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Tight Houses: A Healthy Idea

By HHI Staff

Many people don't like the idea of living in a tight house, even though there are a number of significant disadvantages to the alternative (loose construction). In a loose house, air moves through the cracks, but only part of the time. Most new houses today are too tight to give you the amount of fresh air you really need, but too loose to keep pollutants out effectively.


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When outdoor air moves through the cracks of a house into the living space, it brings with it pollutants contained in the outdoor air. Plus, it picks up additional pollutants along the way as it passes through the cracks, such as tiny particles of insulation, odors from the resin holding the insulation together, and tiny pieces of insects that have died inside the walls. Radon is also often pulled from the soil into the living space through the cracks of a loose house.


Air moving through cracks can also result in hidden moisture condensation, and that can lead to mold growth, rot, termites, or carpenter ants. Here's what happens: All air contains moisture in the form of water vapor. If you cool a given batch of air enough, that vapor condenses into liquid water. So, if air passing through the cracks in a wall reaches a cool surface, it can condense there, hidden inside the wall cavity where you likely won't know anything is wrong until you have a serious problem. This can happen in the winter, when warm indoor air passes through the wall toward the outdoors and hits the back side of the cold sheathing or siding. It can also happen in the summer, when hot, humid, outdoor air passes through the cracks and hits the back surface of the drywall, which is cool because the inside of the house is air conditioned.


Leaky houses can also be energy hogs. In the winter, the warm air leaks out and the cold air leaks in, and your heating bills can skyrocket. In the summer, the opposite happens. Hot, humid air leaks in and cool air leaks out, but the result is the same (high energy bills.) When you analyze the issue, there are absolutely no advantages to loose construction. So, even if you're only remodeling or adding a room, it's a good idea to tighten as much as possible.

The Three Principles of Tight Construction

When considering tight construction, indoor air pollution sources can be grouped into three broad categories: things inside the living space, things outside the living space, and the people in the house.


Some things inside the living space that result in polluted air (and examples of their pollutants) include building materials (formaldehyde), cleaning products (solvents), furniture (volatile organic compounds), books (printing ink), and clothing (chemical treatments).


Some things outside the living space can be a problem if the pollutants they give off find their way indoors. Examples are automobiles (exhaust fumes), the soil (radon), and lawns (herbicides, fertilizers). Some building materials that are outside the living space - insulation being the best example - can be pollutant sources (technically, insulation is outside the living space because it is within building cavities).


People inside the house pollute the indoor air as well. We all release a variety of chemicals as a normal part of our metabolism. If these compounds are allowed to build up indoors, the air can get stuffy and start smelling like a locker room.



To minimize the impact of air pollutants, you have three choices. You can eliminate the polluting material completely, you can separate yourself from the polluting material (build a barrier between you and it), or you can ventilate (dilute the concentration of what's in the air). Eliminate, Separate, Ventilate: These are the three basic principles of a healthy house. (Air filters are also popular, but I place them in a subcategory, as a part of ventilate.)


By far, the most important way to minimize air pollution is to eliminate the source, but that's often easier said than done. What if there's heavy traffic in front of your home? You can't close off the road and eliminate the traffic. What if you're bothered by the formaldehyde resin in fiberglass batt insulation, and a less-toxic insulation is too expensive?

This is where the second healthy house principle of "separate" comes into play. If you build a very tight house by making the drywall layer virtually airtight, then nothing beyond the drywall will be able to get into the living space. You will effectively separate the living space from everything outside that space. Once this is accomplished, in a tight, healthy house you must also ventilate mechanically.


It's important to use all three principles, because each principle works best against a certain group of pollutants. For example, the first principle, eliminate, is most effectively applied to our first category of indoor air pollutants: things inside the living space. In other words, everything you can actually see inside a house should be as non-toxic as possible. This includes the paint, the floor coverings, the cabinets, the decorations, the furniture, the cleaning products.


