Luigi Zanasi - Window Design

The Galleria
Window Frame Design

While making the Galleria, I was faced with the problem of how to fit the window sash into the cedar post-and-beam timber frame. The intent was to fit the windows directly between the posts to maximize the light. While the posts were installed quite accurately at 37-3/16" on the long south wall and 36" on the two shorter side walls, shrinkage soon altered these dimensions, not to speak of slight twisting. I had dried the timbers for about a year before doing any of the joinery, but that obviously was not long enough. The criteria for the sash installation system were:

The sash had to be removable from the inside to clean them (Removable from outside was not an option as there is an acrylic greenhouse roof under the windows.);
Good weatherstripping;
Screens were essential during the summer - the amount of glass would turn the room into a sauna during the summer;
If the sash was removable (à la old-fashioned storm windows), a storage solution for the screens during the winter and the glazed sash during the summer was needed;
I had to use the available recycled double-paned sealed units I had available rather than use new (expensive) glass.

The traditional solution would have been to make double-hung window frames. I rejected that for a number of reasons:

Complicated to make, especially the fit between the two sash;
Difficult to seal/weatherstrip;
I would have had to make jambs, and the width of the jambs would have prevented me from using some of the glass I had;
To make the windows removable from the inside (for cleaning) would have involved the use of complicated hardware and made weatherstripping even more difficult.
Shrinkage and twisting of the timber frame would have meant individually and labouriously fitting each window.

The final solution

The basic solution is as follows:
Two sash per opening (the openings are about 3 feet by 6 feet, with some variation). 1" by 3" brickmold (casing/architrave) was screwed to the posts and the beam. The brickmould protrudes about 1" into the window opening. The sash press against the brickmold. The bottom sash slides into a slot made by a stop and the brickmold. The top sash is held against the brickmold by a stop along the top and a lip that hooks on the bottom sash. Storage for the unused sash is provided by another slot.

Brickmold (architrave) & Weatherstripping
The brickmold is 1" by 3" cedar, screwed into the vertical posts and the horizontal beam. The portion that protrudes is weatherstripped using 3/4" by 1/4" EPDM foam rubber. At first the windows were sticking to the rubber, but silicone spray fixed that. There is also a strip of EPDM foam rubber between the top and bottom sash and along the bottom of the lower sash so that there is no leakage between it and the sill.

Bottom sash
The bottom window sash slides into a slot created by the brickmould and a 1/2" by 3/4" vertical stop. The distance between the brickmold and the stop is 1-9/16" so the sash compresses the EPDM foam set on the brick mould. So the sash is held in place by its stiles (vertical pieces). The top and bottom sort of "float". As is standard practice, the bottom rail of the sash and the sill are bevelled so that water flows. Although the resolution on the picture does not show it, there is a drip slot under the sill.

Top sash
The top sash is held by the horizontal stop (screwed to the beam), the lip that hooks on the bottom window, and the vertical stop which protrudes about 1/2" beyond the top rail of the bottom window. There is no support along the vertical sides. To install the top sash, I push up on it until it clears the bottom sash top rail, push it in place and pull it down so it hooks on the bottom window. The top rail has a small bevel to make it easier to insert into the slot. I did have a problem at first. I had made the stop 3/4" high, which was not enough. The problem stemmed from three causes: the opening was originally 72-1/2", exactly what I needed for two sash exactly 36" high. But shrinkage and the bevel on the sill and bottom rail, as well as the eventual compression of the weatherstripping made one window fall out. Luckily, the glass did not break. So I had to make the horisontal stops bigger.

Storage
I have two sets of top windows: one with glass and the other with screens. So during the summer, the screen is on while the glass goes on during the rest of the year. The unused top sash fits behind the bottom sash. Originally, I had planned on putting 1n a second stop, as the drawing shows. Instead, I ended using screw-in brass shelf supports from Lee Valley. Four of them, two on each post hold the spare sash behind the bottom window. Looks much better than another stop.

For a better view (bigger & clearer) click here

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The background is western red cedar from Romeyn Hough's The American Woods: exhibited by actual specimens and with copious explanatory text.