Mr. G:

There may not be any data directly addressing deterioration of existing wood
siding, sheathing or structure (wood or masonry) under vinyl (or aluminum)
siding, but there is an enormous amount of literature on the movement of
moisture through building envelopes. When I was younger, thinner, and a bit
less grey, I spent many hours searching the literature on this subject --
some of it ultimately made it into a Masters thesis I wrote on the
humidistatic control of temperature and relative humidity in historic
buildings housing museum collections.

Baird Smith wrote a book when he was at the National Park Service on
moisture in buildings -- in fact, I think that is the title of the book. His
approach was rather broad, but I think there was a page or two addressing
this matter. The other source with freely available and accessible
information is the Forest Products Laboratory of the U.S. Dept. of
Agriculture.

In brief, here's the problem: in a northern temperate climate -- such as New
York -- moisture generated inside a building during winter (from people
breathing, showering, cooking food, spilling milk, etc.) creates an
environment with greater relative humidity than the environment on the
exterior of the building (or it should when the seasons behave). Nature
abhors not only a vacuum, but a gross imbalance of resource allocation; in
this case the moisture in the air. So, no matter how well a structure may be
sealed, moisture is going to migrate from the interior to the exterior of a
building -- in a northern climate in winter. The condition in cooling
climates is reversed. We care that moisture is moving through the wall
because at some point the dew point of the air -- the maximum mass of water
vapor it can contain at any given temperature -- will be such that the vapor
will turn to droplets. A wet rotten mess is the result.

So, architects and engineers spent years developing methods to minimize the
movement of moisture by applying moisture barriers to interior surfaces of
the building envelope. The word barriers is generally not used these days as
we've all recognized that they don't really act as barriers. But, for now,
barrier will do. Once all the buildings were well-sealed, problems with
moisture condensing in the wall or wall cavity didn't entirely disappear.
Turns out air currents moving through holes in the walls -- around doors,
windows, electric outlets and switches, etc. -- was carrying moisture laden
air into the wall or wall cavity. Now, openings are regularly sealed to
minimize air movement. And, engineers and architects argue over the value of
minimizing moisture movement via diffusion or the need to minimize air
movement.

Enough background. The problem arises when the moisture that makes its way
into the wall or wall cavity condenses. If it cannot evaporate and move out
through the exterior siding, it accumulates. The wet rotten mess gets bigger
and bigger with each cold day and each long winter. Ultimately, the building
collapses -- as Ralph so elegantly put it. With any moisture or vapor
barrier on the exterior of a building this condition will develop. Moisture
barriers range in type from plywood to polyethylene sheeting to impervious
foam insulations with foil facings to aluminum foil to aluminum siding to
vinyl siding. (I just read a great piece put out by the Douglas Fir Plywood
Association in the late forties on the moisture-resisting characteristics of
DougFir plywood, but that is a matter for another lecture.)

The siding alone -- without the foil-faced foam insulation may not be so
bad; many manufacturers include "weep" holes in what would be the bottom
face of a clapboard that allow some air movement. But I have never
recommended anyone put siding on a new or old building. Except when they
have a lot of old paint to remove, have time to monitor the moisture content
of the siding behind vinyl, and want to play at being clever. If the siding
is removed at just the right time, all the paint -- including all the
information about past color schemes -- may be sitting in a big heap just
above the foundation. Voila! Money well-spent.

Any questions.

Mr. T.




-----Original Message-----
From: Met History [mailto:[log in to unmask]]
Sent: Tuesday, January 04, 2000 12:35 PM
To: [log in to unmask]
Subject: Re: Vinyl siding


Regarding the original query
"Vinyl siding damages the underlying house."

Is this one of those things we just "know" (like "man cannot fly") or is
there real testing or sustained observational data which supports it?

Heidi replies (along with others):
>  There is observational data which supports it, mainly dealing with
moisture
>  control, replacement costs, durability issues....
>  The National Park Service Preservation Brief #8 .....
>   <A HREF="http://www.historichomeworks.com/HHW/pbriefs/pb08.htm">Brief
08:
>  Aluminum and Vinyl Siding</A>
>  -ONEcat, on a cedar shake roof

Heidi, in the Brief cited, I see no hard observational data or case study
references for the famous claim that "trapped moisture rots the underlying
wood".  I agree that hacking off details for a crummy siding job is not so
good, and that siding is no fun to look at, but after decades of vinyl
siding
installation, where is the hard evidence?  I don't doubt it exists, but I'd
like to see it.

Christopher Gray
Office for Metropolitan History
246 West 80th Street, #8, NYC  10024
212-799-0520  fax -0542
e: [log in to unmask]