rarely the only cause of a venting problem. Nevertheless, chimneys in locations such as the one above may be susceptible to wind-induced failure, partly because they were failure-prone to begin with.
The Neutral Pressure Plane
In cold weather the buoyancy of the warm air in a house causes a slight pressure difference from the highest to the lowest point. The pressure high in the house is positive relative to atmospheric pressure and it is negative low in the house. Between the high and low pressure areas is a zone of neutral pressure called the neutral pressure plane. When the air is calm, the NPP is roughly horizontal. The idea of the NPP and its likely position under various conditions can be useful in describing what happens to pressures inside a house in windy weather.
The force of wind blowing around a house produces a positive pressure zone on the windward side and a negative pressure zone on the downwind side. These pressures act on the leaks in the envelope, causing air flow through them and changing the pressures within the house. These pressure changes are best illustrated by looking at their effect on the position of the neutral pressure plane. The NPP can tilt away from the horizontal, but no illustration can properly convey the ragged, messy shape that the zone of neutral pressure can be distorted into by wind effects. Perhaps the best way to visualize the wind-induced pressure variations in a house is to compare the NPP to the surface of rough water. The plane of neutral pressure will have waves, curves, peaks and valleys responding to the aerodynamic influences around the building envelope. This understanding renders inherently inaccurate any simple attempt to define and illustrate the position of the NPP under windy conditions.
In strong winds, the pressures experienced by the building envelope can be very powerful—several times the normal pressures produced in chimneys through natural draft. In gusting winds, the pressures and position of the NPP are in constant change, further complicating the diagnostic process.
The design and setting of a house can influence the pressure environment inside during high winds. Imagine that the house on the left backs onto an attractive ravine and that the architect located most of the windows to take advantage of the view. The majority of leaks in the envelope could be on the exposed two-storey section. When a strong wind blows from the front of the house, the entire interior could be placed under negative pressure. This effect could have disastrous consequences for a hearth system installed inside.
The effects of wind acting on leaks in the building envelope can cause wild fluctuations of the pressure inside. An open window on the downwind side can cause the pressure inside to become extremely negative. Likewise, an open window on the windward side can pressurize the house. This effect can help to explain many venting failures of wood heating systems and illustrates the importance of looking at the whole house, and not just the top of the chimney, when diagnosing venting failures. That is, wind acting on the building envelope can cause smoke to be sucked out of a stove or fireplace due to negative pressure in the house.
The effect of wind on the pressures around and inside a building are complex and unpredictable. In general, however, the leakier the building, the more pronounced and immediate is the effect on pressures inside. The unpredictable effects of wind pressure is one reason why the installation of a specialized chimney cap may not cure a venting problem. The pressure changes inside the house may be either driving or adverse to the desired flow of exhaust gases up the chimney.
The illustration to the left shows why wind is more likely to depressurize than pressurize a house as it flows around it. The air flowing parallel to the sides of the house exerts a pressure lower than atmospheric pressure on the house surfaces. Combined with the negative pressure zone on the downwind side, this means that three of the four sides are likely to experience negative pressur