Should I leave an opening in my vapor barrier?

I live in mid-Michigan and am finishing my basement. I have poured concrete walls and am building 8 foot sections using 2x4 construction. I am stapling a 6 mil plastic vapor barrier on the back of each section that faces the concrete walls. I am leaving about a 1 inch gap between the concrete and wood walls.
A number of people have told me to not extend the vapor barrier the entire wall height but to apply it to approximately 12 inches short of the floor in order to allow ventillation. Others have said to apply it over the entire height but then to cut random slits in the vapor barrier with a utility knife.
I haven't read anything to confirm these tactics. In fact, I believe I read that in Minnesota anyway, a vapor barrier between the wall and concrete must be installed the complete height from ceiling to floor. No mention of any opening or venting.
Can anyone shed any light on the right way to do it?

Thanks.

 

phil mcc,

It was me who wrote what is done in Minnesota and in fact, is starting to be done here in Michigan.

"I guess this is the best and most economical way to construct walls that would be placed on the exterior. I prefer to see 2x4 but as mentioned by others they can get 2x3's. You still need that W/T plate. Doing the framing 16" O.C. provides a solid base for your 1/2" drywall. If using traditional framing method, frame your new wall 1" from the vertical block/masonry surface if using R-13. The reason to keep the wood out from the walls is the moisture that could damage them. If using insulation like R-19 and only 2x4 studs, the insulation would touch the walls. I have stated before that if a homeowner did put thicker insulation in, and the wall was only 1" from the masonry surface, I have recommended hanging a vapor barrier between the back of the wall and masonry surface. This doesn't allow for the insulation to touch the wall and air movement is not restricted but at least you won't create damage to the insulation or wood.
Vapor barrier should be placed directly under the drywall. The warm inside air containing water vapor can get past the wall finish and insulation and condense inside the colder wall cavity. If enough of this happens, and the water cannot escape, wood rot, mold, and other moisture-related problems are likely to occur. For this reason, building codes often require installing a vapor diffusion retarder on the warmest side of the wall cavity. This is what is required in Minnesota;

"A 4 mill poly vapor barrier must be placed against all concrete or block exterior foundation walls prior to applying furring strips for full height of the wall. Another 4 mill poly vapor barrier must be placed over furring strips and insulation prior to covering with finish materials. (State Energy Code Requirement)" - MINNESOTA CODE

***PLEASE NOTE THAT YOU MUST MAKE SMALL SLICES AT GRADE LEVEL ONLY FOR WEEPING IN THE POLY IF POLY IS PLACED AGAINST THE BLOCK - ATTACH THE POLY WITH STAPLES TO YOUR JOISTS/FOUNDATION SILL PLATE*** "

Any questions, e-mail or call me.

 

Great info. Doug. Thanks.

I have acouple more questions.

I often hear that you should allow the walls to "breathe". With 2 vapor barriers it seems that isn't possible. But you also want to keep the moisture out of the walls which is the reason for the vapor barriers. How do you balance it or is moisture control more important than "breathing"?

Also, if moisture, rot, mold are a concern how do you feel about using steel 2x4's in the basement instead of wood?

Thanks.

 

phil mcc,

Surprisingly enough, the "breathing" issue is that the walls don't really breath in a sense. If they did, without vapor barriers you would be in big trouble. Vapor barriers In every household occupants generate moisture that is carried through the air as water vapor. It is generated by such everyday activities as cooking, cleaning, bathing and laundering. A family of four can generate up to 20 lbs. of water (approx. 2 1/2 gals.) in a 24 hour period. Cooking and dish washing alone give off about 5.7 lbs. per day and showers release about 1/2 lb. of water each.

During the heating season this water vapor moves from the heated interior of a home to the cold exterior. If the passage of water vapor into exterior walls is not blocked or retarded by a vapor retarder (any material that limits or restricts the transmission of water vapor), condensation can occur when the vapor contacts a sufficiently cold surface in the wall cavity. Exterior materials, such as siding that are sealed vapor tight with several coats of paint, can also cause condensation to occur. Continued or prolonged condensation can cause wood rot and growth of mold and mildew.

