[GSBN] Straw-Bale Blower Door and Infrared Test Results

[jfstraube]

Glad to say something useful to you Derek.  
Point 1. Condensation hidden in walls can occur anytime any leakage occurs (over 0 ACH50) but the quantity of airflow needed to create widespread condensation is enough that it requires a leaky house, eg over around 3.  
A local leak in a 0.6 ACH50 house will still cause icicles and rot, but over a very limited area in an otherwise nearly perfect house. 

Point 2. as houses leak more and more, the interior moisture production is diluted by cold dry outside air and so the RH drops.  In normal occupancy houses of average 2500 sf and higher size, leakage rates of 5 or 6 result in RHs that are "dry", perhaps 25% or so, which prompts people to buy humidifiers. If they succeed in raising the RH, the high leakage rates can cause some serious interstitial wetting.
As the leakage rates exceed say 12 or 15, the RHs are always so low, even with an additional humdifier that condensation does not occur very much, and the house is uncomfortably dry and drafty (this is a description of an old house).

Point 3. cfm50/sf is a better metric of enclosure performance because it normalizes the results for complex and simple enclosure alike.  ACH only normalized for volume.  CFM50/sf  captures the shape and volume to area ratio of the building.  This really matters with large vs small houses as well as sprawling vs compact forms.  CFM50 total is a very good measure of total building performance, perhaps the best because it rewards small and simple buildings.  But CFM50/sf is the best measure of the enclosure performance (as opposed to building).  Hence I think both numbers are best: total for the whole building performance and CFM50/sf for enclosure tightness.

Hope that explains my comments.

On 2010-12-24, at 11:45 AM, Derek Roff wrote:

> It's a great pleasure and education for me to read messages from John Straube, and all the others who have posted recently.  John, I would like to request clarification of a couple of points from your posting below:
> 
> "... houses over about 3 ACH at 50 tend to have a risk of interstitial condensation. Rates over about 5 or 6 tend to be dry."
> 
> Is this saying that houses between 3 and 5 ACH at 50 have problems, but both above and below that, moisture problems are less likely?  That seems counterintuitive to me, hence I suspect that I don't understand.
> 
> I would also be grateful if you would say a bit more about why "cfm50/sf is better [a better metric]".

John Straube
www.BuildingScience.com