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. <br>
<br><div class="gmail_quote">On Tue, Dec 2, 2008 at 12:02 PM, Jeff P. Waller <span dir="ltr"><<a href="mailto:jwaller@cambridge-eng.com">jwaller@cambridge-eng.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
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<p><span style="font-size: 11pt; color: rgb(31, 73, 125);">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.</span></p>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);"> </span></p>
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<p><b><span style="font-size: 10pt;">From:</span></b><span style="font-size: 10pt;">
<a href="mailto:bldg-sim-bounces@lists.onebuilding.org" target="_blank">bldg-sim-bounces@lists.onebuilding.org</a> [mailto:<a href="mailto:bldg-sim-bounces@lists.onebuilding.org" target="_blank">bldg-sim-bounces@lists.onebuilding.org</a>]
<b>On Behalf Of </b>Justin Spencer<br>
<b>Sent:</b> Tuesday, December 02, 2008 12:34 PM<br>
<b>To:</b> Building Simulation<br>
<b>Subject:</b> Re: [Bldg-sim] Condensation in heritage buildings - best
practice</span></p>
</div><div><div></div><div class="Wj3C7c">
<p> </p>
<p style="margin-bottom: 12pt;">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. <br>
<br>
<br>
</p>
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<p>On Tue, Dec 2, 2008 at 11:25 AM, Daly, Matt <<a href="mailto:Matt.Daly@wesd.org" target="_blank">Matt.Daly@wesd.org</a>> wrote:</p>
<p>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.<br>
<span style="color: rgb(136, 136, 136);"><br>
<br>
Matt Daly<br>
Willamette ESD</span></p>
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