Construction Materials-Rigid Sheet Insulation

Today, we are going to have our first dedicated discussion about a specific building material, sheet insulation.  I originally had this blog named Foam Insulation but changed the name because not all sheet insulations are a type of foam, there are a few non-foam products, one that I will discuss last.

The most common foam insulation used in my area is extruded polystyrene, or XPS.  This is the colorful foam, usually pink, yellow or blue.  All the big box stores and lumber yards stock this type of insulation.  It is easy to work with, can be cut with a knife or saw and will hold an edge well.  This foam has a resistance to heat flow, or R-value of 5 per inch.  It is does not absorb water, making it a good insulation for below grades.  A higher compression strength may be required in areas where the foam is “sandwiched” between two solid objects, such as the ground and the concrete slab of a home.  XPS does have some disadvantages and limitations.  First, this type of foam is the least “green” choice of all the insulations I will be discussing today.  The materials used and methods to manufacture XPS can be harmful to the environment.

XPS insulation

Manufacturers in the United States have changed their production methods to make the product more “green” at an added cost.  Other parts of the world still use chemicals and processes in its manufacturing that are harmful to the environment.  There are recycling options for XPS but finding a recycler that will handle the product may be a challenge in some locations.  This foam has been known to shrink, which can create some air leakage and insulating problems in buildings.  It is best to install in two layers, lapping all joints.  XPS cannot be left exposed.  Sunlight and atmospheric pollutants can cause the foam to deteriorate.  None of the sheet insulations that will be discussed today are durable.  Damage caused by impacts can affect the thermal properties.  Pricewise, XPS is in the middle of all the sheet insulation choices.  At the time of this post, the price for XPS is around $.63 per square foot for one inch material.

Another readily available sheet insulation is expanded polystyrene, or EPS.  A cousin to XPS, expanded polystyrene is the white insulation.  EPS is easy to cut, but the edge after cutting tends to not be sharp.  A closer look at EPS shows the product is made from lots of little white beads.  After working with the product for a day, the little white bead tend be all over the jobsite, making it appear that it recently snowed.  Extruded polystyrene is commonly manufactured into building assemblies.  An example is structural insulated panel, or SIP panels.  This product has oriented strand board, or OSB bonded

EPS insulation

to both sides of the EPS and is used for wall and roof assemblies.  EPS is available in many thicknesses, from 1/4 to 54 inches, though 3/4 to 6 inches are more common as building insulation.  There are also different compression options available, making it another choice for below grade applications.  EPS may be recycled, but again, some recyclers may not handle the insulation.  EPS has an R-4.2 per inch insulating value.  Similar to XPS, EPS must also be protected after installation.  Another disadvantage of both EPS and XPS is the insulation is very toxic when burned.  EPS insulation is the cheapest sheet type available, under $.30 per square foot at one inch.

The third type of insulation is polyisocyanurate, or polyiso.  This type of insulation has a facing on both sides, one facing is typically reflective foil. Polyiso has the highest R-value of all the foam sheet insulations at R-6 to R-7 per inch.  The product usually comes in 4 x 8 sheets, but longer lengths are available.  The insulation has a resistance to burning, and has been used as an interior insulation, especially in retrofit and weatherization projects for years. There are also roof applications for polyiso.  The product cuts and holds an edge similar to XPS, but the foam core is brittle, and can be easily damaged.

Polyiso insulation

The down sides for polyiso is when used on the exterior of a building in cold climates, the insulating value of the foam is reduced.  Polyiso is not rated for direct contact with the ground, it also cannot be buried.  I suggest having a discussion with your local building official if using polyiso as an interior insulation.  There may be restrictions in its use depending on the insulation manufacturer, installation location, and whether or not the product is covered with a finishing material. There are no recycling options with this type of insulation, but there is an option of purchasing used polyiso that has been salvaged from buildings that have been remodeled or re-roofed.  The cost for polyiso is currently between XPS and EPS, around $.50 per square foot for one inch thick insulation.

The last sheet insulation I am going to discuss is one I have not personally used.  Rockboard is a stone wool insulation board that can be used above and below grade, and even on some types of roofs.  It is manufactured from rock and slag that is turned into a fibrous material.  The product does not burn and has a very high melting point.  I was recently at the International Builders Show where Rockwool, the manufacturer of Rockboard, had a display with a torch burning on the surface of the insulation.  The only damage was the

Rockboard insulation

insulation blackened at the point of the flame. The product also is resistant to moisture.  The R-value is around R-4 per inch with thicknesses available between one inch and 4 inches. Because of its weight, Rockboard comes in 2 x 4 panels.  This insulation appears to be the most expensive of the 4 choices.

Most of the insulations I have discussed are commonly available.  There are a few additional sheet insulations that are on the market, such as rigid fiberglass and wood fiberboard insulations, that are more difficult to come by.

If I were building a new home today, and looking into the different insulation types, I would choose EPS because of its cost compared to R-value.  If money wasn’t an factor, I would use Rockboard because of it’s burn characteristics.

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