# Is Vapor barrier needed?



## ti837 (Feb 6, 2020)

I’m finishing my basement right now and I applied xps foam boards all around the walls prior to framing. I know the foam board is supposed to act as a vapor barrier so does this mean that after I install the fiberglass insulation I don’t have to place the clear plastic vapor barrier?


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## Bud9051 (Nov 11, 2015)

One of the benefits of the rigid foam insulation is that it blocks air from reaching the cold concrete. If you have taped all seams you are all set. Rigid foam is actually a vapor retarder so allows a small amount of drying to the inside, which is good. Unless codes require it ( Canada may) no need to add the plastic layer. In fact it is beneficial to allow that small amount of moisture to continue to the inside where it is easily handled by the conditioning system. Note, it is a very small amount.

Bud


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## Windows on Wash (Aug 30, 2011)

Be sure that you air seal the rim/band joist areas.


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## user_12345a (Nov 23, 2014)

vapor barriers don't belong in basements, they create problems by trapping moisture from the foundation in the wall.

Now, your foam has to be thick enough to prevent condensation when you add batts.

The batts make the foam surface cooler and if you only have like R5 and add R11++ batts, you can run into problems.

You may be able to find the required foam r-value to use batts without the vapor barrier in your area.


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## Windows on Wash (Aug 30, 2011)

Never seen moisture in a wall between a 1" or 2" board if it was well sealed and adhered, but he is right that we kinda need to know where the home is.


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## ti837 (Feb 6, 2020)

Thanks for the reply. So now that I’m not going to be putting a vapor barrier do I put a unfaced or faced fiberglass insulation?


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## Nealtw (Jun 22, 2017)




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## Windows on Wash (Aug 30, 2011)

Where is the home?


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## Nealtw (Jun 22, 2017)

We have success with the wall with out the extra foam behind and like he says in the video, it has to be perfect. 

When you add 2 insulation, what will be the temp where the two meet if the inside face of the foam will be cold then you could get condensation on it. Then it would be important to have a VB or at least make every effort to seal the drywall around the edges and any holes in like electrical use exterior wall boxes.


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## user_12345a (Nov 23, 2014)

BC is not humid unlike a good chunk of north america.

Vapour barriers do not work well in basements in humid climates.

With a decent thermal break that stays above dewpoint, vapour barrier is not helpful and just stays cuts drying potential to the inside.

The break completely eliminates condensation risk if it's of sufficient r-value and done right.

Without the thermal break, the barrier is needed to stop condensation on cold surfaces but doing batts against concrete (or with a small gap) with barrier in a basement is very flawed due to trapping moisture in the wall that comes through the foundation. 
R-value is reduced due to moisture from the foundation being trapped in the wall and contractors doing the work say it's fine, it hasn't rotted out and do the same thing again and again.

To have mold/rot, there has to be a severe moisture problem - everything can look okay when r-value is reduced.

Perfect doesn't matter when you have moisture coming in through the foundation.


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## Nealtw (Jun 22, 2017)

user_12345a said:


> BC is not humid unlike a good chunk of north america.
> 
> Vapour barriers do not work well in basements in humid climates.
> 
> ...


 So you have more humidity which would leave a bigger reason to keep air away from a cold surface. I fail to see how that makes moisture pressure in the foundation higher. Damp proofing and drainage on the outside is the key. The system has been successful here for 30 or 40 years. 



I believe this is filmed in Ontario.


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## ti837 (Feb 6, 2020)

I live in New jersey


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## Nealtw (Jun 22, 2017)

ti837 said:


> I live in New jersey


It never hurts to ask the city what they would like to see.


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## Bud9051 (Nov 11, 2015)

Hi NJ, coast or philly area? Spent a bunch of years in Holmdel.
Here's a link from one of the well respected science groups, even a little humor. I try to avoid one person youtube presentations as the often lack the depth of research and science. Too much hand-me-down information. Just my opinion. This link has plenty of sidebar reading as well.
https://www.buildingscience.com/documents/insights/bsi-073-macbeth-does-vapor-barriers

Bud


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## 123pugsy (Oct 6, 2012)

I had planned to do exactly as you without the VB.
My inspector wouldn't accept it unless I could prove the dew point.??


Spray foam, done. No VB.




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## Windows on Wash (Aug 30, 2011)

You do need a VB in your case. Foam board, well sealed, spray foam the rim joists, and install your framing and batt insulation.


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## ti837 (Feb 6, 2020)

I’ve now watched several articles and videos regarding I insulating the basement. In this area I have to use a vapor barrier in the basement, so that is what I’ll do. Many of the videos I’ve watched puts unfaced insulation and then a plastic vapor barrier over it. What’s the difference of doing that and just using a faced insulation. Obviously using the the faced insulation is easier so can I just do that?


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## 123pugsy (Oct 6, 2012)

https://www.buildingscience.com/documents/digests/bsd-103-understanding-basements




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