Low slope Roofs generally range from almost flat to 4-over-12. Deep trusses or beams that are separated from the roof joists are used to frame these roofs. Since the area between the ceiling joists and the sheathing is limited, it can be challenging to insulate a low slope roof properly.
While some builders use the same insulation methods that they would use for a big attic, they should have different tactics. For a vented roof, it is important that it is unvented first to protect the attic from the moisture-laden air. An unvented roof can be efficiently insulated in numerous ways from the interior or exterior or both.
Fiberglass or Cellulose
When it comes to low sloped roofs, ventilation does not work in the same way as it does in steeply sloped roofs, as the air does not travel from the eaves towards the top. This is why ridge or turbine vents are useless when it comes to low slope roof systems.
If there is a space of at least 18 inches between the roof sheathing and the top-floor ceiling, fiberglass or cellulose can be used for insulation and ventilation. This system only works if an adequately sized cupola is included in the middle of the roof.
Closed-cell foam and Vapor-permeable insulation
It is much easier to insulate unvented roofs and there are a few ways you can do it:
- Exterior rigid foam: A thick layer, that is minimum 6 inches, of rigid-foam, is installed on top of the roof sheathing
- Exterior rigid foam paired with interior insulation: A normal layer, about 2-4 inches, of rigid-foam is installed on top of the roof sheathing and that is accompanied with an air-permeable interior insulation (for e.g. fiberglass or cellulose) below the roof sheathing.
- Exterior closed-cell foam: Closed-cell polyurethane foam is sprayed over the roof sheathing and is supplemented by an air-permeable insulation underneath the roof sheathing.
- Interior closed-cell foam: Closed-cell polyurethane foam is sprayed on the bottom side of the roof sheathing
- Interior closed-cell foam pair with an air-permeable insulation: Closed-cell polyurethane foam is sprayed on the bottom side of the roof sheathing and another layer of interior insulation, like blown-in fiberglass or cellulose, touching the cured foam, supplements this insulation.
Note: It is important to make sure that the foam used is adequately thick and can prevent the roof sheathing from dropping below the dew point in winters when installing both closed-cell foam and vapor-permeable insulation together.
Out of the several ways you can insulate a shallow roof, it is always easier to insulate the exterior. No matter which method is used, it is important that the insulation amount achieves the R-values required by the local code.