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Ideal position of main window and storm windows in a thick wall – GreenBuildingAdvisor
I own a 1927 craftsman bungalow house in climate zone 4a. I’m in the process of designing an energy retrofit for the house, which will include adding 6 to 8 inches of extra insulation to the exterior walls. The existing exterior walls will still be there, so the resulting total width will be over 12 inches. The main walls are currently uninsulated, and I may leave them that way, and rely only on the exterior insulation. Maybe. We’ll see. The construction of these walls is as follows:
- three-coat stucco over metal lath and tar paper on the outside
- 1×6 diagonal wood sheathing
- 2×4 studs (old growth)
- traditional wood lath and plaster on the inside
The exterior insulation will be added to all of this, which means that the windows will either need to be very inset, or I’ll need to move them.
I am going to keep the original wooden windows, with their lovely mouth-blown wavy glass, for historical appearance. Currently I have storm windows on both the inside and outside:
- original wooden double-hung windows in very good condition.
- interior Indows brand storm windows (acrylic window with a compression-fit rubber/plastic? frame)
- exterior storm window with glass window and wood frame
The windows actually do a good job of insulating; better than the walls, as a matter of fact! (according to FLIR thermal images I’ve taken). The walls will eventually perform better than the windows, but I’m still interested in getting the best performance out of the windows, within this remodeling effort.
The main question is whether I should move the original windows to the outside wall, or place them in the middle of the thick wall, or leave them where they currently are, which is aligned with the interior wall. Currently the exterior storm windows are aligned flush with the outside of the exterior wall.
I’ve seen energy modeling that shows that putting windows in the center of the wall is more energy-efficient than flush with the inside wall or flush with the outside wall.
The easiest thing to do would be to keep the original windows exactly where they are, and keep the all the storm windows exactly where they are, and create inset windows.
The second easiest thing would be to keep the main windows where they are and move the storm windows to the exterior. That would create a wide gap between the window and the storm window, which would create essentially a greenhouse-like space. That may be a terrible idea… or maybe it may not be that big of a deal, because I actually plan to put old-fashioned retractable window awnings over the windows during the summer, which would severely limit the amount of heat captured through solar radiation. The solar heat gain coefficient would be pretty low with the awnings, because the windows wouldn’t get much direct sun. They’d just convey the ambient heat.
The most difficult option would be to move the original windows, either to the middle of the wall, or to the outside of the wall. Either way is probably about the same amount of effort.
Aside from the amount of effort required, I’m curious: which would actually be the “best” configuration?