I am cross-posting this from the ConsDist List with permission from the
author. I think it is one of the best and most thoughtful accounts on the issues
and experiences involved in doing window film treatments for Historic Houses. In
addition to posting thoughts on Museum-L by all means feel free to respond to
the author, Robert Self, Architectural Conservator at Monticello
([log in to unmask])

Cheers!
Dave

David Harvey
Artifacts
2930 South Birch Street
Denver, CO  80222
303-300-5257
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____________________________________________________
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

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