The second principle, separate, is best applied to our second category: the pollutant sources outside the living space. If you build a tight house, radon from the soil, mold in the crawl space, automobile exhaust, pesticides from your neighbor's fruit trees - none of this will be a problem indoors.


The third principle, ventilate, deals with the third indoor-pollutant category: metabolic pollutants released by the people or animals in the house. [Ventilation can also dilute the concentration of pollutants released by materials found indoors that can't easily be eliminated, such as cooking odors, tobacco smoke on your guest's clothing, or fragrances clinging to your mail.]


Two different types of mechanical ventilation are important in healthy houses. Many houses already have one type, local ventilation. Local ventilation is used to occasionally air out one part of a house quickly. This is what kitchen range hoods and bathroom exhaust fans do. They remove moisture or odors rapidly, before they can travel into the rest of the house. The other type of ventilation - general ventilation - is what's missing in most houses. It's important because we don't spend all of our time in one room. We move throughout the house, so we need fresh air in all the rooms, all the time. It's general ventilation that our third healthy house principle, "ventilate", refers to.


A general ventilation system slowly brings in fresh air, and simultaneously exhausts an equal volume of stale air, so the air in the entire house is kept fresh. [General ventilation systems can be simple or complex, but basically they involve using one or two fans to bring fresh air into the house and, at the same time, expel stale air. The fans generally move this air through ductwork that is connected between the indoors and the outdoors.]


There is also crawl-space ventilation, and attic ventilation, but they benefit attics and crawl spaces (sometimes), not the occupants inside the living space.


Ventilation and Outdoor Pollutants


How to build tightly

Tight construction doesn't involve magic, or any complicated skills, but it does take some training, knowledge, and care on the part of the builder. It involves using gaskets, special airtight electrical boxes, caulking, and expanding foam insulation during various stages of the house-building process. It also requires an understanding of vapor barriers (more correctly called diffusion retarders). Because all houses are different, it's impossible to tell someone in a single article how to handle all the details that will arise when building a house. But one of the easiest techniques to learn, and to implement, is the Airtight Drywall Approach (ADA). With ADA, you make the drywall layer on the exterior walls and ceilings as tight as possible, then seal the drywall to a tightly constructed floor system.

But, you might ask, "Won't ventilation bring in outdoor air pollutants?" Yes and no. If you have a loose house, whatever is outdoors is definitely going to come indoors through the cracks. But if you have a tight, mechanically ventilated house, the outdoor pollution will only enter when the ventilation system is running. And this gives you a tremendous amount of control. If your ventilation system is designed to run 24 hours a day, you can add a filter to clean the incoming air before it enters the house. Or, and this often works extremely well, you can operate the ventilation system intermittently.


In many locales, outdoor pollution is really bad only at certain times of day. For example, wood smoke in winter may only be a problem at night, or there may be heavy traffic only during daylight hours, or lawn mowing may only occur on weekends. In such cases, you can operate your ventilation system with a programmable timer that's set to turn on only during hours when the outdoor air is clean. You can also use a simple on-off switch and operate your ventilation system manually. There are a number of possibilities, but the key word is control. In a tight house with mechanical ventilation, you control your air supply. If you see a neighbor dragging lawn chemicals out of his garage, you can simply turn off your air supply until he's finished and the outdoor air has had a chance to clear.


Of course, you don't want to leave a mechanical ventilation system turned off for several days at a time. But there's enough air in most tight houses that it would take at least 24 hours before they start getting stuffy.


If you build a tight house with healthy materials, and you clean and maintain it with healthy products and decorate with healthy furnishings, then you only need to ventilate it enough to dilute the metabolic byproducts released by the people or animals inside. To arrive at an appropriate ventilation rate, it's been determined that 15 cubic feet of air per minute (cfm) per person is enough to keep a house from smelling stale or stuffy.


In other words, 15 cfm per person is enough to reduce the concentration of metabolic byproducts below a detectable level. Thus, 15 cfm per person is deemed a good ventilation rate for houses if the only pollution source is the occupants. Many people call this a "body-byproducts" standard, not a health standard, and they're right. It's enough ventilation to keep the outgassing of people in check. (Of course, there's some variability to what people can detect with their noses. For 80 percent of us, 15 cfm is sufficient. But some of us might need 18 cfm or 20 cfm.)


