Vapor Barriers

By Gaius Hennin, PE

Someone recently asked me “What is the most common mistake in residential construction?” Without hesitation I responded: “Vapor barriers.” Vapor barriers are the most common mistake in residential construction? Yup-everything about them is often wrong. They are even improperly named. The word ‘barrier’ implies that a vapor barrier stops the movement of vapor, when in reality it only retards that movement. All membranes installed in residential construction as vapor barriers allow some vapor to pass through, for two reasons.

The first is on a molecular level; vapor moisture travels through these membranes via diffusion. This is the process by which water molecules travel from where there are many of them to where there are fewer of them as a result of their ‘kinetic’ properties. Molecules are constantly in motion, bumping into each other, and will therefore mix until evenly distributed. If there is an abundance of vapor moisture on one side of a vapor ‘retarder’, it will work its way between the molecules of the retarder until there is an even quantity on both sides; even a six-mil polyethylene membrane, even a sheet of glass, given enough time. The amount of vapor moisture that passes through the membrane is a function of the discrepancy of vapor moisture and vapor pressure; if one side is very wet and the other dry, more moisture will pass through than if one side is slightly damp and the other dry. Adding heat (and therefore pressure) to the wet side will also accelerate the rate of diffusion.

The second reason is much more practical. Materials manufactured for use as vapor retarders are sold in long rolls (long enough to go around the house several times), but are not wide enough to cover the inside or outside of a house without seams. These seams will leak air and therefore vapor. Also, the vapor retarders are installed with staples, and get pierced thousands of times with drywall screws; more opportunities for vapor to get through. Perhaps most significant, though, are all of the necessary and planned wall penetrations. Every electrical outlet can represent a large hole in the vapor retarder, as do direct vent combustion units used for heating, the all important cable TV wire, the cookstove hood, dryer vent, windows, doors, light switches, air conditioning piping, solar water heater piping and direct vent boiler exhausts, to name a few. Even with careful detailing using acoustical sealant tapes and specialty products that self seal to the vapor retarder, all of these penetrations make it more likely to leak.

In addition to the inherent problems with the vapor retarder itself, there is also significant confusion about when it is appropriate to use a vapor retarder and where to place it in the wall assembly. This confusion is well deserved. The ‘rules’ (by rules, here I mean scientific rules, not building codes which heretofore have been static on the topic of vapor retarders) for vapor retarders change based on the climate in which you build, the type of vapor retarder you use, the type of wall assembly you use, the (variable) climate inside the house, who you ask and which building code is being enforced in your area. It is a contentious subject, with far more misinformation than fact readily available. Figuring out the ‘right’ answers to vapor retarder questions such as whether or not to use one and where to place it in the wall assembly can seem overwhelming and is more critical than ever with today’s highly insulated walls. The fact is there are no short answers or simple generalizations that can be applied to vapor retarders: vapor transport in buildings is complex. However, there is a solid body of knowledge based on partial and full scale testing of different wall assemblies in different climates. The two most important aspects of the wall assembly are the type of vapor retarder and the type of wall cladding (exterior siding). Wall claddings will greatly influence the selection of a vapor retarder. For years, the four model building codes defined a vapor retarder as “a material having a permeance rating of 1.0 (a ‘perm’ of 1) or less when tested in accordance with ASTM E96”, and 4 mil rated polyethylene was the standard bearer. For interest sake, and I know you are interested or you would have stopped reading in paragraph 1, a perm is defined as a hairstyle with long lasting waves or curls produced by treating the hair with chemicals.*

As the result of a five year study at the Oak Ridge National Laboratory and aggressive educating of the establishment figureheads by Joseph Lstiburek and Betsy Pettit of The Building Science Corporation and Achillles Karagiozis from ORNL, the building code has finally been modified to reflect reality. The predominant code in use today is produced by the International Code Council. The ICC has multiple publication (The International Residential Code, The International Building Code, The International Energy Conservation Code, etc) which are collectively referred to as the I-Codes. I will be writing more on these codes later as most states are now adopting the I-Codes and making their enforcement mandatory; it is the most widely used code in our history and it is here to stay.

