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
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-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.
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 email@example.com.
Mr. Jackson’s opinions are solely his own and not necessarily those of this
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