Poly being applied to Basement walls

[David P. Valley]

Gentlemen,

Why am I seeing so many newly constructed homes with poly (6 mil) applied over framed and insulated (faced and unfaced) stud cavities in basements?

I've called this out many times over and I continue seeing this type of install.

Am I missing something?

[William J. Decker]

Originally Posted by dvalley

Gentlemen,

Why am I seeing so many newly constructed homes with poly (6 mil) applied over framed and insulated (faced and unfaced) stud cavities in basements?

I've called this out many times over and I continue seeing this type of install.

Am I missing something?

Vapor barrier.

[Brian K. Doles]

Originally Posted by wdecker

Vapor barrier.

But shouldn't the vapor barrier be installed between the framed/insulated cavity and the concrete/masonry wall instead of the framing and the sheatrock?

Or is it a good practice to encapsulate the moisture in the cavity and damage the framing and insulation?

[Linas I. Dapkus]

Basement Vapor Barrier
Q. Where does the vapor barrier go when finishing a basement? Since we use a vapor barrier on the inside of exterior above-grade walls to keep moisture in, it’s my contention that a plastic vapor barrier should be used against the wall to keep out the moisture in the earth. Many disagree and say the basement is no different than the upstairs. My feeling is that the earth’s moisture will penetrate the concrete and will condense on, or at least be trapped by, the outside of the interior vapor barrier, thereby lowering the insulation’s R-value and possibly producing mildew in the wall cavity.
A. Stephen Smulski responds: I agree with those who feel that a basement is no different than an above-grade living space: The vapor retarder should be installed on the warm side of the wood frame wall. Sandwiching the vapor retarder between the insulation and the interior finish ensures that the vapor retarder is always warmer than the dew point. As a consequence, moisture can’t condense on it regardless of whether the water comes from inside the basement or from the soil outside the foundation.
The vapor retarder inside the wood frame wall works in conjunction with dampproofing (a bituminous liquid, for example) applied to the exterior of the foundation to keep the wood frame wall dry. By clogging micropores in the surface of concrete and masonry, dampproofing hinders both vapor diffusion and capillary transport of soil moisture through the foundation.
Placing a vapor retarder under the floor slab does the same thing. As an added precaution, you can apply a low permeability coating on the inside of the foundation before building the wood frame wall. This is no different than laying polyethylene sheeting over a concrete floor slab that lacks a vapor retarder before putting down a finish floor.
Keep in mind that basement moisture problems can be largely avoided by installing perimeter drains, by applying dampproofing, by sealing cracks, by backfilling with free-draining soil, by grading soils so that they slope away from the foundation, and by installing gutters and downspouts.
Stephen Smulski is president of Wood Science Specialists of Shutesbury, Mass., a consulting firm specializing in wood performance problems in light-frame structures.

[Brian K. Doles]

I can go with that! Thanks for the info Linas!

[William J. Decker]

What he said.

Remember, different areas of the country have different requirements. Around here, the house wrap is a semi-permiable wrap that will allow water vapor to exit the building, not a non-permiable barrier.

In our area, damp proofing works pretty well. In areas further to the south (even just in southern Illinois) or in areas where the water table is high (like in some of the northern Chicago suburbs, near the lake) water proofing is referred (both modified asphault spray, usually two coats, then installation of a rubber membrane).

Again, different areas have different requirements.

Hope this helps;

[Linas I. Dapkus]

Originally Posted by bdoles2

I can go with that! Thanks for the info Linas!

I subscribed to the JLC a couple of months ago. Great information site.

[Brian K. Doles]

Originally Posted by ldapkus

I subscribed to the JLC a couple of months ago. Great information site.

Yep..full of good stuff. I was a member several years ago. Now just a member of Fine HomeBuilding, another place full of good stuff.

[Brian A. MacNeish]

