Can my home be too tight?
No, a home cannot be too tight. However, a tightly constructed home without a planned and measured ventilation strategy is a recipe for disaster! I have seen this many times. The tighter the home, the more important the mechanical ventilation. It’s common sense. People get into trouble when they ignore the “V” in HVAC. Having a tightly constructed home with proper ventilation allows you to maintain good indoor air quality, lower utility bills, increased comfort, and moisture control. With ventilation, you need enough to get the job done and nothing more.
When I mention ventilation, I am not talking about an expensive nuclear powered exhaust system. At a minimum, your home should have local exhaust (fans normally placed above the source of moisture or pollutant) in your home. Each bathroom should have a minimum of a 50 cfm fan, and a 100 cfm exhaust fan in the kitchen. After the fans have been installed, MEASURE the air flowing through them to make sure you are getting the performance you paid for! See, common sense!
IMPORTANT! – Before I go any further, I want to mention that if you are planning on air sealing or weatherizing your home, and you have a natural draft furnace and/or water heater, make sure these heating appliances do not back draft and spill carbon monoxide into the home. Homes with natural draft heating equipment can only be air sealed so far before it becomes potentially unsafe. Have a combustion safety test done on your home by a home performance professional if you think you may be at risk. If you have a power-vented furnace and water heater, geothermal or are all electric, back drafting is not a problem.
Let’s imagine that someone burned a bag of popcorn in the microwave. Smoke has filled the kitchen and dining room and the house smells horrible. What would you do? Would you crack the kitchen window and go back to watching TV? Heck no! You would open every window and door in the house to get the smoke out. You would not leave the windows and doors open all night would you? With ventilation, we need enough to get the job done and nothing more. Leaky homes have too much ventilation.
According to ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers), if a home has a leakage rate greater than .35 ACHnat or 7 ACH50 then ventilation is not necessary. There is enough leakage in the home’s building shell to prevent a buildup of moisture and toxins inside the home to unsafe levels and provide acceptable indoor air quality. If a home has less than that, then mechanical ventilation becomes a necessity.
A blower door is used to measure the leakage rate of a home. We look at air leakage two ways: 1. Natural Air Changes per Hour (ACHnat) – this is the amount of air leakage with a 4 mph wind outside. 2. Air Changes per Hour @ 50 Pascals (ACH50) – this is with the blower door depressurizing the home to 50 Pascals to simulate a 20 mph wind outside.
Let me put this into perspective. Just before sunrise on a cold winter morning, there is very little wind outside. The house is comfortable and the furnace occasionally cycles on and off as you enjoy your morning
coffee. The sun rises and the wind picks up to 20 mph. You feel drafts on your feet and you notice the furnace cycling more often and running longer. The temperature outside is still the same, the only thing that’s different is the wind has picked up. What has happened? The home’s air leakage rate has increased 20 times while you were drinking your coffee! This is why having control over your air infiltration rate is so important.
In closing, whether you choose to let Mother Nature ventilate your drafty home or put effort into air sealing your home and ventilate mechanically, the bottom line is that safety and indoor air quality should be your first priorities!
“You cannot improve what you cannot measure.”