Fifteen cfm per person really isn't a great deal of air, but it's generally enough if you have a healthy house to begin with. So, if you have a tight, healthy house, and it normally contains 4 people, you should have 60 cfm of continuous ventilation.


If you like the intermittent operation option, be sure to design extra capacity into your ventilation system. For example, if you need 60 cfm of continuous ventilation, but only plan to operate the system 12 hours a day, you'll want to overventilate when the system is running at 120 cfm. For added flexibility, you might select a unit capable of 150 cfm and use a variable speed control.


In most cases, you don't want to overventilate 24 hours a day, every day. Too much ventilation in the winter can result in a very dry house; too much in the summer can cause the air conditioner to run too much. But a "just right" rate will be both comfortable and economical.

Balancing Healthy House Costs

Now you're probably wondering, "What's this all going to cost?" The answer is (as usual) "It depends."


Many builders in cold climates have learned that, by building very tight, very energy efficient houses, they can pay for the cost of extra insulation, the cost of tightness, and the cost of a mechanical ventilation system with what they save by installing a much smaller heating/cooling system. That means you can get two healthy house principles (separate and ventilate) for free! Of course, there's a catch: the builder has to train his crew. If he can spread that training over a subdivision full of houses, there's very little investment per house. But if he doesn't specialize in tight, energy efficient houses, and you want him to build one, he'll probably tack that training cost onto your single house.


You can't guess how tight a house is, so if a builder says he builds tight houses, ask him to prove it. To measure the tightness of a house after it's been built, a diagnostic device known

Blower door testing: How to tell if a house is tight


Typically, in a blower door test, a house is depressurized to minus 50 pascals, a pascal being a very tiny unit of pressure measurement. The volume of air blowing out of a house at -50 pascals will be inversely proportional to the tightness (the more air, the looser the house). In a very loose house, you might have to exhaust 2,500 cubic feet of air per minute (cfm) to reach -50 pascals. (This is designated 2,500 cfm50.) An average new house might be in the range of 1,200 cfm50, and many energy-efficient builders construct houses tested at less than 800 cfm50. With care, an extremely tight house can be built that will reach this depressurization at 200 cfm (200 cfm50) or less. While it's impossible to construct a hermetically sealed house, the tighter the better.


as a "blower door" is used. Among other things, a blower door can depressurize a house under very controlled conditions. This allows a technician to tell how tight a house is, compared to other houses. [Some companies that specialize in adding insulation or weatherizing houses routinely use blower doors - but many don't. So, the best way to locate a qualified blower-door contractor is to contact a blower door manufacturer and ask who has one of their devices in your area.]


Once you have a builder who's capable of building tightly, and who understands mechanical ventilation, it's relatively easy to get him to incorporate the first principle and eliminate. This often means using a hard-surface floor covering, preferably ceramic tile or hardwood; low-polluting kitchen cabinets of solid wood or metal; low-VOC paints, finishes, and adhesives; and a healthy heating system. Sometimes, healthy materials can cost more but, in most cases, there are ways to keep cost increases to a minimum.

The Best of All Worlds

In a tight house with mechanical ventilation, you can have just the right amount of air entering and leaving, neither too much to be wasteful nor too little to be effective. So, to sum up, select healthy materials (eliminate the bad ones), put the materials together in a tight manner (separate), and ventilate modestly. You'll have the best of all worlds.


This doesn't mean you should ignore the advantages of windows, they're great to look out of, and they can be opened on nice days to air the house out quickly. But if there's a sudden outdoor pollution problem, they can be shut, and you'll be safe inside your tight house.


(Note: The views expressed in this article are those of the author, and do not necessarily represent those of The Healthy House Institute, LLC.)



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Tight Houses: A Healthy Idea:  Created on April 17th, 2007.  Last Modified on February 28th, 2011


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