UV filtering materials

Betty L. Seifert <seifert [at] dhcd__state__md__us> writes

>A colleague serving on the board for a historic house asked about
>using UV film to cover interior window glass.
>
>   "Film to cover the windows was purchased, but a tinting company
>    informed us that placing film on the interior window surfaces
>    would cause heat build up and wood deterioration in the frames
>    between the inner window and outer storm window. (The museum is
>    in a circa 1925 house). Would this happen? They propose to
>    provide protection on the storm windows at much higher cost"
>
>Does anyone else have experience with this?  The local firm wants to
>charge a high fee for the installation of their product.

We have been experimenting at Monticello with interior storm windows
incorporating solar filtering for the last three years. Although the
main goal has been to come up with a means of reducing window
condensation during the winter months, combining UV and visible
light filtering has been an objective as well. (Since 1993 we have
controlled light levels by suspending tinted UV window shade material
in front of the windows from the inside.) Betty Seifert's initial
query mentioned filming storm windows and I assume that this refers
to exterior storm windows. The message was a little unclear as to
whether a tinted film or clear film was to be applied but the
reference to a "tinting company" suggests a tinted film.

If tinted film is applied to exterior storm windows, the window
sashes will be obscured much the same way as your eyes are obscured
to others when you put on a pair of sunglasses.  This appearance
factor surely would need to be a consideration as it was for us here
where the windows are a major architectural component. On the other
hand, applying a clear UV film that does not provide any visible
light filtering, would have no effect from this visual standpoint
other than the appearance of the units themselves nor would there be
any issues in terms of thermal heat gain. As Dr. Knight notes, IR
energy would simple pass through the clear film.

Although not directly related to the current thread, I'd like to
briefly summarize what we have learned so far regarding interior
storm windows and the effect they can have on condensation. First,
it could probably go without saying that we condition the main floor
spaces for the sake of collections displayed in the house. "Normal"
setpoints are 70 deg. F and 50% RH. In the winter we allow RH to
drop a bit when we use 40% as the setpoint in order to lower the
dewpoint. The temperature setpoint is lowered a bit as well.

We began researching storm window systems about three years ago and
have had prototypes in place for the last two winters. Last winter
we experienced some fairly cold weather with a low of 7 deg. F
recorded on January 19, 2003.  First of all we found, as expected,
that for interior storm windows to successful at all in reducing
condensation they must be sealed as tightly to the interior
architrave so that the humidified air inside is blocked as much as
possible from migrating into the cavity. Second, we have also
discovered that the system works best if there is positive air flow
from the outside. We came to this conclusion based on two separate
installations--one on a window that had been sealed from the outside
due to many years of caulking versus others installed on operable
windows with clearance between the sashes and the jambs as well as
between the meeting rails of adjoining upper and lower sashes thus
allowing air flow to take place. Through comparison with control
windows with no storm windows in the same spaces with the same
exposure we found that the units installed on windows with
ventilation to the outside succeeded in totally eliminating all
condensation. On the other hand the window that was sealed to the
outside, while reducing the degree of condensation somewhat still
experienced the problem to some degree.

At this point, based on our experiments over the last two winters,
we feel fairly confident that we can eliminate virtually all if not
all condensation using interior storm windows sealed to the inside
but with positive air flow from the outside.

The next question, that of heat build-up within the cavity, was
looked at on a preliminary basis this past summer. Monticello
retains most all it it's original Santo Domingo mahogany window
sashes. Approximately 30% of the window glass is original. Clearly,
excess heat could be problematic especially if either wood movement
due to shrinkage; or heat caused glass to break. We have used two
different light-filtering products in our prototype storm windows:
Cyro acrylic sheet, product #311-1, Bronze, 10% visible light
transmission was used as glazing on one window; and CP Films tinted
film, product #N1020B, Bronze, 19% visible light transmission was
applied to tempered glass panels in three other units.

This summer, we have been monitoring temperatures within the
cavities using digital thermometers equipped with sensing probes
that are put into the cavities. So far the maximum temperatures
recorded has only been 121 deg. F. when the window is exposed to
full sunlight. (Two of the test windows have southwest exposure and
experience around 3 hours of direct sunlight a day during the
summer.) Observing the temperature drop immediately when the sun
would go behind a cloud also strongly suggested that the air flow
from the outside was a significant factor in terms of heat build-up.
In addition, we have not seen any difference in the cavity
temperatures between the window with the tinted acrylic storm panel
versus the ones with filmed tempered glass. (It had been suggested
that the acrylic panel would absorb more heat whereas the glass
panels might transmit the heat better leading to possibly cooler
temperatures in those cavities.)

Another factor that we were interested in was actual window glass
temperature. Readings were taken from the exterior using a laser IR
surface temperature thermometer. (A handy tool--and not terribly
expensive.  Good for surface temp. readings as well as monitoring
supply air temps in HVAC systems) We found no significant
differences in readings taken from the windows with interior storm
panels versus those without.

In summary, our experiments have indicated that positive air-flow
from the outside is crucial in preventing condensation and is almost
certainly a major factor in helping to keep the cavity temperature
down as well. The temperatures we have recorded thus far do not seem
high enough to be a concern although we still need more data because
there were a large number of cloudy or rainy days this summer. We
will also be interested in seeing what sorts of readings we get as
the angle of the sun becomes lower. It should be noted that our
results are specific only to the products we used and to our own
particular exposure and climate.

Several questions are still unanswered in my mind that I would like
to pose: At what temperature does damage begin occurring to wood? I
assume there is a point at which a "cooking" process begins that
would cause chemical changes to occur. Scientific principle dictates
that any reactions would increase in relation to temperature. What
chemical changes occur within the structure of wood at increased
temperatures?  What is the observable nature of the degradation? Are
different woods more tolerant of higher temperatures than others?

Any further discussion would be a welcome addition to our research.

Robert L. Self
Architectural Conservator
Monticello
Charlottesville, VA  22902
434-984-9851


                                  ***
                  Conservation DistList Instance 17:34
                Distributed: Thursday, October 16, 2003
                       Message Id: cdl-17-34-001
                                  ***