Originally Posted by ldapkus

Basement Vapor Barrier
Q. Where does the vapor barrier go when finishing a basement? Since we use a vapor barrier on the inside of exterior above-grade walls to keep moisture in, it’s my contention that a plastic vapor barrier should be used against the wall to keep out the moisture in the earth. Many disagree and say the basement is no different than the upstairs. My feeling is that the earth’s moisture will penetrate the concrete and will condense on, or at least be trapped by, the outside of the interior vapor barrier, thereby lowering the insulation’s R-value and possibly producing mildew in the wall cavity.
A. Stephen Smulski responds: I agree with those who feel that a basement is no different than an above-grade living space: The vapor retarder should be installed on the warm side of the wood frame wall. Sandwiching the vapor retarder between the insulation and the interior finish ensures that the vapor retarder is always warmer than the dew point. As a consequence, moisture can’t condense on it regardless of whether the water comes from inside the basement or from the soil outside the foundation.
The vapor retarder inside the wood frame wall works in conjunction with dampproofing (a bituminous liquid, for example) applied to the exterior of the foundation to keep the wood frame wall dry. By clogging micropores in the surface of concrete and masonry, dampproofing hinders both vapor diffusion and capillary transport of soil moisture through the foundation.
Placing a vapor retarder under the floor slab does the same thing. As an added precaution, you can apply a low permeability coating on the inside of the foundation before building the wood frame wall. This is no different than laying polyethylene sheeting over a concrete floor slab that lacks a vapor retarder before putting down a finish floor.
Keep in mind that basement moisture problems can be largely avoided by installing perimeter drains, by applying dampproofing, by sealing cracks, by backfilling with free-draining soil, by grading soils so that they slope away from the foundation, and by installing gutters and downspouts.
Stephen Smulski is president of Wood Science Specialists of Shutesbury, Mass., a consulting firm specializing in wood performance problems in light-frame structures.

Mr. Smulski should have a look at www.buildingscience.com .

BSD-103: Understanding Basements

Basement walls should be insulated with non-water sensitive insulation that prevents interior air from contacting cold basement surfaces – the concrete structural elements and the rim joist framing. The best insulations to use are foam based and should allow the foundation wall assembly to dry inwards. The foam insulation layer should generally be vapor semi impermeable (greater than 0.1 perm), vapor semi permeable (greater than 1.0 perm) or vapor permeable (greater than 10 perm) (Lstiburek, 2004). The greater the permeance the greater the inward drying and therefore the lower the risk of excessive moisture accumulation. However, in cold climates or buildings with high interior relative humidity during cold weather, the upper portion of a basement wall may become cold enough that a vapour permeable insulation will allow a damaging amount of outward diffusion during cold weather. A semi-permeable vapour retarder or foam or a supplemental layer exterior insulation can be used in these situations.

In all cases, a capillary break should be installed on the top of the footing between the footing and the perimeter foundation wall to control “rising damp”. No interior vapor barriers should be installed in order to permit inward drying.

The most cost effective approach involves a combination of rigid insulation and an insulated frame wall assembly (Photograph 4, Figure 10 and Figure 11).


Figure 10: Rigid Insulation/Frame Wall

• Cold concrete foundation wall must be protected from interior moisture-laden air in summer and winter
• Rigid insulation continuous behind wood frame wall
• Rigid insulation is vapor semi-impermeable or vapor semi-permeable (foil facing or plastic facing not present)
• Wood frame wall cavity to be insulated with unfaced fiberglass or damp spray cellulose
• No interior vapor barrier installed

[Ricky K. Kie]

At my parents house I used 6 mil poly on both the front and back of the studs with fiberglass in the middle to stop intrusion, trying to keep the r-factor or the Fiberglass. Also to keep the cost down.

[Linas I. Dapkus]

Mr. Smulski should have a look at www.buildingscience.com

Good point Brian. But who's right and why?? Doesn't it vary by region and other factors?

[Brian A. MacNeish]

Originally Posted by bdoles2

But shouldn't the vapor barrier be installed between the framed/insulated cavity and the concrete/masonry wall instead of the framing and the sheatrock?

Or is it a good practice to encapsulate the moisture in the cavity and damage the framing and insulation?

There is a technique in which 6 mil polyethylene (in this case called a moisture barrier) is installed on the basement wall from grade level to the basement floor. This acts as interior dampproofing since the original exterior applied tar crap does not have much vapour/soil moisture stopping ability after it's a few years old- it has emulsified/dissolved into the soil and you essentially now have a black stain left on your wall which will not stop the slow inward movement of moisture a molecule at a time by diffusion.

After the moisture barrier is installed, the stud walls are built (bring the moisture barrier out under the bottom plate), services and insulation installed. Next, a 6 mil polyethylene air/vapour barrier (AVB) is installed before drywall. This layer has to be truely airtight; if not warm moist house air can get into the cavity where it can condense on the inner side of the moisture barrier (in the wall cavity) to potentially cause rot.

Of course, for this system to work, the initial wood framing must be dry (10-12% or lower) before being closed in or else you trap "construction moisture" in the wall. The system has beeen used 1,000's and 1,000's of times up here but there have been some failures. The system mentioned in the BSC article mentioned elsewhere in this thread is the preferred/better technique but costs more.......funny about that!!!!