A HEALTHY IDEA
Most people have heard that tight houses cause indoor air pollution.Actually, this represents a simplistic view of the problem. Tight constructionis, in fact, part of the solution. This article explains why.
LOOSE CONSTRUCTION VS. TIGHT CONSTRUCTION
Many people don't like the idea of living in a tight house, eventhough there are a number of significant disadvantages to the alternative(loose construction. In a loose house, air moves through the cracks, butonly part of the time. Most new houses today are too tight to give you theamount of fresh air you really need, but too loose to keep outdoor pollutantsout effectively.
When outdoor air moves through the cracks of a house into the livingspace, it brings with it all the pollutants contained in the outdoor air.Plus, it picks up additional pollutants along the way as it passes throughthe cracks - tiny particles of insulation, odors from the resin holdingthe insulation together, tiny pieces of insects that have died inside thewalls. Radon is often pulled from the soil into the living space throughthe 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 watervapor. If you cool a given batch of air enough, that vapor condenses intoliquid water. So, if air passing through the cracks in a wall reaches acool surface, it can condense there ó hidden inside the wall cavitywhere you 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 walltoward 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 throughthe cracks and hits the back surface of the drywall, which is cool becausethe inside of the house is air conditioned.
Leaky houses can also be energy hogs. In the winter, the warm air leaksout and the cold air leaks in, and your heating bills can skyrocket. Inthe summer, the opposite happens. Hot, humid air leaks in and cool air leaksout, but the result is the same (high energy bills. When you really startanalyzing the issue, there are absolutely no advantages to loose construction.So, even if you're only remodeling or adding a room, itísa good idea to tighten as much as possible.
THE THREE PRINCIPLES OF TIGHT CONSTRUCTION
When considering tight construction, indoor air pollution sources canbe 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 (andexamples 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 pollutantsthey give off find their way indoors. Examples are automobiles (exhaustfumes), the soil (radon), and lawns (herbicides, fertilizers). Some buildingmaterials [that are outside the living space] - insulation being thebest example - can be pollutant sources [(technically, insulationis outside the living space because it is within building cavities)].
People inside the house pollute the indoor air as well. Sometimes it'sour breath, sometimes it's our armpits - but the fact is,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 stuffyand start smelling like a locker room.
To minimize the impact of air pollutants, you have three choices. Youcan eliminate the polluting material completely, you can separate yourselffrom the polluting material (build a barrier between you and it), or youcan ventilate (dilute the concentration of what's in the air). Eliminate,separate, ventilate ó these are the three basic principles of a healthyhouse. (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 eliminatethe source, but that's often easier said than done. What if there'sheavy traffic in front of your home? You can't close off the roadand eliminate the traffic. What if you're bothered by the formaldehyderesin in fiberglass batt insulation, and a less-toxic insulation is tooexpensive?
This is where the second healthy house principle of separate comes intoplay. If you build a very tight house - by making the drywall layervirtually airtight - then nothing beyond the drywall will be ableto get into the living space. You will effectively separate the living spacefrom 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 principleworks best against a certain group of pollutants. For example, the firstprinciple, eliminate, is most effectively applied to our first categoryof indoor air pollutants: things inside the living space. In other words,everything you can actually see inside a house should be as non-toxic aspossible. This includes the paint, the floor coverings, the cabinets, thedecorations, 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, pesticidesfrom your neighbor's fruit trees - none of this will be aproblem indoors.
The third principle, ventilate, deals with the third indoor-pollutantcategory: metabolic pollutants released by the people in the house. [Ventilationcan also dilute the concentration of pollutants released by materials foundindoors that can't easily be eliminated, such as cooking odors, tobaccosmoke on your guest's clothing, or fragrances clinging to your mail.]
Two different types of mechanical ventilation are important in healthyhouses. Many houses already have one type, local ventilation. Local ventilationis used to occasionally air out one part of a house quickly. This is whatkitchen range hoods and bathroom exhaust fans do. They remove moisture orodors rapidly, before they can travel into the rest of the house. [Localventilation might also be used in conjunction with a reading box, so odorswithin the box can be blown directly outdoors.] 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 oneroom. 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 houseprincipal, ventilate, refers to.
A general ventilation system slowly brings in fresh air, and simultaneouslyexhausts an equal volume of stale air, so the air in the entire house iskept fresh. [General ventilation systems can be simple or complex, but basicallythey 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 ait throughductwork that is connected between the indoors and the outdoors.]
There is also crawl-space ventilation, and attic ventilation' butthey benefit attics and crawl spaces (sometimes), not the occupants insidethe living space.
