By David Braddy LEED GA
I want to thank all of you who responded with questions and comments on the last article regarding a home being built to tight, its ability to breathe and proper ventilation. So here is some additional information I will share based on those questions.
Now let’s start with a home breathing; even though a home should be as air tight as possible, it doesn’t mean it shouldn’t be well ventilated, it just needs to be ventilated properly and leaky walls or windows is not the proper way. First of all you should find out where you have leakage in your home with an Energy Audit by a BPI or RESNET certified auditor. A blower door test will quickly determine where you have problem areas. You can find an auditor through your local CO-OP, Power Company or look on the internet for one near you.
After you fix (seal) any problem areas it is time to make sure you have proper ventilation.
So how do you ventilate your home properly? A starting point is to have properly sized exhaust fans in all bathrooms and kitchens. Make sure you vent the exhaust to the outside. Do not vent directly into the attic or crawlspace or you are simply adding moisture to another area and creating another set of problems.
The best ventilation system is incorporated into your Heating Ventilation Air Conditioning System and/or the use of an HRV or ARV. This method can also save additional energy cost.
Now let’s examine the most confusing subject; the proper air barrier. The answer could be different depending on the application. Until 2007 the same code applied no matter what the climate. Poly on the inside of an exterior wall was the accepted method regardless of climate. We now know vapor barrier should be used on the warmest side of the wall; this also creates confusion in areas that have mixed climates where both sides can be considered warm sides.
Here is what we now know; “Inward Solar Vapor Drive” in the summer is far more of an issue than vapor drive from the inside out in the winter. Inward solar vapor drive has been linked to problems as far north as Canada. Think about it; we live in an area with hot and humid summers and cold dry winters. Moisture content in the air or on the exterior is driven inward until it hits a cooler surface it cannot penetrate (interior vapor barrier) and forms condensation. This has caused many mold issues in wall cavities and vinyl wallpaper creates the same conditions as an interior vapor barrier. (Ask the hotel industry about the problems they had with this). In the winter the interior of our homes are relatively dry. Any humidity we have is usually minimal and added by cooking, showers, plants, etc and is adequately controlled by exhaust fans. Compare this with the moisture content in the outside air on a hot humid day here in the summertime, there is simply no comparison. So in reality for many years builders have been doing the exact opposite of what should have been done, by applying the vapor barrier to the interior side of the wall.
So what are your options?
· The best option is to have a wall that air & moisture cannot penetrate to begin with. An example would be SIPS panels or closed cell spray foam, but that is not always practical or possible
· In some instances a vapor barrier is recommended on all sides, but extreme care must be taken to make sure wall cavity is completely sealed.
· Rigid Polyisocyanurate foam board is also a great exterior option to stop air movement thru a wall and continuous exterior insulation is now a code requirement in most areas of the country. This option also stops thermal bridging, which is the transfer of cold air through the framing.
· Use a rain channel behind all siding. There is always a possibility water vapor can get in, so make sure it can get out.
· Unless you have a specific reason for doing so do not ever use poly on the interior and if you must, use a “smart” poly that changes its porosity with humidity levels.
Times have changed, in the early seventies this subject was not even on an engineer’s radar yet. No one talked about air leakage or air barriers because no basic research had been done yet. Energy efficient air conditioned homes has changed that. For those of you that would like further information on this and similar green building topics go to: greentite.com