SB: Re: Santa Fe style, breathability

["Robert W. Tom" <ArchiLogic@yahoo.ca>]

10/27/01 4:50:16 PM, David Neeley <dbneeley@earthlink.net> wrote:

> Air permeability is also a factor of the amount of surface area the wall presents.
> Low migration rates of air molecules through the wall could be offset in a large enough 
structure, assuming there is any such migration to begin with.
>
>By the way, if you're searching for any existing studies you should avoid 
>the term "permeance" in your search terms, since I don't believe it is a 
>recognized word. If you look for "permeability" you'll have much more luck.


First of all, getting terminology out of the way...

The term "permeANCE"   (as is the case with "resistANCE", 
"conductANCE" etc.) refers to a value for an entire assembly or component. 
( ie A 2x6 stud wall filled with fibreglass batts is said to have a (nominal) 
thermal resistANCE of R-20.

Similarly, the terms vapour or air "permeANCE" refers to the rate at which 
air or vapour move through a component or entire assembly.
ie 3/4" thick plaster on metal lath is listed as having a vapour permeANCE of
15 perms ... or a mineral wool filled 2x4 stud wall with plaster on lath
on the interior is said to have a vapour permeANCE of  0.97 perms, at
a point on the inside surface of the exterior sheathing.

The term "permeABILITY" ( or "resistIVITY" or "conductIVITY") refers to
the value per standard unit thickness... ie per inch (in Imperial) or
per metre (in SIU).
ie  Septic-fed JoE McCabe's tests on straw yielded thermal resistivity
values of R- 2.1 to 2.9 (or somesuch) per inch depending upon straw 
orientation and on the basis of this resistivity value, that SB walls would
have thermal resistance values of R-40 or so, depending upon moisture
content, density, straw orientation, construction details etc.

                            === * ===
On the first point quoted w-a-ay above, I pretty much dealt with that 
in my first posting to this thread a few days ago.  At the risk of being
annoyingly repetitive...
In the example that was posted, using the air leakage rate (as provided 
by the CMHC testing) through gypsum board, (which is more air permeable 
than cement board (not much different than Portland cement plaster)), 
given in units of litres (volume) per m^2 (unit area) x sec (unit time) 
at a higher-than-normal differential pressure of 75 pascals ...
and calculating the entire wall surface for a building measuring 30 ft
x 60ft (in plan) and 8 ft high,  simple arithmetic told us that the amount
of air moving through the ENTIRE area of the wall surfaces was 
something like 3 litres per sec, for the entire house.

The generally accepted *minimum* make-up air rate (supply) is 5 litres per
second  (about 10 cu ft per minute)  per habitable room, which translates 
to about 0.35 air changes per hour (ACH).   

Hence for an entire 3 bedroom house, a required *mimimum* continuous 
ventilation rate of  50 l/s (100 cfm)  would not be unusual. As is the case with
minimum standards,  the minimum is the worst condition that would be 
acceptable.

All of this (and the absurd plastered/baled head test) was intended to 
point out that attempting to use the walls as a means of providing 
ventilation air (ie as big air exchangers) is not a workable notion and that if
sufficient air changes are occuring, it will be due to air leakage through 
discontinuities in the air barrier. (ie cracks at joints between different 
components, planes or materials).

In superinsulated buildings which are heated or cooled, unintended points
of air leakage are also energy leakage points (ie wasteful) and perhaps more
importantly, points where moisture can find its way into the envelope materials 
potentially causing mould growth and/ or rot.


           ***SUCKER BAIT FOR NORBERT SENF***
                                         \    /
                                           V
>However, in the design I'm working on, I am planning a separate outdoor air 
>supply for the fireplace, so it will not be drawing indoor air to support 
>combustion. 


--- * ---
Rob Tom
Kanata, Ontario, Canada
<FlotsamArchiLogic@yahoo.ca>
(jettison the flotsam before responding)

Please visit http://www.theHungerSite.com daily



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