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

[David Yuill]

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


-----Original Message-----
From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of David Yuill
Sent: Tuesday, December 02, 2008 12:11 PM
To: 'Chris Yates'; 'Building Simulation'
Subject: Re: [Bldg-sim] Condensation in heritage buildings - best practice


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

-----Original Message-----
From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Chris Yates
Sent: Tuesday, December 02, 2008 11:20 AM
To: Building Simulation
Subject: [Bldg-sim] Condensation in heritage buildings - best practice


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
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