Top and Bottom Rails - Compression Seals and Flats
Composed of open-celled foam and a polyethylene liner which provides maximum long-term performance with low compression force, and resistance against compression set. Retains its original shape year after year to maintain an excellent seal.
There are several ways to approach making seals at these two location, each with different advantages and drawbacks. The system above is our favorite for a few reasons. This solution does not require milling seal reliefs in the sash. It also complies with our "Same Plane" philosophy. One drawback is the potential for moisture to be trapped between the bottom bulbs if the window is left open during a rain. That however, poses no problem for the Retainer or Bulb Seals since they are waterproof. Another drawback is that when the sash is fully closed, a small fraction of the Bulb Seal is visible from the exterior under the rail. The solution to the former challenge is to make a few small relief cuts in the exterior Bulb Seal. This will let any moisture drain away, while still minimizing air-infiltration. The latter challenge, visibility of the Bulb Seal at the sill, is not really an issue with a sash and sill with a white exterior. If the exterior is to be painted a dark color, it might be advisable to consider the alternative on the right.
Yet one more advantage of the Bulb Seals and Retainers is that it requires 1/4" to be removed from either the top and bottom rails or 1/2" from just the bottom. This sounds like a drawback but more often than not the bottom of the bottom rail is deteriorated and needs to be removed anyway to get to solid wood. The thickness of the retainers (1/2" total) make up for any height lost to this removal
This method uses the same seals that are used at a the meeting rails. An advantage is that they can be positioned to be completely hidden when the window is closed. Another is that there is no opportunity to trap moisture between bulbs if the window is left open. The disadvantages are that it a) requires more milling, b) if the seals are close enough to the edges for the "Same Plane" sealing philosophy to work, either the seal is then exposed or the edge of the relief is so thin it might break.
All things considered, utilizing option 1 - the Top and Bottom Weather-stripping retainers, is our method of choice.