventilation or you may consider adding a vapor retarder. I would first have the attic’s ventilation evaluated by a qualified contractor. If you do add the vapor retarder, I suggest removing the existing insulation, applying a layer of Kraft faced insulation with the paper facing the inside of the home and then reinstalling the old insulation perpendicular to the new insulation.