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MI EnergyCore Double Hung Window Sill Design: Pocket Sill or Sloped Sill?

There is an age old debate in the window marketplace over whether a pocket sill or a sloped sill design is superior for a double hung window.

The MI EnergyCore double hung window was designed from the ground up using state of the art computer aided design software, finite element analysis, and thermal analysis. Design considerations like water exfiltration were planned and accounted for to make this one of the soundest window systems available on the market today.

The  window uses a hybrid sill design, where the sill has exterior sloped drainage in addition to a taller water column height in the pocket. The weep holes (as shown in the attached image) are only .015” off the bottom wall, so as long as the window is installed properly (level) there should be little to no water left in the sill, and any minute amounts there may be should evaporate with the minimal air movement there will be in the sill.

One of the advantages of the hybrid design is that the bottom sash is trapped in a pocket so that negative load is optimized. In fact, the sill design received a DP 65 (9.82 psf) on water with an overall structural rating of DP40 or DP50 depending on the size of the unit. The air infiltration (AI) value is .09 psf which is 2/3 less than the AAMA allowed .30 psf. Another advantage is that the windows are more secure because the bottom sash is not exposed like it is in standard sloped sill design.

While some window manufacturers use the same frame material to minimize extrusion expense, the MI EnergyCore window's frame has a dedicated hybrid sill extrusion that was designed to take advantage of the benefits of both a sloped and pocket sill. To fabricate the dissimilar frame components to exacting specifications, MI Windows and Doors uses an advanced computer controlled welder and a 4 head, 12 tool CNC corner cleaner.

The MI EnergyCore Window System’s unique design results in a more energy efficient, more secure, stronger and quieter window with traditional style.  To see how the windows are made using state of the art processes, see our Video Library and our How It's Made Video.

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