Indoor-Air Quality and Ventilation for Homes in Hot-Humid Climates
The weather in the Southern United States poses a unique challenge to the issues of ventilation and indoor-air quality. Here’s my opinions and suggestions on how to address these issues if you are building or remodeling in the South.
If you read generally about indoor-air quality and ventilation rates, the theory is that the air inside our homes is “dirty” and needs to be diluted with “clean” air from outside. Does this theory hold true in the hot-humid climate zone of the Southern United States? If so, how do we implement this in a thoughful way? I find the air outdoors to generally be fantastic and “fresh” in January, but I don’t always feel that way in July.
The generally accepted standard in the United States for ventilation is ASHRAE 62.2. This standard says that for every occupant, you need 7.5 CFM of fresh air plus 1CFM per 100 sq. ft. of floor space. For instance, a 3000-sq.-ft. house with a family of four would need (7.5×4) + (3000/100) = 30+30 = 60 CFM, or 60 cu. ft. of fresh air per minute. That’s a lot of ventilation, especially if the air coming into the house is hot and/or humid. This is a pretty broad rule, but is there any better or more specific advice on this topic?
Building-science expert Joe Lstiburek’s advice: “Dilution is not the solution to the indoor pollution.” I saw Joe present at a conference a few months ago, and he did a great visual demonstration to make his point. He took a clear glass and put 1/2 in. of coffee at the bottom. The coffee represents the pollution.
Next, he poured clean water on top till the glass was full. His point is that it takes huge amounts of dilution to make the water pure again.
When it comes to specifics about our Texas climate and ventilation rates, there isn’t a great amount of information available. (See this fantastic blog post by Martin Holladay on this topic.) Here are my thoughts on what to do if you’re in Texas:1. Eliminate as many pollutants as possible
- Use all sealed combustion gas equipment. Gas furnaces should all be 90%-plus efficient units that use PVC vents and a sealed combustion chamber (no metal flue should be present). Water heaters should be mounted outside if tankless, or high-efficiency gas (or heat-pump electric) if inside the house.
- This seems obvious, but don’t smoke in your house. Not so obvious is don’t burn candles. Candles are terrible for indoor-air quality (here’s a great article from CNN on candle emissions).
- Pay attention to introduced pollutants. That “new car” smell is bad for you. Same goes for furniture, cabinets, carpet, and paint. Buy cabinets that meet green standards. I much prefer cabinet cases made from plywood vs. particleboard. Ask for plywood that is formaldehyde-free or CARB compliant.
- Be careful about cleaning supplies and the chemicals they bring into your house.
2. Source control
- Choose good exhaust fans for your bathrooms, put them on countdown timers, and run them for 60 minutes every time you shower. I really like the Panasonic exhaust fans coupled with Leviton countdown timers.
- Choose front-loading washers, which don’t emit much soapy and humid air.
- Run your kitchen exhaust fan when cooking with a gas cooktop (nitrogen dioxide is formed from burning natural gas), or use an induction cooktop, which many of my clients prefer anyhow. They don’t heat the house as much as a gas range.
3. Ventilation equipment
- Build tight, and ventilate right. Even if you’ve followed all the steps above, you still need to ventilate. My opinion is that you can cut the ASHRAE 62.2 standard by 50%. We want some fresh air, but not so much as to pay a huge energy penalty with our hot/humid air coming inside. Using 50% of ASHRAE 62.2 on the 3000-sq.-ft. house example above yields a number of 30CFM of fresh air introduced continuously. My opinion is that the houses I’m building should probably be satisfied with this amount of dilution.
Risinger Homes is a custom builder and whole house remodeling contractor that specializes in Architect driven and fine craftsmanship work. We utilize an in-house carpentry staff and the latest building science research to build dramatically more efficient, healthy and durable homes.
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