[Bldg-sim] Condensation in heritage buildings - best practice

[Jeff P. Waller]

It is very difficult, even with special vapor barrier construction, to
prevent moisture migration through walls. What we really do is slow it
down. Without mechanical intervention, the outside air dew point in
winter is almost always (I will not say never) below the dew point of
the interior spaces. People sweat and that produces moisture. Typically,
people at normal occupant densities do not cause enough moisture to be
in the space to cause condensation, as long as the building is minimally
ventilated (ASHRAE 62) and insulated. Usually there exists other
external producers of moisture that raise the moisture content of the
air to the point of condensation on typical wall temperatures (showers,
poorly ventilated dryers, leaking pipes, etc) and these combined with
poor insulation or ventilation usually cause the problem. Hence the
issues with heritage type buildings.

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Justin
Spencer
Sent: Tuesday, December 02, 2008 12:34 PM
To: Building Simulation
Subject: Re: [Bldg-sim] Condensation in heritage buildings - best
practice

 

Another option is to think about the DOAS route.  This may lower the
cost of providing dehumidification of the outdoor airstream, if you're
trying to maintain a dewpoint indoors that is lower than the dewpoint
outdoors under some conditions.  




On Tue, Dec 2, 2008 at 11:25 AM, Daly, Matt <Matt.Daly at wesd.org> wrote:

Just some thoughts on HVAC basics.  Depending on what can be done with
your heritage building....Increased R-value of the exterior walls will
move the dew point to within the wall.  Combine this with a vapor
barrier - a good coat of paint and sealing to prevent conditioned air
from entering the wall cavity - and the temperature of the inside
surface of the wall is above the dew point.  Efficient glazing will
decrease condensation on the windows, but prepare for the inevitable
condensation anyway.  A dedicated dessicant system after the mixed air,
assuming an air handling system exists or can be installed, would be
another or additional method, and would help prevent condensation within
the wall cavity which will happen to some extent regardless of how well
your walls are sealed.


Matt Daly
Willamette ESD


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