Polyisocyanurate foam is a form of closed-cell insulation that you can spray in place or install as a prefabricated board. The effectiveness of polyiso insulation is rated using the R-value system. Polyiso foam boards typically have an R-value between 5.6 and 8 for each inch of insulation. The amount of insulation you will need and how it is installed, mainly depend on where it is installed. The two main methods used to install polyiso foam boards include mechanical fasteners and adhesive.
Polyiso insulation provides a relatively compact form of insulation that is mainly used in walls and concrete slabs where space is a factor. Exterior walls covered with polyiso insulation are designed to create an insulating sheathe around your home. This type of installation prevents thermal bridging, which allows heat to escape through the wall studs of your home. Interior polyiso wall insulation is typically installed in basements on your foundation walls. This type of installation protects your insulation from moisture and insect damage. Underneath concrete slabs it is intended to prevent heat loss through the floor. This type of installation is done when your home is built and is effective if you have a radiant floor heating system.
There are a broad variety of mechanical fasteners available for attaching rigid foam insulation to different surfaces. Some of the more common types are slender nails with a broad head or a series of push pins that mount to a backing. Mastic adhesive is another common mounting option that avoids puncturing the insulation. Adhesives are often used to mount polyiso boards to concrete walls to avoid drilling holes in the concrete wall for each fastener.
Polyiso insulation is most effective when it is installed as part of a continuous barrier. In exterior walls above the foundation, each board is attached to the exterior sheathe of the home with fasteners. The vertical gaps between each board are sealed using insulated vertical joints, sheathing tape or a mastic adhesive. Insulation installed on the interior or exterior of the foundation walls is typically mounted using a mastic adhesive applied to the wall. This insulation should extend into the ground to the frost line. The frost line is the average maximum depth that the ground freezes during the winter.
Windows in exterior walls can allow cold air to penetrate through your insulation if it is not installed properly. Space your polyiso boards so that a single insulating board will surround the window. Mount the polyiso board over the window and make smooth cuts to reveal the window cavity. Seal the edges of the insulation around the window cavity using flashing and a waterproof sealant.
The insulating boards in your wall should reach up to the underside of the rafters in your roof. If your home uses loose-fill insulation in the attic, you can add gaps for the rafters into your boards that will allow you to extend the polyiso boards into the attic. This will act as a soffit dam to stop wind from pushing your insulation away from the eaves and prevent the insulation from covering your soffit vents.
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