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

[Derek Roff]

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]".

My thanks and greetings,
Derelict

Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm.edu



--On Friday, December 24, 2010 11:04 AM -0500 jfstraube 
<jfstraube at gmail.com> wrote:

There is no doubt that SB can meet the 0.6 standard. To do so 
requires attention to sealing ALL the joints.  Even one crack 1/8" 
long, hidden behind baseboards, window trim, intersection walls etc 
that is maybe 30 or 40 ft long, will essentially mean you meet it.

However, the question should be "Why would we meet 0.6 ACH at 50".
The metric is questionable (cfm50/sf is better although the industry 
is only now shifting away) and that 0.6 target is totally arbitrary.
Why 0.6? Why not 0.5, 1.0 or 1.5 or 2.

The PH people chose the target based on flawed logic (condensation 
control is apparently the reason) and in many climates it is insane 
to expend too much effort to get to 0.6.

In the cold climate anecdotal experience of Canada, 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.  houses under 2 ACH  tend to perform 
quite well and only gross local errors cause condensation problems. 
When the rate falls under 1.5 we note problems with high winter RH. 
This is all just rough numbers but is based on a lot of houses and a 
lot of people in Zones 5, 6 and 7.

Depending on your energy goals and climate, you might want to get 
lower than 2 at 50.  BSC targets 0.1 cfm50/sf for very low energy / Net 
Zero houses in cold climates and hot-humid climates.  But some of the 
production builders we work with have reduced their fleets to under 
2.5 with the occasional one at 1.2 or, and find no problems (except 
the need for mechanical ventilation and controlling high winter RH 
via using HRVs not ERVs).  In milder Zone 3 and 4 climates of the 
south east, getting the houses under 3 has been remarkable for 
improving almost all aspects of performance.


If you can get the house to 0.6 for no cost, by all means do so (and 
of course, this means you cant use normal range hoods in small homes, 
as they will extract 200 cfm + and may cause odd problems).  In my 
experience, airtightness costs money in terms of supervision, 
inspection, testing and a small amount of materials.  There is a 
tremendous benefit to getting as tight as David did on his house. And 
it would be worth hundreds of dollars more to cut it in half.  But 
would it be worth it to cut it to 1/8 at the cost of thousands.  I 
think not: better bang for buck is a small HRV, or an upgraded window.


John