VENTILATION AND OUTDOOR POLLUTANTS
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 definitelygoing to come indoors through the cracks. But if you have a tight, mechanicallyventilated house, the outdoor pollution will only enter when the ventilationsystem is running. And this gives you a tremendous amount of control. Ifyour ventilation system is designed to run 24 hours a day, you can add afilter to clean the incoming air before it enters the house. Or -and this often works extremely well - you can operate the ventilationsystem intermittently.
In many locales, outdoor pollution is really bad only at certain timesof 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 mowingmay only occur on weekends. In such cases, you can operate your ventilationsystem with a programmable timer thatís set to turn on only duringhours when the outdoor air is clean. You can also use a simple on-off switchand 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 chemicalsout of his garage, you can simply turn off your air supply until he'sfinished and the outdoor air has had a chance to clear.
Of course, you don't want to leave a mechanical ventilation systemturned off for several days at a time. But there's enough air inmost tight houses that it would take at least 24 hours before they startgetting stuffy.
If you build a tight house with healthy materials, and you clean andmaintain it with healthy products and decorate with healthy furnishings,then you only need to ventilate it enough to dilute the metabolic byproductsreleased by the people inside. To arrive at an appropriate ventilation rate,it's been determined that 15 cubic feet of air per minute (cfm) perperson is enough to keep a house from smelling stale or stuffy.
In other words, 15 cfm per person is enough to reduce the concentrationof metabolic byproducts below a detectable level. Thus, 15 cfm per personis deemed a good ventilation rate for houses - if the only pollutionsource is the occupants. Many people call this a body-odor standard, nota health standard, and they're right. It's enough ventilationto keep the smell of people in check. (Of course, thereís some variabilityto what people can detect with their noses. For 80 percent of us, 15 cfmis 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'sgenerally enough ó if you have a healthy house to begin with. So,if you have a tight, healthy house, and it normally contains 4 people, youshould have 60 cfm of continuous ventilation.
If you like the intermittent operation option, be sure to design extracapacity into your ventilation system. For example, if you need 60 cfm ofcontinuous 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 150cfm 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. Buta "just right" rate will be both comfortable and economical.
BALANCING HEALTHY HOUSE COSTS
Now you're probably wondering "What's this all goingto 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 withwhat they save by installing a much smaller heating/cooling system. Thatmeans you can get two healthy house principles (separate and ventilate)for free! Of course, there's a catch - the builder has totrain his crew. If he can spread that training over a subdivision full ofhouses, there's very little investment per house. But if he doesn'tspecialize in tight, energy efficient houses, and you want him to buildone, 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 hebuilds tight houses, ask him to prove it. To measure the tightness of ahouse after it's been built, a diagnostic device known as a "blowerdoor" is used. Among other things, a blower door can depressurizea house under very controlled conditions. This allows a technician to tellhow tight a house is, compared to other houses. [Some companies that specializein adding insulation or weatherizing houses routinely use blower doors-butmany don't. So, the best way to locate a qualified blower-door contractoris to contact a blower door manufacturer and ask who has one of their devicesin your area.]
Once you have a builder who's capable of building tightly [seesidebars], and who understands mechanical ventilation, it's relativelyeasy to get him to incorporate the first principle and eliminate. This meansusing 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 materialscan cost more but, in most cases, there are ways to keep cost increasesto a minimum.
THE BEST OF ALL WORLDS
In a tight house with mechanical ventilation, you can have just the rightamount of air entering and leaving - neither too much to be wastefulnor 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 onnice days to air the house out quickly. But if there's a sudden outdoorpollution problem, they can be shut, and you'll be safe inside yourtight house.
SIDEBAR: 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-buildingprocess. It also requires an understanding of vapor barriers (more correctlycalled diffusion retarders). Because all houses are different, it'simpossible to tell someone in a single article how to handle all the detailsthat will arise when building a house. But one of the easiest techniquesto learn, and to implement, is the Airtight Drywall Approach (ADA). With ADA, you make the drywall layer on the exterior wallsand ceilings as tight as possible, then seal the drywall to a tightly constructedfloor system.
SIDEBAR: Blower door testing: How to tell if a house is tight
Typically, in a blower door test, a house is depressurized to minus 50pascals, a pascal being a very tiny unit of pressure measurement. The volumeof air blowing out of a house at -50 pascals will be inversely proportionalto 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 mightbe in the range of 1,200 cfm50, and many energy-efficient builders constructhouses tested at less than 800 cfm50. With care, an extremely tight housecan be built that will reach this depressurization at 200 cfm (200 cfm50)or less. While it's impossible to construct a hermetically sealedhouse, the tighter the better.
Written by: John Bower - The Human Ecologist
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