Minimum Clearance Between Roof Insulation and Cross Purlins

Condensation

• Purlins are horizontal structural beams that cross vertical beams to create a roof's skeletal structure. Exterior roofing material is affixed to purlins. Because purlins are near the roof's exterior, they require insulation to prevent heat loss. Clearance between insulation and cross purlins is necessary to facilitate adequate ventilation. Damaged roof material allows moisture to enter into the space between cross purlins and insulation. Moisture and condensation freeze during cool weather. The expansion and contraction of freezing and thawing water particles damages insulation and can cause pressure shifts that will damage a building's structural integrity. Provide as little as 1 1/2 inch to 3 inches of clearance between insulation and cross purlins. Consult with local building codes for exact clearance recommendations for your area.

Building Envelope

• The term "building envelope" refers to a structure's seal against outdoor climate changes and its ability to retain stable heat or coolness. Window cracks, loose seals, clogged vents and poorly installed or maintained insulation compromise a building's envelope. Before installing roof insulation be certain to provide a vapor seal between the insulation and the building structure. Vapor seals prevent moisture from entering insulation material. Condensation limits insulation's efficacy and encourages the growth of mold during warm seasons. Vapor seals, such as foil wrapped blanket insulation, ensure that condensation remains outside the insulation. When kept outside insulation condensation can be removed from interstitial space by proper ventilation.

Insulating Materials and Installation

• Batting and loose-fill are two types of insulating materials used in attics and in other building areas. Batting is a single, matted piece of fiberglass or rockwool that is placed in sheets within wall cavities and other spaces. Cut slits into batting to allow space for ventilation ducts, wires or plumbing. Install blocks around heat-emitting or electrical fixtures to keep batting from coming into contact with these potentially flammable areas. Blocks are also installed around cross purlins to allow for adequate ventilation between cross purlins and insulation material. Loose-fill insulation is made of the same materials as batting but rather than forming a matted sheet, loose-fill is comprised of separate particles that are blown into prepared spaces. Again, blocks are placed between insulation and fixtures to allow for ventilation space.

Considerations

• Energy auditors and contractors are aware of regional codes. Consult with these professionals prior to installing insulation.