Condensation in heritage buildings - best practice

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I've been looking at the problem of condensation in heritage buildings.
I'm concluding that - especially in winter - best practice would be to
mechanically ventilate to a positive pressure (i.e. supply more than

My rationale is thus:
- The external air is heated, lowering it's RH.
- The positive pressure ensures this air with a more favourable (drier)
psychrometric is continually pushed from the inside out.

Doors and windows need to be reasonably well sealed for this to work.
Obviously, this is no guarantee against condensation build-up. Can
anybody see any error in my rationale? I would be grateful to hear of
alternative methods of control that have been successful.

Most commercially available building simulation codes can't model this:
- I'm uncertain as to what E+ can do at present.
- IES-VE can't
- ESP-r can but is very anachronistic (thar be dragons!)

Can any of the 1D Transient FEA packages model this positive ventilation
scenario? e.g. 1d-HAM, MOIST, WUFI

Best regards

Chris Yates

Chris Yates's picture
Joined: 2011-10-02
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Here's what I see as the problems to the rationale you outlined:

The RH probably isn't the issue; it's the absolute humidity (or
equivalently, the dewpoint). On a psychrometric chart, heating air moves
the state horizontally to the right; no moisture is removed. If the air has
a dewpoint of 10?C, for example, heating it won't change the dewpoint.
Generally, if that air comes into contact with a 9?C surface, you will get
condensation regardless of its temperature.

The outdoor air that you bring in will generally have a dewpoint that's
higher than the coldest envelope component (such as a window), but its
dewpoint will increase inside the building because of latent loads
(breathing, etc.). So when you pressurize the building, you are actually
forcing moister air out through the cracks, potentially forming condensate
inside the walls.

My guess is that you don't need simulation software to analyze your
situation. A psychrometric chart and some information about your project
specifics (design outdoor air condition, latent load, u-values, etc.) should

I hope this is helpful.

David Yuill

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Joined: 2011-10-02
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Oops. I've encountered the read-before-you-send problem. In my third
paragraph I meant that the dewpoint is generally LOWER than the coldest
envelope component, not higher. Apologies.

David Yuill

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Joined: 2011-10-02
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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

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Joined: 2011-09-30
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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.

Justin Spencer's picture
Joined: 2011-10-02
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Hmm, and combine this with HRV (heat recovery ventilation) - without mixing the air streams - to maximize energy savings. Protect your building and save energy $.

Matt Daly

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Chris -

We generally study these building envelope moisture-control issues with
WUFI - a program that uses the hygroscopic properties of component
materials along with moisture and temperature environment variables to
determine if there is potential damage from moisture and condensation.

Dave Bryan

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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.

Jeff P. Waller's picture
Joined: 2011-10-02
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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.

Justin Spencer's picture
Joined: 2011-10-02
Reputation: 1

Moisture in and through construction materials can become a huge problem
(definitely lawsuit class) if the issue is not addressed in a wholistic
fashion. My background is HVAC, not architecture, but I'm know that even a
"good" HVAC system cannot overcome a poor envelope retrofit.

There is an emerging discussion about this topic in the HVAC world; I
recommend looking at past issue of the ASHRAE Journal in the "Building
Sciences" column, written by Joe Lstiburek. I think he's got an excellent
grasp of the physics and is also very practical.

James V. Dirkes II, P.E., LEED AP

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Joined: 2011-10-02
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Since I live in a "heritage building" (100+ year old balloon-framed
house that began as the bunk-house for hired hands and hasn't gained
much insulation since), and I've worked in historic preservation firms
for years, I think a lot on this subject.

Everyone who replied to this subject was mostly right on target, but I
want to add my two cents in re-iterating the cautions in doing poorly
conceived energy retrofits to leaky old buildings without understanding
the building science and how it applies to the particular building. One
should at least figure out where the dewpoint falls in the wall in the
existing structure as well as the proposed retrofit, where the biggest
air leaks are in the building envelope (an infrared camera used during a
blower-door test makes this very graphic), and how one is going to
retard the passage of moist air into the portion of the building
envelope that is near or below dewpoint, and how you will encourage all
portions of the building envelope to dry to either the interior or
exterior faster than they are exposed to moisture. If one does a
haphazard job of tightening the structure and adding insulation, one can
easily cause a century old building that was in decent structural
condition to rot away in less than a decade, and/or make its occupants
deathly ill when you concentrate the humidity escaping into a few
locations and even increase the temperature in the building envelope to
the perfect conditions for micro-organisms to proliferate.

This "heritage" building I'm in (like most such) is a working holistic
building/energy system when operated as originally designed. It is
extremely drafty in winter, so the moisture we generate by breathing,
showering, cooking, etc diffuses right out through the envelope. Yes, it
probably condenses on surfaces within the walls as it does on interior
window panes, but there is so much air movement through the envelope
that it dries out again almost immediately, plus it is probably too cold
in the walls for mold and rot to grow. Part of what made this house (and
many other buildings of its vintage) last so long is that the standards
of human comfort were much lower when it was built. Typically only one
room on the ground floor was heated (with wood) as we are doing now (the
unheated rooms surrounding the heated one reduce heat loss and prevent
ice damming), and humidity levels in winter were very low, due to high
air leakage and a dry heat source (and people bathed infrequently
instead of showering daily). If all future occupants could be content
with cocooning near the woodstove and could deal with the nose bleeds
from dry air as we do, this building could probably last another century
and use relatively little energy to get through winter, but that is
unlikely in the extreme.

