GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted
Building Science

Detailing a Bonus Space in a High-Performance Home

A room above an unconditioned garage and below a vented attic in a cold climate requires special considerations

With today’s high cost of living and building, it is important to maximize a house’s every square foot. As a result, many of our clients want to take advantage of a “bonus space” over the garage. Not all designs suit this type of construction, but many do.

The space over a garage can provide flexibility to be an office, game room, guest suite, or even a self-sufficient VRBO or Airbnb rental. In terms of its construction, some builders say, “Just use attic trusses, install sheets of drywall, paint, and voilà, it’s done.” Sounds easy, and couldn’t cost more than a few bucks, right? Well, not exactly, but adding functional square footage without increasing the size of the home’s footprint is an attractive proposition. It just needs to be done right, and in a high-performance home, some additional detailing is required.

Insulating below a vented roof

Attic trusses are engineered to allow for an open, buildable space within the chords of the truss. Sometimes the floor system is built over the top of a beefed-up (say, 11-7/8-in. LVL) bottom chord; other times an independent floor system is framed. (These spaces can be a great place for a dormer to take advantage of a view from the second level.) It is important to remember that like the rest of the house, this roof-attic space requires venting—unless you plan to use a hot-roof, meaning unvented, strategy, which comes with its own set of pros/cons and cost implications.

Spray foam at baffles
Closed-cell spray foam over cardboard baffles at 45-degree-angle “pinch point”

On a recent project, the truss chord separating the interior space from the vented attic was a 2×4 vertical member. This member extended vertically from the floor level up to 7 ft., where it turned 45 degrees and…

GBA Prime

This article is only available to GBA Prime Members

Sign up for a free trial and get instant access to this article as well as GBA’s complete library of premium articles and construction details.

Start Free Trial


  1. charlie_sullivan | | #1

    There are now more types of XPS than there used to be, some of which are still major offenders in greenhouse gas emissions, and some of which are much, much better. None of the OK ones are blue. Read more at

  2. Expert Member

    I'm interested in how the air-sealing at the awkward junction of the walls and floor/ceiling is kept continuous.

  3. purpleconeflower | | #3

    I’m curious what was used to insulate the ceiling of the unconditioned garage/bonus room floor.

Log in or become a member to post a comment.



Recent Questions and Replies

  • |
  • |
  • |
  • |