[Strawbale] Blower Door test set-up

[Mark Bigland-Pritchard]

Responses in text
Mark

David Neeley wrote:

> What I don't quite understand is how the inside/outside pressure
> comparison fully handles the situation. I can see it may at least
> partly, but doesn't the pressure from prevailing winds vary with such
> differences as wind speed, direction, windbreak locations, and the
> profile of the building?
>
The equipment measures both the difference in total pressure and the 
difference in dynamic pressure between inside and outside.

Something which is sometimes called the static pressure [though I note 
wikipedia disapproves of that nomenclature] is the pressure exerted by a 
fluid - in this case air - at rest.  Dynamic pressure is the pressure 
exerted purely as a result of the movement of the fluid - in this case 
some combination of the air movement caused by the fan and the wind 
outside the building.  Total pressure is the sum of the two components.  
Flow velocity may be calculated from dynamic pressure.

Hence it is possible to find the relationship between total pressure and 
air velocity - this is a relationship which applies at all (reasonable) 
pressure characteristics.  Once you have enough sets of data (each 
consisting of a dynamic and a total pressure) to determine the 
relationship (by curve-fitting), you can calculate the air velocity 
which corresponds to a standard pressure difference value of 50Pa.  From 
that and the volume of the building you can calculate the air changes 
per hour.


> For example, in high winds I would expect the air pressure on the back
> side of a shed roof would be quite different from the air pressure
> behind a hip roof, as one example. Or would this pressure difference
> be too small to have an effect?

In your example here David, the difference is relevant only if there are 
air gaps in the locations you note.  However, other examples could be 
given (in terms of orientations of windows with respect to prevailing 
wind directions, vertical distance between windows, etc) which can 
significantly affect the actual air changes per hour because they change 
the pressure characteristics of the building.
It is because of this that I say that converting the standard figure for 
50Pa to an "average" annual figure is problematic.  The 50Pa figure 
gives you a way of assessing how well sealed your building is compared 
to other buildings or to standards, but it doesn't tell you straight out 
what you can expect in terms of air exchange rates - for that you need 
to know the actual pressure difference across the envelope (something 
which is, of course, always changing).  The "solution" is to apply a 
standard multiplier and hope that it's accurate enough.  As air change 
rates are rarely measured directly over a prolonged period of time, it 
is difficult to tell in any particular case just how accurate it is.


> As far as constructing a near net-zero energy house, it seems to me
> that the best method might be to make one as tightly sealed as
> possible while incorporating an energy recovery ventilator for the
> needed air exchanges inside. That is exactly what I contemplate doing,
> at this point anyway. Would this strategy be wrong?
>
That is the strategy that makes most sense to me too.  The best-planned 
low energy house here in Saskatchewan uses this strategy very 
successfully, and it's also assumed necessary for anyone trying to meet 
the excellent German-initiated Passivhaus standard.  If you can get 
unintentional air infiltration down to a level well below that needed 
for normal living then you can ensure that the majority of your 
ventilation (i) is subject to heat recovery and (ii) is controlled to an 
optimum level.
But it won't work unless you have a truly anal-retentive approach to 
stopping air leaks.
And in some places it's more difficult than others.  In a maritime 
climate, the high humidity values necessitate a higher air change rate 
to avoid condensation, and the mild winter temperatures make HRV less 
economically viable.  So the building industry in places like Britain 
and Ireland has developed habits which make this strategy difficult.  
Not, however, impossible.


> david
>