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

Justin Spencer jspencer17 at gmail.com
Tue Dec 2 11:14:14 PST 2008


I should also point out that the DOAS is also applicable in cases where
you're after a dewpoint that is above the outdoor dewpoint.  I may have left
a different impression in an earlier message.  It can be much cheaper to
dehumidify a portion of the airstream that is already near saturation than
to dehumidify mixed air.

On Tue, Dec 2, 2008 at 12:02 PM, Jeff P. Waller
<jwaller at cambridge-eng.com>wrote:

>  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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