Several materials can be used as vapor retarders. The most common materials are the vapor retarder facings on faced building insulation. Polyethylene sheeting is commonly used when a VAPOR RETARDER is wanted and is applied over unfaced insulation.

For most of the U.S., vapor retarders should be installed on the warm-in-winter side of the insulation (toward the interior). So to make it simple, think about it this way...for plastic to be an AIR BARRIER it needs to be continuous - it is not.

Please keep in mind that when you finish a basement, that even though you are placing a vapor barrier on the walls, this is not "SEALING" it like a bottle. You still have many areas that air movement is obtained. Electrical boxes, voids between the floor joists and the space between the block walls and your new wall. In order to completely stop air movement, you would have to do alot more than what is suggested here. We are talking vapor not air, so the breathing is not an issue. This will and is done naturally.

With regards to your question on wood or steel, it is a matter of choice but it is important to know what to do in each case. As you already read in the previous post, W/T stock is required by code when in direct contact with concrete/masonry - vertical or horizontal. This is why I am against the 1x2/3 furring strips that are placed directly on walls when they should be W/T plus they also destroy waterproofing sealers because you have to penetrate the concrete/block wall if you really want them to be anchored well. Seems rather riduculous to do furring strips when you consider other items, waterproofing, electrical boxes, hanging of heavy cabinets, etc. Without using W/T stock, the issue of mold/mildew, rot would be an issue. Framing the tradiional wall with 2x stock but with W/T plates is all you need. They will not et destroyed if what I wrote is done properly.

The use of steel versus wood, I have attached a link for your review. I prefer not to use them. If you do however, use a wood treated plate so that you can easily anchor your finish trim. Also frame in your door opening with 2x stock for similar reasons as well as easier to hang pre-hung doors. Wood is better for flexibility and the cost is not much different that steel but overall workability is another issue. Just remember when you want to hang pictures, its with a screw and hanging other items will require the same.

Steel framing has several strengths for home building. It's lightweight, uniform, dimensionally stable, easy to install, cost competitive and resistant to fire and decay. A major drawback--and perhaps a fatal flaw--is thermal performance. While this problem is well known, steel industry boosters often remain silent when the issue of heat loss comes up. A trade journal even stated that a steel-framed wall assembly had a higher R-value than a wood-framed one.

Steel studs offer the possibility of thicker walls and thicker insulation. However, heat loss from a wall assembly is affected by framing as well as insulation. Steel conducts heat over 300 times faster than wood. The picture is far from complete, but current information shows that steel framing has a large negative impact on the energy performance of residential buildings. For comparison, take two walls with 2x8 studs and framing spaced 24 in. on-center. With wood studs, the wall would have an insulating value of R-23.8. This example with steel studs would be R-12.3. (Exterior - Above Grade Use). Basements would be similar to wood.

http://www.buildinggreen.com/feature...l_vs_wood.html

I hope this helping?

 

phil mcc,

I'd like to add this and hopefully it will clarify some things that you might have heard. As we progress with more knowledge on what is right or wrong with old methods, we have improved in the construction of homes verus those long past. The issue of "breathing" that you mentioned might better be addressed by this article that I dug up from my files.

