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Finished basement moisture barrier and insulation

Finished basement moisture barrier and insulation

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  #1  
Old 07-16-15, 08:07 PM
L
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Finished basement moisture barrier and insulation

I am moving into a new house and plan to finish the basement

I know typically you would use rigid foam insulation to provide insulation as well as a moisture barrier

With the cost of xps panels I was wondering if it would be acceptable to install r1 xps paneling against the concrete walls ( the stuff that comes 1/4 inch think in 4ft x 50 ft panels) and then use fiberglass insulation over that.

I figured the xps underpayment would provide a good moisture barrier and the fiberglass would give the proper insulation. And this way seems that it would be more cost effective then using the proper r rated panels alone.

The house is in Massachusetts. It is new construction. It is a walkout basement and I'm building a wall down the center and only finishing half. the rear wall is wood framed and is already insulated so esscentially ill only be insulating against the concrete on two walls, one is completely below grade, the other is partially below.

Thanks for any input
 
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  #2  
Old 07-17-15, 05:58 AM
S
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Welcome to the forums.

The foam is not a vapor barrier (it's a vapor retarder) but you do not want a vapor barrier on an underground wall anyway. That said, you need a minimum thickness of the foam to prevent condensation and that's going to vary based on where you are. Hold tight and Bud will be along at some point with a lot more detail than I can provide.
 
  #3  
Old 07-17-15, 06:14 AM
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Thanks for the intro ss and welcome to the forum lowbird.
Brand new poses a concern as we have no moisture history. IF the builder followed standard construction methods, then we can expect most water has been redirected, but moisture vapor will still migrate from below the footings, through the concrete and try to enter your basement. If you add a vapor barrier to block it, the moisture level behind the vb will slowly rise until it is equally as wet as below the footings. If there is anything that can support mold (mold food) it will grow and although fairly well isolated, I don't like mold, and neither do home owners.

My preference is to use a layer of rigid, not the fan fold, and then add studs with batt insulation, but no vapor barrier. That slow migration of moisture will still move through 1" of pink or blue foam, as long as there is no plastic coating, and dry to the inside. It is a moisture management approach.

Before I continue, how new is the concrete?
Also, are you following Mass building/energy codes? I ask because some will still require a vapor barrier. They also have minimum insulation levels.

Bud
 
  #4  
Old 07-17-15, 06:58 AM
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Thank you for the great info.

The foundation was poured about three months ago, the way the yard Is pitched the water should runaway from the foundation and the house is actually built on a hill and from the rear of the house there's about 25 ft of yard before it drops about 15 ft, so I would imagine there wouldn't be much ground water.


The builder did apply the black spray on coating to the outside of the foundation walls before he back filled.

I do plan to follow mass building code
 
  #5  
Old 07-17-15, 07:39 AM
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The moisture I mentioned doesn't have to be water. The moist soils that grow grass hold enough enough moisture to grow mold.

The spray on tar is old school and has proven to fail over time. It also doesn't address below the footings and below the inside slab. The plastic below that slab helps, but those also get holes during the pour, masons have to walk somewhere.

It isn't as if water is going to pour into your basement. It is a case that 10 years down the road you might get a surprise. I underlined might as there are many basements that do well. But choosing to manage any moisture passing through, you lower that risk.

Link below will discuss some of the methods involved.
BSD-103: Understanding Basements — Building Science Information

Do you know if your location is following the 2009 energy codes or the 2012 version?

Bud
 
  #6  
Old 07-17-15, 06:20 PM
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Typically mass follows the 2009 code book, but insulation varies from town to town, i haven't yet moved into the house I'll have to call the building inspectors office and see what they are following
 
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