Volume 11, Issue 6 - July/August 2010

Eye On Energy

Avoiding a “Frame-Ache”
Cavity Fillings Enhance Thermal Performance
by Ric Jackson

If you have a cavity, you go to the dentist to have it filled. That filling protects your tooth from harsh elements and insulates it from temperature changes in your mouth. In the realm of window frames, however, a cavity may be desirable.

The design of these airspace cavities can impact the thermal performance of the window significantly. Larger airspaces permit a certain amount of convection to reduce the efficiency of the framing system. To minimize this transfer of heat, manufacturers may turn to insulating framing systems with filled cavities. Just as dental fillings provide insulating properties, so does insulation within a frame profile.

Despite open cavities within their profiles, metal frames are highly conductive, which makes it difficult to significantly improve their thermal performance. Even with thermal breaks added between the frame and sash, metal-framed windows are very susceptible to heat transfer.

Wood frames offer better insulating properties than metal, but little can be done to improve their thermal performance. The density and conductivity of the wood essentially pre-determines the thermal performance of the frame.

Such cavities exist in framing systems made from vinyl, fiberglass and engineered thermoplastics, giving manufacturers a greater ability to affect thermal performance through the frame design. The cavities provide insulating properties that enable the windows to perform better than those featuring metal or wood frames. Plus, these cavities can be filled to further enhance energy efficiency. With that in mind, let’s examine three primary options for filling vinyl window frames.

Use of Polystyrene
Manufacturers can insert blocks of expanded polystyrene into frame cavities to gain up to a 0.03 U-value improvement in a typical double-hung window compared to the same hollow-framed window. Manufacturers must cut the material or mill it down to profiles that will fit inside the frame cavity. Due to manufacturing variances, the frame cavity will not always be 100 percent smooth, causing potential difficulties and hang-ups when inserting polystyrene blocks. Because the inserts are blocks of solid material, they may hinder water drainage within the frame.

Foam-Filling
Foam-filled frames can provide up to a 0.03 U-value improvement over hollow frames. After extruding the frame, manufacturers spray a liquid polyurethane mixture into the frame cavity. The mixture expands and solidifies into densely packed foam with high insulating properties. However, the process carries the risk of inconsistent filling, as areas within the frame cavity may remain unfilled. Manufacturers also run the risk of overfilling the cavity, which may distort the frame profile. Because the foam fills the entire cavity, manufacturers must engineer channels into the frame design to permit water drainage.

Co-Extruded Fillers
To achieve up to a 0.04 U-value improvement or more than hollow vinyl frames, manufacturers may consider co-extruded fillers like integral insulated air-cell cores, which feature a honeycomb design of alternating solid and hollow areas. These systems are extruded in conjunction with the frame to ensure proper placement within the cavity and reduce secondary manufacturing steps. Compared to polystyrene inserts and polyurethane foam, coextruded fillers provide more insulating airspace within the frame cavity. There is no concern about increased convection currents as the airspaces are very small. In addition, during corner welding, the coextruded core is welded along with the frame material, which adds to the structural strength of the frame. Like foam-filled frames, coextruded frames should be engineered to facilitate water drainage.

Depending on the type of window, frames can represent from 10 percent to 20 percent of the unit’s total insulating factor. That makes the framing choice a critical consideration for any manufacturer. Those that can optimize costs and efficiency by selecting the appropriate frame type and cavity-filling method for their operations will be all smiles.

Ric Jackson is the director of marketing and business development for Truseal Technologies Inc. He can be reached at rjackson@truseal.com. Mr. Jackson’s opinions are solely his own and not necessarily those of this magazine.


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