Weatherization and Reduction of Condensation Build-Up onYour Windows
Minimize Condensation Build-Up Through Proper Re-Installation
Improve Your Home’s Insulation With Properly Restored Windows
Original windows that have been properly restored (including upgraded weather seals and operable storm windows) provide as much insulation as any replacement window. But what about the advantages of insulated glass units (IGU’s)?
It is true that two sets of glass provide more insulation than just one. But the degree of insulation is determined by the distance between the two layers of glass. While insulated glass units can have up to a 2” gap, units designed for residential buildings typically have just a ¼” to ⅜” gap. Original windows combined with storm windows, on the other hand, provide at least a ¾” pocket of air between the storm window and the top sash and anywhere from a 2 to 5 inch insulating pocket between the storm window and the primary window.
Another constraint of the insulated glass units is that the two units of glass are contained within the sash frame. Most air and moisture comes from the many gaps around the edges of the sashes. ISG’s do nothing to fill these gaps! We have devised a complex set of weather-sealing products which we retrofit original windows with to seal them as tight as possibe.
Bedding The Glazing Compound Seals The Gap Between The Glass & The Frame
These weather-sealing procedures begin with properly “bedding the glass.” This procedure entails applying a bead of glazing compound on the rabbet of the window sash into which the glass fits before installing the glass. The glass is then installed and pressed tight so that most of the glaze oozes out on the interior side. This excess glaze gets removed and the edge smoothed tight with the edge of the frame. What remains is a thin, gasket-like band of glaze sealing the gap between the glass and the wood frame. While we have already discussed the aesthetic value of this procedure (see aesthetic restoration of windows) it is significant to also note its insulation-value. This gasket-like seal may be less than a ¼” thick, but it covers every linear foot of the gap between the frame and the glass, thus contributing substantial insulation value.
This picture shows what proper bedding of glass looks like. Notably, we tint the bedding glaze to match the patina/stain color on sashes that will receive a clear finish so as to become nearly invisible.
Upgraded Weather Seals Close The Gap Between The Sash Frame & The Window Jamb.
We employ a variety of weather seals to ensure there is a tight seal around the entire perimeter of each sash, including the meeting rail (the place where the top and the bottom sash meet in a double hung window.
Here is one kind of weather seal that gets installed in the window jamb and interlocks with a kerf that has been cut in the side of the sash. But we don’t just install this weather seal and walk away! A significant source of air infiltration is caused by loose fitting sashes. This gap is the reason windows rattle in the wind. We correct this problem by shimming out the interlocking weather seal when necessary so as to create a tight seal between the sash and the jamb, leaving just enough room for the sashes to ride up and down freely but not enough room for air and moisture infiltration.
Compression Seals and Parting Bead Blocks Seal Gaps at the Meeting Rail.
Because the meeting rail (the place where the top and bottom sash in a double-hung window intersect) is the most vulnerable part of the window for air filtration, we employ a variety of seals to close the gap, including compression seals and, as needed, meeting rail blocks.
Compression Seal
Properly Installed, High Quality Sash Locks Enhance The Seal At The Meeting Rail.
We further enhance the seal at the meeting rail by precise positioning of the sash lock so as to draw the top and bottom sashes at the meeting rail tight. Because most original sash locks were of insufficient quality to achieve a tight seal, we recommend purchasing our traditional, pure brass sash lock that is designed for use on original windows.
Minimize Condensation Build-Up By Establishing Separate Temperature Zones Through Proper Re-Installation
Optimal weather-sealing of historic windows doesn’t stop at sealing the primary windows as tightly as possible. One also needs to have functioning storm windows that are properly installed. Perhaps surprisingly, hermetically sealed storm windows are not the gold standard for weatherization of historic windows. The ideal is to establish a gradation of three different temperature zones in window openings as follows:
- The preferred ambient temperature on the interior side of the primary window.
- A mixture of interior and exterior air temperatures in the space between the primary and storm windows.
- The outside temperature on the exterior of the storm window.
Maintaining these three separate temperature zones is how one keeps the build-up of condensation to a minimum while also providing maximum insulation.
Here’s what one client said about the effectiveness of our weatherization measures after having lived through the 2019 polar vortex in Chicago with a mixture of these original windows we had restored for them and replacement windows which a former owner had installed.
These original windows “…protected us from the Polar Vortex this past January. You’d think that original 1924 windows would leak enough cold air to give us tons of ice inside the house which is what was happening to people all over Chicago over those three days – new and old windows alike, but in the restored windows we didn’t see any of that – so we got original windows that look beautiful but work as well -or better – than some new modern windows.”
– Marnie Goodfriend Old Irving Park Neighborhood | Chicago