You probably don’t think much about it, but it also should not be a shock to you, where the air you breathe in your home comes from. Your fresh air comes in through the cracks and joints and gaps in your home.
- the walls
- up through the crawl space if you have one
- in through an attached garage
- under the sill…
- down through the attic
Bottom line, your interior space is exchanging air in and out depending on the differential pressure gradients inside and out. We talk about pressure gradients acting on your home here.Trying to Get Your Bearings on Indoor Air Quality issues?
Air Leaks are not healthy for many reasons and we will get into that separately but we are talking about what is in the air that we are breathing. when the doors and windows are closed.
The air we are breathing inside the house when the doors and windows are closed pick up the local contaminants coming in plus those in the environment. This is the outside air plus any other molecules the indoor air picks up from mold or critter carcasses in the crawlspace or chemicals stored in the garage… the diagram above is more eloquent that me where your fresh air comes from.In North Texas, air leaks create humidity issues.
We need to discuss the 3 seasons we have in North Texas and their impacts on the air we breathe in our homes.
The heating and cooling systems operate in a scenario where the doors and windows are closed, and the fresh air sources- air leaks- are a function of the air leak pathways available and the differential pressures acting on the air leak pathways in your home. Here is the link again talking about pressure gradients.
We have a heating season.
Doors and windows are shut and the HVAC is running at various times, to give us heat. This heat is floating up on the cooler air and floating right on out of the leaks in the ceiling. This creates negative pressure in the lower reaches of the house, and cold make up air comes in from the outside from airleaks in the lower reaches where the differential pressure is greatest. the sills, the the crawl space if you have one… Cold air coming in is dry air. Cold air carries little moisture by its nature, is heated and the humidity lowers. You see people wanting to add humidifiers in the heating season. This is indicative of an issue with airleaks. Again we address air leaks here.
We have the cooling season
This is the season where the cool conditioned dense air can waft away under the sills and into the crawlspace being more dense and sinking out of the house displacing the hot outside air. The hot air in the attic now backfills through the ceiling from the attic leaks. Hot air leaking in and being cooled can create quite an increase in humidity in your home, and a bigger load for your A/C to dehumidify.This is a critical time to bring up potential issues with your mechanical system.
So we have two different scenarios for looking at indoor air sources across the North Texas year. We have two seasons where the doors and windows are closed and the fresh air is coming in through air leaks from you know where…
and we have the tween seasons
We have such wonderful weather in North Texas, in between the extremes. We can go for months in the fall and Spring when little heating or air conditioning is needed. For most of these days gradually fading into too cold or too hot, the weather is wonderful, and and the doors and windows are open. We are getting our fresh air right through the doors and windows and much less through crooks and crannies because of the really big dominating leaks called screened doors and windows.
When the windows and doors are closed tight.
So lets say we hire someone to cut off the air leaks, to cut down on the contaminants coming in, to help alleviate the humidity issues, to save money on energy and to help us be more comfortable; at what point do we impact the amount of fresh air in the house, when the doors and windows are not open?
If we cut off the air leaks we are lowering the number of air exchanges the indoor air makes in an hour. The indoor air gets more stagnant as a function of the number of air exchanges. The issue of a tight house is more critical with a small, dimensionally simple house. It can be made tighter because the outer shell- the air control layer is just smaller.
Large houses have extensive exterior surfaces which provide myriad paths for air leaks- even with the most strenuous efforts. The stack effect creates greater pressure differentials in 2 story homes.
And so as we make home tighter we must come to terms with the issue of stagnant indoor air. And so you… you yourself get to decide about your fresh air. You may decide you have no trouble with the air in your home. Or you feel a need for cleaner air and you want solutions for getting rid of pollution and pollen and particulates and odors in the house, and you want to know where your incoming air has not been. It is a matter of choice.Make sure you see our broad discussion of airborne contamination.
This brings us to the concept of active ventilation or mechanical ventilation
Active ventilation is having equipment to bring and filter outside air and to exhaust stall indoor air. Heat exchangers help keep our conditioned air from being as big a waste. Bringing in air is just creating air leaks unless you filter and have technology to save the energy you paid for… We talk about options for active ventilation next, but first we have to talk about the tween season in Texas when the air conditioner is off and the doors and windows are open…
What are the implications during the tween seasons?
During the tween seasons our heating and cooling systems are not running much. We are therefore breathing in air that is not being filtered by the HVAC system- you do pay proper attention to your HVAC filtration system? -LINK- and the indoor humidity is as high as the outdoors and goes up as the house cools.
What is this level of humidity? Is it high enough to create problems with mold? Does it make you feel muggy and uncomfortable when the outside air is not creating a breeze in the house? These are issues that you get to determine yourself.
And there are all stages of equipment that can be run to alleviate these conditions… so work fine, others are a waste of time. Running just the fan on the HVAC can actually raise humidity levels…. so this brings us to a discussion on the subject of active ventilation.Make sure you see the comprehensive look at healthy homes and the issues that arise.
Maybe mechanical ventilation is an answer for you.
Active ventilation allows you to bring in outside air directly through a filtered system rather than through the air leaks in your house. We walk in and out of doors. We can have a picnic on the patio on a glorious day with the breeze sans pollen, but the remainder of the time, the windows are closed, the screen door is closed… maybe grandma’s window is open because she was born in a covered wagon… but we basically know where the air comes in we are breathing.
There are a couple critical caveats with mechanical ventilation.
Mechanical ventilation is not a solution for musty smells. Musty smells relate to mold issues and we talk about issues with leaks and humidity issues and moisture problems and unhealthy homes here.
But after the moisture issues are ferreted out and the air leaks are solved and you have a tight house- you just might want to consider mechanical ventilation. It might even save on your energy bills over the course of the year- although you would not consider mechanical ventilation an energy saving option. You are exchanging stale conditioned air for fresh air from the outdoors- that is filtered.. That is, you are exchanging an air leak for filtered air for a bunch of air leaks where you don’t know where the air has been.
If you have comments or questions, just add them below.