Mr. Hosley. I needed a bit of time to see what I was dealing
with so I’m a bit slow to respond to your reply from last week. At least we’ve
isolated the condition down to New England like climate and the condition that
it depends on what you have stored. So the original statement really isn’t a
general statement about the value of HVAC for all materials in all
places.
Not being familiar with New England climates, I had a look at
the last three years of outdoor climate data from Noaa for some more or randomly
selected locations (although I did intentionally try to cover as many states as
I could in my selection.) So I looked at Nantucket, Bennington, Portland,
Concord, and New Haven. I have to admit that I was quite surprised at how
similar the data was.
Temperatures were generally pretty moderate peaking mostly in
the high 60s (19 or 20 C – ish) so for materials that tolerate room
temperature//human comfort temperatures, it really isn’t bad, although there
were days into the high 80s and even almost 100 F (around 37 C. I’m not
sure how international this list membership is, but Si units are included for
the non-Americans.) So some cooling is needed, but as you point out, probably
not needing as much as we might in a place like upstate New York.
At the other end, winters bottom out in the low 30s (around
+1 or +2 C) and for about five months in each of the three years of data, the
temperature was below 35 F (+2 C) so winter heating is probably
necessary..
All year round, the outdoor humidities sit around the very
high 60s (68%, 69%) to mid-70s %. Between the end of the heating season and the
start of the next, humidity control will be an issue.
The dew point temperature changes quite rapidly between the
summer and winter. In all five locations and three years, the dew point
temperature reaches about 55 F (13C) around the beginning of June and sits at a
peak between about 65 F (18 C) and 69 F (21 C) for about three months before
finally dropping below 55 F ( 13 C) around the end of September. All other
things being equal, a dew point temperature of 55 F (13 C) will produce a
humidity of about 59% at 70 F (21 C). The peak dew point produces a
humidity between about 84% and 93% RH.
Cooling could bring down the humidity, but every small air
conditioner that I’ve ever looked at (and even my central air conditioner at
home) all work on thermostatic control while a New England institution is less
likely to require cooling, but needs dehumidification. It’s possible that a
compromise might be reached by using dehumidifiers, although they add to the
heat load that needs to be removed along with heat generated by people and
lights. Opening windows to bring down the heat load brings up the humidity so
monitoring and testing is required to find an appropriate compromise if one
wants to avoid installing a “large and invasive” system.
Around mid-November, the dew point temperature drops below 35
F (2 C) and stays below there through to May. Ignoring the up and down spikes,
the dew point tends to sit between about 15 F and 20 F (about -7 C and -9 C).
Heating the building to about 70 F (21 C), would leave the
humidity at about 11% to 14% RH for about three months so occupied buildings
require winter humidification.
Rick Kerschner has gotten around this by allowing buildings
to be minimally heated leaving them around 40 F (+4 C) for the winter. However,
this approach has several conditions: The building can’t be occupied (or
occupants are willing to wear winter clothing indoors), there can’t be plumbing
in the building or it must be possible to drain it for winter, and finally
museum objects in the building must tolerate the colder temperatures.
Humidification is also required for the winter if the
existing conditions don’t allow the low winter heating option to be used.
Even in New England, HVAC can’t be completely dismissed, but
there may be lower cost solutions depending on the size of the institution, how
it operates, the design and construction of the buildings and collection objects
housed there.
-Doug
Douglas Nishimura
Image Permanence Institute
Rochester Institute of Technology
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