In many existing buildings, the only economical way to improve the thermal resistance of the roof is by having insulation installation against the underside of the roof surface. This is a popular term by name of “under roof insulation”. This method has a number of major advantages, but one potentially disastrous disadvantage.
You can use various types of insulation underneath roof decks, including boards, batts, and spray-applied materials. Each type of material and each method of insulation installation involves its own set of details. These details are important. Follow the manufacturer’s instructions carefully. This measure covers the key issues for all methods of insulating underneath roof surfaces. Make sure that each of these issues is covered in adequate detail by the manufacturer’s instructions, and that the installer pays attention to all of them. The proper under roof insulation installation surely will conserve energy in your home.
As with all roof deck insulation, this method should not be used if there is a vented attic space below the roof.
Advantages of under roof insulation installation
This method has several significant advantages:
• Ease of installation. In many buildings, the underside of the roof is the only reasonably accessible surface for insulation installation.
• Unrestricted thickness. There is typically plenty of space to add as much insulation thickness as you want.
• Low cost. Cost is relatively low because of the ease of insulation installation, and because it is not necessary to remove and replace other roof structure and components.
• Longevity. Under roof insulation is well protected from sunlight, weather, and impact.
Risk of Moisture Damage
The overwhelming disadvantage of this method is the risk of serious moisture damage if the building is located in a climate where the outside temperature can be much colder than the inside temperature. In such locations, the under roof insulation must have an effective vapor barrier on its inner surface. If the outer roof surface is also impermeable, a region is created between the outer roof surface and the vapor barrier that can trap and hold moisture. Persistent moisture will eventually destroy any type of roof structure, and may even cause roof structure collapse. You may not be able to provide drains and/or vents that are adequate to get rid of trapped moisture.
Because of this potential problem, this method of insulation is safe only if you can design a reliable moisture venting for the space above the insulation, or if the outside air dew-point temperature is always higher than the inside temperature.
This method under roof insulation is least likely to have trouble in climates that are warm and dry, where the insulation is used primarily to reduce cooling load and conserve energy in home.
How to Avoid Moisture Damage and Roof Leak
Insulation installed underneath the roof is much better protected from precipitation than insulation installation on top of the roof. However, even under roof insulation is vulnerable to water damage from roof leak. Expect any leaking roof occasionally during its life. Moisture inside the insulation may do more damage to the roof structure than to the insulation itself.
The best protection against roof leak is to use insulation that is highly permeable, such as glass fiber board. This allows any moisture to evaporate freely and escape through the vents.
If you decide to install under roof insulation in a colder climate, you face a serious challenge in avoiding moisture damage. In that case, two things are needed:
• A very reliable vapor barrier. You must keep water vapor from entering the insulation from the inside of the building. Use a very effective, reliable vapor barrier, such as thick plastic film with overlapped, taped joints. If you cannot be sure of the vapor barrier, do not use under roof insulation. The exception is climates that are warm and dry, where it may be desirable to omit the vapor barrier.
• A path to vent any accumulated moisture to the outside. Aside from the issue of roof leak, no vapor barrier can completely block the flow of water vapor into the insulated space. The installation must provide a path for water vapor to vent to the outside before it reaches a concentration that allows it to condense. In a cold climate, the insulated space must vent to the outside of the building, not to the inside.
Adequate venting to the outside is not possible with typical commercial flat roofs that use an impermeable outer membrane. The best case occurs with a highly permeable outer roof surface, such as tiles on a sloped roof. However, standard practice involves laying an impermeable base under the tile. Even if an impermeable base does not presently exist, one may be installed later during a renovation. Adequate venting may occur with a corrugated or channeled sheet metal roof, in which the channels act as vapor vents. This requires keeping the channels open to the outside at the top and bottom. But again, an impermeable underlayment under the sheet metal will block adequate venting.
