Tuesday, January 18, 2011

No Stack Effect in Passive Houses

During our Passive House Consultants Training in Summer 2010, we were told empirical evidence is revealing is there is no stack effect in Passive Houses.  I'm referring to the way heat rises.  In normal buildings, the upper floors are warmer than the lower floors of buildings.  And tall rooms can be expensive to keep warm in winter because all the heat rises to the ceiling, while the occupants remain on the floor.

I found this claim of no stack effect fascinating but didn't know why it might be so.  After some thinking, I feel the answer is in the surface temperatures.   For stack effect to occur, there must be differences in air temperatures in a building.  In passive houses, all the interior surface temperatures are within 3 degrees C, throughout the year.  From our trainings, I understand this is even on the coldest day in winter, and on the hottest day in summer.  This means the stratification of air in the house is minimal, and that there is very little drive for stack effect to occur.  I would think in fact it does occur, but to a small degree only.

If you imagine a room full of air and the air is of varying temperatures, you expect that the warmer air will migrate slowly upwards while the cooler air stays lower, but what about all the air in-between?  If regardless of the room's height, all the air is within 3 degrees, one would expect a linear progression from one temperature extreme to the other.....so in a room 10ft high, the floor-air might be 18deg, while the ceiling-air is 21deg. At 5ft, the air might be 19.5deg.  All of this would be with no motion of the air at all.  Now if the heat source were near the floor and blowing a slight amount, there will be minute currents of air moving throughout the room.  You can see how this is getting pretty un-important, with these low temperature differences.  On the other hand, with a 10-degree C differential or more like in a regular building, the stack effect (and drafts of cool or warm air) might reasonably become much more noticeable.



  1. You're confusing a couple of different phenomena. Stack effect pressures are due to the difference between inside air temperature and outside air temperature as well as the overall height of the building. The same stack effect pressures exist in a PH as in any house of the same height in the same climate.

    Interior air temperature stratification, however, is due to the static buoyancy of warmer air, which requires either a source of warmer air (such as a heating register) or a source of cooler air (such as envelope leakage or cold surfaces - both of which are negligible in a PH).

  2. An excellent and insightful comment from Robert Riversong. Thanks!
    The wikipedia on stack effect is very good, as usual. No stack effect if interior air temperatures and moisture contents are the same as that for the exterior air. As Robert stated, Passive Houses will have stack effect, but temperature stratification within the building will be negligible. So the next Q is this - can cooling via stack effect actually take place in the summer, when exterior air temperatures are higher than interior? Seems it will serve to reduce the temperature differential, resulting in a loss of cooling in the building, until the interior and exterior are at equilibrium with each other.