As I have mentioned in earlier articles, post 1940 modern windows have not had to
withstand the same long tests of time that their well maintained 100-300 year old
ancestors, still in service today, have. Not withstanding the good intentions for lower cost
modern post war materials and methods have not always had a great track record for
holding up to weather, making them a replaceable instead of a permanent building
element. We therefore, before implementing new technologies, use testing to determine
the anticipated performance and longevity. The new materials, joinery methods, glazing
and weather-stripping are first bench tested by the prototype craftsman then sent out to a
testing lab for independent confirmation. The American Society for Testing and Materials
(ASTM) has established guidelines for air and water penetration as well as structural
endurance at different levels of wind pressures. These are known as Design Pressures
(DP) which currently run from the minimum DP-15 at 78MPH wind speed to the
maximum DP-100 at 200MPH. Most residential windows are in the DP-30-40 range,
109-126MPH. In addition, the operating hardware is opened and closed thousands of
times in a life cycle test plus a structural test after which the air and water tests are
repeated. After being satisfied that the target DP has been achieved we run the tests up to
the point of failure which allows us to set limits on how large a particular window design
can be safely built.

The upshot of this rigorous testing is that we find out the flaws in our new window
designs before we put them into production. I have observed that most of the new joinery
methods that hold up best are most often the ones that harken back to traditional trial and
fail based craftsmanship. As an example look at the 18th century lighthouses of the British
coastline, still standing, the oldest of which were designed by the multi-generational
Stevenson (think Robert Luis) engineering firm of Glasgow who based their, innovative
for that time, storm surf resistant towers on the parabolic curves of the 500 year old
coastal brown oak trees standing on the cliffs nearby. The art of designing structures from
successful natural examples cannot be over looked, as the forces of nature are the
ultimate test chamber for determining what endures and what does not stand the test of
time.

Pictured below is a DH window with a new fully concealed weather-stripping system
being tested. The window passed initially with no leaks at DP-55 (148MPH). We learned
where we could improve the design so it will pass DP-65 (161MPH) or better. The
changes were relatively simple to make but we would not have seen where to make them
without the extreme lab testing.

Thermal performance is mostly influenced by the insulated glass design. All the major
glass manufacturers have the U-Values, Heat gain coefficients, Visible light
transmittance and other important performance factors posted on their websites for their
various I.G. Products. We use computer thermal modeling of the entire glass and wood
window assembly to make sure that the overall U-Value will pass the project
requirements. Note that in the thermal simulation image below there is also a dew point
line so we can adjust weather-stripping and thermal break locations to make sure that
condensation will occur towards the exterior. Also to this end the new thermally broken
warm-edge insulated glass spacers help with controlling the dew point much better than
the older mono aluminum ones. These new spacers combined with a wide array of
application specific Low-E coatings give the HVAC engineers the ability to fine tune the
I.G. in a building to either resist or enable solar heat gain while still limiting nighttime
heat loss. The end result is an enjoyable increase in natural light without compromising
energy efficiency.

Exterior wood finishes are periodically tested in house by placing samples out in the sun
and rain. Now that the high performance but highly toxic lead paints are in the past we
have new generation water borne finishes which perform as well but without the toxins.
When trying a new exterior finishing system we first try to make the finish fail by
performing adhesion tests, freeze/thaw tests and water emersion at our shop. When
satisfied with the results we send samples out for accelerated weatherization testing at an
ASTM certified lab. We have seen the new European water borne urethane paints holding
up without discoloring or failure for the equivalent of 20 years exposure.
Clear finishes and stains on the other hand usually require frequent maintenance recoating
especially in direct sun. As any boat owner will tell you keeping up the bright
work is an annual ritual. There are some promising new finishes that would require only
an cleaning and replenishing coat instead of scraping and sanding down to bare wood and
starting over. We have started bench testing some of these for our door thresholds and
will keep you posted as the results come in.

Sound Transmittance Coefficient (STC) is again mostly a property of the glazing. How
ever when required as in some urban projects we will send a window to an acoustical lab
to be tested in a sound chamber for over all STC performance. As you might imagine the
wall construction of the project also has an influence on how well the window will
perform. Masonry of course is best because of the mass, wood framed walls on the other
hand are more finicky because the harmonizing between exterior sheathing and interior
wall board. If the wall is harmonizing in C flat then the exterior and interior glass
thicknesses must not only be tuned to help reduce direct sound transmission but must also
not harmonize in C flat with the wall sheathing. Working with sound engineers over the
years has opened our ears as to how the smallest choices in building materials can effect
the ultimate acoustical enjoyment of the owner’s interior spaces.

In conclusion methodical empirical scientific bench and lab testing is the slow but steady
path to a window designed and built to be a permanent building component. The path to
window design failure is based on unsubstantiated engineering assumptions. Not least of
which are the use of unproven “cost saving” fad materials and methods resulting in a
disposable commodity that comes with a built in shelf life. At the end of the day the most
expensive window is the one that has to be replaced every generation or two.
Steve Benson