[GSBN] Straw-Bale Blower Door and Infrared Test Results
derek at unm.edu
Fri Dec 24 10:45:26 CST 2010
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
"... 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
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,
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.
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