Two layers of vapor barrier???

[mherm]

I am in the midst of remodeling a basement bedroom with ensuite bath and have run into a bit of a conundrum. The following is a quotation from a handout provided by the city building department in Andover, MN.

"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)"

This seems to be a recipe for moisture entrapment that must almost certainly result in mold issues sooner or later. Does anybody out there have a rational explanation for this requirement?

[concretemasonry]

The first and most important question is - Are finishing the entire basement or just a portion. If not all, are you separating the finished and unfinished areas by insulation and vapor barriers?

A little strange, but may have some merits - A 4 mil poly is really not a vapor barrier by most definitions. Usually, you have to have 6 mil to be classified as a "vapor barrier". Poly is not a vapor barrier, but a vapor retarder and it is a variable depending on the perm rating (usually relative to the thickness).

If they are talking about rigid insulation, it may be OK depending on the amount of insulation required. If fiberglass, it could be a problem.

The prescriptive standard is unusual that looks unique. IS THERE ANY OPTIONAL METHOD? My son did not have to do anything like that in Andover.

The Minnesota energy and structural codes have gotten pretty strange lately and there are some obvious errors in the new recommendations that the new bureaucrats and politicians will have to sort out. Maybe the governor will veto the politicians building code changes.

Most people over-do the vapor situation and try to take a good method from one area to another where it will not work. I hope you do not have to over-insulate. - That is one think that was never enforced previously because of common sense.

Dick

[mherm]

The basement is entirely finished now and I have demo'ed a portion of it for the remodel. The stuff I took out had no vapor barrier at all on the block walls. I believe the wood frame portion of the walkout is a standard fiberglass with vapor barrier but am not anxious to tear out the sheetrock to verify that.
My plan on these block walls is to use inch and a half polystyrene insulation and frame it up with two by twos.
I guess as long as I use the two layers of poly, I should be okay with the code and the polystyrene will sure be better than fiberglass, but whether you want to consider 4 mils a vapor barrier or vapor retarder, whatever moisture gets in between the layers is going to be a long time getting out!

[concretemasonry]

Since you are not changing the use of the room or the exterior, you should decide whether you need a permit to change the wall surfaces. If you do not to be concerned with the prescriptive code requirements, you could do what would be appropriate for for your specific situation.

The 1 1/2" of extruded polystyrene is certainly adequate and putting in one layer of the "vapor barrier" would be a improvement over what the original construction was.

[berrywise]

Here is a great link I found while doing research on this very topic.

http://www.buildingfoundation.umn.edu/

I plan on using polystyrene against the concrete, tapes at the seams, then a 2x4 wall with unfaced insulation and then 1/2" drywall on top of that. Using some spray foam and calk to to take care of the top where the ceiling meets the wall. As far as I can tell my codes in St. Paul are pretty non-descript and my calls to three different building inspectors in my area gave me three different answers.