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

Christina A. Snyder CASnyder at ic.org
Tue Dec 2 17:27:15 PST 2008


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 www.passivehouse.us
&www.passivehouse.com), 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,
Christina 


The original messages are below:
>    4. Condensation in heritage buildings - best practice (Chris Yates)
>    6. Re: Condensation in heritage buildings - best practice
>       (David Yuill)
>    8. Re: Condensation in heritage buildings - best practice
>       (David Yuill)
>    9. Re: Condensation in heritage buildings - best practice
>       (Daly, Matt)
>   11. Re: Condensation in heritage buildings - best practice
>       (Justin Spencer)
>   12. Re: Condensation in heritage buildings - best practice
>       (Daly, Matt)
>   13. Re: Condensation in heritage buildings - best practice
>       (David Bryan)
>   14. Re: Condensation in heritage buildings - best practice
>       (Jeff P. Waller)
>   16. Re: Condensation in heritage buildings - best practice
>       (Justin Spencer)

> ----------------------------------------------------------------------

> Message: 4
> Date: Tue, 02 Dec 2008 17:20:11 +0000
> From: Chris Yates <Chris at zed-uk.com>
> Subject: [Bldg-sim] Condensation in heritage buildings - best practice
> To: Building Simulation <bldg-sim at lists.onebuilding.org>
> Message-ID: <49356E4B.4080703 at zed-uk.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> Hi
> 
> 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 
> extract).
> 
> 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
> 
> 
> ------------------------------
> 
> Message: 6
> Date: Tue, 2 Dec 2008 12:10:48 -0600
> From: "David Yuill" <dyuill at cox.net>
> Subject: Re: [Bldg-sim] Condensation in heritage buildings - best
> 	practice
> To: "'Chris Yates'" <Chris at zed-uk.com>,	"'Building Simulation'"
> 	<bldg-sim at lists.onebuilding.org>
> Message-ID: <26B865D1756E451E9B96A515D29E0DF8 at bsiuo0xemntizk>
> Content-Type: text/plain;	charset="iso-8859-1"
> 
> Chris,
> 
> 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
> suffice.
> 
> I hope this is helpful.
> 
> Regards, 
> David Yuill
> 
> ------------------------------
> 
> Message: 8
> Date: Tue, 2 Dec 2008 12:18:28 -0600
> From: "David Yuill" <dyuill at cox.net>
> Subject: Re: [Bldg-sim] Condensation in heritage buildings - best
> 	practice
> To: "'David Yuill'" <dyuill at cox.net>, "'Chris Yates'"
> 	<Chris at zed-uk.com>,	"'Building Simulation'"
> 	<bldg-sim at lists.onebuilding.org>
> Message-ID: <D05CCEC4C8B749E1A1B134FE46C84D89 at bsiuo0xemntizk>
> Content-Type: text/plain;	charset="iso-8859-1"
> 
> 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
> 

> ------------------------------
> 
> Message: 9
> Date: Tue, 2 Dec 2008 10:25:09 -0800
> From: "Daly, Matt" <Matt.Daly at wesd.org>
> Subject: Re: [Bldg-sim] Condensation in heritage buildings - best
> 	practice
> To: 'Building Simulation' <bldg-sim at lists.onebuilding.org>
> Message-ID:
> 	<06D9FD985A42704BB0E8F483ACA3FE2E3EE4828AA2 at miranda.wesd.org>
> Content-Type: text/plain; charset="us-ascii"
> 
> 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
> 
> ------------------------------
> 
> Message: 11
> Date: Tue, 2 Dec 2008 11:33:53 -0700
> From: "Justin Spencer" <jspencer17 at gmail.com>
> Subject: Re: [Bldg-sim] Condensation in heritage buildings - best
> 	practice
> To: "Building Simulation" <bldg-sim at lists.onebuilding.org>
> Message-ID:
> 	<4e1ecbb70812021033q267756bfq6ff008dbf25d3068 at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
> 
> 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.
> 
> ------------------------------
> 
> Message: 12
> Date: Tue, 2 Dec 2008 10:46:11 -0800
> From: "Daly, Matt" <Matt.Daly at wesd.org>
> Subject: Re: [Bldg-sim] Condensation in heritage buildings - best
> 	practice
> To: Building Simulation <bldg-sim at lists.onebuilding.org>
> Message-ID:
> 	<06D9FD985A42704BB0E8F483ACA3FE2E3EE4828B07 at miranda.wesd.org>
> Content-Type: text/plain; charset="us-ascii"
> 
> 
> 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
> Willamette ESD
> 
> ------------------------------
> 
> Message: 14
> Date: Tue, 2 Dec 2008 13:02:15 -0600
> From: "Jeff P. Waller" <jwaller at cambridge-eng.com>
> Subject: Re: [Bldg-sim] Condensation in heritage buildings - best
> 	practice
> To: "Justin Spencer" <jspencer17 at gmail.com>,	"Building Simulation"
> 	<bldg-sim at lists.onebuilding.org>
> Message-ID:
> 	<2A2F02A39DAC794BB1A77F7B94221592BF61F2 at exchange-stl.cambridge-eng.stl>
> 	
> Content-Type: text/plain; charset="us-ascii"
> 
> 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.
> 
>  
> ------------------------------
> 
> Message: 16
> Date: Tue, 2 Dec 2008 12:14:14 -0700
> From: "Justin Spencer" <jspencer17 at gmail.com>
> Subject: Re: [Bldg-sim] Condensation in heritage buildings - best
> 	practice
> To: "Building Simulation" <bldg-sim at lists.onebuilding.org>
> Message-ID:
> 	<4e1ecbb70812021114k49a56c8lea073efcb35f60d0 at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
> 
> 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.
> 
> ------------------------------
> 
> Message: 18
> Date: Tue, 2 Dec 2008 14:21:36 -0500
> From: "James V. Dirkes II  P.E." <jvd2pe at tds.net>
> Subject: Re: [Bldg-sim] Condensation in heritage buildings - best
> 	practice
> To: "'Justin Spencer'" <jspencer17 at gmail.com>,	"'Building Simulation'"
> 	<bldg-sim at lists.onebuilding.org>
> Message-ID: <3A98EA665F5249FF873FACA0B6E46E01 at BPT>
> Content-Type: text/plain; charset="us-ascii"
> 
> 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.
>  
> 
> The Building Performance Team
> James V. Dirkes II, P.E., LEED AP
> 1631 Acacia Drive NW
> Grand Rapids, MI 49504
> 616 450 8653
-- 
  Christina A. Snyder
  casnyder at ic.org
  voice: 734-428-9249




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