Wet insulation does not insulate. Make sure there is a vapor barrier in place. A vapor barrier is a layer of moisture resistant material between the inside of the house and the insulation. The principle behind a vapor barrier is simple: Warm air holds more moisture than cold air, and warm air is always on the move, so as the warm air moves toward the cold space it cools and sheds moisture along the way. If it sheds its moisture inside your insulation then your insulation will get soggy, no longer provide insulation value, and possibly lead to mold buildup and rotting of structural members. The vapor barrier shields your insulation from moisture. Vapor barriers can be made out of many types of materials and come with a “vapor permeance value” called aperm rating" which should be no higher than “1”. There are several materials used as vapor barriers including plastic sheeting, foil sheeting and treated papers. You can purchase batt, blanket and rigid insulation with a vapor barrier already attached. A simple rule to apply so that you don’t install it backwards is that if you are looking at the insulation from the heated space (from inside the room) you should see the vapor barrier, but if you are looking at it from the other side such as from the attic, you should NOT see the vapor barrier.

Attic. If you are adding extra insulation on top of existing insulation in your attic then purchase insulation without the vapor barrier or peel off the barrier (make a slit down the middle with a knife and pull it away) because you don’t need or want a vapor barrier between layers of insulation. Insulation without a vapor barrier should be less expensive than with a vapor barrier. Install the added layer perpendicular to the first layer. If you have designed your attic to be well ventilated, however, with roof and soffit vents so that the attic has a continuous current of air flowing through it which can evacuate moisture that is shed from the warm air cooling in the insulation, then a vapor barrier in the attic is not crucial.

Walls. Ideally the inside cavity between your interior and exterior walls should be completely sealed, so applying this principle it is extremely important that a vapor barrier be used, and installed correctly (with the barrier toward the inside of the house, between the heated area and the insulation) to prevent moisture from building up inside the wall cavity which could lead to mold and rot. One technique is to insert unfaced insulation in the cavities between the studs and then attach a continuous layer of a vapor barrier material, like 6-mil (1 mil + 1/1000 of an inch) plastic sheeting, to the studs overlapping all the seams.

Ceilings with no attic or crawl space, like cathedral ceilings or Brownstone attics that have had the attics roofs raised just enough to make adequate ceiling heights but not enough to also allow for a crawl space for thick insulation (in NYC often due to limits placed by the Landmark’s Preservation Commission) should apply a vapor barrier in the manner described for walls: insert unfaced insulation in the cavities between the studs and then attach a continuous layer of a vapor barrier material, like plastic sheeting, to the studs overlapping all the seams.

Floors: Typically floors do not have vapor barriers because sub-flooring made of plywood or other sheeting materials usually are manufactured with some kind of waterproof substance that acts as a vapor barrier, however if you have a crawl space under your house or extension – typical in brownstones – put a vapor barrier in place on the ground under the house, such as a layer of 6-mil plastic laid directly on the dirt floor. Check back to our 168 Clinton St. blog to see how we deal with the sagging ground level floor of our extension where it seems the joists may have rotted.