"It seems simple enough. Breathing, how hard can it be? You breathe in. You breathe out. That's also what your house is supposed to do. With a house, we call it infiltration and exfiltration. Open a door or a window and the house breathes in (infiltration). Light a fire in the fireplace, turn up the heat or turn on the dryer and your house breathes out (exfiltration). The trouble is, in winter, we keep the house sealed so tight our homes start gasping for air. In heating contractor talk, we say the house is suffering from negative air pressure.
Before we started sealing our houses to save energy, air came in easily through cracks in walls and loosely fit windows. Now, with everything caulked, fitted and insulated baggie tight, our homes have an increasingly difficult task breathing. Water condenses on windows, air grows stale and polluted and zebra stripes form on walls and ceilings. When the furnace or drier goes on, air is forced out of the house, creating a partial vacuum. The house tries desperately to find "make-up" air to make the internal air pressure equal to the outside air pressure. To do this, air is sucked in through the attic insulation, through the drier vent or down the fireplace chimney. By the time the clean outside air is pulled through the cobwebs, mouse droppings, dusty insulation, carbon encrusted fireplace chimney or lint laden drier vent line, it is filthy. Unfortunately, that is what most of us are breathing in all winter long. Installation of passive air make-up systems is the remedy. Whenever the house "goes negative" they can automatically open a little vent that permits fresh air to come into the house and make up the air pressure deficit.

That is fine as far as it goes, but neither system assures that a continuous supply of fresh air enters the house unless the furnace or some other appliance is exfiltrating old air out of the house.
If you want a continuous supply of fresh air, you have to have a mechanical method of pushing stale air out and pulling fresh air in. As companies become aware of the importance of make-up air, they are developing many techniques to accomplish this purpose.

Ninety Plus efficient furnaces help to prevent negative air because they bring in as much fresh air as they exfiltrate during operation. Heat Recovery Ventilators available from most heating and cooling contractors do the same thing with the additional benefit that they use the heat of the air being exfiltrated to warm the incoming air making the heating system more efficient.

The problem with these two systems is that they only satisfy the furnace's need for air. They do nothing to assure a stream of fresh air for the home's inhabitants or to make up air lost by operation of dryer vents, bathroom and kitchen fans or fireplaces.

Air make-up units called Energy Recovery Ventilator's bring in fresh air continuously. They also dehumidify indoor air when the humidity outside is lower than inside. Some of the most popular ventilators are the Honeywell HR200, (800) 328-5111; the Carrier V-series, (800) 227-7437; Research Products' April Aire 8100, (608) 257-8801; Broan-Nutone Guardian ERV100 HC, (888) 882-7626; and the Stirling Technology Recoupaerator, (800) 535-3448. Broan-Nutone also makes the Guardian Plus, which is a heat recovery ventilator, a whole-house fresh air ventilator and a HEPA stand alone by-pass filter. The combination of the three needed features in one economical package makes the unit increasingly popular.
The term stand alone by-pass filter means that the filter is not attached to the furnace and has its own power, bringing in fresh air while filtering a third or more of all of the the air going through the home's duct work.
The Amaircare, Air Wash System, (877) 839-3036, www.amaircare.com, is another highly efficient stand alone HEPA filtration system that brings in fresh air and operates continuously cleaning the home's air.
If your home has a damp basement, Therma-Stor Products, (800) 533-7533, a Wisconsin manufacturer of high efficiency dehumidifiers has introduced the Ultra-Air A.P.D.. The unit dehumidifies, filters and brings in fresh air. It is so efficient that it virtually eliminates mold, mildew, dust mites and bacteria that need a humidity level of 50 percent or above. As you can see, air make-up units are definitely not one size fits all.

One more thought on this, "Air Exchangers" or a HRV system (Heat Recovery Ventilation) is the best method for a home that is of new constuction. They ensure fresh filtered and moisture controlled air for the occupants. Briefly, an air-exchanger (HRV) brings fresh air into the home exchanging it with stale interior air while capturing 80% of the heat from the outgoing air. They are recommended because a properly built home of any type, will be very tight with minimal air-infiltration. Older homes relied on air leaks to provide fresh air. In addition, the issue of Energy Efficiency was not of any major concern nor addressed as an important issue in todays world. A properly built home will control this with an air-exchanger for much greater efficiency. Basically you can build a house as tight as you want but you must install an air-exchanger to control how the house breathes. The old adage that a house has to breath is a correct one, but to do it efficiently and mechanically, not unevenly with leaks."

Hope all this is more understandable in the complete scope of things mentioned before.