Understanding Roof Insulation Methods
A reliable energy-efficient roofing system needs a solid roof membrane, in many cases a vapor barrier, and beneath that an insulation suitable for space, pitch, and material needs. The purpose of insulation is to provide a barrier between the exterior and interior of a building to limit the thermal exchange between the two spaces. This is a crucial consideration for energy efficiency in areas that experience extremes of hot or cold.
When undertaking a roofing insulation project, there are a number of things to consider before you choose materials and begin installation:
- Thermal Resistance is measured in R-Value with higher numbers representing greater degrees of insulation. If you choose an R-value that is too low, you will add little energy-saving benefit compared to cost. However, more is not always better. Too much insulation can actually increase thermal shock on roof membranes and lead to more rapid deterioration. When choosing your insulative material, you may also want to keep in mind the fact that R-values decrease over time as insulation degrades.
- Water penetration is the bane of all building owners, and the first line of defense is, of course, the roof membrane. But insulation is your second layer of protection. Wet insulation not only spreads moisture away from the point of entry so that leaks are hard to find, but some types quickly lose their insulative effectiveness when wet. Depending on the type of roof and the climate in your area, you may want to consider insulation products that offer more resistance, such as composite materials or nonabsorbent materials, such as certain polystyrenes.
- Fire resistance is another major consideration in your choice of roofing insulation. Materials that have been properly fire-tested and certified can both help prevent fires and limit flame damage and also add to the life of your roof. Look for materials that have a UL Class A fire-resistance rating or FM Class 1 rating.
Once you have a sense of the characteristics you need for your particular roof, you can begin choosing insulation materials.
Types of Roofing Insulation
As you begin shopping for insulation, understand that materials are categorized as a rigid board, non-rigid, and sprayed polyurethane foams. Each has its pros and cons that you will have to consider as you choose.
Rigid boards, or board stock insulation, are thin but sturdy layers of insulation applied between the roof membrane and the primary insulative source. They provide an even surface for membrane application, improve fire resistance, create compressive strength to reduce foot traffic wear and tear and impact damage, resist wind uplift (which is important in high-wind areas) and prevent blistering and shrinkage.
Perlite boards, which are often used as cover boards, have a low R-value and often lose compressive strength when wet. Because they increase the likelihood of blistering when used with hot asphalt, they are used less than other materials.
Polyisocyanurate is a form of foam insulation often used in low slope roofs. It offers higher R-values and is more fire-resistant than polystyrene. It, however, absorbs water where leaks occur and must be replaced, whereas polystyrene can be reused after moisture exposure.
Composite boards present two layers of different insulation combining a rigid material such as perlite, plywood, or gypsum with a foam-based material, such as polyisocyanurate or other polystyrene materials.
You can also find boards offered of two types of polystyrene material. Expanded polystyrene is a low-density material with medium R-values that doesn’t thermally age. It is, however, more prone to water damage and requires a cover board, particularly when used with hot asphalt or solvent-based adhesives. Extruded polystyrene is much more water-resistant and cheaper to replace than many other boards, but it does come with the caveat that it shouldn’t be used in areas exposed to extremely high temperatures or where hot asphalt or solvents are used.
Finally, there are the insulations that are used beneath board covering. Batt and blanket insulations are precut bundles or rolled bundles, respectively, that are usually made of fiberglass. These materials are cut to fit and often laid out in attic spaces. You can also purchase both styles in mineral wool or cellulose. Sprayed polyurethane foam is often applied to the underside of roof decks where it adheres and acts as an insulator.
Now that you have a handle on the basics of insulating your roof, it’s time to put up the scaffolds, pull out your tools, and start laying a roof that you know will offer you water resistance, energy efficiency, and the security of knowing it will give you the particular protections you need for your area.