The IRC (which governs construction of 1 and 2 family dwellings) recognizes 8 zones that the Department of Energy has developed for moisture control recommendations (see accompanying map from the DOE). The 2004 changes to the code have eliminated the requirement for vapor retarders in zones 1 (Southern Florida), 2 (the gulf Coast Region), 3 (such as Oklahoma, Arkansas, South Carolina) and 4 (such as Missouri, Kentucky, Virginia, Maryland). The 2007 (to the IRC and the IECC) change moves from the old definition of a vapor retarder (see above) to three classes of vapor retarders :

• Class I - Less than or equal to 0.1 perm (such as polyethylene)
• Class II - Greater than 0.1 perm but less than or equal to 1.0 perm (such as kraft-facing)
• Class III - Greater than 1 perm but less than or equal to 10 perm (such as latex paint)

The code now requires that zones 5, 6, 7 and 8 have a Class I or Class II vapor retarder. There are several exceptions to this, the most far reaching is that a vapor retarder is not required on walls constructed of materials that cannot be damaged by moisture or freezing.

For the first time we have rules regarding vapor barriers not dictated by politics, but actually based on fact. Getting any building code to change is a colossal undertaking, and usually involves going toe-to-toe with some of the largest and most established organizations in the country (the National Association of Homebuilders, for example, who fought this change to the very end). The facts of the studies, however, and common sense make the necessity of this change obvious. We have known for years that a one size fits all approach to vapor retarders simply doesn’t work, and finally it is no longer a part of the document that governs how we build.

Saw Sale

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Pat's Perspective

Maintaining an Oil Boiler by Pat Hennin

Right now is the perfect time to prepare for what will be the most expensive winter of our lifetime. The U.S. owes 90 trillion dollars to the rest of the world, a world which is not likely to continue to buy worthless American paper to continue to fund our questionable lifestyle, so let’s set about taking care of ourselves.

Over the next few months I will post strategies that I personally use which, taken together make my home considerably more affordably heated and comfortable. It is my experience that all people can learn to be competent and self-sufficient. Do learn from many sources before diving in. In my 45 years of adulthood I've found that whether dealing with oncologists, oil burner "technicians", estate lawyers, auto mechanics, veterinarians, flight instructors, physicians, builders, etc., I've always had to fix their errors and pay and pay. There is great joy in discovering how much can easily be learned and applied personally. Don't trust anything I say or write but use it critically to refine your knowledge. Please tell me of my misconceptions! And of course government people have never made life better and always more time consuming and costly...the ultimate mafia (Al Capone is wishing in his grave that he had simply become a code enforcement officer or Senator)

Let’s start with a simplified look at oil boilers and furnaces since there will be a crunch on these services this fall. Oil, no matter the price, still produces about 100,000 BTU’s of heat per gallon if the furnace is working right, which it never does without considerable help. Read the following and assess how much of it you can do or want others to do. Anybody can screw it up.

1. Only clean oil will squirt into the boiler or furnace and burn hot. Most dirt is created in the oil tank by moisture that condenses over the summer in partially empty tanks. This condensation creates rust, sludge and sulfuric acid. I keep my tank full and warm. If a tank is sweating, the basement is too cool, I keep my tank away from walls so I can keep it painted and rust free. I use “bullet” or similar rust-proof paint.


2. I clean the oil on its way to the burner gun by changing the filter now. There is a model number on the filter can and my local hardware store carries most filters. I turn off the valve at the bottom of the tank (counter clockwise) and tap it down, (it sometimes stay stuck up and open despite an unscrewed valve handle). I put a one gallon steel or plastic pan under the filter assembly to catch spillage. I unscrew the canister, usually by turning the bolt on top of the canister counterclockwise, clean the canister and replace the filter and rubber seal, carefully lining the seal up in its groove. I hold the canister up in place while screwing the bolt on top clockwise until finger tight, then usually another ½ turn or so with a wrench until it feels seated and tight. (Tightness is a feeling one develops with practice of putting different things together and observing them work or not). I keep the pan under the filter in case of a leak and dispose of the dirty filter. I turn the tank valve clockwise until it stops; I then gently back off one turn. The valve handle is soft metal designed to melt in a fire, so the valve can pop shut and not feed a fire with the fuel. Some newer furnaces have a more modern spin-on filter that is easier to replace, costs considerably more, but supposedly lasts several seasons. I run the boiler once a month off season to keep sludge from congealing in the filter and pump.


