Windows in Double Wall Construction

By Michael Goetinck

There is a lot of information about window performance and how it can affect the R-value of the wall assembly, but it boils down to a few things:

1. As soon as you add windows to a wall assembly the performance normally goes down.
2. The more windows you add, the lower the performance of the assembly.
3. The higher the performance of the window the higher the performance of the assembly.

This article will focus on considerations of where to place windows the within the double wall assembly. There are three choices:

1. All the way to the exterior.
2. All the way to the interior.
3. Somewhere in the middle.

Where you decide to place the windows will depend on how much “weight” you give to each of the variables. In addition to performance there are aesthetic choices and ease of installation (or in the case of retro-fits; the ease or difficulty of moving the existing windows).

Installing windows all the way to exterior is the most common method and most windows are designed to be installed in this fashion. The windows will look “right” from the outside and the flashing and exterior trim details are familiar to most builders. Most people like the deep window sill on the inside especially if the sides of the openings are flared to an angle of 45°. The easiest way to handle the interior trim is to use drywall returns to create the flared extension jambs, although in some situations the flares need to be made from paint- grade plywood in order to attach the window stops. A head jamb that blends into the wall above it and a wood sill makes for a very nice finished look.

Installing the windows all the way to the interior preserves the familiar interior trim details of wood casing. The exterior details include a deeper drainage pan and wider exterior trim. This placement is useful if you want to protect the windows from the elements. I would consider flaring the exterior opening especially if you want to let the most amount of light come into the room.

Installing the windows in the middle gives you the benefits of both of the previous options. It also allows you to increase the thermal performance of the window units by adding rigid insulation to both sides of the unit frame which will reduce thermal bridging across the window frame. Additional advantages are that the windows are more protected from the elements because they are recessed into the wall assembly and you still get the opportunity to have a deeper window sill and flared interior sides. A good rule of thumb is to install the units a third of the way in from the exterior. More time will be necessary to install the window pan, the wider exterior trim, and the interior details of the flared sides and deeper sill.

Michael Goetinck is the owner of Snowdog Construction, Ltd, in Norwich, VT. This series will continue in Green Energy Times, where the author will cover other topics that can help achieve the deep energy measures which help buildings’ energy performance, and so can benefit us all.