We need to understand the forces that empower the air leaks in our home. Our homes leak air for two reasons. Your home provides pathways for air to leak in and out. Second, these forces we describe here create the differential air pressures that empower the movement of air through the leak pathways in your home.
First up is the stack effect.
The stack effect is a straightforward as well as a confusing term. The effects are much less confusing. Warm air is less dense than cold air, so warm air floats up- and if left undisturbed would stack up into layers of air by temperature, but this does not happen in the house, because so many things are stirring the air, cooling the air heating the air that the convection currents are mixing it up… but if you have an upstairs you will just might have a sense of the stack effect on your skin… on your comfort zone. In fact a lot of effort is spend solving problems with hot upstairs where the problem is all kinds of issues exacerbated by the stack effect.
Lets talk about the seasonal impacts of the stack effect.
Here is North Texas we have only two seasons and they can appear for any length of time, except in late summer when the cooling seasoin is in full swing, and in deep winter when we can run into periods of heating season. The stack effect operates between these two extremes in our flamboyant North Texas weather.
Heating season and the stack effect on the homestead.
Hot air floats up. In the cooling season this warm conditioned air creates pressure at the ceiling level as your warm conditioned air wants to float up right through the ceiling into the attic where cold ambiant air is more dense. Air passages in the ceiling allow your warm conditioned air to float up and float away.
Here is a sophisticated diagram of the interior of your house. It has been reduced to one big leak in your attice and all the little streams of leaking air have been reduced to just one stream. You will notice that high pressure at the ceiling creates low pressure in the lower parts of the house- I don’t see any reason why this cannot be described as cold ambient air sinking into the interior space through leaks and all the conditioned warmer air in the house floating up on this cold infiltrating air.
The stack effect in the cooling season.
Here the hottest air in the house is unconditioned air in the attic. The coldest air in the neighborhood is inside your house settling out on the ground floor because cold air is more dense. This cold air sinks out through leaks along the sill and floor joints, out through the crawl space and all points lower. This allows hot air to come down through the ceiling leaks to make up for the air sinking out of the lower levels. We have given you another elegant illustration to demonstrate this. Again, all leak paths in the ceiling are collected into one leak as well at the leaking out at floor level.
How can we nullify the stack effect?
We can stop the leakage of air empowered by the stack effect by sealing the passages for air to get out. If we have a well sealed attic, much less air is able to sink into the house, and so very little conditioned air can sink away through the sills and floor joints because it cannot overcome the vacuum effect of the ceiling being sealed. The pressure would equalize between inside and outside and no flow would go…
The wind effect on Air Leakage.
When the wind blows your home leaks more air. Wind effects are very dynamic and every changing which creates air movement in and out of the building do to the ever changing dynamics of the wind loading. We will take all these dynamic forces and turn them into static easy to understand forces.
Wind directly into the side of a building increases the pressure there depending on the wind speed, this is like wind loading for building construction it has to take so much of a direct wind load. Wind also slides by a wall running along its edge. This reduces the pressure along that wall according to Bernoulli’s Law- which is basically that a wind flowing horizontally along a plane- in this scenario that plane is the wall of our house- the faster the wind, the lower the pressure along that plane.
Amy is going to demonstrate Bernoulli’s Principle for you.
So the dynamics of wind acting on your house will distribute a pattern of lower and higher pressures on the exterior of your home and these differential pressures will facilitate air leaks in and out, in relation to the speed and direction of that wind. So when you get a chill in your North Texas home when the heating season arrives and the cold north wind is blowing, you know that the problem is cold air leaking in and your nice warm air leaking out and you are in the zone of air leak heat transfer. Go find a sweater.
So how can we nullify the wind effect on air leakage?
Fix the leak pathways and your conditioned air stays where it belongs with you and not running off to the see the world outside the air control layer of your home.
Mechanical Fans in your home can create negative pressure inside your home.
Fans and blowers exhausting bathrooms or kitchens or clothes dryers through ducting to the outside, cause outside air to leak back into the house as make up air to again equalize pressure.
This is an important subject for two totally contradictory reasons.
On the one hand, by running fans in your home you are sending conditioned air out of the house, but in most cases, unless you forget to turn the fan off, you are exhausting hot moist air from a shower in a bathroom which would only increase the cooling load in the house, or add moisture- or, in the case of a kitchen fan you are moving hot moist air as you sent cooking odors out of the house.
A clothes dryer is casting off moist hot air which would add to the cooling load too…so these are all good- even though the make up air is through the air leaks in the house,and bringing in the whatever is happening outside- inside the house.
On the other hand, if you seal up the air leaks in the house, and run the same fans, you can create negative pressure in the house that can create a pressure gradient making it hard to evacuate air from the house.
Incidentally, this is not the reason why your bathroom vent is not working as well as it should right now… you need to track down that flex ducting, it has an issue, or you have a clogged filter… using my feminine intuitions side… We talk about exhaust fans- kitchen and bathroom here.
This becomes a bigger deal when you have combustion appliances in the house, and your home has the potential to create back pressure for lack of make up air. Some range hood fans can be very powerful, and require a large volume of make up air.
You and your family are in potential danger if some combination of your fans running and some combustion appliance burning can create a situation where Carbon Monoxide is not safely sent up the chimney but is allowed to waft about the house. Carbon monoxide can make you chronically sick at low levels- at lower levels than some carbon monoxide testers even register.
Does your home have combustion appliances? Do you use powerful fans to exhaust cooking odors or have central vacuuming facilities? Do you have a fairly tight house? Does your large range hood have a make up air system added to it? If this raised questions or doubts I would investigate further. We talk about combustion issues more fully here. The general idea is called CAZ testing – Combustion Appliance Zone.
These are the forces which drive air in and out of your house.
The air will not drive if you seal up the air leaks, and this is a good thing for all kinds of reasons as we discuss here, but if you get your home air tight- small homes are a bigger risk than a huge home just because of the surface are of your large home…. small is better, but small and air tight can create a ventilation problem.
You might have concerns about healthy indoor air this can create an interest in active or mechanical ventilation technology- systems which bring in a regular dose of outside air through a heat exchanger to give you fresh air through a filtered source. Have you heard of these before? We talk about them here.
Any questions? Any thoughts come to mind? Feel free to add them below.