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Making Room for Roof Insulation

Whether you use trusses or rafters, creating space above exterior walls will improve energy performance

May 02, 2018
professional remodeler adding a space between exterior wall and roof for efficiency

In cold climates, a thick blanket of insulation in the attic keeps heat in conditioned space and helps prevent ice dams by keeping the roof cold. In warmer climates, attic insulation helps prevent the A/C unit from working overtime. Framing with standard rafters and trusses provides plenty of clear space for insulation in the middle of the span, but not so over the exterior walls. Here’s a look at ways to improve that situation. 

[1] With trusses, the space over the exterior wall is limited to the height of the bottom chord. With rafters, it’s limited to the height of a plumb line above the seat cut.

[2] Raised-heel trusses improve on standard truss designs by raising the top chord. The simplest solutions insert a 2x6 block or a short vertical web between the top and bottom chords.

[3] Gaining more height requires more engineering but can be done by adding a taller vertical web between the top and bottom chords over the exterior walls.

[4A] If you’re framing with rafters instead of trusses, you can still raise the heel to gain space for insulation. Instead of running ceiling joists next to rafters, build the ceiling frame as if it were a floor, and set a plate along the outer edge.

[4B] Now you can set rafters on the plate for a solid roof with plenty of room for insulation. With a little more attention to layout, you can eliminate the plate and align the rafters directly over the ceiling joists.

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How would you attach hurricane straps in this situation?

Hi Miller,
You can attach the hurricane straps on the rafters to the raised top plate. The assembly is structurally sound with the plywood spanning the framing members.

As ceiling joists are not directly tied to the rafters as it is "traditionally" done, what is preventing the rafters from spreading outward in this case? Only the nails driven through the rafters to the raised top plate?

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