Consequently, though I preach energy efficiency at every turn, I've not
done anything to the "heritage" building I live in yet, mostly due to
other unrelated changes in its environment that now make the home an
undesireable place to live. But even if this home was desireable to
salvage, I'd wait until I had sufficient funds to retrofit the whole
building with a super-efficient retrofit such as one that would meet or
at least approach the Passive House Standard (see
&, else I could easily make things worse instead of
better. From all of my study and research, I've come to the conclusion
that unless one is prevented by historic preservation restrictions, the
safest/ best energy retrofit to old buildings is to wrap them on the the
exterior with a new air barrier which must be carefully sealed to the
highest performance windows and doors available (preferably ones that
can meet the Passive House certification standards) to create a very
tight continuous air barrier, which must then be wrapped with a
continuous superinsulation layer on the exterior of the air barrier. The
cheapest solution is typically to decide that basement and crawlspaces
are outside the conditioned space (air barrier on underside of original
joists, with superinsulation below that), and move all mechanicals and
plumbing into the conditioned space if possible, superinsulating
plumbing/ mechanical runs that can't be moved inside the insulated
envelope. One can deal with insulating basements and slab on grade
construction, but it is much more challenging and costly to do it well.

I say it is safest to energy retrofit on the exterior because one can
often attempt to replicate the original facades on the new exteriors
easier than one can succeed in creating a tight air barrier on the
inside of the insulation layers if one is limited to retrofitting on the
inside of the building. But if you are forced to retrofit from the
inside, you'd better check for concentrated air leaks thoroughly with
infrared camera and blower door test until you get a VERY tight
building, and calculate the temperature at that air barrier to ensure
that it is above the dewpoint. The Passive House Institute has
demonstrated that even if you are forced to create the air barrier and
to superinsulate on the inside (this inevitably results in more air
leaks and lots of thermal bridging), superinsulating to the Passive
House Standard typically prevents condensation on the interior surfaces
of thermal bridges because it attenuates the path of heat loss which
typically raises the interior surface temperature of the thermal bridge
to above the dewpoint. But retrofitting to the Passive House Standard
often costs as much as building new, and needs to be done in one
building season - incremental "improvements" can be worse than doing
nothing, as I described above.

Best wishes if you are trying to salvage existing buildings, and keep
your professional insurance up to date,

The original messages are below:

Christina A. Snyder's picture
Joined: 2011-09-30
Reputation: 0

We have successfully used WUFI to recreate heritage and new building
envelope components to help determine condensation/moisture issues.

Andy Hoover

Andy hoover's picture
Joined: 2011-10-02
Reputation: 0

Dear Bldg-Sim community,

Many years ago, I used "A Simplified Energy Analysis Method" software, which
was a bin analysis energy modeling tool. I'm interested in finding it
again, primarily for existing building models, but can't locate it.

Has anyone got current contact info for that program?

Thanks in advance!

James V. Dirkes II, P.E., LEED AP

James V. Dirkes II  P.E.'s picture
Joined: 2011-10-02
Reputation: 0

Happy holiday;
I am trying to use water heater mat with solar collector. Is there any suggestions on how to simulate this or if there is any literature on this subject.
thank you in advance.
h. abaza

Abaza Hussein's picture
Joined: 2011-10-02
Reputation: 200

Hi JV;

This question was asked and resolved about 3 years ago.
As I remember the person who had the Program only had 5.25" Floppy Disks.
We still have 5.25" Floppy capability but we are about to shed our old
support, no one ever calls for it. We have QIC-120, QIC-250, 3010,
TR-1, TR-2, TR-3, TR-4
tape capability. We have ZIP-100, Zip-250 capability. We expect to
abandon this already offline hardware in
the next 30 days !!! We have not had 8" floppy capability for several
years. There are 5 or 6
different ways to get the timing holes in 8" floppy disks so unless you
had DSDD's things could
be sparse.

JRR's picture
Joined: 2011-10-01
Reputation: 0

Dear Bldg-Sim list,

I'm still unable to track down ASEAM (although several of you provided
helpful leads), so I'll phrase the question differently:

I want to create a simple energy model of:
1) existing buildings
2) which use more than one HVAC system
3) using the ACTUAL weather data from the last 12 months.

Any suggestions?

James V. Dirkes II, P.E., LEED AP

James V. Dirkes II  P.E.'s picture
Joined: 2011-10-02
Reputation: 0

John Ross wrote;

The Commonwealth of Virginia and its wonderful State Corporation Commission
allows some very heinous, nasty billing practices concerning electric bills,
I'm "all electric" here as far as the utility company is concerned.........

They allow the number of days in a month's bill to vary between 28 and
35 days;
AND any two same months ie December 2008 and December 2009 DO NOT
have the same number of days billed in general....There is a listing of
the kwhr
used for each of the previous 12 months on each month's bill but there
is no mention
of days in the billing cycles, HDDs or CDDs for those months. ... the
days billed is
only given for the current month and requests for old records from the power
company were returned without days billed also.

Every State is supposed to have a Climatology Record, Virginia's record
is at
The University of Virginia. Look at the NOAA website to track down the
records for the state in question. I suggest on-site daily readings for
performance buildings. I got 120 gallons of 143 F water from my "Oversquare"
array today at my house and it's December 30 just 9 days off Dec 21....
At a customer site just 12 miles away they were 40% overcast between noon
and 2 pm, critical hours this time of year.

Do you want to stage the multiple heating systems ?? My biggest problem
with E+
is the lack of "innovation" available in the heating systems to be
modeled. I have
tricks up my sleeve that just destroy conventional heating systems. You
might not
want to do some of these on a large commercial building, but on a
residence with
telecommuting from home... it's Solar Sailing in an unlimited 10 meter

JRR's picture
Joined: 2011-10-01
Reputation: 0