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Innie vs. Outie Sill Details for Thick Foam

June 12, 2015

The WRB is a BFD when choosing where to put the window

These architectural details are from Measure Guideline: Wood Window Repair, Rehabilitation, and Replacement by Peter Baker, PE.

They show how to integrate a new window into an old wall without introducing new water leaks or air leaks. In this case, four inches of foam was added to the outside; these details show how to set a window to the inside or the outside (innie or outie).

Either one is fine and each has its own charm.

  • An innie is easier to incorporate into an existing drainage plane behind the exterior insulation
  • An outie is easier to incorporate into a drainage plane in front of the exterior insulation.

So, the WRB is pretty much the decider.

Other considerations:

  • Innies set the windows into a recess, which protects windows from the weather.
  • Outies add deep window sills—great for plant shelves, cat perches, and bowling trophies.

What all of these details have in common is that there is a continuous air barrier, a continuous water barrier, and a continuous thermal barrier for the wall.

Don't know what that is? Take the Pen Test


  1. Building America Solutions Center's Complete Window Replacement Guide (16 pages, public domain)
  2. NREL's Measure Guideline on Wood Window Repair and Replacement by Peter Baker at Building Science Corporation (94 pages, public domain).

2012 IRC: N1102.3.6 (R402.3.6) Replacement fenestration

2012 IECC: R402.3 Fenestration (Prescriptive)


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