Insulating A Vented Roof

Vented roofing the typical vented roof has a vent at the lower eave and a vent at the higher.

Insulating a vented roof.

A layer of spray polyurethane foam can be installed against the underside of the roof sheathing with the rest of the rafter cavity filled with air permeable insulation. Heat from the house rises up to the attic but when there isn t one the heat rises up to the roof. You must meet the same minimum code requirements for the spray foam layer and the total assembly as for the rigid foam plus air permeable insulation approach. All insulation materials require proper venting.

While fiberglass insulation is an excellent and effective product for insulating most building cavities in areas where there is extra risk of trapping moisture and thus rot or mold infections such as crawl spaces and cathedral ceilings where roof venting may be absent or minimal we prefer to use closed cell foam insulation products or spray in icynene foam insulation. Air can move through the attic space freely and the floor of the attic is insulated to thermally separate the unconditioned attic from the living spaces below. Then insulate below with fiberglass or cellulose add a vapor retarder and your finish ceiling. When hvac equipment is in the attic insulation and an air barrier at the roof line brings that equipment within the conditioned enclosure envelope thereby allowing it to operate in more favorable temperature and humidity conditions without air leakage and heat loss gain directly to the exterior.

Spray foam with air permeable insulation. Because of limited space between the roof sheathing and ceiling joists a low slope shed roof can be difficult to insulate properly. Foam insulation can play an important role in both vented and un vented roof assemblies. In this energy smart homes article senior editor martin holladay explains how to insulate a vented low slope shed roof using a combination of intake vents and cupolas.

That is create a vent channel above the insulation with low and high vents. Cathedral roofs especially need a lot more insulation between the ceiling and the rafters because there usually isn t any attic space. Covering up the soffit vents with loose fill or batts which can happen if you stuff insulation along the eaves is a huge no no. For the same reason insulation shouldn t touch the roof s underside.

The airflow from the soffits to the ridge vent keeps the roof cool and prevents ice dams and the material will block that flow. In an un vented roof foam can serve as the air barrier and vapor retarder and prevent moisture from getting in to the roof cavities.