I'm finishing my basement. I am wondering about the north wall which is a 2x6 stud wall (walkout basement). I removed the fiberglass insulation an put 2 layers of 3/4" rigid insulation (sealed edges with silicone)... each has a R4. I will then put the fiberglass back. I am wondering about putting the plastic vapor barrier back. I don't khow how permeable the rigid insulation is and I don't want to trap moisture in the wall. It was -10 below last night and I didn't have any condensation on the rigid insulation. ( I haven't put back the fiberglass yet).The rigid insulation sealed up all the leaking seams and eliminated all the drafts.

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  • I would be concerned about whether moisture developed between the two layers of the Rigid before continuing. Feb 10, 2019 at 13:01

4 Answers 4


Foam board, especially the one in your pics, act as a vapor barrier. The only issue with it is when you cut it and install between joists or studs. Then it needs to be taped, caulked, or foamed. You said you caulked them so you have completed your air seal. Adding another vapor barrier will only cause moisture problems and wood rot. I have been insulating homes for over 30 years and have seen the entire sides of homes that have had to be tore out and rebuilt because of the second vapor barrier.


The pink foam board (XPS) is permeable, so it acts as a vapor retarder but not a vapor barrier, unless it's backed with something impermeable. I am not sure what the number for two 3/4" sheets stacked would be but I believe it would be classed "semi-permeable."


A polyethylene vapor barrier should never be placed on a wall. This will trap moisture, which will rot out the wood. Water from poor drainage, flashing issues, and other moisture areas should be your focus. It's more important for a wall to be able to dry itself than to trap water vapor with a plastic barrier.

  • Finally, someone understands vapor barriers and how walls need to dry out. Here’s a link that explains that concept: energyvanguard.com/blog/75130/…
    – Lee Sam
    Feb 10, 2019 at 13:42
  • 1
    Just to clarify...we’re talking about NOT installing a plastic vapor barrier on the INSIDE of a wall. You still need a MOISTURE BARRIER on the exterior of the wall.
    – Lee Sam
    Feb 10, 2019 at 13:47
  • The Minnesota code reads: A vapor retarder and air barrier shall be applied to the warm in winter side of the wall with a permeance of not greater than 1.0 in accordance with ASTM E96 procedure. I am going to skip the poly and apply a coat of vapor-retarder paint to the drywall. Vapor retarder paint is considered a 'Class II vapor retarder” — perm rating between 0.1 perm and 1.0 perm. which meets the code. my take after reading a lot on this, is that air leakage is far more likely to lead to problems than vapor diffusion.
    – CD Bosak
    Feb 11, 2019 at 18:58
  • It's country specific Jerry. There are lots of vapour barriers on walls, on the warm side of the insulation here in Norway. Current trends are to put the barrier 50mm or so into the insulation, with 200mm or so on the outside. This protects the barrier from most fasteners etc.
    – handyman
    Feb 12, 2019 at 22:12

It is a universal construction specification to have: the exterior walls above ground coated in water barrier Tyvek, concrete foundations beneath the ground coated in asphalt, major perimeter drainage around the footing of the home, then the siding, house wrap, sub wall or OSB, then the studs, THEN installing the bats or blown in insulation and THEN a vapor barrier on TOP of the bats. One needs ventilation designed into their foundation and walls. Lots of stuff actually allows some air flow but you have to understand when and where that works, what is allowed in the city or county buildings and code specifications.

For a basement one needs a proper perimeter or foundation drain, perforated pipe sitting just off the footing of the walls, not on the footing. Landscape fabric covering the entire perimeter drain pipe and then drain rock back fill. No water allowed to flow TOWARDS the home's foundation!

Ventilation between crawl space or basement is critical. Attic fans, exhaust fans pull the air through the walls to stop mold and fungus and dry rot. Fans are not necessary if one understands heat rises and cold air comes in at the bottom. Ventilation is oh so important!

Drainage, designed and executed properly for your foundation is the top most important thing you need to do. Otherwise, everything else is either assisting or being a band aid.

The picture shows you are ready to put up the bats of insulation or spray insulation then install plastic as a vapor barrier.

My first worry is your drainage outside of this basement. If there is any indication of moisture on those 2X6's or concrete floor, you have to take care of that problem first. Do you have a low spot in your basement floor that has a DRAIN?

Look at your perimeter of your home closely. ALL slopes have to slope AWAY from your foundation. Check to make sure that the contractor actually installed a proper foundation drainage system. Check to make sure that contractor applied asphalt on the foundation's concrete between the outside of the foundation and between any soil or rock.

Why did you remove the old insulation? Was it wet? Big red flag!

Note: I am very for ventilation! But the entire reason for vapor barriers is condensation. A plastic barrier will attract the condensation AWAY from your siding or interior wall board. Condensation will happen no matter what between two surfaces of different temperatures. A vapor barrier protects your wall board by collecting the condensation versus having the wall board collecting the condensation. The caulking was probably a no no for air flow.

  • The insulation was not wet. There was one area where there was some mold on the insulation up against the plastic- basement cool in the summer and hot & humid outside. I am finishing a unfinished basement and removed the insulation to run wiring. I notice a lot of the insulation was dirty from air flow so I added the ridged to seal it up so that the fiberglass would be more effective. I did put some fiberglass back (as a test) with out a air or vapor barrier (-10 outside) and I did get frost on the rigid. I am going to try it again with an air barrier to see if I still get frost.
    – CD Bosak
    Feb 21, 2019 at 18:15
  • This is a walkout basement wall (2x6 with OSB sheathing - no house wrap- steel siding) - there is a patio on the outside of this wall at about the same elevation as the basement floor. I have good drainage and drain tile on both the inside and outside of the wall with pea rock around the tile. I have never had any water in the sump basket. I have good drainage all around the house. My basement is very dry I even put poly under the concrete floor
    – CD Bosak
    Feb 21, 2019 at 18:22
  • Thank you for this information! I won't be able to get back to this until tomorrow the soonest. A daylight basement, very cool. Do you know if you have a perimeter drain around the foundation? Do you have an 'As Built' in your mortgage paperwork? Property lines and underground utilities, set backs? Any elevation numbers included on a rough plan (the as built) in your paperwork for at least 3 spots? on your property? Receipts for any improvements?
    – stormy
    Feb 22, 2019 at 0:03
  • What is the insulation product you've installed? 3/4 inch with what R value? Looked it up and R5? Pretty Puny. If you are concerned about the exterior foundation walls then you need to dig down and find that perimeter drain; 4" pvc drain pipe covered with landscape fabric installed just off the footing not ON the footing and then back filled with drain rock. All of the exterior concrete wall below any soil should have a nice asphalt coating. Your basement floor should have vapor barrier between the soil and gravel. If there was a water leak in your basement where would that water go?
    – stormy
    Feb 22, 2019 at 0:22
  • The dark area on the wall about 6' from the floor? Are the 2X4s of the floor forms wet in that back corner or is this just photo stuff? You've gotten excellent advice from these other answers! I am kind of nit picky. Disaster lies in the details, grins!
    – stormy
    Feb 22, 2019 at 0:25

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