[Strawbale] OSB or Not?

RT ArchiLogic at yahoo...
Mon Dec 6 20:25:06 CET 2010

On Mon, 06 Dec 2010 01:41:35 -0500, Michel Van Mulders  
<michelvanmulders at siol...> wrote:

> The question comes back to the basics; too much protection against  
> moisture movement sounds dangerous.
> The fact that the plaster started coming off with this man in Austria  
> could as well be of too dry....

Just to clarify a few points:

One needs to distinguish between "air permeance" and "vapour permeance"  
when talking about materials' ability to "breathe".

With any well insulated building, the construction detailing needs to be  
such that air leakage through the building envelope materials is minimised.

In heating situations, bulk moisture transport piggy-backed on air leakage  
 from conditioned interiors will lead to large amounts of water vapour  
being deposited in the envelope materials where it will condense into  
liquid water at the dew point within the cross section.

Once that moisture is in liquid form, heat loss through the well insulated  
building component will seldom be enough to turn that liquid back into  
vapour to be driven out through plaster finishes no matter how "vapour  
permeable" they may be. You may have witnessed this problem on the inside  
of Gore-tex garments in sub-zero temperatures.

The air permeance of most plasters at the thicknesses at which they are  
typically applied to walls and ceilings is low enough that it may be  
considered to be "negligible". (ie less than 0.001 litres per second per  
square metre of surface area at 75 Pascals pressure difference).

The minimum air change rate that is recommended by the American Society of  
Refrigeration & Air-conditioning Engineers (ASHRAE) is 0.35 complete  
changes of the total air volume of the building every hour (ie 0.35 ACH) .  
Baubiologists prefer a higher 1 ACH.

These are the minimums below which are considered unhealthy.

I'll leave it to you to do the arithmetic but it should be obvious from  
the above that if one relies upon the air permeance of a wall or ceiling  
assembly to allow sufficient air movement to provide the required air  
changes, one would be seriously mistaken.

The only other means that natural air movement through the envelope  
materials would occur is though air leaks at discontinuities in the air  
barrier. Since those locations would also be the same place that moisture  
is being leaked into the envelope materials, there is almost a certainty  
that there will be moulds etc. at those spots so that unhealthy and  
potentially deadly microbes will be transferred to the ventilation  
airstream (such as it is).

That is to say, the air exchange rate would be too low to be considered  
healthy (ie polluted) and what little air exchange that does occur would  
be contaminated by mould spores, crud and if combustion appliances are  
used for heating, quite likely combustion by-products (ie combustion  
Another point that bears mentioning re: OSB or Not ?

About 15 or more years ago, blower door testing was carried out on a  
number of SB houses built in maritime provinces Atlantic)here in Canada.  
They were all wet-applied plaster on SB.

The tested air leakage rate was low enough to be considered "air tight"  
for the purposes of the R-2000 performance standard. Houses built to the  
R-2000 standard were/are some the most energy-efficient, heathiest homes  
on the planet.

In the buildings which did not meet the required air-tightness standard,  
the weak points in the envelope were the "usual" suspects ... penetrations  
in the ceiling air barrier for electrical boxes. Once they were sealed and  
the building was blower door tested again, they passed.

=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at  Y a h o o  dot  c a >
manually winnow the chaff from my edress if you hit "reply"
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 

More information about the Strawbale mailing list