3. Next stop is the burner gun. People in the trade and lawyers will tell you not to touch these. It is yours, it is up to you. Here are some principles: a gun is a fan and a pump run by an electric motor which pumps oil from the tank and squirts it into the boiler chamber with the right amount of air while also creating a constant electric spark to keep the oil burning. There is a little photo cell in there which, if it does not detect an adequate flame in the boiler within a few seconds of start-up, will shut off the pump to prevent flooding the furnace and basement with unburned oil. Three things must be right. (A) Nozzle through which the oil squirts must not be clogged. The spin-on-filters pretty much prevent this. These nozzles control the amount of fuel burned (e.g. .6 gallon/hour and the shape of the flame to fit the shape of the boiler chamber, e.g. cone shape 60 degree spread. Nozzles are cheap and commonly replaced yearly. (B)The right amount of air is controlled by adjusting a plate covering the air intake by adjusting a screw or sliding a plate back and forth until the flame goes from orange or yellow (bad) to white or blue (good). Older boilers have a peep hole for this. Watching the exhaust gas out of the direct wall vent or chimney top go from black smoke to clear is the general objective. Today this process is refined by inserting a probe into the exhaust pipe and measuring the by products of combustion and by sucking exhaust gas through cotton filters to observe soot quantity. Air in-take is adjusted accordingly. (C) The two electrodes in front of the nozzle must be the right distance apart for electricity to jump from one to another to create sufficient spark at the right place to ignite the squirting atomized fuel. These electrodes (two wires about ¼-in apart) burn away over time, and must be adjusted forward and closer together over the nozzle or replaced yearly (see manual for exact calibration). A turbo shaped plate near the electrodes will have soot on it which must be cleaned. It swirls incoming air. Nozzle and electrodes are on an assembly that is easily removed from the gun for service. The manual illustrates this. One screwdriver and one wrench usually do it. I unplug the photo cell before I remove the assembly and check for soot on the cell. If it can’t see, boiler won’t ignite. Air intake is usually on the side of the gun. Again, to access all this, I shut the valve on the tank before disassembly. After reassembly, I turn the valve at the tank back on, but oil will have been replaced with air while I was changing the nozzle and points. Absence of oil in the pump will prevent it from pumping (remember the song . . . You must have faith and believe . . . ) so it will have to be primed by loosening a bleed nipple on the pump and letting air squirt out when the furnace is trying to start. Shut the nipple when a steady stream of fuel is squirting out hopefully not on your shoe. I put a foot of 3/16 vinyl tube over the nipple into a can to prevent spillage.
4. But the real fun is about to start. Since no one has done the previous steps correctly in years, yellow orange dirty flames have created huge amounts of soot that have completely coated the inside of the boiler so that no matter how good the flame is, it can’t get heat past all that soot to the water in the boiler or air in the furnace. A boiler is like a cast iron radiator through which water is pumped to other radiators in the house (these could be individual cast iron radiators, cheesy baseboard, expensive porcelain wall radiators or plastic in-floor tubing). If that first radiator in the boiler or furnace is soot covered, the heat from the gun just goes up the chimney. So, the entire gun is easily removed, and usually a side or two of the boiler. Then, bottle cleaner-like brushes are used to scrub the boiler radiator clean of soot and scale. A vacuum cleaner you don't care for is useful. Know your boiler before attacking it. There may be a fragile pot at the bottom into which the flame is directed and there may be pieces of sheet metal among the boiler sections to spread the flame. The general idea is to remove all material that was not there when the boiler was new that might prevent heat from getting to the water inside the cast iron. This is a very dirty job with very choking sulfuric dust. I’ve learned to enjoy it every year for the past 40 years because I’ve found no one cleans my boiler as well as I do.

Obviously, all of this is fraught with potential danger, damage, explosion, personal injury, pollution and many other possible calamities, jail sentences and liabilities. But, it is done every day by people with a hugely varied amount of understanding, training and give-a-damnedness.

Make your own decisions about participating (don’t take chances), only hire people who will let you watch and learn, but at least understand and supervise what needs to be done. The difference in fuel cost between correctly and badly running furnaces is huge. And remember that to live a long and happy life, take no pharmaceutical drugs, avoid animal based foods and eat very varied fruits, nuts, and vegetables. If a visit to a doctor results in a prescription, run! Patsy would be here today. Donations are always welcome to the PKH Cancer Research Fund, managed by the Maine Community Foundation. The can be mailed to the Maine Community Foundation 245 Main St, Ellsworth, ME 04605

Design Build Class Schedule

Spaces are still available in our July Two Week Design Build Class. Come to Maine at the most beautiful time of year and enjoy two weeks of learning the best practices of housebuilding from the ground up! We'll cover everything from laying out your site to take advantage of its natural features and passive solar gain to foundation and septic design, framing, architecture, engineering, wiring and plumbing, insulation and energy efficiency. We'll even cook your lobster for you! Whether you are building a new house to avoid the energy crunch or trying to make your existing home better, this is the course for you! Check out the daily course schedule on our web site.