Rolf, 48%

[Rolf von Walthausen & Nancy Larson]

Ruth,
Good point about guarding against excessive moisture build-up.   Those
of us in northern climes maintaining higher levels of indoor humidity
during winter months have to be especially careful to make sure we have
adequate amounts of insulation, ventilation and air circulation
throughout the house.   I have known situations where it wasn't possible
or wise to maintain even 35% rh indoors, but other types of construction
and preferences where 50% rh was both healthy and safe.  Most literature
I've read on the subject indicates that, after considering variables
such as home construction, energy efficiency and air quality, indoor
levels of 40-50% rh are generally acceptable.

As a guideline I have found window condensation to be a good indicator:
if you start to see your windows fog up or moisture condense around
window panes then the indoor levels are probably too high for your house
in its present condition.

For anyone interested there's a good publication available from the Cold
Climate Housing Center in Minnesota.  It advocates taking all variables
into consideration and balancing ones needs (such as protecting musical
instruments, furniture, flooring, etc.) with issues of comfort and
health.   A portion of the material is copied below.  For the complete
publication:
http://www.extension.umn.edu/abstracts/nonweb/abstract.html?item=03415

Rolf von Walthausen

---------------------------------------------------

HOME INDOOR WINTER RELATIVE HUMIDITY:
                            WHAT IS ACCEPTABLE?
                            This publication is part of the Cold Climate
Collection from
                            the Cold Climate Housing Information Center
at the
                            University of Minnesota.

                            William J. Angell, professor and extension
specialist in
                            housing, Department of Design, Housing, and
Apparel,
                            University of Minnesota

                            This publication addresses the issue of
indoor relative
                            humidity in cold climate housing according
to the following
                            variables:

                            - Window condensation
                            - Other visible interior surface
condensation, mold, and dust
                            - Human health considerations
                            - Furniture considerations
                            - Concealed condensation
                            - Occupant comfort

Health Considerations

                            In general, it is agreed that too much
humidity is as
                            undesirable as too little. The effects of
relative humidity (rh)
                            appear to reflect the following pattern (see
Figure 1):

                            - bacteria increase at 30 percent or less
and 60 percent or
                            more rh.
                            - viruses increase at 50 percent or less and
70 percent or
                            more rh.
                            - fungi increase at 60 percent or more rh.
                            - mites increase at 50 percent or more rh.
                            - respiratory infections increase at 50
percent or less and
                            unknown above 50 percent rh.
                            - asthma difficulties increase at either
less than or higher
                            than 50 percent rh.
                            - chemical interactions increase at 30
percent or more rh.
                            - ozone production increases at 75 percent
or less rh.




Ruth Phillips wrote:

> Hi Rolf,
> I saw your message about your Schimmel.  I'm concerned about
> that 48%, especially where you live now.  The piano loves it I'm
> sure, but it's really high for the health of your house.  At least,
> depending on the structure and insulation, you can have a vapor
> lock with ice forming inside the walls when it's fairly cold out.
> It may be something you've never encountered because of moving
> before damage showed up.
> Hope everyone had a good Thanksgiving,
> Ruth P.