Environment in historic house

Christine Cross <ccross1 [at] execpc__com> writes

>I am currently working on the restoration of a 1930s home in
>Wisconsin, USA. We have recently upgraded the existing HVAC system.
>My question concerns RH levels. I had always been told that optimum
>RH in a historic house was 50% with a temperature between 65 and 70
>degrees Fahrenheit year round. The project architect feels this RH is
>too high and will infiltrate the plaster and begin to rot the wall
>studs. What should we do? Any suggestions for sources concerning
>this topic?

I agree with your project architect.

First of all, 50% RH is kind of a false promise. The 50% RH number
has crept into the conservation vernacular as if something magical
happens if any object is kept at that RH year-round. The fact is,
your RH value should be driven by the needs of the materials in your
collection and the realities that the collection faces. All other
things being equal, 50% RH is maybe a bit too high for 20th century
paper and reactive metals like silver, but too low for materials
like ivory, bone and some inlaid furniture. So, even in many museum
showcase exhibits (where the RH value can be set and maintained
pretty readily) the 50% RH number is often a compromise on what's
ideal for most materials.

Depending on the type of collection in your house, I doubt any
damage would occur if you allowed your winter-time RH to drift as
low as 30% RH for the duration of the heating season. Even at that
low level of humidification, it would be advisable to shut your
humidifier off entirely on the coldest days of the year (does it get
below zero there?--ever?) to prevent condensation. Any part of the
collection that might require a higher RH could probably be enclosed
in a micro-climate to see it through the heating season.

It is often the case with historic houses, that the largest and most
significant object in the collection is the house itself. Delivering
enough humidification to maintain 50% RH at 65 to 70 degrees F--in
Wisconsin--for the duration of the heating season--would likely have
disastrous results for an older building. (In cold climates, it
isn't even very easy to provide such conditions for newly
constructed buildings that are meticulously insulated, double-glazed
and well-sealed!)

Air at that temperature and RH has a fairly high dewpoint--if the
ambient temperature is around 65 to 70 degrees and the RH is 50%,
then you will achieve condensation on just about every surface that
is below 48 degrees F in temperature! This means that your windows
will fog up throughout the house, but you will also get condensation
in uninsulated wall cavities and this can cause protracted physical
damage that you will not be able to detect until it is far too late.

Don't sacrifice an important structure for the sake of sustaining a
humidity level that won't really be of great benefit to any one part
of the collection. You will succeed in doing little good and could
cause a lot of harm. Marion Mecklenberg gave a great report on this
topic at AIC this year (noting how hygroscopic materials behave
differently in reality than they do under test conditions, due to
the magnitude of RH cycling and the hysteresis of the material). You
may also want to peruse the resources at the Building Research
Institute and the University of Illinois at Champaign-Urbana. Feel
free to contact me off-list for specific links and more information.

Craig Oleszewski
Hygrospec
Rhinebeck, NY.
845-876-3311


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                  Conservation DistList Instance 14:36
                Distributed: Wednesday, January 3, 2001
                       Message Id: cdl-14-36-003
                                  ***